Hypervalent chromium oxidation of carbohydrates of biological ...

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2011 Simplex Academic Publishers. All rights reserved. Hypervalent chromium oxidation of carbohydrates of biological importance. Luis F. Sala*, Juan C.
GLOBAL JOURNAL OF INORGANIC CHEMISTRY

Hypervalent chromium oxidation of carbohydrates of biological importance Luis F. Sala*, Juan C. González, Silvia I. García, María I. Frascaroli, María F. Mangiameli Área Química General, Departamento de Químico Física, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Instituto de Química de Rosario (IQUIR-CONICET), Suipacha 531 (2000) Rosario, Argentina *

Author for correspondence: Luis F. Sala, e-mail: [email protected] Received 28 Jul 2010; Accepted 14 Sep 2010; Available Online 11 Nov 2010

Abstract A revision of the kinetic and mechanistic results of the interaction of carbohydrates of biological importance with hypervalent chromium done by the Bioinorganic Chemistry Research Group at the General Chemistry Area Department of the Biochemistry and Pharmaceutical Science Faculty at the National University at Rosario, during the last ten years is reported. Keywords: Hypervalent chromium oxidation; Uronic acids; Monosaccharides; Polysaccharides

1. Introduction Compounds of CrVI represent a potential environmental hazard because of their mammalian toxicity and carcinogenicity [1-4]. The observation of CrV and CrIV intermediates in the selective oxidation of organic substrates by CrVI and their implication in the mechanism of Cr-induced cancers [1,5-7] has generated a considerable amount of interest in the chemistry and biochemistry of this element [8-13]. We are investigating the possible fate of CrVI and CrV in biological systems by examining reactions of CrVI with low-molecular-weight neutral [14-26] and acid saccharides [27-29]. The present review deals with the chromic oxidation of acid and neutral substituted monosaccharides and polysaccharides. 2. Kinetics and mechanism of chromic oxidation of uronic acids (URO) Naturally occurring acid saccharides are suitable ligands for stabilization of CrV, since they possess the 2-hydroxycarboxylato and vicdiolato sites for potential chelation of CrV, carcinogenic agent [27-29]. URO are essentials saccharides in vegetal and animal physiology, being one of the metabolites involved in the formation of polygalacturonic acid present in

pectins (D-galacturonic acid moiety: GALUR) of the plant kingdom [30] and in polyglucuronic acid present in hyaluronic acid (D-glucuronic acid moiety: GLUCUR) (Scheme 1). In the animal physiology, URO are vitals to the elimination of xenobiotics substances [31]. The determination of the ability of URO to reduce or stabilize high oxidation states of chromium will contribute to unravel its potential role in the biochemistry of this metal. It is then necessary to determine the ability of URO to bind and reduce CrVI / CrIV / CrV under conditions where interference from other reaction products is negligible as well as to assign the coordination modes of URO in the CrV species formed in solution. In vitro studies on the chemistry of CrV complexes can provide information on the nature of the species that are likely to be stabilized in vivo. In particular, the EPR pattern of URO-CrV species can be used as a “finger print” to identify CrV complexes formed in biological systems [32,33]. No previous, full report of chromic oxidation of URO was reported on literature. The study of URO helped us to understand the role played by the carboxylic and hemiacetalic groups on the oxidative process. The oxidation of URO by CrVI yielded D-sacaric acid (SACAR) when a 15-fold or higher excess of sugar over CrVI was used [32,33] (Scheme 2).

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY HO 2C

HO2C O

O

HO

OH HO

HO

OH

OH HO

GALUR

OH

GLUCUR

Scheme 1. Structural formula for GALUR and GLUCUR O

HO 2C

OH OH

O 3

HO

OH

+

2

Cr

VI

OH 3

HO

+

2

Cr

III

OH OH O HO

OH

URO

SACAR

Scheme 2. Stoichiometry for chromium oxidation for URO

The kinetics of the redox processes involving the CrVI / CrIII, CrV / CrIII and CrIV / CrIII couples with URO were determined and a general mechanism was proposed for the chromic oxidation of URO. The presence of CrIV as an intermediate of the redox reaction has been proposed on the basis of experimental results obtained through the oxidation of URO with CrVI and detecting the presence of CrII through the formation of superoxoCrIII ion (CrO22+). It is known that CrIV oxidizes alcohols as a twoelectron oxidant to yield CrII and the oxidized organic product. The fact that CrII is involved in the oxidation mechanism of several alcohols by CrIV and CrVI in HClO4 acid medium, was demonstrated by conversion to CrO22+ upon reaction with dioxygen [34-38]. In appropriate experimental conditions, such as high [O2] –1.26 mM- and low [CrVI] –6.5 x 10−4 mM- the reaction of CrII (if any) with O2 to give CrO22+ can compete successfully with: a) the reaction of CrII with CrVI and b) the autocatalytic consumption of CrO22+ by CrII. If CrII is an intermediate specie in the redox reaction, CrO22+ should be detected [35,39]. A periodic scanning of the O2-saturated solution (1.26 mM) of the URO/CrVI reaction mixture in 1.0 M HClO4 showed two absorptions bands at 290 and 247 nm characteristic of CrO22+ (Figure 1). These results indicated that CrII forms in the URO/CrVI reaction and can be taken as evidence that CrIV is involved in the redox mechanism of the reaction between URO and CrVI. Inset Figure 1 shows evolution of [CrO22+] with time. Absorbance values were calculated in the following way: Abs (CrO22+) = Abs247 – Abs350 x (1) −1 x (2) where 1 and 2 represent the molar absorption coefficient of CrVI at 350 nm and 247 nm respectively. In these experimental conditions, 1 = 1550 M−1cm−1 and 2 = 1900

M−1cm−1. Taking into account that the molar absorption of CrO22+ at 247 nm [35] is 7000 M−1cm−1, the maximum concentration of CrO22+ (tmax = 26.0 min) was 4.7 x 10−4 mM (yield 72%). When the absorbance at 350 nm was negligible, 0.150 mM of Fe2+ was added to bring about this reaction: CrO22+ (ac)+ 3 Fe2+ (ac)+ 4 H+ (ac) (ac) + 3 Fe3+ (ac) + 2 H2O (l)

Cr3+

The spectrum of the reaction mixture was recorded every 2 min. Each spectrum was subtracted from the one prior to Fe2+ addition. What remains is a difference spectrum between the final solution and that after the addition of Fe2+. As shown in Figure 2, there is a negative absorbance difference around 290 nm, consistent with the presence of CrO22+. The redox reaction of intermediate CrIV with the sugar compound has been considered always a fast reaction, which has to be confirmed in order to establish a true mechanism. The reaction of URO with CrO2+ (CrIV generated in situ) in acid media and O2saturated solutions was followed by CrO22+ formation at 290 nm. UV-Vis spectra of mixtures URO/CrO2+ confirmed that the increase of the absorbance at 290 nm was the result of the increment in the [CrO22+]. The spectra obtained revealed max at 247 and 290 nm and Abs247/Abs290 ratio was 2.2, characteristic of CrO22+. At the end of the reaction URO/CrIV, Fe2+ was added and the spectra showed a negative absorbance difference at 290 nm confirming the identity of the CrO22+. The monotonic increase of absorbance was found to follow first-order kinetics. The experimental rate constants, k4exp, were calculated by nonlinear least-square fit of absorbance-time data using Equation 1:

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

[CrO 22+] (mM)

0.0005

0.03

0.02

0.0004

0.0003

0.0002

Abs

0

15

30

45

60

75

90

105

120

t (min) 0.01

0.00 250

300

350

400

450

500

 (nm)

Figure 1. CrO22+ formation from the reaction between URO and CrVI. [URO] = 0.13 M, [HClO4] = 0.1 M, [O2] = 1.26 mM, [CrVI] = 6.5 x 10−4 mM, I = 1.0 M, T = 25°C an 10.0 cm quarz cell. Spectra 1-4 and 5-15 recorded every 2.0 and 10 min respectively. Inset Evolution of [CrO22+] with time.

0.04

0.05

0.03

0.02

 Abs

0.04

(b) 0.03

Abs

0.00

-0.01 250

0.02

0.01

0.01

300

350

400

 (nm)

(a)

0.00 300

350

400

 (nm)

Figure 2. Difference spectra from the reaction of 0.13 M URO with 6.5 x 10–4 mM CrVI and 150 M Fe2+ at [HClO4] = 1.0 M. I =1.0 M, [O2] = 1.26 mM and T = 25°C. The Fe2+ was added after reaction URO/CrVI was complete as a regent for CrO22+ (a) spectrum before the addition of Fe2+ (b) spectra after the addition of Fe2+. Inset: the spectral changes shown represent the absorbances differences (Abs) before and immediately after the addition of Fe2+. Global Journal of Inorganic Chemistry | Volume 2 | Issue 1 | February 2011

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY Abst = Abs + (Abs0 - Abs) e(−k4exp t) (1) where Abs and Abs0 means absorbance at infinite time and initial absorbance respectively. Figure 3 shows spectral scanning of mixtures URO/CrIV and the right fitting of the absorbance data vs. time (at 290 nm) employing Equation 1. It is important to notice that – in eq. 1 – CrO22+ is considered as a final redox product: in our experimental conditions CrO22+ forms very fast and then decays slowly ( 20 times slower than the URO/CrIV reaction) to final CrIII through a URO-independent pathway. At T, [H+] and I fixed, the variation of [URO] between 1.0-2.5 mM did not affect the decay rate of CrO22+ indicating that URO does not react with CrO22+. An independently prepared solution [40] of CrO22+ decays at the same rate as CrO22+ formed in the URO/CrIV reaction if experimental conditions are the same. Table 1 summarizes values of k4exp for various concentrations of URO in HClO4. Chosen experimental conditions allowed the URO/CrIV reaction to compete successfully with CrIV disproportionate into CrVI and CrIII. In the absence of URO or its concentration was too low, disproportion of CrIV was evident by the appearance of typical CrVI spectra which has a characteristic band at 350 nm (Figure 4). Disproportion should be avoided because the CrVI formed absorbs at 290 nm. CrVI was not detected using concentrations of URO shown in Table 1. As represented in the Figure 5, plots of k4exp vs. [URO], when URO was GLUCUR gave good straight lines from which slopes values of k4H were determined (Equation 2). The bimolecular rate constant, k4H, varied linearly with [H+]−1 with a positive interception k´ = 47.9  2.7 s−1M−1 and slope k´´ = 11.5  0.5 s−1 (Equation 3) (Inset Figure 5). k4exp = k4H [URO] k4H = k´+ k´´ [H+]−1

(2) (3)

absorbance decay values. Considering the CrV absorption superimposition, the absorbance at 350 nm, at any time during the redox reaction, is given by Equation 5: Abs350 = VI [CrVI] + V [CrV]

(5)

Combining Equation 5 with rate expressions derived from Scheme 3 yields: Abs350 = Abs0 e−2k6t (6) + k6 V [CrVI]0 (e−k5t – e−2k6t) / (2k6 – k5) In Equation 6, V refers to the molar absorptivity of oxo-CrV-URO at 350 nm. Parameters k6 and k5 refer to the rate of disappearance of CrVI and CrV, respectively. It must be noted that in eq. 6, k6 appears in the numerator of the pre-exponential term and 2k6 appears in the denominator and in the exponential terms because, according to the proposed reaction Scheme 3, only half of the CrVI reaches CrIII through a CrV intermediate. In the range of protons employed in this work, plots of k6 vs. [URO], when URO was GLUCUR, gave good straight lines from which values of k6H were determined (Figure 6 and Equation 7). k6 = k6H [S]

(7)

The bimolecular rate constant, k6H, varied with the [H+] with quadratic dependence as shown in inset Figure 6, according to Equation 8. k6H = kS6 [H+]2

(8)

k6 = kS6 [H+]2 [S]

(9)

where kS6 = (1.52  0.15) x 10−2 s−1M−3 At [H+] constant, plots of k5 vs [URO], when URO was GLUCUR, exhibited a linear dependence on [URO] with positive intercept, Equation 10 and Figure 7, from which the bimolecular rate constants, k5H, were calculated. Plot of k5H vs. [H+] revealed a linear dependence (Equation 11 and inset Figure 7).

The complete rate laws for the CrIV disappearance are given by Equation 4:

k5 = kH0 + k5H [S]

(10)

k4exp = (k´+ k´´ [H+]−1) [URO]

k5H = kS5 [H+]

(11)

k5 = k5H + kS5[H+] [S]

(12)

(4)

Absorbance curves vs. time at 350 nm of the URO/CrVI mixtures exhibited a monotonic decrease which cannot be described by a single exponential decay. These kinetics profiles were appropriately described by the set of consecutive first-order reactions of Scheme 3. It is known that CrV species absorb strongly at 350 nm and may superimpose CrVI absorbance yielding the wrong interpretation of spectrophotometric

where kH0 = (6.4  0.6) x 10−4 s−1 and kS5 = (1.07  0.11) x 10−2 s−1M−2. (9) The rate constants for the CrVI and CrV disappearance are then given by Equation 9 and 12 respectively.

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

0.90

0.36

247 nm

Abs at 290 nm

0.32

0.75

0.60

0.28

0.24

Abs

290 nm 0.20

0.45

0

10

20

30

40

t (s) 0.30

0.15

0.00 240

280

320

360

400

 (nm)

Figure 3. Formation of CrO22+ from the reaction 1.0 mM URO, [H+] = 0.30 M, [O2] = 1.26 mM, I = 1.0 M, T = 15°C, [CrIV] = 0.07 mM. Inset: absorbance at 290 vs. time. Fitted lines were calculated using Equation 1.

Table 1. Observed pseudo-first-order rate constants (k4exp) for different [HClO4] and [URO].a 103 x [URO] (mM)

1.00

1.26

2.00

2.50

10 x k4expb (s1)

[HClO4] (M)

a

1.50

0.1

1.7  0.08

2.0  0.10

2.4  0.12

3.3  0.16

3.9  0.04

0.15

1.4  0.07

1.6  0.08

2.1  0.10

2.6  0.13

3.4  0.04

0.20

1.1  0.05

1.3  0.06

1.6  0.08

2.0  0.10

2.6  0.04

0.40

0.85  0.04

1.0  0.05

1.2  0.06

1.5  0.07

1.9  0.04

0.60

0.70  0.03

0.83  0.04

1.0  0.05

1.3  0.06

1.7  0.04

0.80

0.65  0.03

0.81  0.04

0.95  0.05

1.2  0.04

1.5  0.04

T = 15°C, [CrIV]0= 0.07 mM, I = 1.0 M. Mean values from multiple determinations

b

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

0.4

Abs

0.3

0.2

0.1

0.0 200

250

300

350

400

 (nm) Figure 4. Disproportion of CrIV in absence of URO, T = 15°C, I = 1.0 M and [H+] = 0.20 M, [CrIV] = 0.07 mM

150

0.60

-1

-1

k4H (s M )

120

0.45

90 60

(a)

30

(b)

-1

k4exp (s )

0 0

2

4

+ -1 6 -1

8

10

[H ] (M )

0.30

(c) (d) (e) (f)

0.15

0.00 0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

[GLUCUR] (mM) Figure 5. Effect of [URO = GLUCUR] on k4exp at 15°C, I = 1.0 M and [H+] (a) 0.10, (b) 0.15, (c) 0.20, (d) 0.40, (e) 0.60 and (f) 0.80 M. Inset linear dependence of k4H on [H+]−1.

2 Cr

VI

V

Cr + CrIII

Cr

III

Scheme 3. Sequential reduction of Cr (VI) to Cr(III).

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0.016

0.008 k6H (s -1 M-1)

0.012

0.006

0.008

(a) 0.004

-1

k6 (s )

0.000 0.0

0.2

0.4

0.6

0.8

1.0

+

[H ] (M)

0.004

(b)

(c) 0.002

(d) (e)

0.000 0.0

0.1

0.2

0.3

0.4

[GLUCUR] (M)

Figure 6. Effect of [Glucur] on k6 at 33ºC, I = 1.0 M and [H+] (a) 0.96, (b) 0.80, (c) 0.60, (d) 0.40, (e) 0.20 M. Inset: dependence of k6H on [H+].

0.012

0.009

k5H (s -1 M-1)

0.008

0.006

0.006

0.003

(a) -1

k5 (s )

0.000 0.0

0.2

0.4

0.6

0.8

1.0

(b)

+

0.004

[H ] (M)

(c) (d) 0.002

(e)

0.000 0.0

0.1

0.2

0.3

0.4

[GLUCUR] (M)

Figure 7. Effect of [Glucur] on k5 at 33ºC, I = 1.0 M and [H+] (a) 0.96, (b) 0.80, (c) 0.60, (d) 0.40, (e) 0.20 M. Inset: dependence of k5H on [H+].

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY The reduction of CrVI by URO is strongly dependent on pH. The redox reaction is fast when [H+] > 0.1 M but is very slow at pH > 1. The time-dependent UV/Vis spectra of the reaction mixture showed that the absorbance at 350 nm and 420-470 nm decreased with time, while absorbance at 570 nm increased, without an isosbestic point. The lack of an isosbestic point indicated that there are two or more competing reactions at any time and that in the reduction of CrVI to CrIII intermediate chromium species are present in appreciable concentrations. Formation of CrV and/or CrIV intermediates by the redox reaction of CrVI with the substrate has been observed previously for a number of substrates [41]. The fact that CrO22+ was detected in the reaction of URO with CrVI, coupled with the observation of relatively longlived oxo-CrV species in the EPR experiments, and the successful trapping of organic radicals using acrylamide, indicated that both CrIV and CrV intermediate species were formed in the reaction URO/CrVI. Besides, the CrV-IV detected intermediates suggested that the reaction URO/CrVI occurred through both one and two electron pathways. However, under conditions employed in the kinetics measurements, the reaction between CrIV and URO was very fast as demonstrated previously. Using the experimental rate laws, eqs. 4 and 12, the CrIV reaction with URO (at 15°C) was, at least, 3.1 x 103 times higher than CrV reaction at 33 °C. Therefore, CrIV should be involved in fast steps and does not accumulate in the mixtures URO/CrVI. The simulated profiles of the Cr species concentration indicated that the maximum and minimum [CrV] was 26% (tmax = 260 s) and 7.0% (tmax = 2000 s) respectively. The time dependence of the reaction absorption data at several wavelengths can be fitted with the sequence proposed in Scheme 3. By taking account of a) the absence of an isosbestic point in URO/CrVI mixtures, b) the detection and the yielding of CrO22+ in mixtures URO/CrVI, c) the detection and characterization of oxo-CrV species with URO by EPR, d) the positive test of organic radicals by polymerization of acrylamide, e) the kinetic results of URO redox reaction with CrVI, CrV and CrIV, f) the detection of an intermediate CrVI ester at high pH, and g) the formation of SACAR as the only organic reaction product, we are now able to propose a possible mechanism, Scheme 4, which combines CrVI  CrIV  CrII and CrVI  CrIV  CrIII pathways for the reaction of URO with CrVI. In the [H+] range under study, CrVI 

exists [42] as HCrO4 and this species is proposed as the reactive form of CrVI, which is also in agreement with the first order

dependence of the reaction rate on [CrVI]. It is known that oxidation of alcohols, glycols dicarboxylic and URO by CrVI is preceded by the formation of a chromate ester [32,33,43,44]. The observation of the absorbance bands characteristic of chromate oxy-esters around 377 nm a few minutes after mixing URO and CrVI under conditions where the redox reaction is slow, indicates that at least three intermediate CrVI complexes are formed rapidly prior to the redox steps (Figure 8). Thus, the first step of the mechanism proposed in Scheme 4 involved the formation of URO-CrVI monochelates with URO acting as a bidentate ligand, which is also consistent with the first order dependence of the reaction on [CrVI]. Several co-ordination modes are possible for the CrVI-URO species. Since the oxidation of URO occurs only on the anomeric hydroxyl group, the complex with the anomeric hydroxyl group bound to CrVI is the only redox active intermediate. The slow step proposed in Scheme 4 for the CrVI consumption involves the intramolecular two-electron transfer within the active CrVI-URO species to yield CrIV and SACAR. The formation of the chromate ester is followed by the conversion of intermediate SCrVI into CrIV and oxidized substrate. This slow step is proposed to involve a two electrons intramolecular transfer to yield CrIV and SACAR (detected by HPLC) and requires two protons, so that the redox reaction URO/CrVI should be favored in acid medium, such as observed. The rate law for the CrVI consumption derived from eqs. 13-14 in Scheme 4 is given by Equation 21, where [CrVI]T refers to the total [CrVI] in the reaction mixture. -d[CrVI]/dt = k6 KVI [H+]2 [URO] [CrVI]T / (1 + KVI [URO])

(21)

If KVI [URO] 3 the redox reaction between CrV with Glc3Me becomes slow, CrV species remain in solution for longer periods of time and

the shf pattern of the CrV EPR signals can be resolved using a modulation amplitude lower than the 1H shf splitting. With this in mind, CrVGlc3Me complexes formed upon addition of 100-times molar excess of Glc3Me to CrV generated in either the one-electron reduction of CrVI by glutathione or the ligand-exchange reaction of [CrVO(ehba)2]− were investigated in the 5.5-7.5 pH range by EPR spectroscopy. The CrV EPR signals are more intense at pH 5.5 than at pH 7.5, probably because CrV disproportionation is slower at the lower pH [48]. In this pH range, the EPR spectra taken at the beginning of the reaction were dominated by a triplet at giso1 1.9792 (aH = 1.0 x 10−4 cm−1), with a minor component at giso2 1.9794 (quintet, aH = 0.9 x 10−4 cm−1). Fifteen minutes after mixing, the two components at giso1 and giso2 were present in 71% and 29%, respectively (Figure 13a). It is known that the EPR spectrum of a CrV-diolato species of six-membered ring cis-diols yields a doublet, since only one proton is in the plane of the unpaired electron density of the CrV ion. Consequently, the EPR spectral multiplicity of bis-chelate CrV-diolato2 species formed between CrV and pyranosic cis-diols exhibit a triplet with two (one from each chelate ring) carbinolic protons coupled to the CrV electronic spin. In the present case, the major component of the EPR signal can be attributed to the bis-chelate [Cr(O)(cis-O1,O2-Glc3Me2]− (VI, Figure 12). This result is in line with the reported higher ability of the cis- vs. the trans-diolato for binding CrV [2,14,46]. The minor component at giso2 1.9794 can be attributed to the bis-chelate formed with the pyranose and furanose forms of Glc3Me: [Cr(O)(cis-O1,O2-Glc3Me)(O5,O6Glc3Me-furanose)]− (VII, Figure 12), with four protons coupled to the electronic spin of CrV. The assignment of the minor component to was [Cr(O)(cis-O1,O2-Glc3Me-furanose)2]− disregarded based on the aH values  aH values expected for CrV-diolato2 species of fivemembered ring cis-diols are lower than found here [49,50]. With time, the EPR spectral pattern

OH HOH2C 3

HOH2C

O

HO MeO

+ 2 Cr OH Glc3Me

OH

VI

O

3

OMe

+ 3 HCO2H + 2 Cr

III

HO Ara2Me

Scheme 6. Stoichiometry for Chromium oxidation for Glc3Me.

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

VI

Cr

0.09

6'

0.06

Abs

15'

30'

0.03

45' 75' 2h

2+

CrO2

0.00 200

3h

300

400

500

 (nm)

Figure 10. Formation of CrO22+ (max 293, 247 nm) from the reaction between 0.21 M Glc3Me, 1.26 mM O2 and 4.9 x 10−2 mM CrVI, in 0.84 M HClO4. T = 22°C.

Peak-to-peak height (mm)

75

60

45

30

15 3500 3520 3540 3560 G

0 0

50

100

150

200

250

t (min)

Figure 11. Peak-to-peak heights of CrV EPR signals vs. time. [CrVI] = 0.025 M, [Glc3Me] = 1.225 M, [H+] = 0.3 M, I = 1.02 M, T = 20°C. Inset: time evolution of X-band EPR spectra from the reaction mixture in 0.3 M HClO4.  = 9.766887, mod. ampl. = 4 G. HOH2C

+

O O w Cr O w w

O

OH

HOH2C

O O O Cr O O

O

HO

O

-

OH

OMe

OMe

CH2OH

OMe VI

V

HOH2C

-

O O O Cr O O

O

HO

O

OH

OMe MeO

OH

VII

O

HO

HO

-

O O O Cr O O

O

MeO

OMe

OH

OH

VIII

Figure 12. Structures of CrV complexes formed with Glc3Me. Global Journal of Inorganic Chemistry | Volume 2 | Issue 1 | February 2011

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

a

3495

3500

G

3505

3510

3515

b

3500

3505

3510

G

3515

3520

3525

Figure 13. Experimental (▬) and simulated (▬) X-band EPR spectra from mixtures of 1 mM glutathione + 1mM CrVI and 100 mM Glc3Me, pH 5.5; taken (a) 15 min ( = 9.705912 GHz) and (b) 1 h ( = 9.728655) after mixing. Mod. ampl. = 0.2 G. T = 20°C.

slowly changed, the proportion of the components varied and a third component appeared. The EPR spectrum taken 1 h after mixing, shown in Figure 13b, could be deconvoluted into a triplet at giso1 1.9792 (aH = 1.0 x 10−4 cm−1), a quintet at giso2 1.9794 (aH = 0.9 x 10−4 cm−1) and a septuplet at giso3 1.9793 (aH = 1.0 x 10−4 cm−1), in 6:3:1 ratio. Other minor species were also present, but they were not included in the simulations to avoid overparameterization. Based on the shf coupling, the three components could correspond to [Cr(O)(cisO1,O2-Glc3Me2]− (VI), [Cr(O)(cis-O1,O25 6 Glc3Me)(O ,O -Glc3Me-furanose)]− (VII) and (VIII) [Cr(O)(cis-O5,O6-Glc3Me-furanose)2]− (Figure 12). At longer times, the proportion of the species at giso1 decreased and the spectra were dominated by species at giso2 and giso3. These five-coordinate CrV bis-chelates were still observed 15 h after mixing. Thus, initially, CrV bis-chelate is formed with CrV bound to the 1,2cis-diolato moiety of the pyranose form of Glc3Me (kinetic control); but, with time, it transforms into bis-chelates with CrV bound to the 5,6-vic-diolato moiety of the furanose form of the ligand. The kinetic profiles for the growth and decay of the CrV EPR signal peak-to-peak height vs. time could be adequately fitted with the rate expression derived from a two-step reaction

sequence involving two consecutive first-order reactions (Scheme 3 and Equation 23) [11,44]. EPRarea = A ka {exp(- 2 ka t) - exp(- kb t)}/(kb – 2 ka)

(23)

where A depends on the spectrometer settings, and ka and kb refer to the rate of disappearance of CrVI and CrV, respectively, and were evaluated from a non-linear least-squares fit of eq. 23. In the 0.3 to 0.5 M HClO4 range, the calculated values of ka were more than tentimes lower than kb. For conditions in Figure 11, ka = 0.00496 min−1 and kb = 0.1224 min−1. The fact that kb > ka implies that the slow redox step involves the reduction of CrVI. The simulation of the kinetic profiles employing ka and kb values obtained from EPR data in 0.3 M HClO4 (Figure 14), shows that the maximal [CrV] represents 3% of the total Cr in solution. These results confirm that [CrV] is low throughout the reaction and should not interfere in absorbance at 350 nm, which essentially reflects changes in [CrVI]. Rate constants calculated by this technique were consistent with those obtained from the spectrophotometric measurements at 350 nm, under the same experimental conditions. For 1.225 M Glc3Me and 0.3 M HClO4, the value determined spectrophotometrically for kexp was 1.90 x 10−4 s−1, that compares well with 2ka = 1.65 x 10−4 s−1 obtained from EPR measurements.

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

0.024

0.018

[Cr] (M)

V

Cr VI Cr III Cr

0.012

0.006

0.000 0

50

100

150

200

250

t (min)

Figure 14. Simulated kinetic profiles for Cr species. [Cr] calculated using ka = 0.00496 min−1 and kb = 0.1224 min−1 obtained from EPR data in 0.3 M HClO4. [Cr]T = 0.025 M.

Sequential absorption spectra of the reaction mixture showed the appearance and growth of two d-d bands at max = 409 nm and 572 nm (Figure 15). Kinetics traces at 570 nm showed that at this wavelength, the absorbance grew to intensities higher than expected for the free Cr3+ and then slowly decayed to the value corresponding to [Cr(H2O)6]3+ (409 nm,  = 21.1 M−1cm−1 and 572 nm,  = 17.3) [51], as shown in the inset of Figure 15. This behavior suggests the formation of an intermediate CrIII-Glc3Me complex, which then hydrolyses to the final product [47, 52]. EPR spectra taken at the end of the reaction confirm that Cr3+ is the final Cr species in the reaction of CrVI with excess Glc3Me in acid medium. In order to determine the rate of CrIII formation relative to that of the CrVI consumption, the formation of CrIII was followed at 570 nm, in the presence of excess of Glc3Me

in the 0.40–1.02 M HClO4 concentration range. Under these conditions, the first order dependence of the rate upon [CrVI] was verified. For [Glc3Me]0 = 0.4 M, values of kexp calculated at this wavelength were plotted against [H+] (Figure 16). The rate constant calculated by nonlinear least-square fit of kexp vs. [H+] data pairs −4 −2 −1 was found to be 25.4 x 10 M s , which is the same as that calculated from data at 350 nm. This indicates that the rate of formation of CrIII equals the rate of consumption of CrVI and that the slow redox path effectively implies the reduction of CrVI, which should react slower than CrV does. This is also consistent with the observation of an isosbestic point at 527 nm in Figure 15 and implies a low CrV concentration throughout the reaction in the [H+] range used in the kinetics studies. In the range of substrate and acid concentration used in the kinetics measurements,

570

0.16 Abs

1.5

0.12 0.08 0.04 0

6000 12000 t (min)

Abs

1.0

0.5

0.0 300

400

500

600

700

 (nm)

Figure 15. Time evolution of the UV-Vis spectra of a mixture of 0.4 M Glc3Me and 0.01 M CrVI over a period of 16 h. [HClO4] = 1.02 M, T = 33°C. Inset: growth and slow decay of Abs570 over a period of 9 days Global Journal of Inorganic Chemistry | Volume 2 | Issue 1 | February 2011

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0.003

kexp

570

-1

(s )

0.002

0.001

0.000 0.00

0.25

0.50

0.75

1.00

+

[H ] (M)

Figure 16. Effect of acidity on kexp calculated from absorbance data at 570 nm. [CrVI] = 8 x 10−3 M, [Glc3Me] = 0.4 M, I = 1.02 M, T = 33ºC.

the oxidation of Glc3Me by CrVI is a complex reaction that yields CrIII, HCO2H and Ara2Me as final redox products. The fact that CrO22+ is detected in the reaction of Glc3Me with CrVI together with the observation of CrV species and the successful trap of organic radicals using acrylamide indicates that the reaction occurs through both one- and two-electron pathways involving CrIV and CrV intermediate species. However, under conditions used in the kinetic measurements, CrIV and CrV react with Glc3Me faster than CrVI and do not accumulate in the reaction mixture. Therefore, CrIV and CrV, although formed in the CrVI + Glc3Me, should be involved in fast steps of the reaction pathway. In Scheme 7, we propose a mechanism that combines CrVI  CrIV  CrII and CrVI  CrIV  CrIII pathways, and takes into account: a) kinetic results, b) the polymerization of acrylamide added to the reaction mixture, c) detection of intermediate of CrVI esters and oxochromate(V) species, d) observation of CrO22+, and e) the reaction products. The first step of the mechanism proposed in Scheme 7 involves the formation of a Glc3Me-CrVI mono-chelate (Equation 24), in agreement with the observation of the absorption band at 373 nm characteristic of chromate oxoester, immediately after mixing Glc3Me and CrVI under conditions where the redox reaction is extremely slow. Therefore such a chromate ester should be formed rapidly prior to the redox steps. The formation of the chromate ester is followed by the slow redox step where the C-C bond cleavage is proposed to occur through an acid catalyzed two-electron redox process to yield CrIV, HCO2H and Ara2Me (eq. 25). The initial two-electron reduction of CrVI by Glc3Me is in agreement with previous reports on a number of oxygenated compounds that were selectively oxidized by CrVI to the lower homologue [47,53]. In the mechanism, we have

included two competitive one- and two-electron reductions of CrIV by Glc3Me. Thus, CrIV is proposed to react with excess Glc3Me to yield CrIII, HCO2H and Glc3Me radical, or CrII, HCO2H and Ara2Me, through two alternate fast steps (eqs. 26 and 27). The first is supported by the observed polymerization of acrylamide when it is added to the CrVI/Glc3Me reaction mixture, while the second, by the observation of CrO22+ (the product of the reaction of CrII with O2). CrV can form by fast reaction of CrII with CrVI (eq. 28) and, alternatively, by rapid reaction of the Glc3Me radical with CrVI (eq. 29). CrV can further oxidize Glc3Me to yield CrIII, HCO2H and Ara2Me as final redox products (eq. 30). In the mechanism, one half of the CrVI reaches CrIII through the CrV intermediate, in accordance with Scheme 3 used to fit the time evolution of the CrV EPR signal. In O2-saturated solutions (1.26 M) and [CrVI]0 < 0.1 mM, reactions 28 and 29 can be and CrII neglected because Glc3Me intermediates formed in reactions 26 and 27 should be rapidly trapped by O2 (reactions 31 and 32) [54]. The proposed mechanism is in accordance with the observation that O2 does not have kinetic effect on this reaction, because when [CrVI]0  0.5 mM (as employed in the kinetic measurements), both CrII and Glc3Me react with CrVI faster than they do with O2, and reactions 31 and 32 can be neglected [17,55,56]. The reaction of Glc3Me with CrVI strongly depends on pH. In acid medium, redox reaction occurs and reactive CrV, CrII and CrIV intermediate species are generated in the redox process, together with free radicals, HCO2H and Ara2Me. At pH > 3, Glc3Me oxidation by CrVI or CrV is very slow and long-lived oxo-CrVI or species form. EPR oxo-CrV-Glc3Me spectroscopy shows that at pH 5.5-7.5 Glc3Me is able to trap CrV through the 1,2-diolato moiety to yield a CrV bis-chelate that then slowly converts

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY fast

Glc3Me + CrVI Glc3Me-CrVI

Glc3Me-CrVI k6, 2 H

+

fast CrIV

+ Glc3Me

fast

fast

II

Cr + CrVI

fast

CrVI + Glc3Me

Glc3Me + CrV

fast (kb)

fast CrII + O2

(24)

CrIV + HCO2H + Ara2Me

(25)

CrIII + HCO2H + Glc3Me

(26)

CrII + HCO2H + Ara2Me

(27)

CrV + CrIII

(28)

CrV + Ara2Me + HCO2H

(29)

CrIII + HCO2H + Ara2Me

(30)

CrIII

CrO22+

(31)

fast Glc3Meox

O2 + Glc3Me

(32)

Scheme 7. Redox reaction of CrVI with excess of Glc3Me.

into species with CrV bound to the 5,6-diolato moiety of its furanose form. The selective C1-C2 bond cleavage of Glc3Me upon reaction with high valent chromium in acid medium, distinguishes the 3-OMe derivative from glucose that, under the same experimental conditions, is oxidized to the gluconic acid at comparable rate [19]. 5. Chromic oxidation of methyl glycosides (Gly1Me) The oxidation of Gly1Me by CrVI is a complex multi-step reaction yielding [Cr(OH2)6]3+(ac) and the methyl glycuronolactone (Gly1MeURO) as the final redox products. In Scheme 8a mechanism is proposed that takes Gly1Me + CrVI VI

fast VI

Gly1Me-Cr slow

CrVI + Gly1Me V

Cr + Gly1Me

Cr

VI

+ diald

IV

+ diald

III

+ Gly1Me

Cr

Gly1Me-Cr

CrIV + Gly1Me

account the several experimentals results related to the chromic oxidation process of Gly1Me. The trend in the reactivity of the Gly1Me substrates (Figure 17) reveals that methyl -D-ribofuranoside (-Rib1Me) reacts with CrVI faster than any of the other studied glycopiranosides, a fact probably related to the furanosic form of Rib1Me. The tendency observed in the reactivity of the Gly1Me with CrVI can be interpreted in terms of the Gly1Me chelate instability induced by the nonbonden 1,3-diaxial interactions [16]. As an example, the O4a : H4a steric interaction

fast

fast

fast

fast

Cr

V

Cr + diald III

Cr

+ diald

CrIV + Gly1MeURO

diald = dialdoglicósido Scheme 8. Redox reaction of CrVI with excess of Gly1Me. Global Journal of Inorganic Chemistry | Volume 2 | Issue 1 | February 2011

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0.03

-1

-1

kh, (M s )

0.02

0.01

0.00 -Gal  -Gal -Glc  -Glc -Man  -Man -Rib  -Rib

Figure 17. Redox reactivity of Gly1Me toward CrVI in 1.00 M HClO4 and T= 40°C.

O OH O H

H HO

O

Cr HO MeO

H

O

H

-Gal1Me-CrVI

-Gal1Me-CrVI

H OH

HO

H OH

H Cr

O

O

O

H

HO MeO

H

H

H OH

-Glc1Me-CrVI H OH

OH Cr

O

O

H MeO

O

H OMe

Cr

O

O

O

H H

H

-Man1Me-CrVI

-Man1Me-CrVI

H

H O

O MeO

O

H

-Glc1Me-CrVI OH

Cr

O

O

H

OMe

Cr

H

H

OMe

H

O OH O H

H

Cr

MeO

O

O

O

H OH

-Rib1Me-Cr

Cr O

H OH

VI

-Rib1Me-CrVI

Scheme 9. 1,3-Non-bonding interactions in 1,2-diolato quelates of Gly1Me with Cr(VI).

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY

VI

k6, H

+

PEC + Cr

fast CrIV

+ PEC

fast

CrII + CrVI

CrVI + PEC

V

fast

CrIV + HCO2H + PECox

Cr

III

+ CO2 + PEC

II

Cr + HCO2H + PECox

V

Cr + Cr fast

III

V

Cr + PECox

fast (k5)

PEC + Cr

fast II

Cr

III

+ HCO2H + PECox

2+

CrO2

Cr + O2

III

Cr

fast O2 + PEC

PECox

Scheme 10. Redox reaction of CrVI with excess of PEC.

in the methyl D-glactopyranoside (Gal1Me) stereoisomer should have the highest rate accelerating effect, followed by the O2a : H4a interaction in methyl D-mannopyranoside (Man1Me). Methyl D-glucopyranoside (Glc1Me), with all the ring substituent in pseudo equatorial position, forms the most thermodynamically stable chelate and is oxidized more slowly than the other isomers. The relative reactivity of the α- and anomers of each Gly1Me is explained in terms of the relative instability of the respective Gly1Me-CrVI chelates. Scheme 9 shows the additional syn 1,3 non-bonding interactions generated by the axial methoxy group in α- and -O4,O6-Gly1Me-CrVI. For Gal1Me and Glc1Me, the α-anomer has two additional (Oa : Ha) steric interactions. These interactions run counter to and exceed the electronic stabilization of the axial methoxy group of this anomer [57], so that the α-anomer is expected to be more reactive than the -anomer, which is consistent with the results in Figure 17. In Man1Me the axial hydroxyl group on C2 increases the magnitude of the anomeric effect [57] and the result is that the -anomer is less stable than the α-anomer. The free energy difference in favour of the α-Man1Me over βMan1Me is smaller than that found for the over the α-anomer in Glc1Me and Gal1Me. This explains why that -Man1Me is only slightly more reactive than α-Man1Me (Figure 17). For Rib1Me, the magnitude of the destabilization due to syn-1,3-interactions is larger than the stabilization gained when the methoxy group on

C1 assumes a quasi-axial orientation, so that the -anomer will be less stable and should react faster than the α-anomer, as is observed [14]. 6. General pattern of reactivity saccharides/CrVI The relative reactivity of functional groups in saccharides toward CrVI follows – (H)C(OR)OHhemiacetal [15,19,21] (or aldehyde) > -CO2H [27,28,32,58] > -H2COH(primary) [59] >> (H)COHsecondary [57] –(H)COR glycoside [48] RCO2R ester [60,61]. This tendency was applied in order to determine the mechanistic chromic oxidation process of the natural biopolymer Pectin (PEC), which afforded the following mechanism, Scheme 10. In the polymer, terminal hemiacetalic groups sites and free carboxyli groups of GALUR residues are the most reactive sites toward CrVI. However, excess of –CO2H in the polymer compared to –(H)C(OR)OHhemiacetal favors the reaction of CrVI with –(H)C(OR)CO2H moieties of GALUR residues. Therefore, C-C bond break in –(H)C(OR)-CO2H moieties prevails over C-H cleavage of terminal hemiacetals, to yield HCO2H/CO2 and oxidized PEC. 7. Summary A generalization of mechanistic behavior can be summarized as follows: 1) In all the redox reactions the electrontransfer occurs in the slow steps. 2) The total number of electrons transferred in the slow steps is always two.

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GLOBAL JOURNAL OF INORGANIC CHEMISTRY 3) A redox precursor complex is always formed. 4) The electrons are always transferred intramolecularly within the complex. 5) The formation of the redox precursor complex (es) is not acid catalyzed. 6) The slow redox steps occur through several acid and non-acid catalyzed parallel paths. 7) For substituted aldoses, the kinetic parameters belonging to acid catalyzed paths are higher than the respective ones for the non-catalyzed steps. 8) In every case, the highly reactive CrIV formed in the slow redox steps, rapidly reacts with sugar, as has been demonstrated starting with CrVI or CrIV as oxidizing agent. Acknowledgement This work was supported by Research Council at Argentina (CONICET) (PIP 0075), and the National University at Rosario (UNR), Santa Fe, Argentina (BIO 145). References Klein, C.B., Toxicology of Metals, Ed. L. W. Chang, CRC-Lewis Publishers, New York (1996) p. 205. 2. Codd, R., Dillon, C.T., Levina, A., Lay, P.A., Coord. Chem. Rev. 216/217 (2001) 533. 3. Katz, S.A., Salem, H., The biological and environmental chemistry of chromium, VCH Publishers, New York (1994). 4. Barnhart, J., J. Soil Contam. (Special Issue) 6 (1997) 561. 5. Grevat, P.C., Toxicological Review of Hexavalent Chromium (CAS No. 18540-299), U. S. Environmental Protection Agency, Washington DC (1998). 6. Shi, X., Chiu, A., Chen, C.T., Halliwell, B., Castranova, V., Vallyathan, V., J. Toxicol. Environ. Health Part B. (1999) 87. 7. Costa, M., Crit. Rev. Toxicol. 27 (1997) 431. 8. Gould, E.S., Coord. Chem. Rev. 135/136 (1994) 651. 9. Geiger, D.K., Coord. Chem. Rev. 164 (1997) 261. 10. Levina, A., Lay, P.A., Dixon, N.E., Inorg. Chem. 39 (2000) 385. 11. Signorella, S., Palopoli, C., Santoro, M., García, S., Daier, V., González, J.C., Roldán, V., Frascaroli, M.I., Rizzotto, M., Sala, L.F., Trends Inorg. Chem. 7 (2001) 197. 12. B. Gyurcsik, L. Nagy, Coord. Chem. Rev. 203 (2000) 81. 1.

13. Ciéslak-Golonka, M., Polyhedron. 15 (1996) 3667. 14. Rizzotto, M., Levina, A., Santoro, M., García, S., Frascaroli, M.I., Signorella, S., Sala, L.F, Lay, P.A., J. Chem. Soc., Dalton Trans. (2002) 3206. 15. Roldán, V., González, J.C., Santoro, M., García, S., Casado, N., Olivera, S., Boggio, J.C., Salas-Peregrin, J.C., Signorella, S., Sala, L.F, Can. J. Chem. 80 (2002) 1676. 16. Signorella, S., Lafarga, R., Daier, V., Sala, L.F, Carbohydr. Res. 324 (2000) 127. 17. Signorella, S., Frascaroli, M.I., García, S., Santoro, M., González, J.C., Palopoli, C., Daier, V., Casado, N., Sala, L.F, J. Chem. Soc., Dalton Trans. (2000) 1617. 18. Rizzotto, M., Moreno, V., Signorella, S., Daier, V., Sala, L.F, Polyhedron. 19 (2000) 417. 19. Signorella, S., Daier, V., García, S., Cargnello, R., González, J.C., Rizzotto, M., Sala, L.F, Carbohydr. Res. 316 (1999) 14. 20. Signorella, S., García, S., Sala, L.F, J. Chem. Educ. 76 (1999) 405. 21. Daier, V., Signorella, S., Rizzotto, M., Frascaroli, M.I., Palopoli, C., Brondino, C., Salas-Peregrin, J.M., Sala, L.F, Can. J. Chem. 77 (1999) 57. 22. Rizzotto, M., Frascaroli, M. I., Signorella, S., Sala, L.F, Polyhedron. 15 (1996) 1517. 23. Signorella, S., Rizzotto, M., Daier, V., Frascaroli, M.I., Palopoli, C., Martino, D., Boussecksou, A., Sala, L.F, J. Chem. Soc., Dalton Trans. (1996) 1607. 24. Rizzotto, M., Signorella, S., Frascaroli, M.I., Daier, V., Sala, L.F, J. Carbohydr. Chem. 14 (1995) 45. 25. Sala, L.F, Palopoli, C., Signorella, S., Polyhedron. 14 (1995) 1725. 26. Sala, L.F, Signorella, S.R., Rizzotto, M., Frascaroli, M.I., Gandolfo, F., Can. J. Chem. 70 (1992) 2046. 27. Signorella, S., Santoro, M., Palopoli, C., Brondino, C., Salas-Peregrin, J.M., Quiroz, M., Sala, L.F, Polyhedron. 17 (1998) 2739. 28. Signorella, S., García, S., Sala, L.F, Polyhedron. 16 (1997) 701. 29. Signorella, S.R., Santoro, M.I., Mulero, M.N., Sala, L.F, Can. J. Chem. 72 (1994) 398. 30. Thibault, J.F., Carbohydr. Res. 155 (1986) 183. 31. Chan, T.S., Wilson, J.X., Selliah, S., Bilodeau, M., Zwingmann, C., Poon, R., O'Brien P.J., Toxicol. Appl. Pharmacol. 232 (2008) 456. 32. González, J.C., Daier, V., García, S., Goodman, B.A., Atria, A.M., Sala, L.F., Signorella, S., Dalton Trans. 15 (2004) 2288.

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