glycine with Cu(II)

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Aug 21, 2015 - glycine with Cu(II) in Solution, Based on Viscosity Measurements .... Two calibrated Ostwald viscometers were used. Before each measurement ...
P. U. SA K E L L A R ID IS E T A L . • Cu(gly), AND Cu(glygly) IN SOLU TIO N

83

A Study of the Structure o f the Complexes o f Glycine and Glycylglycine w ith C u(II) in Solution, Based on V iscosity M easurem ents P. U.

S a k e l l a r id is ,

E. K .

K arag eo r g o po ulo s,

an d J . M.

T s a n g a r is

N a tio n a l T ech n ical U n iv e rs ity o f A th en s, G reece (Z. Naturforsch. 29b, 83-85 [1974]; received August 4, 1973)

C opper, glycine, gly cy lgly cin e I n th e re la tio n rjr = 1 -f- B m + D m 2 th e B v a lu e s o f th e co m p lex es b is(g ly cin ato )C u (II) a n d (glycylgly cin ato )C u (II) b e tw e e n 10 a n d 40 °C in d ic a te t h a t th e s e com plexes are h y d ra te d b y tw o m olecules o f w a te r a t th e ir c a rb o x y l o x ygens. L a rg e v a lu es of D in d ic ate so lu te-so lu te in te ra c tio n s v ia th e a x ia l p o sitio n s o f th e c u p ric sq u a re m ic ro sy m m e try , as th e m olecules are m oving in th e liq u id stre a m lin es. F o r b o th co m p lex es d B /d T w as fo u n d to be p o sitiv e in th e e x a m in e d te m p e ra tu re ran g e . T h is p re s u m a b ly in d ica tes t h a t th e ex am in ed com plexes h a v e a s tru c tu re b re a k in g effect on th e w a te r s tru c tu re .

1. Introduction Dipolar ions behave as non-electrolytes and a linear relationship exists between partial molal quantities and concentration, when dilute solutions are investigated. For interpreting viscosity data of aqueous solutions of several dipolar ions T s a n g a r i s and M a r t i n 1 used the Eqn. ( 1 ) : rjT = 1 +

+ Dm 2

(1)

where nr is the relative viscosity, and m the molal concentration. The coefficient B is related with the solvent-solute interaction and is always positive, whereas D measures the solute-solute interaction. I t has also been proposed 1 that the plus or minus sign of dB/dT in Eqn (1) indicates the structure breaking or structure making ability of the dipolar ions on the water structure, respectively. Sofar, based on this criterion, seven dipolar ions, i.e. sulfamic acid, taurine, glycine, betaine, serine, glycyl-glycine and triglycine, have been shown to be structure breakers (dB/dT > 0 )1 and five dipolar ions, i.e. sarcosine, hydroxyproline, proline, eaminocaproic acid and glutathione, to be structure makers (dB/dT cO )1. The dipolar character of the amino acids and peptides disappears when these compounds are chelated with transition metal ions. R e q u e sts for re p rin ts sh o u ld be se n t to P ro f. P . U . N a tio n a l T echnical U n iv e rs ity of A th e n s, L a b o ra to ry o f G eneral C h em istry , 42, P a tission S tre e t, Athens, 147, G reece. S a k e l l a r id is ,

The aim of the present research is to investigate the action of the chelated amino acids on the pseudo­ crystalline structure of water, as it is manifested by viscosity measurements. I t is interesting to clarify if the chelated amino acids continue to have the same kind of action on the water structure as before chelation. Glycine and glycyl-glycine chelates of Cu(II) were selected for this task, because glycine is without any am biguity a structure breaker 1'3 and the solid structures of both complexes bis (glycinato)Cu(II) and (glycyl-glycinato)Cu(II) are very well established 4 which may help the interpretation of the obtained results. Viscosity measurements of aqueous solutions of chelated compounds are rare. C h a r l e s 5 studied the viscosities of aqueous solutions of several metal chelates derived from the EDTA as ligand. It was found th a t a relationship exists between viscosity B coefficient of the metal chelates and the radius of metal cation as well as the cation electronegativity and the second ionisation potential. Temperature variations of the B coefficient were not examined. 2. Experimental All materials used were from Fluka Co, in puriss grade. Cis-bis(glycinato)Cu(II) monohydrate and (Glycyl-glycinato)Cu(II) trihydrate were prepared according to A b d e r h a l d e n and S c h n i t z l e r 6 and were recrystallized twice from a mixture of alcohol, water and ether. Solutions were prepared by weighing the appropriate amounts of the prepared complexes in filtered distilled water of conductivity

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P. U. S A K E L L A R ID IS E T A L . • Cu(gly)2 AND Cu(glygly) IN SOLUTION

84

C u(gly )2 C u(glygly)

B m ole '1

a

2.20

2.62

25 °C

A

D m ole '2

B m o le '1

7.0 7.4

53.12 36.30

2.30 2.75

a

30 °C

A

D m o le '2

B m ole "1

a

A

D m ole '2

B m ole '1

a

A

D m ole "2

7.1 7.5

53.00 43.33

2.40 2.85

7.2 7.6

48.40 47.94

2.75 3.10

7.5 7.9

45.00 45.14

less than IO 6 c n r1. The prepared solutions were measured within two days after their preparation. Densities wTere measured in different tem peratures in 25 ml pycnometers with a capillary neck and a sindered stopper. Viscosities and densities were measured in a constant tem perature bath at 10, 25, 30 and 40 °C with a variation of tem perature within the range ± 0.01 to ± 0.03 °C as repeatedly was checked by a Beckman thermometer. Low concentrations were used, ranging from 0.002 to 0.02 M because the complexes have limited solubility in water and in addition low concentra­ tions are im portant in evaluating the results. Two calibrated Ostwald viscometers were used. Before each measurement the viscometers were carefully and repeatedly cleaned with a number of filtered solvents. The viscometers were fitted into the glass-baths in vertical reproducible position. Verti­ cal it y was checked by a cathetom eter and the observation of the dropping liquid surface in the capillary of the viscometer was effected by the cathetometer telescope. Viscometers were filled with the same volume of liquid for each experiment. The number of measurements of the efflux times were twenty for each tem perature and each con­ centration and the mean deviations were ± 0.05 to i 0.09 sec. If the mean deviations wrere greater than 0. 1 % the run was discarded. Small kinetic energy corrections w'ere made for each run. The calibration of viscometers wras made by water at the temperatures 10, 25, 30 and 40 °C. The absolute viscosities and densities of water of these temperatures w'ere taken as follows7.

t

[°C] 10

25 30 40

>?w nip

dw [g.cnr3]

13.097 8.949 8.004 6.536

0.99973 0.99707 0.99567 0.99224

o

p o

C om plex

O

T ab le I. B . D coefficients o f th e E q n ??r = f(m ) for th e co m p lex es (C u(gly )2 a n d C u(glygly); a ra d iu s o f th e ir h y d ro d y n a m ic sp h ere.

For the measurement of efflux times an electric timer was used giving an accuracy ± 0.01 sec.

3. Results Eqn (1) was used in evaluating B and D coeffi­ cients. For each complex and tem perature a plot of r/T— 1/m vs molality m was constructed. Straight lines wrere obtained using four concentrations. B coefficient was calculated from the intercept and D coefficient from the slop of obtained lines. In Table I B and D coefficients are reported for each complex and temperature. B coefficients can be correlated w ith the radius a in A of the hydrodynamic sphere of the species in solution, by the relation (2 ) provided th at spherical shape pre­ dom inates1. a = 5.417 B1/,s

(2)

Values of a are reported for each complex and tem perature in Table I. For both complexes, dB/dT was found positive for all tem perature intervals examined, as it is showTi in Table II. T a b le I I . d B /d T C oefficients of th e co m plexes C u (gly )2 a n d C u(glygly) in a q u eo u s so lu tio n s a n d in d ifferent te m p e ra tu re in te rv a ls. d B /d T m ole' i T -i C om plex C u (g ly )2 C u(glygly)

10-25 °C + 0.0067 + 0.0087

25-30 °C + 0.02 + 0.02

25 -4 0 °C + 0.03 + 0.023

4. Discussion Supposing spherical shape for the molecule of cis-bis(glycinato)Cu(II), a value a = 7.1 Ä at 25 °C for its hydrodynamic sphere was found. This value is large enough compared with the value 4.1 Ä of the distance between copper and the more distant atom of the complex, in the solid state, which is the carboxyl oxygen. The value 4.1 Ä w'as calculated from the data of F r e e m a n et al.1. The excess radius

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P. U. S A K E L L A R ID IS E T A L . • Cu(gly)2 AND Cu(glygly) IN SO LU TIO N

of 3 A of the hydrodynamic sphere is certainly due to the hydration. Relative viscosities can be related 8 with the relative volume concentration of solutes 9p by Eqn 3: rjj. = 1 + ci^p -f a2cp2 + a 3cp3 + . . .

(3)

The first virial coefficient of the Eqn (3) is as­ sociated with the shape of the dissolved particles and is independent of its size. Comparing Eqn (3) with (1) the relation (4) is obtained: 1000 B

where Veff is the effective volume of the dissolved particles in A 3 and N is the L o s c h m id t’s number. Assuming th a t one molecule of water is attached by hydrogen bond to the carboxyl oxygen of the Cu(gly )2 molecule in solution, the complex may be represented by an ellipsoid of revolution with semi­ axis 6.96 Ä and 1.98 Ä. 6.96 Ä is the distance be­ tween the Cu atom and the hydrogens of the mole­ cule of water which is attached to the carboxyl oxygen of the complex and 1.98 A is the distance between Cu and the nitrogen of the glycine molecule in the complex. The volume of this ellipsoid was calculated Veff = 401 A3, therefore from (4) a 1 = 9.45 was obtained. Simha 9 derived a relation (5) of a 1 with the axial ratio (J) of the dissolved particles having the shape of ellipsoids. J2 01 = 15(ln2J— |)

+

J2 5(ln2J— |)

14 + 15

(o)

The axial ratio of the considered ellipsoid is J = 3.57. This value upon substitution to the Eqn (5) gives a x = 4.49. The obtained values of a x

by two different ways are in desagreement because the B coefficient of the complex is subjected to greater shape factor influence. Furtherm ore, assuming th at instead of one, two molecules of water are attached by hydrogen bond 1 J . M. T s a n g a r i s a n d R . B . M a r t i n , A rch. B iochem .

B io p h y s. 11*2, 267 [1965]. 2 L . S. M a s o n , P . M . K a m p m e y e r , a n d A. L. R o b i n ­ s o n , J . A m er. chem . Soc. 74, 1287 [1952]. 3 H . J . V. T y r r e l l a n d M . K e n n e r l e y , J . chem . Soc. [L o n d o n ], Ser. A, 1968, 2724. 4 H . C. F r e e m a n , M . R . S n o w , I. N i t t a , a n d K . T o m i t a , A c ta cry stallo g r. [C openhagen] 17, 1463 [1964]; B . S t r a n d b e r g , I. L i n d q u i s t , a n d R . R o s e n s t e i n , Z. K ristallo g r. 116, 266 [1961].

85

to the carboxyl oxygen of the complex, an ellipsoid of revolution with semiaxis 9.72 A and 1.98 A is considered. This ellipsoid has a volume Veff = 799 A 3 giving a 1 = 4.75. Using S i m h a ’s Eqn (5) a ± — 5.70 was obtained. Analogous results were obtained considering the complex Cu(glygly). From the above discussion it is possible to consider the molecules of both complexes moving in the fluid streamlines hydrated mainly by two molecules of water attached to the carboxyl groups of the complexes of hydrogen bonds. The small positive dB/dT values for both com­ plexes in the range of tem peratures examined give some evidence th a t these compounds exhibit an almost structure breaking effect to the water structure. This effect is more remarkable between 25 and 30 °C. The high values of D coefficients in both examined complexes, even though low concentrations were considered, indicate strong solute-solute inter­ actions which may be arisen from the cupric-cupric interactions of the dissolved complexes via axial positions. The carboxyl oxygen atom in both complexes is the most distant from the copper atom. Since it is considered th a t the hydration takes place in this particular atom, it is impossible considering the B coefficient to detect any other hydration site in the complexes which is closer to the copper atom. Therefore the possible hydration in the axial posi­ tions around the square microsymmetry of copper cannot be detected. The large value of D coefficient, which lead to the assumption of strong solute-solute interactions, indicate presumably th a t the axial positions of the copper atom are not occupied by water molecules. In contrary if these positions were occupied copper-copper interactions would be much weaker and consequently small values of D coeffi­ cient would be obtained. 5 R . G. C h a r l e s , J . A m er. ch em . Soc. 78, 3946 [1956]. 6 E . A b d e r h a l d e n a n d E . S c h n i t z l e r , Z. P h y sio l.

C hem . 168, 94 [1927]. 7 In te rn a tio n a l C ritical T ab les, V o l. 8 a n d V o l. 5,

M cG raw H ill, N .Y . 1930.

42, 409 [1949]: J . Coll. Sc. 5, 386 [1950]. 9 R . S i m h a , J . p h y sic. C hem . 44, 25 [1940]. 8 R . S i m h a , J . R es. N a t. B u r. S ta n d .

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