Accepted Manuscript Title: Volumetric and optical properties of ACE inhibitor captopril drug in aqueous-alcoholic mixtures Authors: Santosh D. Deosarkar, Meenakshi P. Pawar, Atmaram D. Arsule, Rajendrakumar T. Sawale, Tukaram M. Kalyankar PII: DOI: Reference:
S1658-3655(17)30026-2 http://dx.doi.org/doi:10.1016/j.jtusci.2017.02.003 JTUSCI 363
To appear in: Received date: Accepted date:
25-10-2016 23-2-2017
Please cite this article as: Santosh D.Deosarkar, Meenakshi P.Pawar, Atmaram D.Arsule, Rajendrakumar T.Sawale, Tukaram M.Kalyankar, Volumetric and optical properties of ACE inhibitor captopril drug in aqueous-alcoholic mixtures (2010), http://dx.doi.org/10.1016/j.jtusci.2017.02.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Volumetric and optical properties of ACE inhibitor captopril drug in aqueous-alcoholic mixtures Santosh D. Deosarkara*, Meenakshi P. Pawara, Atmaram D. Arsulea, Rajendrakumar T. Sawalea, and Tukaram M. Kalyankarb a*
School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded-
431 606 (MS) India b
School of Pharmacy, Swami Ramanand Teerth Marathwada University, Nanded-431 606
(MS) India
Corresponding author: Santosh D. Deosarkar;
[email protected]
Abstract Captopril is an angiotensin converting enzyme (ACE) inhibitor which is used for the treatment of hypertension and congestive heart failure. The article deals with the accurate measurements of densities and refractive indices of solutions containing captopril in pure solvents like water/methanol/ethanol/1-propanol and aqueous mixtures of methanol, ethanol and propan-1-ol of 30, 50 and 70vol % in wide concentration interval of drug at 26oC. It also includes the evaluation of apparent molar volume, partial molar volume at infinite dilution and transfer volumes. Concentration dependence of refractive indices studied and respective fitting parameters have been reported. Different properties are interpreted in terms of intermolecular interactions, effect of drug on structure of solvent/solvent mixture and overall structural fittings in solutions.
Keywords: Drug · Solvation · Molecular interactions· Alcohols
1. Introduction Water-co-solvent mixtures are widely used in the pharmaceutical sciences in order to enhance the solubility of drugs. These mixtures are highly non-ideal due to existence of intermolecular interactions. The drug solutions in these water-co-solvent systems have applications in the drug solubility and design of homogeneous pharmaceutical dosage forms like syrups and
elixirs etc. The physicochemical behaviour of captopril (CPT) drug in water-co-solvent mixtures therefore is highly significant in view of its pharmaceutical applications. Study of molecular interactions in solutions containing alcohols is of great interest as alcohols are -OH group containing highly polar organic molecules and self associated through hydrogen bonding [1-3]. They are polar compounds in which hydrogen bonding or dipole-dipole Van der Waals forces are present. Hydrophobic character of alcohols increases with length of chain and therefore, hydrophobic effects are studied using alcohol solvents. Studies on physicochemical properties of drugs in aqueous-alcoholic mixture are of interest due to peculiar results in different solvent systems. Volumetric data of drugs can provide clues to the interactions occurring in cellular fluids. Literature survey reveals that the studies on volumetric properties of drugs show increasing interest by a number of workers in this field of research. Drugs are the organic molecules containing hydrophobic as well as hydrophilic groups. Pharmacological properties of drug molecules are highly dependent on their solution behavior [4-5]. Drug may break or preserve the structure of solvent or solvent mixture through the interactions due to presence of hydrophobic and hydrophilic groups. These interactions are specific and electrostatic interactions. Drug transport, anesthesia and protein binding processes are important where drug and biomacromolecules interacts which affects their physicochemical behaviour. Volumetric properties are depend on different interactions such as solute-solute, solutesolvent and solvent-solvent and structural effects due to volume difference among different components of solution [6]. Systems containing hydrogen bonds are of great interest due to vital role of these bonds in chemical, physical and biological processes [7]. Pure and mixed (with water) alcohols are widely used in fields like pharmaceutical industry, ecology, cosmetics, and energy source [7-8]. Density, refractive index and viscosity and derived properties are useful in collecting analytical information required for industrial purposes [9]. Study of refractive indices of different systems containing various solutes has been carried out by many workers [10-15]. Densities and refractive indices of drug solutions are helpful for understanding molecular interactions and structural fittings in solution.
Molecular
interactions in solution are understood from studies based on measurements of refractive indices of solutions [16-19]. Therefore, in continuation with our earlier work to understand the solution behaviour of different systems [20-24], the systematic study of volumetric and refractometric properties of
CPT drug in water, alcohols and in alcohol-water mixtures of different vol % is carried out in present work.
2. Experimental Captopril (CPT) was received as a gift sample from Wockhardt Ltd. Aurangabad (MS) India and it was used as it is. Deionized distilled water (HPLC grade, pH=6.91) obtained from Millipore prefiltration kit (Direct-QTM system series) was used. Methanol (MeOH, Merck, 99.0 %), ethanol (EtOH, SD fine, 99.9%) and 1-propanol (1-PrOH, SD fine, 99.0 %) were used. Solutions of drug having different concentrations were prepared in pure solvents and solvent mixtures of different vol %. Density of solution was measured using single capillary calibrated pycnometer. Pycnometer was calibrated by ethanol at experimental temperature. Weighing was done on single pan electronic balance (±0.001g). Refractive index of solutions was measured using thermostatically controlled Cyber LAB-Cyber Abbe Refractometer (Amkette Analytics, ±0.0002, 1.3000 to 1.7000). Averages of three readings of density and refractive index are reported. CPT has different interaction sites as shown in Figure 1. It has hydrophilic and hydrophobic parts (functional groups) along with hydrogen bonding site. Amide (-CONH) group can shows hydrogen bonding interaction with polar molecules, acid (-COOH) group is polar and ionic; therefore it shows interesting interactions with polar solvent molecules.
3. Results and Discussion Experimental densities of drug in pure solvents and solvent mixtures are reported in table 1 and 2. It is seen that, the density increased with drug concentration in given solvent system and it largely decreased with increase in vol % of MeOH, EtOH and PrOH for given drug concentration which is consistent with the densities of these alcohols. Behaviour density with drug concentration indicates change in volume, strengthening of drug-solvent interactions and modification of structural orientation in solution. Apparent molar volume (v, AMV) of drug in different solvent systems is calculated from accurately measured densities of solution [25] using following Equation (1): v
M2
1000 ( 0 ) c 0
(1)
Where, c is molar concentration, 2=molecular weight of solute, 0=density of solvent and =density of solution. AMV values of drug in pure solvents and aqueous-alcoholic mixtures with different composition are presented in figure 2 and 3.
It is seen that AMV decrease with drug concentration in each solvent/solvent mixture and increase with increase in vol% of alcohols in respective systems. Decrease in AMV with drug concentration is attributed to solvophobic hydration between polar groups of CPT and solvent and compression in volume due to modified mean distance [26]. Increase in AMV with vol% of alcohols is due to relative weakening of drug-solvent interactions and reduction in electrostriction. AMV of drug in pure MeOH, EtOH and PrOH are greater than in water which is due to presence of additional alkyl groups (-CH3, -CH2CH3 and -CH2CH2CH3 respectively) in these alcohols. AMV is due to the sum of solute molecules geometric volume and changes in volume occurred due to interaction of solute with solvent [27]. Literature survey on model compounds revealed that the volumes of the water-soluble compounds are generally smaller in aqueous medium compared to in non-aqueous medium.
AMV data is fitted to following Massons relation [28-29] Equation (2):
v v0 S v c
(2)
The v (partial molar volume, PMV) and Sv are determined as intercept and slope of the linear plots for drug + pure alcohols and drug + mixed solvents. Values of v and Sv are obtained from the graph by linear fitting and are reported in table 3. The v is independent on solute-solute interactions in solution; its values for CPT are positive and large in all the solvent systems due to strong drug-solvent interactions because of reduction in the electrostriction, change in the volume and solvation behaviour of drug. It is seen that the v values in MeOH-H2O and EtOH-H2O are nearly same but are greatly differ from the values in 1-propanol-water. Positive v values also suggests that no restriction on the molecular motion in solution [30]. The trend of v values with solvent composition is: MeOH-H2O ≈ EtOH-H2O > 1-PrOH-H2O as shown in Figure 4. This observation suggests that relatively more structure is disrupted in aqueous solutions of 1-PrOH due to hydrophobic chain of alcohols [25]. The v increased with alcohol content which is attributed to the solvation behaviour of drug and change in volume due to reduction in the electrostriction and migration of solvent from second layer around drug to bulk solvent.
Sv values are found to be negative (negative slopes) for all the solvent systems which indicate presence of weak ion-ion or solute-solute interactions and insufficient counter ion binding of drug molecule due to its hydrophobic nature [26]. At infinite dilution the drug molecules are away from each other and could not interact strongly which results in the negative slopes and weak drug-drug interactions and strong drug-solvent interactions. A negative Sv value also suggests that the drug molecule try to occupy the void space of solvents. There is no appreciable change in the Sv with solvent composition even from water to 1-propanol. Partial molar volume of transfer (standard transfer volume of drug, trv) from aqueous solution to aqueous-alcoholic solutions is calculated [31] from following Equation (3) and is reported in table 3. tr v0 v0 (Aqueous) v0 (Water )
(3)
In all the cases, the trv are positive. trv increased with increase in the alcohol content. These values are large and positive in pure solvents like MeOH, EtOH and 1-PrOH. trv values are smaller in 1-PrOH-H2O solutions compared to MeOH-H2O and EtOH-H2O. The trend of trv values with solvent composition is: MeOH-H2O ≈ EtOH-H2O < 1-PrOH-H2O. Hydration number (the number of water molecules hydrating drug) of the drug decreases in the presence of alcohol in aqueous drug solution and further decreases with increase in the alcohol content in solution due to the existence of drug-alcohol polar-hydrophilic interactions and intermolecular interactions (intermolecular hydrogen bonding) between water and alcohol [32]. Positive trv values is attributed to decrease in volume of shrinkage because interactions between drug and alcohols and existence of hydrophilic-hydrophilic and ionhydrophilic interactions and reduction in the electrostriction (less hydration of drug in presence alcohol) due to intermolecular hydrogen bonding between water and alcohol. These effects increase with increase in the alcohol content of solution. This also suggests the presence of dominating hydrophilic-hydrophilic interactions in solution. The increase in the values of v and positive trv values are due to strong-drug-solvent interactions and decrease in the shrinkage volume of water by the drug in presence of alcohols i.e. alcohols have dehydrating effect on the hydrated drug [33]. Apart from the interactions of -OH group with polar/hydrophilic/ionic groups of drug, additional interactions between hydrophobic
alkyl groups (–CH3, –CH2CH3 and –CH2CH2 CH3) of MeOH, EtOH and PrOH and hydrophobic part of drug molecule occurs in aqueous-alcoholic solutions. The experimental refractive indices of drug in pure and mixed solvents are reported in table 1 and 2. It is seen that, the refractive index increase with drug concentration in each solvent system. It also increases with vol% of MeOH, EtOH and 1-PrOH for given drug concentration. In pure MeOH, the refractive index values are small compared to H 2O and aqueous-MeOH mixtures. But in the case of aqueous-EtOH and aqueous-1-PrOH, the refractive index values increased with increase in alcohol content for given drug concentration. In general, the refractive index data in different aqueous-alcoholic mixtures follows the following trend: Aqueous-1-PrOH > Aqueous- EtOH > Aqueous- MeOH. And in pure solvents the refractive index data follows the following trend: 1-PrOH > EtOH > H2O > MeOH. Trends of refractive index in various systems are in accordance with the refractive indices of respective solvents and solvent composition. Relation between refractive index and concentration of solute depends on nature of solute, solvent, temperature and wavelength. Temperature and wavelength dependence is eliminated by using thermostat and monochromatic light respectively. Linear relationship between refractive index and concentration of drug is checked [34] using following Equation (4) and graphical parameters, namely refractive index at infinite dilution, no and constant, K which depends on nature of solute are determined graphically and are reported in table 4 along with r2 values of the respective plots:
nD K c n 0
(4)
Where, n=refractive index of solution, n0=Infinite dilution refractive index and K=Constant.
It is seen that the refractive index at infinite dilution increase with vol % of MeOH, EtOH and 1-PrOH for given drug concentration except in the case of pure MeOH. The r2 values of various plots indicate that relationship between refractive index and concentration of drug is linear in most of the cases [35]. Further, linear relationship between refractive index and density of solutions is checked. From the r2 values of various plots, the relationship between refractive index and density of solutions seems to be linear.
4. Conclusions It is concluded from volumetric and refractometric properties of CPT in pure solvents and solvent mixtures of different compositions, that the strong drug-solvent interactions are present in each solution. Large positive values of v confirms reduction in electrostriction, change in volume and solvation behaviour of CPT. Relatively more structure disruption is observed in aqueous solutions of 1-PrOH due to hydrophobic chain. Drug-drug interactions are weak in all aqueous-alcoholic systems and are not greatly affected by nature of alcohol. Standard transfer volume, trv indicate reduction in hydration number of drug with increase in alcohol content.
Hydrogen bonding interactions and ionic/hydrophilic-hydrophilic
interactions between drug (-C=O, amide group and -COOH, acid group) and solvent/cosolvent (-OH) exists in solution. Refractive index data suggests that the packing of drug molecules become tighter with increases in concentration of drug in each system due to strengthening of drug-solvent interactions and structural cause of change in density and existence and modification of molecular interactions. Present work is helpful for prediction of absorption and permeability of CPT drug through membranes which finds applications in the field of medicinal and pharmaceutical chemistry.
Acknowledgements Authors are thankful to Wockhardt Ltd. Aurangabad Ltd. Aurangabad (MS) India for generous gift of drug sample.
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Fig. 1: Hydrophilic, hydrophobic and hydrogen bonding sites in CPT.
Fig. 2. Plots of v (cm3·mol–1) as a function of drug concentration in different pure solvents
Fig. 3. Plots of v (cm3·mol–1) as a function of drug concentration in different solvent mixtures
Fig. 4. Plot of v (cm3·mol–1) vs. vol % of alcohol content in different systems
Table 1: Density, g·cm–3 and refractive index data for CPT in pure solvents at 26oC
c
n
H2O
MeOH
EtOH
1-PrOH
H2O
MeOH
EtOH
1-PrOH
0.005
0.9969
0.7850
0.7994
0.7975
1.3318
1.3306
1.3615
1.3812
0.010
0.9970
0.7854
0.7999
0.7982
1.3320
1.3309
1.3617
1.3818
0.030
0.9976
0.7871
0.8017
0.8003
1.3327
1.3316
1.3620
1.3830
0.050
0.9984
0.7891
0.8037
0.8023
1.3334
1.3321
1.3622
1.3835
0.070
0.9992
0.7913
0.8057
0.8044
1.3340
1.3327
1.3625
1.3840
Table 2: Density, g·cm–3 and refractive index data for CPT in aqueous-alcoholic mixtures at 26oC
c
n
MeOH +
EtOH +
1-PrOH +
MeOH +
EtOH +
1-PrOH +
H2O
H2O
H2O
H2O
H2O
H2O
30vol % 0.005
0.9576
0.9592
0.9559
1.3365
1.3435
1.3505
0.010
0.9581
0.9595
0.9563
1.3367
1.3443
1.3507
0.030
0.9593
0.9606
0.9581
1.3375
1.3464
1.3515
0.050
0.9604
0.9617
0.9597
1.3383
1.3480
1.3520
0.070
0.9615
0.9628
0.9614
1.3390
1.3490
1.3535
50vol % 0.005
0.9275
0.9320
0.9227
1.3390
1.3545
1.3615
0.010
0.9279
0.9322
0.9232
1.3393
1.3550
1.3620
0.030
0.9289
0.9331
0.9251
1.3402
1.3555
1.3622
0.050
0.9301
0.9342
0.9269
1.3410
1.3560
1.3630
0.070
0.9311
0.9354
0.9287
1.3415
1.3565
1.3632
70vol % 0.005
0.8874
0.8944
0.8755
1.3400
1.3592
1.3697
0.010
0.8877
0.8946
0.8761
1.3403
1.3595
1.3700
0.030
0.8888
0.8956
0.8778
1.3410
1.3600
1.3708
0.050
0.8900
0.8968
0.8796
1.3416
1.3605
1.3714
0.070
0.8917
0.8983
0.8815
1.3421
1.3610
1.3721
Table 3: The v (cm3·mol–1), Sv (cm3·dm3/2·mol–3/2) and transfer volumes, trv (cm3·mol– 1 ) of CPT in pure solvents and aqueous-alcoholic mixtures PMV, v
Experimental slope, -Sv
Transfer volume, trv
205.25 255.82 257.17
85.36 208.71 249.67
50.57 51.92
232.24
189.38
26.99
211.93 220.29 232.33
179.64 151.91 171.81
6.68 15.04 27.08
EtOH + H2O 30vol % 50vol %
214.89 222.32
169.33 140.75
9.64 17.07
70vol %
235.50
173.12
30.25
1-PrOH + H2O 30vol % 50vol % 70vol %
207.65 212.80 220.99
253.67 253.52 213.08
2.40 7.55 15.74
Binary/ternary system Pure solvents H2O MeOH EtOH 1-PrOH MeOH + H2O 30vol % 50vol % 70vol %
Table 4: Graphical parameters of concentration dependence of n, {n=K (dm3·mol-1) × c (mol·dm-3) + n0} for CPT in pure solvents and aqueous-alcoholic mixtures Infinite dilution refractive index, n0
Constant, K
r2
H2O
1.3317
0.0340
0.9987
MeOH
1.3305
0.0313
0.9909
EtOH
1.3615
0.0144
0.9788
1-PrOH
1.3813
0.0414
0.9340
30vol %
1.3363
0.0388
0.9992
50vol %
1.3389
0.0389
0.9859
70vol %
1.3399
0.0319
0.9897
30vol %
1.3434
0.0849
0.9750
50vol %
1.3546
0.0285
0.9698
70vol %
1.3592
0.0266
0.9923
30vol %
1.3502
0.0433
0.9626
50vol %
1.3616
0.0251
0.9350
70vol %
1.3696
0.0361
0.9943
Binary/ternary system Pure solvents
MeOH + H2O
EtOH + H2O
1-PrOH + H2O
