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The density, ρ, and the speed of sound, u, of γ-aminobutyric acid (GABA) in water and in (0.005, 0.01, 0.02, 0.04, and 0.06) mol·kg–1 aqueous caffeine (CAF) solutions were measured at temperatures,...

The densities (ρ), viscosities (η), and 1H nuclear magnetic resonance (NMR) studies for caffeine, theophylline, and theobromine in water and in aqueous solutions of 0.10, 0.25, 0.50, 0.75, and 1.00 mol·kg–1 sodium chloride over a temperature range T = 288.15–318.15 K and at p = 101.325 kPa have been carried out using vibrating-tube digital densimeter, micro-Ubbelohde type capillary viscometer, and Bruker (AVANCE-III, HD 500 MHz) NMR spectrometer, respectively. From the density and viscosity data, apparent molar volume (V2,ϕ), partial molar volume at infinite dilution (V2,ϕ0), viscosity B-coefficient, corresponding transfer (ΔtrV2,ϕ0 and ΔtrB) and other related parameters have been calculated. The trends in transfer parameters reveal the dominance of hydrophilic–ionic interactions at lower molalities of NaCl while hydrophobic–ionic interactions at higher molalities of NaCl. The expansibilities and dB/dT data show the structure-breaking behavior of theophylline and theobromine in water and in aqueous soluti...

Binding studies of pharmacologically active drugs-caffeine and theophylline to bovine serum albumin (BSA) in phosphate buffer solution, pH = 7.4 have been investigated using spectroscopic (UV–visible, fluorescence and nuclear magnetic resonance) and calorimetric (isothermal titration calorimetry) techniques. The derived parameters obtained from all the techniques suggest that the magnitude of binding affinity of BSA-drug follows the order: theophylline > caffeine, which may be attributed to greater acidity of former at physiological pH. UV–visible and fluorescence spectroscopy suggests 1:1 stoichiometry of drug-protein interactions, while calorimetric results support two binding sites. Apart from the tryptophan binding site, a low affinity site also exists which is assessed from the calorimetric measurements. The negative enthalpy and positive entropy, both favors the binding process, and the feasibility of binding is also supported by negative value of Gibbs free energy. The positive heat capacity, consistent with the positive entropy, may be attributed to the exposure of hydrophobic protein surfaces. The observed positive changes in chemical shifts, ∆ δ may be due to the deshielding of drug protons owing to hydrophilic-ionic/hydrogen bonding interactions with BSA.

To have an insight into the effect of preservatives on various ingredients of processed items, it is important to study their thermodynamic, transport and spectroscopic properties in aqueous solutions to elucidate various solute-co-solute interactions. The densities, viscosities and enthalpies of dilution of l-serine and l-proline have been determined in water and in aqueous solutions of sodium propionate and calcium propionate at different temperatures. The derived parameters elucidate the changes in taste quality and hydration number of l-serine and l-proline in the presence of the studied preservatives. Predominance of dehydration effect has been observed from calorimetry and changes in chemical shifts from nuclear magnetic resonance spectroscopy also support the above results.

Caffeine, a widely used stimulant is a purine based xanthine alkaloid which can lead to mutagenic effects in DNA helix via pairing errors. In order to study the interactions between caffeine and the pyrimidine based model compounds of nucleic acids, densities ρ, viscosities η, and enthalpies of dilution, q of aqueous solutions of some nucleic acid bases and nucleosides have been determined at different concentrations of caffeine over a temperatures range (288.15–318.15) K. Apparent molar volumes at infinite dilution (Vo2,ϕ), viscosity B-coefficient (B), and standard molar enthalpy of dilution (∆dilHo), and the corresponding transfer parameters at infinite dilution have also been calculated. The nature of solute–co-solute interactions is exothermic. UV–visible and NMR spectroscopic studies have been carried out and the shifts in absorption spectra and NMR spectra signify the role of hydrophobic interactions. An effort has been made to compare the present work with the similar properties in other co-solutes.

Viscosity, η and enthalpy of dilution, q of studied nucleic acid bases (uracil, cytosine, thymine) and nucleosides (uridine, cytidine and thymidine) in water and in aqueous solutions of (0.10, 0.25, 0.50, 0.75 and 1.00) mol · kg −1 guanidine hydrochloride (GnHCl) have been determined over a temperature range (288.15-318.15) K at (101.3 ± 0.5) kPa. These data have been used to calculate viscosity B -coefficients and standard molar enthalpy of dilution, Δ dil H o which have further been used to evaluate corresponding transfer parameters Δ tr B and Δ tr Δ dil H o , respectively. Positive Δ t r B values and negative Δ tr Δ dil H o values indicate the predominance of hydrophilic–ionic interactions in these systems. Temperature dependence of B -coefficient and Δ dil H o has also been discussed. Spectroscopic studies have been carried out to further investigate the structural changes and the interactions involved in the mixed aqueous solutions.

Apparent molar volumes (V2,ϕ) have been determined from density measurements for some nucleic acid bases (uracil, cytosine, thymine), and nucleosides (uridine, cytidine and thymidine) in water and in aqueous solutions of (0.10, 0.25, 0.50, 0.75 and 1.00) mol ⋅ kg−1 guanidine hydrochloride (co-solute) over the temperature range (288.15 to 318.15) K, at atmospheric pressure using vibrating-tube digital densimeter. These data have further been used to calculate apparent molar volumes V 2 , ϕ o at infinite dilution and the corresponding apparent molar volumes of transfer Δtr V 2 , ϕ o at infinite dilution from water to aqueous solutions of guanidine hydrochloride (GnHCl). Apparent molar expansibilities ( E 2 , ϕ o ), second-order derivative ∂ 2 V 2 , ϕ o / ∂ T 2 p and volumetric interaction coefficients have also been calculated. Co-sphere overlap model has been used to rationalize the data in terms of molecular interactions (hydrophilic, hydrophobic and ionic interactions). The present data have been compared and discussed for the systems studied with those reported in the presence of other co-solutes. The contributions of –CH2 group to V 2 , ϕ o have also been calculated at different temperatures and concentrations of GnHCl.

Thermodynamic and transport properties of aqueous solutions are very useful in the elucidation of solute-solvent and solute-solute interactions, which help to understand the hydration and taste behavior of solutes. The densities and viscosities of L-glycine, β-alanine and L-leucine have been determined in water and in aqueous solutions of sodium propionate (NaP) and calcium propionate (CaP) at temperatures 298.15 and 308.15K. From these data, apparent molar volumes (V2,ϕ), viscosity B-coefficients and corresponding transfer parameters (ΔtrV2,ϕo and ΔtrB) have been calculated. The dB/dT values suggest that L-glycine and β-alanine act as structure-breaker, while L-leucine acts as structure-maker both in water and in aqueous solutions of NaP and CaP. The decrease in hydration number and change in taste behavior have also been observed with increasing concentration of the cosolute.