Effect of sodium caproate on the volumetric and conductometric properties of glycyl-l-glutamine and l-alanyl-l-glutamine in aqueous solution at 298.15 K

The apparent molar volume (V2,ϕ) of glutamine dipeptides (glycyl-l-glutamine and l-alanyl-l-glutamine) and the molar conductivity (Λ) of sodium caproate (NaC6) have been determined from density and conductance measurements, respectively. The values of V2,ϕ were utilized to deduce the standard partial molar volumes V 2 , ϕ o , standard partial molar volumes of transfer for glutamine dipeptides from water to aqueous sodium caproate solutions (ΔtVo), hydration number (nH) and volumetric interaction coefficients. An increase in transfer volumes and a decrease in hydration numbers of the glutamine dipeptides with increasing sodium caproate concentration have been explained due to stronger interactions of sodium caproate with the charged center and polar groups of the glutamine dipeptides compared to the sodium caproate-nonpolar group of dipeptide interactions. From the molar conductivity of sodium caproate, the limiting molar conductivity of sodium caproate (Λ0) and Walden products (Λ0η0) were calculated. The decrease in Λ0 and Walden products with increase in glutamine dipeptide concentration is attributed to the interactions of sodium caproate with glutamine dipeptides and the friction resistance of the solvent medium. Both the hydration number for glutamine dipeptides and Walden products values in peptide–sodium carboxylate–water systems are in the order: sodium acetate > sodium butyrate > sodium caproate. This can be ascribed to the fact that the interactions of dipeptides with sodium carboxylate increase with the increase in the size of side chain of carboxylate anions.