Volumetric, conductometric and fluorescence probe studies of interactions between glycyl dipeptides and sodium caprylate in aqueous media

The interactions of glycyl dipeptides (glycylglycine, glycyl-l-valine and glycyl-l-leucine) with sodium caprylate (NaC8) in aqueous solution have been investigated by a combination of density, conductivity and fluorescence methods. Apparent molar volumes (V2,ϕ  ), standard partial molar volumes (V2,ϕ∘), and standard partial molar volumes of transfer from water to aqueous NaC8 solutions (ΔtVo) are evaluated using density data. The limiting molar conductivity of sodium caprylate (Λo) and its anion contribution λo(C8-), as well as the Stokes’ radii of caprylate anion in aqueous glycyl dipeptide solutions have been calculated using conductivity data. Pyrene fluorescence spectra were used to estimate the critical micellar concentration (ccmc) of NaC8 in aqueous dipeptide solutions. The analysis shows that the interactions of sodium caprylate with charged and polar groups of glycyl dipeptides are dominating over the sodium caprylate-non-polar group interactions over the entire concentration range of sodium caprylate. From the volumetric data, it is suggested that the glycyl dipeptides are solubilized in the palisade layer of sodium caprylate micelles. The decrease in Λo values of sodium caprylate with an increase in dipeptide concentration is attributed to the interaction of sodium caprylate with the dipeptides and increasing viscosity of solvent. The addition of dipeptide in water decreases the ccmc of NaC8. The interactions of glycyl dipeptides with sodium caprylate increase with the increase in the size of side chain of carboxylate anions and of glycyl dipeptides.

► The values of V2,ϕ∘ and ΔtV° values are positive.
► The interactions of NaC8 with charged and polar groups of glycyl dipeptides are dominate.
► Addition of glycyl dipeptide in water decreases the limiting molar conductivities and ccmc of NaC8.
► The Stokes’ radii of caprylate anion are much higher than their crystallographic radii.
► The size of alkyl chain of glycyl dipeptides affects the interaction between NaC8 and glycyl dipeptides.