Some Physical Chemical Aspects of Albumin — Alkali Halide Interaction

Abstract

The interaction of bovine serum albumin (BSA) with alkali halides (AH) have been investigated considering both the partial unfolding of BSA macromolecules in ionic solution and the effect of the Debye-Huckel surrounding ionic atmosphere on the π-electronic structure of peptide groups lying in the nearest proximity to BSA aromatic radicals. Results of MO (PPP and AMI) computation support the idea of a reduced π-electron delocalisation between these peptide groups and the aromatic radicals. In the UV absorption spectra of BSA-AH aqueous solutions, the short wavelength side of the p-benzenoid band, with the maximum centred at λ ≅ 206 nm, exhibits a marked hypochromic effect, depending on AH concentration. From a molecular model of the peptide-AH interaction, the relation ∆A=∆A_max c / (a + c) was obtained and well verified, in which ∆A is the modification of absorbance at a given λ, c is the AH concentration, a and ∆A_max are constants, function of λ. Some interferometrical results support, too, the assumption that a significant interaction took place between BSA and AH, a factor proportional with the thermodynamic activity coefficient of AH being determined in BSA solution of various concentrations. A significant modification of some thermodynamic parameters (T_transition,, ∆G, ∆H, ∆S) was obtained in a dilatometric study of BSA thermal denaturation in aqueous solution and in the presence of Nal ions, which was new evidence of these interactions.