Organic solvents-induced difference spectra of bovine serum albumin

Abstract

Difference spectra of tyrosyl residues in bovine serum albumin produced by three organic solvents (ethanol, n-propanol, and dioxane) have been studied at three different pH values (pH 2.2, pH 5.4–5.8, and pH 11.6). The measurements were taken at 25 °C and ionic strength 0.03 and in a concentration of organic solvent from 5% to 25% by volume. The spectra are compared to those of a model chromophore, N-acetyl-L-tyrosine ethyl ester. Both n-propanol and dioxane cause a change in the conformation of the protein, while ethanol does not. Near the isoelectric point the difference spectra produced by n-propanol (at high concentration) and dioxane appear to be the results of a superposition of denaturation blue shifts on solvent-induced red-shifts; those produced by ethanol are characteristic of perturbation of tyrosyl and tryptophyl residues. An empirical equation which correlates Δε (the molar difference absorption coefficient) with n (refractive index) and D (dielectric constant) is proposed to interpret the results qualitatively.