THE pH DEPENDENCE OF THE ADSORPTION ISOTHERM AND ABSORPTION SPECTRUM OF METHYL ORANGE BOUND TO HUMAN AND BOVINE SERUM ALBUMIN

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Abstract
The difference between adsorption isotherms and absorption spectra of methyl orange bound to human and to bovine albumin at pH 6.8, 9.1, and 11.0 has been studied. Exaltation of the spectrum of methyl orange bound to human albumin is not necessarily correlated with total binding capacity. However, above pH 6.8, heterogeneity of the binding sites for methyl orange on human albumin increases so markedly that it is reflected in an appreciable increase in extinction coefficient for the first three or four anions bound. The exaltation is accompanied by an increased −ΔF° of binding. Increase in ionic strength diminishes exaltation, and denaturation of the protein destroys it. These effects were not observed for bovine albumin. Results are interpreted in terms of a limited reversible expansion of the human protein molecules, without unfolding, due to intramolecular, electrostatic repulsion between groups.