Effect of Binding of Ions and Other Small Molecules on Protein Structure. VIII. On Isomerization of Serum Albumin in Acidic Media

The complex electrophoretic patterns of BSA in 0.02M NaCl‐HCl are due to reaction boundaries arising from a set of rapidly established isomerization equilibria and a slow, apparently irreversible reaction. The slow reaction is of only minor importance for electrophoresis in acetate buffer, although a second irreversible process occurs in this solvent at protein concentrations of 0.2% and lower. This latter reaction accounts for the bimodality of the patterns obtained at low field strength with 0.2% BSA in acetate buffer. Resolution of those peaks corresponding to the rapidly established isomerization equilibria is a consequence of changes in either pH or conductance in the reaction boundary during nonideal electrophoresis. Equilibrium constants can, nevertheless, be computed from extrapolated values of the various electrophoretic mobilities without recourse to area measurements. Our observations suggest the following set of consecutive isomerization equilibria: $$\begin{array}{c}{\mathrm{N+nH}}^{+}\mathrm{\rightleftarrows F}\\ {\mathrm{F+mH}}^{+}\mathrm{\rightleftarrows I}\end{array}$$ where, as in the reaction scheme of Aoki and Foster, the F form of the protein has a larger electrophoretic mobility than the N form, whereas that of the I form is intermediate between those of the N and F forms.