The determination of the membrane potentials of protein solutions and the valence of protein ions

Abstract

A method is described which can be applied to systems where the potentials are small and the volumes of solutions are restricted. Hb, serum albumin, acid hematin, kathemoglobin and edestin were equilibrated with dialy-sates with pH values ranging from 2.0 to 7.8 and conc. from 0.005 to 0.16[image]. The observed relationship between the membrane potential E and the conc. Cv, (gm. protein per ml. solvent) closely approximated a straight line when E was less than 2 mv. The hypothesis is advanced that, from such measurements, it is possible to calculate the mean valence of the protein ions, symbolized np, by the formula np=0.00425 [image]J(E/Cv)o, in which M = molecular wt. of the protein, J=the sum of the concs. of the ions in the dialysate multiplied by the squares of their valences, and (E/Cv)o = the limiting value of the ratio at Cv = 0, detd. by extrapolation. The hypothesis is consistent with measurements of the relationships between membrane potentials and protein conc., and with evidence obtained from chemical analyses and osmotic pressure measurements on dilute solutions of edestin and on Congo red. In the case of serum albumin, the results obtained can be reconciled with investigations of the distribution of ions on the assumption that the protein exists as a zwitterion and that some of the positive charges are neutralized by phosphate ions.