Ionic salt permeabilities and membrane potentials of asymmetric polypeptide membrane

Asymmetric membrane potentials and transport properties of polypeptide membrane consisting of two layers with poly(L-glutamic acid) and poly (γ-methylL-glutamate) were studied in the pH range of 2 to 6 at 25 °C. Under the condition adopted, the poly (L-glutamic acid) layer of the membrane underwentα helix to coil transition which was confirmed by ATR-IR measurements. The membrane potentials of the asymmetric membrane between two identical solutions of KCl,Δψ _asym, which is called asymmetric potential, were observed.Δψ _asym values were effected by the structural transition of poly (L-glutamic acid) layer. As a result, the effective charge density of the membrane, which was derived by the asymmetric membrane potential measurements, had a maximum at the transition region of pH=4.8. On the other hand, the permeation coefficient of KCl,P _i, was higher in the direction from poly (L-glutamic acid) side to poly (γ-methylL-glutamate) side, “G →M direction”, than in the oppositeM →G direction. Furthermore,P _i inG →M direction was dependent on the solution pH, that is,P _i decreased when pH was increased to 4.8 and increased on further increasing of pH These membrane behaviors were described in terms of the competition between structural transition and variation of fixed charge density owing to the helix to coil transition of the asymmetric polypeptide membrane.