Influence of cation binding on the electro-optically determined electric moments of purple membranes.

Analysis of the electro-optically determined permanent dipole moment and electric polarizability of purple membrane fragments reveals the complex nature of the membrane electric moments. The problem to distinguish between the contribution of the membrane structural charges (charged groups of the polypeptide chain and polar lipid headgroups), bound cations and the electric double layer structure deserves particular attention not only because of its importance for electro-optics but also in respect to the relation of the membrane surface electric properties to the membrane transport function. The removal of divalent cations (Ca2+ and Mg2+) bound to purple membrane in the native state induces a cation-free species of purple membrane (deionized--blue membrane) with drastically changed spectroscopic properties and function. The present paper summarizes our study on the electric moments of blue membrane and their changes during the blue to purple transition. We intended to provide an insight into the possible regulation of this reversible transition (purple-to-blue and blue-to-purple) through changes of the asymmetric charge distribution and the importance of the asymmetric interfacial charge distribution for the proton transfer in purple membranes. The changes in the electric moments (permanent and induced dipole moments) of purple membrane fragments upon di- and trivalent cations binding to cation-depleted purple membranes were studied by electric light scattering (rotational electrokinetics) in d.c. and a.c. electric fields, and by electric pulses with reversing polarity. The results show a recovery of the membrane charge asymmetry (permanent dipole moment) though not of the induced dipole moment.