Estimation of the permanent dipole moment of bacteriorhodopsin

Abstract

The static electric dipole moment persisting in bacteriorhodopsin was defined from electro-acoustic measurements of the dried films of purple membranes and compared with the value estimated from quantum chemical calculations. The projection of this value normal to the membrane surface is experimentally estimated to be equal to 40 D and oriented from the cytoplasmic side to the extracellular side of the membrane. This value is almost independent of the environment pH. QM/MM calculations were also performed for the known structures of the ground and intermediate states of bacteriorhodopsin. According to calculations the dipole moment is mainly determined by the cytoplasmic and extracellular coils, while the contribution from the transmembrane helices is smaller and of the opposite direction, and this value corresponding to the active centre is small. The calculated values of the dipole moment of bacteriorhodopsin in the intermediate states K, L, and M provide understanding about the origin of the driving force for the proton pumping. Employing the values of the dipole moments corresponding to the ground and intermediate states of bacteriorhodopsin, defined by means of QM/MM calculations, the experimentally determined photoelectric response of the dried films is explained. Keywords: dried films of purple membranes, electrostriction, photo-electric response, pH changes, molecular mechanics calculations, quantum chemical calculations