Electrooptical measurements on purple membrane containing the wild-type and 10 different bacteriorhodopsin mutants have shown that the direction of the permanent electric dipole moment of all these membranes reverses at different pH values in the range 3.2-6.4. The induced dipole moment and the retinal angle exhibit an increased value at these pHs. The results demonstrate that the bacteriorhodopsin protein makes an important contribution to the electrooptical properties of the purple membrane.
Mostafa, H. I. A., Varo, G., Toth-Boconadi, R., Der, A., & Keszthelyi, L. (1996). Electrooptical measurements on purple membrane containing bacteriorhodopsin mutants. Biophysical Journal, 70(1), 468–472. https://doi.org/10.1016/S0006-3495(96)79590-5