BIPHASIC M DECAY OF HIGH pH DEHYDRATED PURPLE MEMBRANE STUDIED WITH FOURIER TRANSFORM INFRARED SPECTROSCOPY: A MODEL ACCOUNTING FOR FUNCTIONAL DIFFERENCES BETWEEN DIFFERENT M FORMS

Abstract— A purple membrane film treated with a pH 10 buffer and subsequently dehydrated exhibits an M intermediate that consists of two clearly separable fast‐and slow‐decaying forms at 275 K; 45% decays thermally within the first 10 min but most of the remaining is stable even after 25 min. Fourier transform infrared spectroscopy is applied to investigate differences in molecular structural changes accompanying thermal decay of the fast‐decaying form and photochemical decay (M bacteriorhodopsin back photoreaction) of the slow‐decaying form. When difference spectra are taken of the two decay transitions, small but highly reproducible differences are observed. These suggest differences in molecular structural changes accompanying the two decay transitionas. However, the high degree of similarity in the spectra also suggests that these forms of M are not in thermal equilibrium with other intermediates such as N. We propose a model to account for functional differences between two M forms.