Studies of an acid-induced species of purple membrane from Halobacterium halobium

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Abstract
A new spectral species of the purple membrane of H. halobium was observed below pH 3.2. The formation of this new species is temperature-dependent and is favored by increasing temperatures up to the physiological range of the organism. The rate of formation at pH 3.0 and 22.degree. C is 7.9 .times. 10-3s-. The spectral distribution and temperature-dependence of the new species suggest that it may be phototransient O, stabilized by low pH. Flash-photolytic experiments in the pH range 7.2-2.7 show a pH-dependence corresponding to the static events and are consistent with a single protonation of bacteriorhodopsin below pH 3.22. These results can also be interpreted in terms of the stabilization of phototransient O at low pH. The temperature-dependence of the formation of the acid-induced species may reflect a relationship with the phase transition of the membrane.