The formation and reversal of the acid species of purple membrane generated below pH 4.00 (22 degrees C) is studied together with the photochemical cycle over the pH range 6.40–3.20. The buffering capacity of the membrane reaches a peak at pH 4.30, indicating the possibility of a conformational change taking place. The generation of the new spectral species can take place in the dark and is unaffected by the addition of reducing agents. Kinetic parameters measured indicate that the group being titrated below pH 4.00 could be the same as that protonated in the formation of intermediate O. The temporal placement of intermediate O after M in the photochemical cycle is shown to be incompatible with the data presented here. Reneutralization of acidified purple membrane shows that the spectral changes in acid are reversible but the phototransient properties are altered.

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