The quantum efficiency for the photochemical conversion of the purple membrane protein.

The quantum efficiency for the formation of M(412), an intermediate product in the photoconversion of the purple membrane protein of Halobacterium halobium, was determined to be 0.30 +/- 0.03 at -40 degrees C. This photochemical reaction was photoreversible to the original pigment and the ratio of the quantum efficiencies gamma PM(568 leads to M(412)/gamma M(412) leads to PM(568) was 0.39 +/- 0.02. No change was seen in either value when exciton interaction between chromophores was eliminated. The sum of gamma PM(568) leads to M(412) plus gamma M(412) leads to PM(568) was 1.07 +/- 0.10, approximately 1, suggesting that the pigment and its primary photoproduct share a common excited state.