Energy transfer in the purple membrane of Halobacterium halobium.

The absorption spectrum of the primary photoproduct (the bathoproduct, or K) of the purple membrane protein (PM) at-196 degrees C has a maximum at 628 nm and an extinction coefficient of 87,000. Knowing the absorption spectrum allowed us to calculate the quantum efficiencies for PM to K and K to PM conversion at -196 degrees C. Direct measurements of these quantum yeilds at -196 degrees C gave 0.33 +/- 0.05 and 0.67 +/- 0.04, respectively. Determination of relative quantum efficiencies for PM to K and K to PM conversion by analysis of the absorption spectra of several photostationary-state mixtures of PM and K at -196 degrees C, however, gave wavelength-dependent quantum efficiencies that appear to be greater than 1. These anomolous results can be readily explained in terms of energy transfer from PM to K within the trimer clusters of pigment molecules which exist in the purple membrane. A model for such a transfer predicts an efficiency of energy transfer from PM to K of about 43%.