Determination of Q(A)-content in bacterial reaction centers: an indispensable requirement for quantifying B-branch...

We have been able to determine the occupancy of the quinone site at the A-branch (Q(A)) of a reaction center preparation with an accuracy of 2%. This is achieved by accumulating the P(+)Q(-)(A) state after multiple actinic excitation and monitoring the extent of the 30 ms ground state bleaching. This bleaching is corrected for deviations from complete saturation due to competing charge separation to the B-branch. On the other hand, knowledge of the Q(A) content is indispensable for determining the yield of B-branch charge separation from nanosecond transients associated with the recombination of P(+)H(-)(B), which have to be corrected for the nanosecond signal originating from P(+)H(-)(A) of RCs having lost Q(A).