SENSITIVE ANALYSIS OF THE OCCUPANCY OF THE QUINONE BINDING-SITE AT THE ACTIVE BRANCH OF PHOTOSYNTHETIC REACTION CENTERS

A sensitive method for determining the degree of the occupancy of the primary quinone site in reaction centers of photosynthetic bacteria is described. By measuring the transient absorbance bleaching of the primary electron donor during the time interval 1 ns to 1 ms after an exciting laser pulse, small amounts of quinone-depleted reaction centers can be detected with an accuracy of about +/- 3%. In the case of Rb. sphaeroides, standard preparations typically contain 20% reaction centers in which the quinone binding site at the active branch is not occupied.