Chromophore organization in photosynthetic reaction centers: High-resolution magnetophotoselection.

The electron spin resonance spectrum of the triplet excited state of Rhodopseudomonas spheroides R-26 reaction centers has been studied after excitation with the polarized narrow-bandwidth output of a tunable dye laser from 520 to 670 nm. A theory is developed relating experimental observables to the angles between the electronic transition dipole moment of the excited chromophore and the principle magnetic axis system of the triplet state of the dimeric trap. Data is presented which demonstrates that the treatment is correct and useful, and angles are obtained for the Q(x) transitions of bacteriopheophytin. High-resolution magnetophotoselection data in the region of the bacteriochlorophyll Q(x) transitions can be combined with polarized photobleaching experiments to provide direct information on the structure of dimeric trap.