The functional sites of chlorophylls in D1 and D2 subunits of photosystem II identified by pulsed EPR.

The functional site of ChlZ, an auxiliary electron donor to P680+, was determined by pulsed ELDOR applied to a radical pair of YD * and Chlz+ in oriented PS II membranes from spinach. The radical-radical distance was determined to be 29.5 A and its direction was 50 degrees from the membrane normal, indicating that a chlorophyll on the D2 protein is responsible for the EPR Chlz+ signal. Spin polarized ESEEM (Electronin Spin Echo Envelop Modulation) of a 3Chl and QA - radical pair induced by a laser flash was observed in reaction center D1D2Cytb559 complex, in which QA was functionally reconstituted with DBMIB and reduced chemically. QA -ESEEM showed a characteristic oscillating time profile due to dipolar coupling with 3Chl. By fitting with the dipolar interaction parameters, the distance between 3Chl and QA - was determined to be 25.9 A, indicating that the accessory chlorophyll on the D1 protein is responsible for the 3Chl signal.