MAGNETIC FIELD-SENSITIVE STAGE OF ELECTRO N-TRANSFER IN PHOTOSYSTEM-II PRIMARY REACTIONS OF CHLAMYDOMONAS-REINHARDII MUTANTS

Magnetic field-sensitive stage of electron transfer in photosystem II reaction ceters (PS II RCs) of unicellular green alp Chlamydomonas rei nhardii mutants (ACC-66, ACC-98, ACC-238)containing only PS II RCs and antenna core complex was studied. The following conclusions are made: a) the magnetic field-induced change of fluorescence yield is due to charge recombination processes in the radical pair (RP) (P680+ I-); b) exchange interaction in the RP is 12 Oe (3.4 . 10(7) s-1), correspond ing to the distance between P680 and I of about 10 angstrom; c) the ra te constants for charge recombination of RP to the lowest excited sing let, ground, and triplet states of the P680 at 293 K are k(s)=0.30-0.3 2 ns-1, k(g)=0.1 - -0.12 ns-1, k(t)=0.9-1.1 ns-1, correspondingly. Act ivation energy of the reaction (P680+I-)-->P680I is 0.040-0.046 eV; d ) Fluorescence yield increase resulting from PS II RC transitions both to the state P680 I Q(A)- and to the state P690 I Q(A)2- is due to co ntribution of delayed luminescence arising from charge recombination o f the RP (P690+I-) and to an increase in the antenna chlorophyll fluor escence yield, induced by a decrease of the exitation energy trapping efficiency by the reaction centers. It is suggested that the electron transfer from I- to Q(A)H. is possible (the P690 I Q(A)H. state is for med in the dark under sodium dithionite treatment at concentrations ex ceeding 20 mM).