THEORETICAL INVESTIGATION OF THE RATES OF ELECTRON-TRANSFER PROCESSESQ(I)(-)-]Q(I)+Q(II)(-) AND Q(I)(-)+Q(II)(-)-]Q(I)+Q(II)(2-) IN PHOTOSYNTHESIS(Q(II))

A theoretical investigation on the rates of electron-transfer processe s Q(I)(-) + Q(II) --> Q(I)(-) + Q(II)(-) and Q(I)(-) + Q(II)(-) --> Q( II)(-) + Q(II)(2-) was carried out by using the Marcus theory of long- range electron transfer in solution. The molecular reorganizational pa rameter lambda, the free-energy change Delta G(0) for the overall reac tion, and the electronic matrix element H-DA for these two processes w ere calculated from the INDO-optimized geometries of molecules Q(I), Q (II), and histidine. Q(I) and Q(II) are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe2+ ion. The plastoquinone representing Q(I) is additionally flanked by two peptide fragments. Each of the species (Pep)(2)Q(I) . His and His . Q(II) has been considered to be immersed in a dielectric continuum that represents the surrounding molecules a nd protein folds. INDO calculations confirm the standard reduction pot ential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be cl ose to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ . His](-) and the net charge in [PQH . His ](-). The electron is transferred effectively from the plastoquinone p art of [(Pep)(2)Q(I) . His](-) to the plastoquinone moiety of Q(II). H is in the first step and to the plastoquinone fragment of HisH(+). Q(I I)(-) in the second step. Therefore, we made use of the formula for th e rate of through-space electron transfer from Q(I) to Q(II) (and to Q (II)(-)). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular br idge consisting of histidines and also Fe2+. The through-bridge effect is inherent in our calculation of Delta G(0), H-DA, and the reorganiz ation parameter A. We investigated the correlation between half-times for the transfer and (D-op(-1) - D-s(-1)), where D-op and D-s are, res pectively, optical and static dielectric constants of the condensed ph ase in the vicinity of the plastoquinones. We found that with reasonab le values of D-op (2.6) and D-s (8.5) the experimental rates are adequ ately explained in terms of transfers from the plastoquinone moiety of Q(I) to that of Q(II). The t(1/2) values calculated for the two proce sses are 247 and 472 mu s in the absence of Fe2+ and 134 and 181 mu s in the presence of Fe2+. These are in good agreement with the observed values which are approximate to 100 and approximate to 200 mu s when Fe2+ is present in the matrix and which are known to be almost twice a s large when the Fe2+ is evicted from the matrix. The present work als o shows that the Marcus-Hush theory of long-range electron transfers c an be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. (C ) 1997 John Wiley & Sons, Inc.