Excited state energy transfer pathways in photosynthetic reaction centers.4. Asymmetric energy transfer in the heterodimer mutant

In bacterial photosynthetic reaction centers, ultrafast singlet excited sta te energy transfer occurs from the monomeric bacteriochlorophylls, B, and b acteriopheophytins, I-I, to the homodimer special pair, a pair of strongly interacting bacteriochlorophylls. In the M202HL mutant, one of the bacterio chlorophylls comprising the special pair is replaced by a bacteriopheophyti n, and this is called the heterodimer special pair or D. We report the dire ct observation of spontaneous fluorescence from B-1 in the heterodimer muta nt. In contrast to results for the homodimer special pair where 1B decays w ith a rate constant of (similar to 160 fs)(-1) (King, B. A.; McAnaney, T. B .; de Winter, A.; Boxer, S. G. J. Phys. Chem. B 2000, 104, 8895-8902), B-1 decay in M202HL exhibits two components with rate constants (similar to 700 fs)(-1) and (similar to 190 fs)(-1); these are similar to what we reported earlier for the rise of D-1 spontaneous fluorescence (King, B. A.; Stanley , R. J.; Boxer, S. G. J. Phys. Chem. B 1997, 101, 3644-3648). In the double mutant M202HL/M182HL, where the accessory bacteriochlorophyll on the M sid e is replaced by a bacteriopheophytin, the absorption bands corresponding t o the chromophores in the B-L and B-M binding sites are quite well resolved , and it is possible to preferentially excite the chromophore on either the L or the M side. Analysis of the rise of D-1 fluorescence in the double mu tant supports the earlier assignment of the slower similar to 700 fs energy transfer component to B-1(L) --> D, while the faster similar to 190 fs ene rgy transfer component is assigned to B-1(M) --> D. Replacement of bacterio chlorophyll by bacteriopheophytin in the BM binding site does not alter the time constants of the two energy transfer pathways. Excited state energy t ransfer to D is the same in Q(A)-depleted and Q(A)-reduced reaction centers , suggesting that electron transfer processes that be might sensitive to a charge on Q(A), such as B-1(L) --> BL+HL- do not compete with relatively sl ow B-1(L) --> D energy transfer. The results support earlier findings that singlet energy transfer from the monomeric chromophores along the L and M b ranches to the heterodimer special pair is asymmetric and is faster along t he M side, in contrast to the homodimer special pair in wild type where the energy transfer rates along the two branches are-very similar. Thus, conve rsion of the special pair homodimer to a heterodimer breaks the symmetry of ultrafast energy transfer along the two branches of chromophores. These fi ndings may provide information on differences in the electronic interaction s on the L vs M sides of the RC that is relevant to unidirectional electron transfer.