Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 4. Asymmetric Energy Transfer in the Heterodimer Mutant
| Autor: | Alex de Winter, Brett A. King, Steven G. Boxer, Tim B. McAnaney |
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| Rok vydání: | 2001 |
| Předmět: | |
| Zdroj: | The Journal of Physical Chemistry B. 105:1856-1862 |
| ISSN: | 1520-5207 1520-6106 |
| Popis: | In bacterial photosynthetic reaction centers, ultrafast singlet excited state energy transfer occurs from the monomeric bacteriochlorophylls, B, and bacteriopheophytins, H, to the homodimer special pair, a pair of strongly interacting bacteriochlorophylls. In the M202HL mutant, one of the bacteriochlorophylls comprising the special pair is replaced by a bacteriopheophytin, and this is called the heterodimer special pair or D. We report the direct observation of spontaneous fluorescence from 1 B in the heterodimer mutant. In contrast to results for the homodimer special pair where 1 B decays with a rate constant of (160 fs) -1 (King, B. A.; McAnaney, T. B.; de Winter, A.; Boxer, S. G. J. Phys. Chem. B2000, 104, 8895-8902), 1 B decay in M202HL exhibits two components with rate constants (700 fs) -1 and (190 fs) -1 ; these are similar to what we reported earlier for the rise of 1 D 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 side is replaced by a bacteriopheophytin, the absorption bands corresponding to the chromophores in the BL and BM 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 1 D fluorescence in the double mutant supports the earlier assignment of the slower 700 fs energy transfer component to 1 BL f D, while the faster 190 fs energy transfer component is assigned to 1 BM f D. Replacement of bacteriochlorophyll by bacteriopheophytin in the BM binding site does not alter the time constants of the two energy transfer pathways. Excited state energy transfer to D is the same in QA-depleted and QA-reduced reaction centers, suggesting that electron transfer processes that be might sensitive to a charge on Q A, such as 1 BL f BL + HL - , do not compete with relatively slow 1 BL f D energy transfer. The results support earlier findings that singlet energy transfer from the monomeric chromophores along the L and M branches to the heterodimer special pair is asymmetric and is faster along the 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, conversion of the special pair homodimer to a heterodimer breaks the symmetry of ultrafast energy transfer along the two branches of chromophores. These findings may provide information on differences in the electronic interactions on the L vs M sides of the RC that is relevant to unidirectional electron transfer. |
| Databáze: | OpenAIRE |
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