Femtosecond kinetics of electron transfer in the bacteriochlorophyllM -modified reaction centers from Rhodobacter sphaeroides (R-26)

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Abstract
Formation of the vibronic wavepacket by 90-fs excitation of the primary electron donor P in bacteriochlorophyllM-modified reaction centers is shown to induce nuclear motions accompanied by (1) oscillation of the stimulated emission from excited primary electron donor P∗ and (2) wavepacket motions leading to electron transfer at 293 K from P∗ to bacteriochlorophyll (BL) and then to bacteriopheophytin (HL). The latter motions have low frequency (about 15 cm−1) and are related to protein-nuclear motions which are along the reaction coordinate. When the wavepacket approaches the intersection of the reactant (P∗BL) and product (P+BL−) potential energy surfaces (∼1.5 ps delay), about 60% of P∗ is converted to the P+BL− state. The P+HL− state formation is delayed by ∼2 ps with respect to that of P+BL−. It is suggested that the wavepacket is transferred to and moves also slowly on the P+BL− potential energy surface and approaches the intersection of the surfaces of P+BL− and P+HL− within ∼2 ps (∼8 cm−1), indicating the electron transfer to HL.

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