Nuclear wave packet motion between P* and P(+)B(A)(-) potential surfaces with a subsequent electron transfer to H(A) in bacterial reaction centers at 90 K. Electron transfer pathway.

In Rhodobacter sphaeroides R-26 reaction centers (RCs) the nuclear wave packet induced by 25 fs excitation at 90 K moves on the primary electron donor P* potential energy hypersurface with initial frequency at ∼130 cm - 1 (monitored by stimulated emission measurement). At the long-wavelength side of P* stimulated emission at 935 nm the wave packet is transferred to the surface with P + B A - character at 120, 380, 1.2 fs, etc. delays (monitored by measurement of the primary electron acceptor B A - band at 1020 nm). However, only beginning from 380 fs delay and later the relative stabilization of the state P + B A - is observed. This is accompanied by the electron transfer to bacteriopheophytin H A (monitored by H A band measurement at 760 nm). The most active mode of 32 cm - 1 in the electron transfer and its overtones up to the seventh were found in the Fourier transform spectrum of the oscillatory part of the kinetics of the P* stimulated emission and of the P + B A - and P + H A - formation. This mode and its overtones are apparently populated via the 130 cm - 1 vibrational mode. The deuteration of the sample shifts the fundamental frequency (32 cm - 1 ) and all overtones by the same factor of ∼ 1.3. This mode and its overtones are suppressed by a factor of ∼ 4.7 in the dry film of RCs. The results obtained indicate that the 32 cm - 1 mode might be related to a rotation of hydrogen-containing groups (possibly the water molecule) participating in the modulation of the primary electron transfer from P* to B A - in at least 35% of RCs. The Brookhaven Protein Data Bank (1PRC) displays the water molecule located at the position HOH302 between His M200 (axial ligand for P B ) and the oxygen of ring V of B A which might be a part (∼35%) of the molecular pathway for electron transfer from P* to B A .