Vibrational coherence in bacterial reaction centers with genetically modified B-branch pigment composition

Femtosecond absorption difference spectroscopy was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of the HM182L mutant of Rhodobacter (Rb.) sphaeroides. In this mutant, the composition of the B-branch RC cofactors is modified with respect to that of wild-type RCs by replacing the photochemically inactive BB accessory bacteriochlorophyll (BChl) by a photoreducible bacteriopheophytin molecule (referred to as ΦB). We have examined vibrational coherence within the first 400 fs after excitation of the primary electron donor P with 20-fs pulses at 870 nm by studying the kinetics of absorbance changes at 785 nm (ΦB absorption band), 940 nm (P*-stimulated emission), and 1020 nm (BA− absorption band). The results of the femtosecond measurements are compared with those recently reported for native Rb. sphaeroides R-26 RCs containing an intact BB BChl. At delay times longer than ∼50 fs (maximum at 120 fs), the mutant RCs exhibit a pronounced BChl radical anion (BA−) absorption band at 1020 nm, which is similar to that observed for Rb. sphaeroides R-26 RCs and represents the formation of the intermediate charge-separated state P+BA−. Femtosecond oscillations are revealed in the kinetics of the absorption development at 1020 nm and of decay of the P*-stimulated emission at 940 nm, with the oscillatory components of both kinetics displaying a generally synchronous behavior. These data are interpreted in terms of coupling of wave packet-like nuclear motions on the potential energy surface of the P* excited state to the primary electron-transfer reaction P* → P+BA− in the A-branch of the RC cofactors. At very early delay times (up to 80 fs), the mutant RCs exhibit a weak absorption decrease around 785 nm that is not observed for Rb. sphaeroides R-26 RCs and can be assigned to a transient bleaching of the Qy ground-state absorption band of the ΦB molecule. In the range of 740–795 nm, encompassing the Qy optical transitions of bacteriopheophytins HA, HB, and ΦB, the absorption difference spectra collected for mutant RCs at 30–50 fs resemble the difference spectrum of the P+ΦB− charge-separated state previously detected for this mutant in the picosecond time domain (E. Katilius, Z. Katiliene, S. Lin, A.K.W. Taguchi, N.W. Woodbury, J. Phys. Chem., B 106 (2002) 1471–1475). The dynamics of bleaching at 785 nm has a non-monotonous character, showing a single peak with a maximum at 40 fs. Based on these observations, the 785-nm bleaching is speculated to reflect reduction of 1% of ΦB in the B-branch within about 40 fs, which is earlier by ∼80 fs than the reduction process in the A-branch, both being possibly linked to nuclear wave packet motion in the P* state.