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

Abstract

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 B B accessory bacteriochlorophyll (BChl) by a photoreducible bacteriopheophytin molecule (referred to as Φ B). 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 (Φ B absorption band), 940 nm (P*-stimulated emission), and 1020 nm (B A − absorption band). The results of the femtosecond measurements are compared with those recently reported for native Rb. sphaeroides R-26 RCs containing an intact B B BChl. At delay times longer than ∼50 fs (maximum at 120 fs), the mutant RCs exhibit a pronounced BChl radical anion (B A −) 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 +B A −. 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 +B A − 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 Q y ground-state absorption band of the Φ B molecule. In the range of 740–795 nm, encompassing the Q y optical transitions of bacteriopheophytins H A, H B, and Φ B, the absorption difference spectra collected for mutant RCs at 30–50 fs resemble the difference spectrum of the P +Φ B − 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 Φ B 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.