Electrochromic detection of a coherent component in the formation of the charge pair P⁺H(L)^- in bacterial reaction centers

We demonstrate coupling of an intraprotein electron transfer reaction to coherent vibrational motions. The kinetics of charge separation toward the radical pair state P⁺H(L)^- were studied in reaction centers of Rhodobacter sphaeroides at 15 K. The electrochromic shift of the bacteriochlorophyll monomers is the most prominent spectral feature associated with this charge displacement. The newly reported absolute absorption spectrum of the P⁺H(L)^- state is discussed in terms of this shift. In wild-type reaction centers, the rise kinetics of the electrochromic shift display a small but significant 30 cm^-1 periodic modulation (period of ~1 ps). This modulation is also present in FL181Y mutant reaction centers, where overall charge separation is somewhat more rapid than in the wild-type reaction center. In contrast, in YM210L mutant reaction centers, where the charge separation is much slower, the modulation is absent. The conclusion that the motion along the reaction coordinate has a 30 cm^-1 coherent component is discussed in light of possible mechanisms of electron transfer.