PHOTOCHEMICAL HOLE-BURNED SPECTRA OF PROTONATED AND DEUTERATED REACTION CENTERS OF RHODOBACTER-SPHAEROIDES

Photochemical hole-burned spectra with improved signal-to-noise ratio (X20) are reported for the protonated and deuterated reaction center of the purple bacterium Rhodobacter sphaeroides. Spectra obtained as a function of burn frequency (omega(B)) establish that the lifetime of P870*, the primary electron-donor state, is invariant to location of omega(B) within the inhomogeneous distribution of P870 zero-phonon line transition frequencies. For both the protonated and deuterated RC, which exhibit P870 absorption widths at 4.2 K of only 440 and 420 cm-1, the zero-phonon holes yield a lifetime of 0.93 +/- 0. 10 ps. This lifetime is independent of temperature between 1.6 and 8.0 K (range over which the zero-phonon hole could be studied). The invariance of the P870* lifetime to omega(B) and other data indicates that the nonexponential decay of P870* (Vos et al. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 8885) is due neither to a distribution of values from the electronic coupling matrix element associated with electron transfer, which one might expect from the normal glasslike structural heterogeneity of the RC, nor to gross heterogeneity. The higher quality of the hole spectra has allowed for more stringent testing of the theoretical model previously used to simulate the P870 hole profiles and absorption spectrum. Although the essential findings reported earlier (see, e.g., Reddy et al. Photosyn. Res. 1992, 31, 167) are not altered, it is concluded that the modeling of the distribution of low-frequency phonons (mean frequency approximately 30 cm-1), which couples to P870*, in terms of a Debye distribution is inadequate. The anomalous low-frequency modes of glasses and polymers are suggested to be important also for proteins.