Selective enhancement of resonance Raman spectra of separate bacteriopheophytins in Rb. sphaeroides reaction centers

Tunable dye laser excitation of carefully prepared samples of Rb. sphaeroid es reaction centers provides richly detailed resonance Raman (RR) spectra o f the bacteriopheophytins, H, and the accessory bacteriochlorophylls, B. Th ese spectra demonstrate selective enhancement of the separate bacteriopheop hytins on the active (H-L), and inactive (H-M) sides of the reaction center s. The spectra are assigned with the aid of normal coordinate analyses usin g force fields previously developed for porphyrins and reduced porphyrins. Comparison of the H-L and H-M vibrational mode frequencies reveals evidence for greater polarization of the acetyl substituent in H-L than H-M. This p olarization is expected to make H-L easier to reduce. thereby contributing to the directionality of electron transfer from the special pair, P. In add ition, the acctyl polarization of H-L is increased at low temperature (100 K), helping to account for the increase in electron transfer rate. The pola rizing field is suggested to arise from the Mg2+ of the neighboring accesso ry bacteriochlorophyll, which is 4.9 Angstrom from the acetyl O atom. The 1 00 K spectra show sharpening and intensification of a number of Rn bands, s uggesting a narrowing of the conformational distribution of chromophores, w hich is consistent with the reported narrowing of the distribution in elect ron transfer rates. Excitation at 800 nm produces high-quality RH spectra o f the accessory bacteriochlorophylls, and the spectral pattern is unaltered on tuning the excitation to 810 nm in resonance with the upper exciton tra nsition of P. Either the resonance enhancement of P is weak, or the bacteri ochlorophyll RR spectra are indistinguishable for P and B. (C) 2000 John Wi ley & Sons. Inc.