Temperature Dependence of the Q y Resonance Raman Spectra of Bacteriochlorophylls, the Primary Electron Donor, and Bacteriopheophytins in the Bacterial Photosynthetic Reaction Center ² Nerine J. Cherepy, ‡,§ Andrew P. Shreve, ‡,| Laura J. Moore, ⊥ Steven G. Boxer, ⊥ and Richard A. Mathies* ,‡ Department of Chemistry, UniVersity of California, Berkeley, California 94720, and Department of Chemistry, Stanford UniVersity, Stanford, California 94305 ReceiVed January 3, 1997; ReVised Manuscript ReceiVed April 9, 1997 X ABSTRACT:Q y -excited resonance Raman spectra of the accessory bacteriochlorophylls (B), the bacte- riopheophytins (H), and the primary electron donor (P) in the bacterial photosynthetic reaction center (RC) of Rhodobacter sphaeroides have been obtained at 95 and 278 K. Frequency and intensity differences are observed in the low-frequency region of the P vibrational spectrum when the sample is cooled from 278 to 95 K. The B and H spectra exhibit minimal changes of frequencies and relative intensities as a function of temperature. The mode patterns in the Raman spectra of B and H differ very little from Raman spectra of the chromophores in Vitro. The Raman scattering cross sections of B and H are 6-7 times larger than those for analogous modes of P at 278 K. The cross sections of B and of H are 3-4 times larger at 95 K than at 278 K, while the cross sections of P are approximately constant with temperature. The temperature dependence of the Raman cross sections for B and H suggests that pure dephasing arising from coupling to low-frequency solvent/protein modes is important in the damping of their excited states. The weak Raman cross sections of the special pair suggest that the excited state of P is damped by very rapid (<30 fs) electronic relaxation processes. These resonance Raman spectra provide information for developing multimode vibronic models of the excited-state structure and dynamics of the chromophores in the RC. The photophysical properties of the chromophores in the photosynthetic reaction center (RC) 1 have been extensively studied with the goal of understanding how they mediate efficient electron transfer reaction across the photosynthetic membrane (Boxer et al., 1989; Rees et al., 1989; Fleming & van Grondelle, 1994; Friesner & Won, 1989; Kirmaier & Holten, 1987). The bacterial reaction center of Rhodobacter sphaeroides has an approximate C 2 axis of symmetry about which lies a bacteriochlorophyll (BChl) dimer P, two monomeric BChls called B, and two bacteriopheophytin (BPheo) molecules called H. The lowest energy Q y elec- tronic transitions of the P, B, and H chromophores lie in the near-infrared, with peaks at room temperature near 860, 800, and 760 nm, respectively (Figure 1). Charge separation is complete between the special pair and a bacteriopheophytin within about 3 ps at room temperature and in about half that time at 100 K. Excitation of B or H leads to ultrafast (100- 200 fs) energy transfer to P, although the details of the excited-state dynamics in B are as yet unresolved (Jia et al., 1995; Martin et al., 1986; Stanley et al., 1996). The electronic transitions comprising the near-infrared absorption bands of the RC need to be better characterized and the nature of their vibronic coupling determined in order to better understand the rapid and efficient photochemistry in the RC. Resonance Raman spectra and their temperature depen- dence can provide insight into the electronic dynamics and reveal the normal coordinates along which the chromophores distort when electronically excited. Comparisons with model ² This work was funded by NSF Grant CHE 94-19714 to R.A.M. A.P.S. was supported by a National Institutes of Health Postdoctoral Fellowship (GM 14298). S.G.B. thanks the National Science Founda- tion Biophysics Program for its support. * Author to whom correspondence should be addressed. ‡ University of California at Berkeley. § Current address: Department of Chemistry, University of Califor- nia, Santa Cruz, CA 95064. | Current address: Los Alamos National Laboratory, CST-4, MS G755, Los Alamos, NM 87545. ⊥ Stanford University. X Abstract published in AdVance ACS Abstracts, June 15, 1997. 1 Abbreviations: RC, reaction center; BChl, bacteriochlorophyll; BPheo, bacteriopheophytin; SERDS, shifted excitation Raman differ- ence spectroscopy; FT, Fourier transform; OD, optical density. FIGURE 1: Near-IR absorption spectra of Rb. sphaeroides reaction centers at 298 and 77 K. Excitation wavelengths of 890, 870, and 850 nm were used to acquire the Raman spectra of P while 800 and 760 nm excitation were used to acquire the B and H spectra. 8559 Biochemistry 1997, 36, 8559-8566 S0006-2960(97)00024-X CCC: $14.00 © 1997 American Chemical Society