Electronic and Nuclear Dynamics of the Accessory Bacteriochlorophylls in Bacterial Photosynthetic Reaction Centers from Resonance Raman Intensities

Abstract

Resonance Raman spectra and absolute Raman scattering intensities of the Franck−Condon coupled vibrational modes of the accessory bacteriochlorophylls (B) in the photosynthetic reaction center from Rb. sphaeroides have been obtained with excitation in their 800 nm Q y absorption band. Although the relative Raman intensities are unchanged when the temperature is reduced from 278 to 95 K, the absolute Raman scattering intensity is found to increase by a factor of ∼4 at 95 K. A self-consistent multimode vibronic model for the excited-state properties of B has been developed that provides a unique fit to the absorption band and the Raman cross sections at both temperatures using a total S (linear electron−nuclear coupling factor) of 0.32 and an effective electronic dephasing time of ∼54 fs at 278 K and ∼210 fs at 95 K. The Raman scattering cross sections are consistent with an electronic structure model where the two B chromophores absorb and scatter independently. The Raman cross sections scale as 1/T 2 in the 95−298 K range, suggesting that the pure dephasing time, if modeled as a Gaussian (slow-modulation limit), decreases linearly with temperature. At room temperature, the electronic dephasing time is controlled by bath-induced dephasing processes rather than the energy transfer time from the excited state of B. The electronic and nuclear relaxation parameters derived here provide a more quantitative picture of the time scale and structural relaxation of the chromophores involved in energy transfer in reaction centers.

Ultrafast Dynamics and Spectroscopy of Bacterial Photosynthetic Reaction Centers

“…Here (a 1 ; a 2 ; a 3 ; a 4 ) represent the diagonal matrix elements, while (b; b 14 ; b 23 ; b 34 ) represent the off-diagonal matrix elements [67]. As shown in Introduction, a main feature of the P band is that its absorption maximum shows a pronounced temperature shift [42,52]. According to the displaced oscillator model, the absorption maximum is independent of T. Although the distortion effect of potential surfaces will introduce some temperature shift, the effect cannot be as large as that shown in Fig.…”

“…(2.38) we can see that in order to analyze the absorption spectra it is necessary to know the vibrational modes involved and their Huang-Rhys factors. Fortunately for the B band, these data have been obtained by Mathies and Boxer (see Table I) by analyzing the resonance Raman spectra [42,46,47]. But they only analyzed the B band.…”

“…But they only analyzed the B band. The data shown in Table I for the P band are from the literature [42,46,47] and from our fitting of absorption spectra [68].…”

“…These bands have conventionally been assigned to the Q y electronic transitions of the P (870 nm), B (800 nm), and H (870 nm) components of RCs. By considering that the special pair P can be regarded as a dimer of two 2 s. h. lin et al ultrafast dynamics and spectroscopy 3 bacteriochlorophylls, the P band can be assigned to the lower excitonic band of P. By taking into account some difference in the protein environment in the L and M branches [2,3,[6][7][8][9][10][17][18][19][20][21]23,[30][31][32][33][34]41,42,46,47,52], the 800-nm band can be attributed to B L and B M [30][31][32][33][34][35][36][37][38][39][40][41][42]46,47]. The vibrational frequency of the special pair P and the bacteriochlorophyll monomer B have also been extracted from the analysis of the Raman profiles [39,40,42,44,51].…”

Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin

“…capsulatus F(L121)H/F(L97)V and L(M212)H RCs with excitation in the 770 -785 nm region are due almost exclusively to ␤ with little or no interference from the accessory BChls. With excitation at 800 nm, which probes the accessory BChls, 27,28,32,35,37,38 the RR spectra obtained for the two Rb. capsulatus ␤-containing mutants are essentially identical to one another and identical to that of the wild type.…”