Rapid-flow resonance Raman spectroscopy of bacterial photosynthetic reaction centers.

Abstract

Rapid-flow resonance Raman vibrational spectra of bacterial photosynthetic reaction centers from the R-26 mutant ofRhodobacter sphaeroides have been obtained by using excitation wavelengths (810-910 nm) resonant with the lowest energy, photochemically active electronic absorption.The technique of shifted excitation Raman difference spectroscopy is used to identify genuine Raman scattering bands in the presence of a large fluorescence background. The comparison of spectra obtained from untreated reaction centers and from reaction centers treated with the oxidant K3Fe(CN)5 demonstrates that resonance enhancement is obtained from the special pair. Relatively strong Raman scattering is observed for special pair vibrations with frequencies of 36, 94, 127, 202, 730, and 898 cm-'; other modes are observed at 71, 337, and 685 cm-'.Qualitative Raman excitation prordes are reported for some of the strong modes, and resonance enhancement is observed to occur throughout the near-IR absorption band of the special pair. These Raman data determine which vibrations are coupled to the optical absorption in the special pair and, thus, probe the nuclear motion that occurs after electronic excitation. Implications for the interpretation of previous holeburning experiments and for the excited-state dynamics and photochemistry of reaction centers are discussed.Bacterial photosynthetic reaction centers (RCs) are the pigment-protein complexes in which the light-induced chargeseparation reactions of photosynthesis occur (1-4). The primary electron donor is a dimer of bacteriochlorophyll (Bchl) molecules called the special pair (P). After optical excitation, electron transfer from lP, the lowest-energy singlet excited electronic state of P, to a bacteriopheophytin acceptor occurs within about 3 ps at room temperature. Electroabsorption measurements suggest that 'P contains a large amount of charge-transfer character (4-8), and holeburning studies and theoretical models indicate that strong coupling exists between electronic and vibrational motions in P (9-20). The electronic-nuclear coupling determines the nuclear motion that occurs after electronic excitation, but the description of this motion, including its role in possibly mediating the primary charge transfer, has been hampered by the lack of information about the vibrational modes of P that are coupled to optical excitation.Resonance Raman spectroscopy can provide information necessary to address many of the unresolved questions about the electronic and vibrational dynamics of RCs. The vibrational modes that appear in a resonance Raman spectrum are just those modes that are coupled to the optical excitation (21), so the ground electronic state frequencies of the coupled modes are determined. In addition, quantitative analysis of the resonance Raman intensities and assignment of the resonance Raman-active modes can provide a picture of the nuclear distortions that occur upon electronic excitation (21). Because these distortions reflect the underlying excited electronic st...

Low frequency vibrational modes in proteins: Changes induced by point-mutations in the protein-cofactor matrix of bacterial reaction centers

“…Spectra obtained with the two techniques show good general agreement (10,45,46,47); so it is of interest to compare the results of the present work with those from resonance Raman investigations.…”

Coherent nuclear dynamics at room temperature in bacterial reaction centers.

“…However, it should be noted that the present technique is unique in its ability to resolve the vibrational structure of the excited state underlying the P-* P* transition at room temperature. Two notable differences between the observations at 10 K and at room temperature are (i) the apparent upshift of the lowest-frequency band to =30 cm-' at room temperature (observed with 100-fs pulses), which is near the lowest resolvable peak in the resonance Raman spectrum of P at 36 cm-' (30), and (ii) the congestion of two peaks, which at 10 K were observed at 122 and 153 cm-', to a single broad band at room temperature peaking at =145 cm-' (observed with 30-fs pulses), presumably a manifestation of frequency broadening of those bands by inhomogeneity and anharmonicity.…”

The nature of the low energy band of the Fenna-Matthews-Olson complex: Vibronic signatures

“…The FMO complex is a trimer, with individual subunits composed of eight strongly coupled chromophores that serve as a link between the Chlorosome antennae and the reaction center of green sulfur bacteria 7,8 . Chromophores have a spatial extension that supports the existence of intramolecular vibrations [9][10][11][12] . These vibrations play an important role when the chromophore structures are perturbed due to the electronic charge redistribution upon photon excitation.…”