Near-infrared resonance Raman spectra of chlorosomes: Probing nuclear coupling in electronic energy transfer

Resonance Raman spectra of chlorosomes, isolated from the green bacteria Chloroflexus aurantiacus and Chlorobium tepidum, have been obtained with excitation in their near-infrared (Q(y)) absorption bands by using shifted-excitation Raman difference spectroscopy. Both spectra exhibit strong low-frequency (50-300 cm(-1)) modes, although these modes are relatively more intense in the Chloroflexus aurantiacus spectrum than in the Chlorobium tepidum spectrum. These intensity differences indicate that the low-frequency electron-nuclear coupling is strongest in the Chloroflexus aurantiacus chlorosome. The Raman spectrum of the Chloroflexus aurantiacus chlorosome is independent of excitation wavelength, and the scattering intensity tracks the absorption band and relative Raman intensities. A model comprised of more than one transition having similar vibronic parameters most successfully reproduces the Chloroflexus aurantiacus Q(y) absorption band shape. The resonance Raman mode frequencies and intensities agree qualitatively with the power spectrum generated by Fourier transforming the oscillations seen in femtosecond stimulated emission measurements on both chlorosomes. The greater intensity of the low-frequency Raman modes observed for the Chloroflexus aurantiacus chlorosome is consistent with the increased amplitudes of the stimulated emission oscillations seen for that chlorosome. These data provide a more quantitative understanding of the vibronic properties of chlorosomes that can now be used to explore the possible role of vibrational coherence in energy transfer.