The photosynthetic purple sulfur bacterium Allochromatium vinosum (Alc. vinosum) has six pairs of pucBA genes that encode the α and β polypeptides of the LH2 complex [Carey et al. Biochim. Biophys. Acta 2014, 1837, 1849]. These genes lead to different LH2 antenna complexes based on growth conditions: B800-820, B800-840 and B800-850, where the usual 850 nm absorbing band shifts to shorter wavelength in low-light growth conditions. It was recently observed that the B800-850 LH2 antenna complex exhibits an unusual spectral splitting of the B800 absorption band and recent transient absorption spectroscopy measurements suggested that the band split originates from two spectral forms of the associated monomeric bacteriochlorophyll a (BChl) molecules bound within LH2 [Niedzwiedzki et al. Biochim. Biophys. Acta 2012, 1817, 1576]; though different explanations also exist in the literature [van Dijk et al. Spectrochim. Acta A 1998, 54, 1269; Kennis et al. J. Phys. Chem. 1996, 100, 2438; Kennis et al. J. Phys. Chem. B 1997, 101, 390]. We propose that various protein conformations lead to either strong or weak hydrogen bonds between the protein and B800 pigments. That is, we suggest that BChls contributing to the B800R and B800B bands (near 804 and 792 nm, respectively) differ only in the position of the proton in the C=O···H-protein hydrogen bond. Our data indicate that in Alc. vinosum not only selective laser excitation, but also white light illumination into the B800B and B850 bands can lead to photo-transformation of the B800R molecules. It appears that proton tunneling along the hydrogen bond is the major mechanism of the hole burning (HB) process in the B800R band. In addition, temperature-dependent absorption spectra of B800-B850 LH2 show the positions of several isosbestic points within the B850 band; indicative of multiple states. Therefore, an exciton dichotomous model is applied in an attempt to explain the unusual B800 spectral splitting and complicated shapes of HB spectra of the B800-850 LH2 complex.

On the B800-850 LH2 antenna complex of the purple sulfur bacterium Allochromatium vinosum

249th National Meeting (2015)

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