The photosynthetic purple sulfur bacterium Allochromatium vinosum (Alc. vinosum) has six pairs of pucBA genes that encode the a and � polypeptides of the LH2 complex. It was recently observed that this LH2 antenna complex exhibits an unusual spectral splitting of the B800 absorption band and recent transient absorption spectroscopy measurements suggested that the band splitoriginates from two spectral forms of the associated monomeric BChl a molecules bound within LH2 [Niedzwiedzki et al. Biochim. Biophys. Acta 2012 , 1817, 1576-1587]; though different explanations also exist in the literature [van Dijk et al. Spectrochim. Acta A 1998 , 54, 1269-1278; D. Garcia 1986; Kennis et al. J. Phys. Chem. 1996 , 100, 2438-2442; Kennis et al. J. Phys. Chem. B 1997 , 101, 390-395]. We suggest that the split of the 800 nm band is not attributed to heterogeneity among the B800 BChls within LH2, rather, B800 BChls can be located in slightly different protein environments leading to either strong or weak hydrogen bonds to the protein and two distinct B800 peaks. We propose that several BChls contributing to the B800 band have hydrogen bonds between BChl a and amino acid residues of the apoproteins. 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 suggest thatin Alc. vinosum selective excitation into the B800B band leads to uncorrelated excitation energy transfer and subsequent photo-transformation of the B800R molecules. It appears that proton tunneling along the hydrogen bond is the major mechanism of the hole burning process in the B800R band. Future studies should provide more insight into the �intrinsic� (host matrix) and �extrinsic� (e.g., hydrogen bond) related two-level systems in the protein landscape of these important LH2 complexes.

New insight into unusual splitting of the B800 absorption band in the LH2 antenna of the purple sulfur bacterium <i>?Allochromatium vinosum</i>

248th National Meeting (2014)

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New insight into unusual splitting of the B800 absorption band in the LH2 antenna of the purple sulfur bacterium ?Allochromatium vinosum