Influence of the protein binding site on the absorption properties of the monomeric bacteriochlorophyll in Rhodobacter sphaeroides LH2 complex.

Resonance Raman spectroscopy was performed on peripheral light-harvesting proteins from Rhodobacter sphaeroides in which the residue betaArg-10 has been modified by site-selected mutagenesis. We show that this residue is indeed involved (as proposed by X-ray crystallographic studies on the LH2 complex from Rhodopseudomonas acidophila), in an H-bond with the acetyl carbonyl of the 800 nm-absorbing BChl in these proteins (B800), and that the presence of such an H-bond induces a ca. 10 nm red shift of the lowest energy transition (Qy) of this molecule. Moreover, other parameters involved in the fine tuning of the absorption of the B800 molecules may be determined from our experiments, and we propose that the local electromagnetic properties of the B800 binding site may induce an additional 10 nm red shift of this transition. These results constitute the first experimental evidence for the parameters able to modify in vivo the absorption of "monomeric" BChl molecules, i.e. BChl not involved in strong excitonic interactions, and will be of great help for understanding the absorption properties of such pigments in other light-harvesting systems.