Structure and protein binding interactions of the primary donor of the Chloroflexus aurantiacus reaction center.

Soret resonance, QX resonance, and QY near-infrared Fourier transform (FT) (pre)resonance Raman spectroscopies were used to determine pigment-protein interactions of specific bacteriochlorin molecules in the reaction center from Chloroflexus aurantiacus. FT Raman spectroscopy, using 1064 nm excitation, was used to selectively obtain preresonance and resonance vibrational Raman spectra of the primary donor (P) of reaction centers (RCs) from Chloroflexus aurantiacus in the Po and P.+ states, respectively. The FT Raman spectrum of RCs in their neutral P (Po) state exhibits bands at 1605, 1632, 1648, and 1696 cm-1 which are attributable to P in its resting neutral state. Specifically, the latter three Raman bands can be assigned to the conjugated C2 acetyl and C9 keto carbonyl groups of the bacteriochlorophyll (BChl) molecules constituting P. The observation of at least three such bands is indicative of a non-monomeric nature of P, consistent with the proposal that it is a dimer of BChl molecules. The 1632 cm-1 band is consistent only with a hydrogen bonded BChl acetyl carbonyl, while the 1648 cm-1 band is assigned to a non-hydrogen bonded acetyl carbonyl. The 1696 cm-1 band is consistent only with a non-hydrogen bonded keto carbonyl group; from the unusually high intensity of this latter band compared to the others, we propose that the 1696 cm-1 band contains contributions from two keto carbonyl groups, both free of hydrogen bonds. From published protein sequence alignments of the L and M subunits of Rhodobacter (Rb.) sphaeroides and Chloroflexus aurantiacus we assign the 1632 cm-1 band as arising from the C2 acetyl carbonyl of the analogous PM constituent of P, which is hydrogen bonded to tyrosine M187 in the Chloroflexus RC, and propose a pigment-protein structural model for the primary donor of Chloroflexus aurantiacus. The FT Raman spectrum of RCs in the P degrees+ state indicates that one component of the 1696 cm-1 band has upshifted 21 cm-1 to 1717 cm-1. Compared to Rb. sphaeroides which showed a 26 cm-1 upshift for the corresponding band, the 21 cm-1 upshift indicates that the + charge is more delocalized over the P.+ species of Chloroflexus; we estimate that ca. 65% of the + charge is localized on one of the two BChl molecules of the Chloroflexus primary donor as compared to ca. 80% for Rb. sphaeroides. The consequences of the proposed structure of the Chloroflexus primary donor in terms of its Po/P.+ redox midpoint potential are discussed.