title: Fourier transform infrared study on the primary donor P798 of Heliobacterium modesticaldum: cysteine S-H coupled to P798 and molecular interactions of carbonyl groups.
abstract: Light-induced Fourier transform infrared (FTIR) difference spectra of the primary donor P798 upon its cation formation (P798(+)/P789) were measured using the membranes and purified RC complex of Heliobacterium modesticaldum. A differential signal at 2550/2560 cm-1 was observed in the difference spectra and assigned to the S-H stretching mode of cysteine by an isotopic shift to 1854/1861 cm-1 upon deuteration. The observed frequencies indicate that this S-H forms a strong hydrogen bond and that the bond is further strengthened upon P798(+) formation. Polarized FTIR difference spectra showed that this S-H group is oriented at <40 degrees with respect to the membrane normal. It was proposed that the cysteine S-H is coupled to P798 through a hydrogen-bond network or by direct hydrogen bonding to either a P798 carbonyl or a ligand histidine. In the carbonyl stretching region, differential signals were observed at 1741/1737, 1725/1718, 1702/1693, and 1687/1666 cm-1. In a dry membrane film, the signal at 1687/1666 cm-1 was mostly lost and hence was assigned to the amide I bands arising from the protein conformational change, which was suppressed upon dehydration of the membranes. The 1702/1693 cm-1 signal was assigned to the 13(1)-keto C&dbd;O of P798, which was free from hydrogen bonding and had a nearly parallel orientation to the membrane plane. The upshift by 9 cm-1 upon P798 oxidation, which is much smaller than upshifts of monomeric (bacterio)chlorophylls [(B)Chls] in organic solution, indicates that the positive charge on P798(+) is significantly delocalized in a BChlg dimer. The signals at 1741/1737 and 1725/1718 cm-1 were assigned to a free and a hydrogen-bonded ester C=O group, respectively. The dichroism measurement showed that the C=O of 1741/1737 cm-1 was oriented nearly parallel to the membrane plane while that of 1725/1718 cm-1 was considerably tilted by <31 degrees to the membrane normal. It was proposed that one of the two ester signals arose from the 13(2)-carbomethoxy C=O of P798 while the other arose either from the 17(2)-ester C=O of P798 or from an ester C&dbd;O of adjacent BChlg or 8(1)-OH-Chla that was electrostatically influenced by oxidation of P798.
authors.affiliation: Photosynthesis Research Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Noguchi
authors.forename: Y
authors.initials: Y
authors.lastname: Fukami
authors.forename: H
authors.initials: H
authors.lastname: Oh-oka
authors.forename: Y
authors.initials: Y
authors.lastname: Inoue
mesh_terms: D000600:Amino Acids, Dicarboxylic / Q000737:chemistry*; D001419:Bacteria / Q000737:chemistry*; D003545:Cysteine / Q000737:chemistry*; D006860:Hydrogen Bonding; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013438:Sulfhydryl Compounds / Q000737:chemistry*
chemical_list: D000600:Amino Acids, Dicarboxylic; D045322:Photosynthetic Reaction Center Complex Proteins; D013438:Sulfhydryl Compounds; D003545:Cysteine