Symmetric structural features and binding site of the primary electron donor in the reaction center of Chlorobium.
The protein binding interactions of the constituent bacteriochlorophyll a molecules of the primary electron donor, P840, in isolated reaction centers from Chlorobium limicola f thiosulphatophilum and the electronic symmetry of the radical cation P840+. were determined using near-infrared Fourier transform (FT) Raman spectroscopy excited at 1064 nm. The FT Raman vibrational spectrum of P840 indicates that it is constituted of a single population of BChl a molecules which are spectrally indistinguishable. The BChl a molecules of P840 are pentacoordinated with only one axial ligand on the central Mg atom, and the pi-conjugated C2 acetyl and C9 keto carbonyls are free of hydrogen-bonding interactions. The FT Raman spectrum of P840+. exhibits a 1707 cm-1 band attributable to a BChl a C9 keto carbonyl group vibrational frequency that has upshifted 16 cm-1 upon oxidation of P840; this upshift is exactly one-half of that expected for the one-electron oxidation of monomeric BChl a in vitro. The 16 cm-1 upshift, thus, indicates that the resulting +1 charge is equally shared between two BChl a molecules. This situation is markedly different from that of the oxidized primary donor of the purple bacterial reaction center of Rhodobacter sphaeroides, (i) which exhibits a 1717 cm-1 band that has upshifted 26 cm-1, indicating an asymmetric distribution of the resulting +1 charge over the two constituent BChl a molecules, and (ii) whose H-bonding pattern with respect to the pi-conjugated carbonyl groups is asymmetric.(ABSTRACT TRUNCATED AT 250 WORDS)
Feiler
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U
Section de Biophysique des Protéines et des Membranes, DBCM, CEA and URA 1290 CNRS, Centre d'Etudes de Saclay, Gif-sur-Yvette, France.
Albouy
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D
Robert
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B
Mattioli
T A
TA
D001419:Bacteria / Q000737:chemistry*; D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D004563:Electrochemistry; D004579:Electron Transport; D006860:Hydrogen Bonding; D045342:Light-Harvesting Protein Complexes; D015394:Molecular Structure; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman
D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins