Fourier transform infrared study of the primary electron donor in chromatophores of Rhodobacter sphaeroides with reaction centers genetically modified at residues M160 and L131

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Abstract

Structural changes in chromatophores of Rhodobacter sphaeroides reaction center mutants associated with the substitution of amino acid residues near the primary electron donor P have been investigated by light-induced FTIR difference spectroscopy. The single-site mutations Leu-L131 to His and Leu-M160 to His and the corresponding double mutation were designed to introduce a proton-donating residue that could form a hydrogen bond with the keto carbonyl of ring V of each bacteriochlorophyll (PL and PM) of the dimer. The presence of large positive bands at approximately 1550, 1480, and 1295 cm-1, as well as at 2600-2800 cm-1 in the light-induced P+QA-/PQA FTIR difference spectra, corresponding to the photooxidation of P and the photoreduction of the primary quinone QA, demonstrates that the BChl dimer state of P+ is preserved in the LH(L131), LH(M160), and LH(M160)+LH(L131) mutants, although frequency shifts and amplitude changes can be observed, notably for LH(M160). Compared to wild type, these changes are thought to reflect a different charge repartition over the two BChls in P+. Large frequency downshifts in the 9-keto C=O stretching region of the P+QA-/PQA FTIR difference spectra of chromatophores are observed in the mutant samples relative to wild type. For the LH(M160) mutant, a large differential signal at 1678/1664 cm-1 is assigned to a shift, upon photooxidation, of the 9-keto C=O of PM hydrogen-bonded to His-M160, while that at 1718/1696 cm-1 corresponds to the free 9-keto C=O of PL.(ABSTRACT TRUNCATED AT 250 WORDS)

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