Electronic and vibrational structure of the radical cation of P840 in the putative homodimeric reaction center from Chlorobium tepidum as studied by FTIR spectroscopy

Takumi Noguchi, Noriaki Kusumoto, Yorinao Inoue, Hidehiro Sakurai

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Abstract

Light-induced FTIR difference spectra of P840 upon its oxidation (P840+/P840) have been measured with the reaction center complex from the green sulfur bacterium Chlorobium tepidum. A broad band centered near 2500 cm-1 was observed in P840+, which is comparable to the band near 2600 cm-1 previously observed in P870+ of purple bacteria and assigned to the electronic transition in the bacteriochlorophyll a (BChla) dimer [Breton et al. (1992) Biochemistry 31, 7503-7510]. The intensity of this electronic band found in P840+ was about the same as that in P870+. The P840+ spectrum also showed several intensified vibrational modes, which are characteristic of the P840+ spectrum as well. These similar features of the electronic transition and the intensified lines indicate that P840+ is a BChla dimer whose electronic structure is similar to P870+. Based on the previous theoretical works, the possibility that P840+ has an asymmetric structure as P870+ was suggested. Also, two strong positive bands at 1707 and 1694 cm-1 probably assigned to the keto C9=O stretching modes of P840+ were observed in the P840+/P840 spectrum. Three different interpretations are possible for the presence of the two C9=O bands: (i) P840+ is an asymmetric dimer cation. (ii) P840+ has a symmetric structure, and the time constant of positive charge exchange between the two BChla molecules coincides with that of IR spectroscopy (10-13 s). (iii) The electric field produced by the positive charge on P840+ affects the C9=O frequency of the neutral BChla in P840+ itself (when the charge exchange time is slower than the time scale of 10-13 s) or of a BChla in the close proximity of P840+. The negative bands at 1734 and 1684 cm-1 were assigned to the ester C10=O and the keto C9=O of neutral P840, respectively, both of which are free from hydrogen bonding. These results and interpretations regarding the structural symmetry and the molecular interactions of P840 and P840+ are discussed in the framework of the 'homodimeric' reaction center of green sulfur bacteria.

Original languageEnglish
Pages (from-to)15428-15435
Number of pages8
JournalBiochemistry
Volume35
Issue number48
DOIs
Publication statusPublished - 1996 Dec 17

ASJC Scopus subject areas

  • Biochemistry

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