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.
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