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

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.