Electronic and vibrational structure of the radical cation of P-840 in the putative homodimeric reaction center from Chlorobium tepidum as studied by FTIR spectroscopy

Light-induced FTIR difference spectra of P-840 upon its oxidation (P-840(+)/P-840) 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 P-840(+), which is comparable to the band near 2600 cm(-1) previously observed in P-870(+) 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 P-840(+) was about the same as that in P-870(+). The P-870(+) spectrum also showed several intensified vibrational modes, which are characteristic of the P-870(+) spectrum as well. These similar features of the electronic transition and the intensified lines indicate that P-840(+) is a BChla dimer whose electronic structure is similar to P-870(+). Based on the previous theoretical works, the possibility that P-840(+) has an asymmetric structure as P-870(+) was suggested. Also, two strong positive bands at 1707 and 1694 cm(-1) probably assigned to the keto C-9 = O stretching modes of P-840+ were observed in the P-840(+)/P-840 spectrum. Three different interpretations are possible for the presence of the two C-9 = O bands: (i) P-840+ is an asymmetric dimer cation. (ii) P-840(+) 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 P-840(+) affects the C-9 = O frequency of the neutral BChla in P-840(+) 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 P-840(+). The negative bands at 1734 and 1684 cm(-1) were assigned to the eater C-10 = O and the keto C-9 = O of neutral P-840, respectively, both of which are free from hydrogen bonding. These results and interpretations regarding the structural symmetry and the molecular interactions of P-840 and P-840(+) are discussed in the framework of the ''homodimeric'' reaction center of green sulfur bacteria.