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 P870+ 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.