Stark effect spectroscopy of the 1250 nm P + band of Rhodobacter sphaeroides reaction centers and related model compounds

The absorption and Stark effect spectra of the 1250 nm hand associated with the oxidized special pair (P+) in Rhodobacter sphaeroides reaction centers have been measured. The absorption spectrum in the 1000-1450 nm region is essentially identical whether P+ is generated by chemical oxidation or continuous illumination. The Stark effect on the 1250 nm band of P+ is relatively small compared to the Stark effect seen for the Qv transition of P. A quantitative analysis indicates that the change in dipole moment for the 1250 nm band is very small. These results are compared with data for two model systems: a lanthanide-bridged porphyrin dimer, Eu(OEP)2, and a mixed valence coordination complex, the Creutz-Taube ion. The Stark effect spectrum of the 1250 nm P+ band is found to be similar to both of these systems. This contrasts with valence-trapped systems whose mixed valence transitions exhibit a large change in dipole moment (Oh, D. and Boxer, S.G. (1990) J. Am. Chem. Soc. 112, 816; Oh, D. and Boxer, S.G. (1991) J. Am. Chem. Soc. 113, 6880). These results, together with other observations on the spectrum of reaction centers in the P+ state, are consistent with the view that the 1250 nm electronic transition of P+ is delocalized over the two bacteriochlorophyll molecules comprising the special pair.