Jw. Stocker et al., STARK-EFFECT SPECTROSCOPY OF THE 1250 NM P-SPHAEROIDES REACTION CENTERS AND RELATED MODEL COMPOUNDS( BAND OF RHODOBACTER), Biochimica et biophysica acta, 1144(3), 1993, pp. 325-330
The absorption and Stark effect spectra of the 1250 nm hand associated
with the oxidized special pair (P+) in Rhodobacter sphaeroides reacti
on centers have been measured. The absorption spectrum in the 1000-145
0 nm region is essentially identical whether P+ is generated by chemic
al oxidation or continuous illumination. The Stark effect on the 1250
nm band of P+ is relatively small compared to the Stark effect seen fo
r the Q(Y) transition of P. A quantitative analysis indicates that the
change in dipole moment for the 1250 nm band is very small. These res
ults are compared with data for two model systems: a lanthanide-bridge
d porphyrin dimer, Eu(OEP)2, and a mixed valence coordination complex,
the Creutz-Taube ion. The Stark effect spectrum of the 1250 nm P+ ban
d is found to be similar to both of these systems. This contrasts with
valence-trapped systems whose mixed valence transitions exhibit a lar
ge 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, 6
880). 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 b
acteriochlorophyll molecules comprising the special pair.