Evidence for protein dielectric relaxations in reaction centers associatedwith the primary charge separation detected from Rhodospirillum rubrum chromatophores by combined photovoltage and absorption measurements in the 1-15 ns time range

Fast photovoltage measurements in Rhodospirillum rubrum chromatophores in t he nanosecond time range, escorted by time-resolved absorption measurements , are described. Under reducing conditions, the photovoltage decayed signif icantly faster than the spectroscopically detected charge recombination of the radical pair P+HA-. This indicates the occurrence of considerable diele ctric relaxations. Our data and data from the literature were analyzed by m eans of a reaction scheme consisting of three states, namely, A*, P* and P( +)HA(-). A time-dependent DeltaG(t) was introduced by assuming a time-depen dent rate constant of the back-reaction, k(-1)(t). With the exception of th e latter rate constant, all ether parameters of the model are reliably know n within narrow limits. This allowed us to distinguish between the three ca ses assumed for DeltaG degrees (t): (1)DeltaG degrees (t) = constant; (2)De ltaG degrees (t) as published by Peloquin et al. [Peloquin, J. M., Williams , J. C., Lin, X. M., Alden, R. G., Taguchi, A. K. W., Allen, J. P., and Woo dbury, N. W. (1994) Biochemistry 33, 8089-8100]; and a (3)DeltaG degrees (t ) that fits the present data. The assumption that (1)DeltaG degrees (t) = c onstant is incompatible with our photovoltage data, and (2)DeltaG degrees ( t) is incompatible with the constraint that the ratio of fluorescence yield s in the closed and open state is F-m/F-o approximate to 2. We specify a (3 )DeltaG degrees (t) that should be valid for photosynthetic reaction center s in vivo. Furthermore, the overall kinetics of the electric relaxation, e( t), in response to the primary charge separation were determined.