Photoacoustic diagnostics of laser-induced processes in reaction centers of Rhodobacter sphaeroides

摘要

A theoretical model of photoacoustic signal generation in a suspension of bacterial reaction centers (RCs) is suggested. The existing technique of separation of the two contributions to photoinduced volume changes (thermal expansion and structural rearrangement), based on their different temperature dependence, is extended to take into account the fact that they may also have different time dependence. In the practical implementation of the technique, it was found that electron transfer in the RC isolated from Rhodobacter sphaeroides is associated with a fast (< 100 ns) contraction, 32 ± 1 ?3 which has the same value within the accuracy of our measurements for wild-type and R-26 preparations. The enthalpy of the P+Q?A state deduced from our measurements is 0.56 ± 0.04 eV. The quantum yield of photochemistry, was measured in a broad range of background light intensities. Its value for dark-adapted RCs, φ ≈ 1.0, decreased continuously to φ ≈ 0.3 at background light intensity φ≈ 1 photon/(RC · s). Reduced values of observed after long background light illumination were attributed to reversible structural changes, which relax in about 15–20 min after charge recombination. In the photochemically ‘closed’ reaction center, a deactivation process, following laser excitation, involves a long-lived intermediate state characterized by an exponential decay with a lifetime of 620 ± 80 ns and an amplitude of 0.14 ± 0.04.