The charge separation in a stable and fully active D1-D2-cyt b-559 reaction center (RC) complex containing 4 Chi a/2 Pheo a was analyzed using laser-induced optoacoustic spectroscopy (LIOAS) under anaerobic conditions. One microsecond after an 8-ns laser pulse, the volume change associated with the formation of the tripler (3)[P680 Pheo] by recombination of the tripler radical pair (3)[P680(+)Pheo(-)] was determined to be an expansion of 4.4 +/- 1.0 mL/mol. This expansion is attributed to reorganization of the protein around the radical pair within its lifetime of ca. 100 ns and persists for times longer than 1 mu s. A yield of greater than or equal to 0.5 resulted for the triplet state of this "minimal" reaction center, higher than that reported previously for the reaction center complex containing 6 Chl, on the basis of absorption measurements. Thus, charge separation is highly efficient in this reaction center. Near-IR emission measurements with the D1-D2-cyt b-559 reaction center complex in the presence of air afforded a value for the quantum yield of O-2((1) Delta(g)) production, Phi(Delta) = 0.02, for low-energy excitation, much lower than the tripler yield. The increase of this value at higher O-2 concentrations is discussed in terms of the microheterogeneity of the system.