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Spontaneous emission from reaction centers of photosynthetic bacteria has been recorded with a time resolution of 50 fs. Excitation was made directly into both the special-pair band (850 nm) and the Qx band of bacteriochlorophylls (608 am). Rhodobacter sphaeroides R26, Rhodobacter capsuleas wild type, and four mutants of Rb. capsulatus were studied. In all cases the fluorescence decay was not single exponential and was well fit as a sum of two exponential decay components. The short components are in excellent agreement with the single component detected by measurements of stimulated emission. The origin of the nonexponential decay is discussed in terms of heterogeneity, the kinetic scheme, and the possibility of slow vibrational relaxation.The mechanism of the initial electron transfer step in the reaction center (RC) of photosynthetic bacteria has been the subject of intense study over the past 10 years. This initial step is ultrafast, occurring in about 3 ps at room temperature (1). As the understanding of the RC improves the need arises for more precise kinetic data. In particular, questions arise as to the exponentiality of the observed kinetic signals (2-8), the possibility of differing behavior at different wavelengths (3, 4), the existence of oscillatory components (5), and the existence of spectral shifts (3, 6) accompanying the excitation and subsequent electron transfer processes.The primary method used for ultrafast studies of the primary charge separation step has been time-resolved absorption spectroscopy, generally with low-repetition-rate (10-30 Hz) relatively high-power (excitation pulse energies in the range 1-30 pJ) laser systems. In addition to the limited dynamic range and signal/noise ratios of such measurements, precise determination of kinetics requires that accurate account be taken of all the competing absorptions and bleachings at the detection wavelength. In measurements of the decay of the excited state of the special pair (P*) by stimulated emission, most workers have made measurements at or near the isosbestic point in the spectrum consisting of ground state (P) bleaching and absorption of the radical cation of P (P+) and P*. However, such a procedure makes it difficult to observe longer decay components in the stimulated emission and to look for the presence of spectral evolution or wavelength-dependent kinetics. Zinth and coworkers (7) could not rule out the presence of a 10-to 20-ps component within their experimental accuracy. More recently Vos et al. (5), after significantly improving their signal/noise ratio, reported that the stimulated emission in Rhodobacter sphaeroides R26 (R26) did not decay exponentially but was well described by two decay times (2.9 and 12 ps) with relative amplitudes of 65% and 35%. This observation is very significant for kinetic analyses of absorption changes in other portions of the spectrum, in particular for discussion of whether the primary process should be described by a one-step superexchange or two-step sequential mechanism (2-15).An...

The initial electron tranfer in reaction centers from Rhodobacter sphaeroides R26 was studied by a subplcosecond transient pump-probe technique. The measured kinetics at various wavelengths were analyzed and compared with several mechanisms for electron tra er. An unambiguous determination of the initial electron transfer mechanism in reaction centers cannot be made by studying the anion absorption region (6404690 nm), due to the spectral congestion in this region. However, correlations between the stimulated emission decay of the excited state of the special pair, P*, at 926 nm and bleaching of the bacteriopheophytin Qx absorption at 545 mu suggest that the electron transfer at 283 K is dominated by a two-step sequential mechanism, whereas one-step superexchange and the two-step sequential mechanism have about equal contributions at 22 K.The highly efficient primary charge separation in photosynthesis occurs in a membrane-bound protein complex called the reaction center (RC). The x-ray crystal structure of RCs from Rhodopseudomonas (Rps.) viridis and Rhodobacter (Rb.) sphaeroides has been determined and reveals a pseudo C2 symmetry (1-3). In the RC, two strongly interacting bacteriochlorophylls form the special pair, denoted P. Along each side of the C2 axis there is a monomeric bacteriochlorophyll (BA and BB) adjacent to the special pair, followed by a bacteriopheophytin (HA and HB) and then by a quinone molecule (QA and QB). There is also a single non-heme iron on the C2 axis between the two quinones.Electron transfer could occur along either branch related by the C2 symmetry, yet it occurs only down one branch (branch A) from P* to HA in about 3 ps at room temperature. One unresolved question concerns the role of BA in the initial charge separation process between P* and HA. Early picosecond and femtosecond measurements on RCs from Rb. sphaeroides and Rps. viridis at room temperature and at 10 K (4-9) did not detect any intermediate state involving B-, and the decay of P* and the bleaching of HA had the same time constant. These observations indicated that the initial electron transfer from P*BAHA to P+BAH-occurred either by a single-step superexchange mechanism or by a two-step hopping process with the second step much faster than the first step. The one-step superexchange mechanism has been supported by measurements by Kirmaier and Holten (10,11). Using an electric field to perturb the electron transfer, Lockhart et al. (12) concluded that (at 77 K) the two-step mechanism was unlikely. The same conclusion was reached by Ogrodnik et al. (13) (19) suggests that for reasonable parameter values, the electron transfer is dominated by the two-step sequential mechanism at room temperature, while the contribution of the one-step superexchange mechanism becomes important at low temperature.In this paper, we address the mechanism of the initial electron transfer by using kinetic data for Rb. sphaeroides R26 RCs obtained at several wavelengths where the various species involved absorb. The analysis of our da...

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