A time-resolved iron-specific X-ray absorption experiment yields no evidence for an Fe2+ f Fe3+ transition during QA- f QB electron transfer in the photosynthetic reaction center

ABSTRACT: Previous time-resolved FTIR measurements suggested the involvement of an intermediary component in the electron transfer step QA- f QB in the photosynthetic reaction center (RC) from Rhodobacter sphaeroides [Remy and Gerwert (2003) Nat. Struct. Biol. 10, 637]. By a kinetic X-ray absorption experiment at the Fe K-edge we investigated whether oxidation occurs at the ferric non-heme iron located between the two quinones. In isolated reaction centers with a high content of functional QB, at a time resolution of 30 ís and at room temperature, no evidence for transient oxidation of Fe was obtained. However, small X-ray transients occurred, in a similar micro- to millisecond time range as in the IR experiments, which may point to changes in the Fe ligand environment due to the charges on QA-and QB-. In addition, VIS measurements agree with the IR data and do not exclude an intermediate in the QA- f QB transition. The photosynthetic bacterial reaction center (BRC)1 rep-resents one of the best studied bioenergetic model systems both with respect to its structural characterization by crystal-lography (1) and with respect to the elucidation of its light-driven electron and proton transfer reactions (2). In the R