Rapid-scan FTIR difference spectroscopy applied to ubiquinone reduction in photosynthetic reaction centers: Role of redox mediators

Rapid-scan FTIR difference spectroscopy was used to investigate light-induced reduction of the ubiquinone Q B , and the oxidation kinetics of Q B – and QH 2 by external mediators in isolated reaction centers (RCs) from R. sphaeroides . As redox mediators, a Ferrocyanide/N,N,Nʹ,Nʹ-tetramethyl- p -phenylenediamine (TMPD) mixture and an Ascorbate/2,3,5,6-tetramethyl- p -phenylenediamine (DAD) mixture are compared. Results show that TMPD red rapidly reduces the photoproduced P 870 + primary donor. The process is fast enough to record rapid-scan FTIR spectra devoid of P 870 + bands down to 260 K. Results show also that TMPD ox oxidises both Q B – and QH 2 faster than DAD ox . In particular, Q B – is oxidised faster than QH 2 at all temperatures studied. Results are discussed in the framework of time-resolved infrared studies on R. sphaeroides RCs, showing advantages/drawbacks of the proposed experimental approach.