Temperature dependence of electronic energy transfers within B850 antennae of the NF57 mutant of the purple bacterium Rhodobacter sphaeroides Original Research Article

Two-color absorption difference experiments were performed using numerous pump-probe wavelength combinations within the B850 absorption band of NF57 (LH1-deficient) mutant chromatophores from the purple bacterium Rhodobacter sphaeroides. Bacteriochlorophyll a photobleaching/stimulated emission rise features are observed at 870 nm for pump wavelengths between 830 and 86- nm. They typically exhibit 80–100 fs kinetics at room temperature, and 250–300 fs kinetics at 19 K. For other wavelength combinations, the time evolution in the B850 absorption difference profiles is somewhat reminiscent of BChl a spectral equilibration in FMO trimers, antenna complexes with well-defined structure that are known to exhibit diagonal energy heterogeneity. Our results suggest the presence of diagonal disorder within the B850 band of individual LH2 antennae, possibly due to distortions from Cn symmetry. The temperature dependence of B850 spectral equilibration is considerably less marked than that of spectral equilibration in FMO trimers.