Energy landscape of the intact and destabilized FMO antennas from C. tepidum and the L122Q mutant: Low temperature spectroscopy and modeling study.

ABSTRACT

We discuss the excitonic energy landscape of the typically studied wild-type (WT) Fenna-Matthews-Olson (FMO) antenna protein from the green sulfur bacterium Chlorobaculum tepidum (referred to as WTM), which is described as a mixture of intact (WTI) and destabilized (WTD) complexes. Optical spectra of WTM and the L122Q mutant (where leucine 122 near BChl 8 is replaced with glutamine) are compared to WTI FMO. We show that WTM and L122Q samples are mixtures of two subpopulations of proteins, most likely induced by protein conformational changes during the isolation/purification procedures. Absorption, emission, and HB spectra of WTM and L122Q mutant are very similar, in which the low-energy trap (revealed by the nonresonant HB spectra) shifts to higher energies as a function of fluence, supporting a mixture model. No fluence-dependent shift is observed in the WTI FMO trimers. New Hamiltonians are provided for WTI and WTD proteins. Resonant HB spectra show that the internal energy relaxation times in the WTM and L122Q mutant are similar, and depend on excitation frequency. Fast average relaxation times (excited state lifetimes) are observed for burning into the main broad absorption band near 805nm. Burning at longer wavelengths reveals slower total dephasing times. No resonant bleach is observed at λB≤803nm, implying much faster (femtosecond) energy relaxation in this spectral range in agreement with 2D electronic spectroscopy frequency maps.