The nature of the low-energy 825 nm band of theFenna-Mathews-Olson (FMO) protein complex from Chlorobaculum tepidum at 5 K is discussed. We provide a criticalassessment of published and recently measured FMO optical spectra. FMO complexeswere found to destabilize over time producing spectral shifts, withdestabilized samples having significantly higher hole burning efficiencies;indicating a remodeled protein energy landscape. Observed correlated peakshifts suggest possible correlated (protein) fluctuations. We show, using bothnonresonant and resonant hole-burned (HB) spectra and modeling studies, that thenature of the so-called 825 nm absorption band of the FMO trimer cannot beexplained by a single transition. To explain the shape of emission and HBspectra downward uncorrelated excitation energy transfer (EET) between trimersubunits should be taken into account. Modeling studies of absorption spectra showthe presence of three sub-bands within the 825 nm band, in agreement with HBand emission spectra. The sub-bands (for intact complexes), with maxima at 12097,12118 and 12142 cm-1, correspond to bacteriochlorophylls having two,one, and zero donors, respectively. We argue that after light inducedcoherences vanish within each monomer, uncorrelated EET between the lowestexciton levels of each monomer takes place due to static structural inhomogeneitiesin the trimer.

On the controversial nature of the 826 nm exciton band in FMO complexes

247th National Meeting (2014)

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