Evidence of Simultaneous Spectral Hole Burning Involving Two Tiers of the Protein Energy Landscape in Cytochrome .

Cytochrome , with one chlorophyll molecule per protein monomer, is a simple model system whose studies can help achieve a better understanding of nonphotochemical spectral hole burning (NPHB) and single-complex spectroscopy results obtained in more complicated photosynthetic chlorophyll-protein complexes. We are reporting new data and proposing an alternative explanation for spectral dynamics that was recently observed in cytochrome using NPHB. The relevant distribution of the tunneling parameter λ is a superposition of two components that are nearly degenerate in terms of the resultant NPHB yield and represent two tiers of the energy landscape responsible for NPHB. These two components likely burn competitively; we present the first demonstration of modeling a competitive NPHB process. Similar values of the NPHB yield result from distinctly different combinations of barrier heights, shifts along the generalized coordinate , and/or masses of the entities involved in conformational changes , with parameter different by a factor of 2.7. Consequently, in cytochrome , the first (at least) 10 h of fixed-temperature recovery preferentially probe different components of the barrier- and λ-distributions encoded into the spectral holes than thermocycling experiments. Both components most likely represent dynamics of the protein and not of the surrounding buffer/glycerol glass.