Spectral hole burning, recovery, and thermocycling in chlorophyll-protein complexes: Distributions of barriers on the protein energy landscape

40 Pags. The definitive version, with tabls. and figs., is available at: http://pubs.acs.org/journal/jpcbfkChlorophyll–protein complexes are ideal model systems for protein energy landscape research. Here pigments, used in optical spectroscopy experiments as sensitive probes to local dynamics, are built into protein by Nature (in a large variety of local environments; without extraneous chemical manipulations or genetic engineering). Distributions of the tunneling parameter, λ, and/or protein energy landscape barrier heights, V, have been determined for (the lowest energy state of) the CP43 core antenna complex of photosystem II. We demonstrate that spectral hole burning (SHB) and hole recovery (HR) measurements are capable of delivering important information on protein energy landscape properties and spectral diffusion mechanism details. In particular, we show that tunneling rather than barrier hopping is responsible for both persistent SHB and subsequent HR at 5–12 K, which allows us to estimate the md2 parameter of the tunneling entities as 1.0 × 10–46 kg·m2. The subdistributions of λ actually contributing to the nonsaturated spectral holes (and affecting their recovery) differ from the respective full true distributions. In the case of the full λ-distribution being uniform (or the barrier height distribution 1/√V, a model which has been widely employed in theories of amorphous solids at low temperatures and in HR analysis), the difference is qualitative, with λ subdistributions probed in the HR experiments being highly asymmetrical, and barrier V subdistributions deviating significantly from 1/√V. Thus, the distribution of λ for the protein energy landscape tier directly probed by SHB is likely Gaussian and not uniform. Additionally, a Gaussian distribution of barriers, with parameters incompatible with those of the landscape tier directly probed by SHB, contributes to the thermocycling results.Financial support from NSERC, CFI, and Concordia University is gratefully acknowledged. R.P. thanks the MINECO of Spain (Grant AGL2011-23574, partially financed by the EU FEDER Program), and M.S. acknowledges the U.S. Department of Energy’s Photosynthetic Systems Program within the Chemical Sciences, Geoscience, and Biosciences Division of the Office of Basic Energy Sciences under NREL Contract #DEAC36- 08-GO28308 for support. R.J. acknowledges support from the NSF ARRA Grant (CHE-0907958). M.S. also acknowledges partial support from NREL pension program.Peer reviewe