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Conformational changes in pigment-protein complexes at low temperatures - spectral memory and a possibility of cooperative effects

AuthorsNajafi, Mehdi; Herascu, Nicoleta; Shafiei, Golia; Picorel Castaño, Rafael CSIC ORCID ; Zazubovich, Valter
Issue Date2015
PublisherAmerican Chemical Society
CitationNajafi M, Herascu N, Shafiei G, Picorel R, Zazubovich V. Conformational changes in pigment-protein complexes at low temperatures - spectral memory and a possibility of cooperative effects. Journal of Physical Chemistry B 119 (23): 6930-6940 (2015)
AbstractWe employed non-photochemical hole burning (NPHB) and fluorescence line narrowing (FLN) spectroscopies to explore protein energy landscapes and energy transfer processes in dimeric cytochrome b6f, containing one chlorophyll molecule per protein monomer. The parameters of the energy landscape barrier distributions quantitatively agree with those reported for other pigment-protein complexes involved in photosynthesis. Qualitatively, the distributions of barriers between protein sub-states involved in the light-induced conformational changes (i.e. - NPHB) are close to glass-like ~1/√V (V is the barrier height), and not to Gaussian. There is a high degree of correlation between the heights of the barriers in the ground and excited states in individual pigment-protein systems, as well as nearly perfect spectral memory. Both NPHB and hole recovery are due to phonon-assisted tunneling associated with the increase of the energy of a scattered phonon. As the latter is unlikely for simultaneously both the hole burning and hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume towards the pigment. Entities involved in the light-induced conformational changes are characterized by md2 value of about 1.0.10-46 kg.m2. Thus, these entities are protons or, alternatively, small groups of atoms experiencing sub-Å shifts. However, explaining all spectral hole burning and recovery data simultaneously, employing just one barrier distribution, requires a drastic decrease in the attempt frequency to about 100 MHz. This decrease may occur due to cooperative effects. Evidence is presented for excitation energy transfer between the chlorophyll molecules of the adjacent monomers. The magnitude of the dipole-dipole coupling deduced from the delta-FLN spectra is in good agreement with the structural data, indicating that explored protein was intact.
Description40 Pags.- 9 Figs. The definitive version is available at: http://pubs.acs.org/journal/jpcbfk
Publisher version (URL)http://dx.doi.org/10.1021/acs.jpcb.5b02845
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