title: Conformational Changes in Pigment-Protein Complexes at Low Temperatures-Spectral Memory and a Possibility of Cooperative Effects.
abstract: We employed nonphotochemical 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 substates 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 the hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume toward the pigment. Entities involved in the light-induced conformational changes are characterized by md(2) value of about 1.0 × 10(-46) kg·m(2). 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 Δ-FLN spectra is in good agreement with the structural data, indicating that the explored protein was intact.
authors.affiliation: †Department of Physics, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Najafi
authors.affiliation: †Department of Physics, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada.
authors.forename: Nicoleta
authors.initials: N
authors.lastname: Herascu
authors.affiliation: †Department of Physics, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada.
authors.forename: Golia
authors.initials: G
authors.lastname: Shafiei
authors.affiliation: ‡Estacion Experimental de Aula Dei (CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Picorel
authors.affiliation: †Department of Physics, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada.
authors.forename: Valter
authors.initials: V
authors.lastname: Zazubovich
mesh_terms: D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000528:radiation effects; D019281:Dimerization; D004583:Electrons; D055668:Photochemical Processes; D011487:Protein Conformation; D011522:Protons; D013050:Spectrometry, Fluorescence; D018724:Spinacia oleracea; D013696:Temperature
chemical_list: D011522:Protons; D045346:Cytochrome b6f Complex