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.

Monte Carlo Modeling of Spectral Diffusion Employing Multiwell Protein Energy Landscapes: Application to Pigment–Protein Complexes Involved in Photosynthesis

“…Overall, the HGK are multiexponential and are in qualitative agreement with experiments and modeling based on the NPHB master equation, 11,12,31,35,36 . One has to remember that at the present stage our software is incapable of properly modeling the evolution of the width of a narrow hole.…”

“…The evidence comes not only from SPCS, but also from the analysis of the spectral hole recovery. 12,31 Dependence of the hole recovery (HR) rate on the fractional depth of the originally burnt hole is a clear signature of a significant degree of spectral memory. The functions describing the distributions of spectral shifts ("antihole" or "photoproduct" functions) in both glasses 35,36 and proteins 11,12 are phenomenological and not straightforwardly related to the features of the energy landscape.…”

Probing Energy Landscapes of Cytochrome b6f with Spectral Hole Burning: Effects of Deuterated Solvent and Detergent

“…In this case, all observed barrier distributions appeared to be Gaussian, and there was no evidence for ~V / 1 distributions predicted by the theories of amorphous solids and indeed apparently observed in glasses 4,5 and some proteins 6 . On the other hand, in the case of cytochrome (Cyt) b6f 25 "glasslike" ~V / 1 barrier distributions were observed 15 . This raised a question of intactness of our Cyt b6f samples.…”

“…The system may make the transitions between two wells by tunneling through the potential barrier (dashed 3 diagonal arrow) or by hopping over the barrier (solid bent arrow). It is generally believed that at temperatures below 10 K the transition occurs by means of tunneling 14,15 . At higher temperatures both mechanisms may be at work.…”

A simple and efficient method to prepare pure dimers and monomers of the cytochrome b 6 f complex from spinach

“…In that respect, some spectroscopic works have been published using the Cyt b 6 f isolated from cyanobacteria (Petermen et al 1998;Dashdorj et al 2005;Yan et al 2008 laboratory of Prof. V. Zazubovich (Najafi et al 2015) has used the bulk dimeric form of the Cyt b 6 f isolated from spinach to characterize the thermodynamics and energy landscape of this system using high resolution hole burning and site-selection fluorescence spectroscopy of the Chl a at cryogenic temperatures. A comparison of pure dimers and monomers seems to be necessary to reach more solid conclusions.…”