Nonequilibrium Heating in LHCII Complexes Monitored by Ultrafast Absorbance Transients

Abstract

Evidence of nonequilibrium local heating in transient spectra of LHCII, the main light-harvesting complex of plants, was studied by using various excitation intensities over a wide temperature range, from 10 K to room temperature. No obvious manifestation of local heating was found at room temperature, whereas at 10 K, the local heating effect is discernible when more than 10 excitons per LHCII trimer per pulse are generated. Under these conditions, a major part of the excitation energy is converted into heat as a result of exciton-exciton annihilation. Initially, the heat energy is allocated on chlorophyll a molecules, reaching hundreds of degrees at the highest excitation intensities, which correspond to almost 100 excitons per trimer generated by a single excitation pulse. The decay of the nonequilibrium temperature is characterized well by two exponentials. The initial phase of cooling, which is most likely caused by the spreading of heat over the protein, corresponds to a characteristic time constant of approximately 20 ps. Later, the cooling rate decelerates to approximately 200 ps and is related to heat transfer to the solvent.

Membrane Crystals of Plant Light-Harvesting Complex II Disassemble Reversibly in Light

“…The light-induced disassociation of LHCII from the membrane crystal, reported here, is consistent with the thermooptic mechanism, according to which fast thermal transients due to dissipated excitation energy can lead to elementary structural transitions in the close vicinity of the site of dissipation (Cseh et al 2005, Gulbinas et al 2006, Garab 2014. The elementary structural changes are made possible by the presence of inherent structure instabilities near the site of dissipation, while the reversibility stems from an intrinsic, molecular self-association of the complexes into oligomers and ordered aggregates.…”

Fluorescence Spectral Dynamics of Single LHCII Trimers

“…Although the excitation energy density was substantially higher than under in vivo conditions, local heating was negligible (Supporting Material) (38,39). However, the high excitation rate may drive the protein into conformational states that occur less frequently under natural conditions but are still intrinsic to the system (16).…”

Structural Changes and Non-Photochemical Quenching of Chlorophyll a Fluorescence in Oxygenic Photosynthetic Organisms

“…Garab 2014 ). Dissipation of excitation energy causes a thermal transient with a sizeable temperature jump in the close vicinity (~1 nm) of the dissipation site (Cseh et al 2000 ) and with a decay time of about 20-200 ps (Gulbinas et al 2006 ). The relaxation of the protein might not be complete, and might thus, with some (usually very low) probability induce an elementary structural change, especially if the heat (temperature) jump induces a cation release (as has been observed both for LHCII and for isolated thylakoid membranes; Garab et al 1998 ;Garab et al 2002, see Garab 2014.…”