Nonequilibrium heating in LHCII complexes monitored by ultrafast absorbance transients

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.