Light-induced excitation quenching and structural transition in light-harvesting complex II

Light-induced fluorescence quenching of chlorophyll a in light-harvesting complex II (LHCII) incorporated into liposomes was examined. The rate of fluorescence quenching was found to depend on the incubation temperature. The effect was almost not observed at liquid nitrogen temperature, demonstrated a lag phase after onset of light at temperatures below 25 °C and was most distinctly pronounced at temperatures above 25 °C. Energetic uncoupling of accessory xanthophylls and chlorophyll a, and energetic uncoupling of chlorophyll b and chlorophyll a were observed as accompanying the excitation quenching. The observed changes were reversible during dark incubation. Similar energetic uncoupling was also observed in darkness, induced by the increase in temperature. Additionally, the temperature characteristics of fluorescence measurements displayed a pronounced transition in the region of 22–25 °C. The experiments carried out with the monomolecular layer technique in dicated a structural transition of LHC II in the same temperature region as demonstrated by an increase in the mean molecular area of LHC II at the argon-water interface. Alterations in surface topography induced by temperature changes could also be observed with scanning force microscopy of LHC II monolayers deposited as Langmuir-Blodgett films onto glass slides. The transition was found to be associated with the enhanced excitation energy consumption by the protein, monitored by calorimetric measurements. It is proposed that the observed transition efficiently protects LHC II against overexcitation-related damage and is therefore of physiological importance.