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

Light-induced fluorescence quenching of chlorophyll a in light-harvesting c omplex II (LHCII) incorporated into liposomes was examined. The rate of flu orescence 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 degrees C and wa s most distinctly pronounced at temperatures above 25 degrees C. Energetic uncoupling of accessory xanthophylls and chlorophyll a, and energetic uncou pling of chlorophyll b and chlorophyll a were observed as accompanying the excitation quenching. The observed changes were reversible during dark incu bation. Similar energetic uncoupling was also observed in darkness, induced by the increase in temperature. Additionally, the temperature characterist ics of fluorescence measurements displayed a pronounced transition in the r egion of 22-25 degrees C. The experiments carried out with the monomolecula r layer technique indicated a structural transition of LHC II in the same t emperature 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 m icroscopy of LHC II monolayers deposited as Langmuir-Blodgett films onto gl ass slides. The transition was found to be associated with the enhanced exc itation energy consumption by the protein, monitored by calorimetric measur ements. It is proposed that the observed transition efficiently protects LH C II against overexcitation-related damage and is therefore of physiologica l importance.