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Extraction of plant light-harvesting complex II (LHCII) from the native thylakoid membrane or from aggregates by the use of surfactants brings about significant changes in the excitonic circular dichroism (CD) spectrum and fluorescence quantum yield. To elucidate the cause of these changes, e.g. trimer-trimer contacts or surfactant-induced structural perturbations, we compared the CD spectra and fluorescence kinetics of LHCII aggregates, artificial and native LHCII-lipid membranes, and LHCII solubilized in different detergents or trapped in polymer gel. By this means we were able to identify CD spectral changes specific to LHCII-LHCII interactions, at (−)-437 and (+)-484 nm, and changes specific to the interaction with the detergent n-dodecyl-β-maltoside (β-DM) or membrane lipids, at (+)-447 and (−)-494 nm. The latter change is attributed to the conformational change of the LHCII-bound carotenoid neoxanthin, by …