Characterisation of LHC II in the aggregated state by linear and circular dichroism spectroscopy

Absorption, linear dichroism and circular dichroism spectroscopy at 293 and 77 K have been used in order to further explore the process of energy quenching in LHC IIb, the main light-harvesting complex of photosystem II. Upon aggregation there was an enhancement of linear dichroism bands in the Q(y) absorption region of chlorophyll b. The absorption spectrum at 77 K of aggregates revealed new bands around 656 nm and 680 nm, characterised by positive linear dichroism and negative circular dichroism signals, In the circular dichroism spectrum of aggregates a characteristic change was seen in the carotenoid and chlorophyll b regions, an increase of the chlorophyll a transition at 438 nm (-) and decrease of the red most negative band at around 677 nm. The amplitude of this band was in a tight correlation with a fluorescence quenching occurring upon LHC II aggregation. A new transition appeared at 505 nm with positive linear dichroism signal. It is suggested that protein aggregation causes a change in conformation and association of some chlorophyll a, chlorophyll b and xanthophyll molecules. These features of the linear dichroism spectrum of the aggregates were also detected for thylakoids in which they were particularly enhanced at low pH, suggesting that at least part of the light harvesting complex in the thylakoid membrane is in an aggregated form and the extent of aggregation in vivo can be controlled by the thylakoid pH gradient. (C) 1997 Published by Elsevier Science B.V.