COMPARISON OF THE ABSORPTION-SPECTRA OF TRIMERS AND AGGREGATES OF CHLOROPHYLL A B LIGHT-HARVESTING COMPLEX LHC-II/

The absorption spectrum of a suspension containing aggregates of LHC I I, the light-harvesting chlorophyll a/b-protein complex associated wit h photosystem II, when corrected for distortions introduced by scatter ing and mutual shadowing of trimers within a single aggregate, turns o ut to be almost superposable on the absorption spectrum recorded after disrupting the aggregates by the addition of a detergent at a concent ration close to its critical micelle concentration (CMC). The correcti on for scattering is effected by implementing a strategy proposed in 1 962 by Latimer and Eubanks; that for shadowing, by using a relation de rived by Duysens in 1956, which also furnishes an estimate of the aggr egate size. The standard procedure for bringing down scattering-relate d distortions, namely the use of an opal-glass plate, is found to be u nsatisfactory for LHC II samples. Extinction spectra (i.e. scattering- contaminated experimental absorption spectra), recorded over a limited range of the detergent concentration (lying between zero and the CMC) , are found to pass through two isosbestic points, which differ from t heir counterparts in true absorption spectra: being points at which to tal extinction stays constant, their locations depend on the instrumen tal geometry as well as on the size of the aggregates. (C) 1997 Elsevi er Science B.V.