ADMR STUDY OF THE EFFECT OF OLIGOMERIZATION ON THE CAROTENOID TRIPLETS ANA ON TRIPLET-TRIPLET TRANSFER IN LIGHT-HARVESTING COMPLEX-II (LHC-II) OF SPINACH

Carotenoid triplets in isolated light harvesting complex (LHC) II of s pinach at different concentrations were studied by absorbance-detected magnetic resonance (ADMR). Going from high to low LHC II concentratio ns, a change was observed in the intensity of the ADMR spectra of the \D\+\E\ transition recorded at 507 nm relative to that recorded at 525 nm, from approx. 0.5 to approx. 1.0. The relative intensity of the 2\ E\ transition did not change. The change in relative intensity of the ADMR signal is due to a change of the ADMR signal intensity of the \D\ +\E\ transition that is detected at 525 nm. The effect is ascribed to an aggregation of trimeric LHC II into an oligomeric form of LHC II, T aking into account the narrowing of the zero-field resonance bands wit h oligomerisation, and the absence of bandshifts, the relative increas e of the signal intensity of the \D\+\E\ transition detected at 525 nm can be explained by assuming that the oligomer consists of a multiple of trimers, between which 'inter-trimer' energy transfer occurs. This yields an increase in the number of triplets that is transferred to t he carotenoid having its triplet absorption maximum at 525 nm. Our new results indicate that the carotenoids are bound to Chl a monomers, an d not dimers as proposed earlier (Van der Vos, R., Carbonera, D. and H off, A.J. (1991) Appl. Magn. Res. 2, 179-202).