ADMR study of the effect of oligomerisation on the carotenoid triplets and on triplet-triplet transfer in light harvesting complex II (LHC II) of spinach

Abstract

Carotenoid triplets in isolated light harvesting complex (LHC) II of spinach at different concentrations were studied by absorbance-detected magnetic resonance (ADMR). Going from high to low LHC II concentrations, 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 singla 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. Taking into account the narrowing of the zero-field resonance bands with oligomerisation, and the absence of bandshifts, the relative increase 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 the carotenoid having its triplet absorption maximum at 525 nm. Our new results indicate that the carotenoids are bound to Chl a monomers, and not dimers as proposed earlier (Van der Vos, R., Carbonera, D. and Hoff, A.J. (1991) Appl. Magn. Res. 2, 179–202).