Microwave and optical spectroscopy of carotenoid triplets in light-harvesting complex LHC II of spinach by absorbance-detected magnetic resonance

Abstract

Absorbance-detected magnetic resonance (ADMR) of the light-harvesting complex LHC II of spinach revealed two triplet contributions, having differentD values, but equalE value (|E|=0.00379 cm−1). The two triplets are assigned to two of the three carotenoids present in LHC II: lutein (|D|=0.03853 cm−1) and neoxanthin (|D|=0.04003 cm−1). The ADMR-detected Triplet-minus-Singlet (T—S) optical difference spectrum of the carotenoid (Car) triplet transition of LHC II showed, apart from bands in the Car absorption region, a contribution in the chlorophyll (Chl) absorption region due to a change in interaction between lutein and Chla at 670 nm, and neoxanthin and Chla at 670 and 677 nm. From Linear Dichroic (LD-)ADMR-detected LD-(T—S) spectra we have determined that the tripletz-axis (which corresponds roughly to the polyenal axis) of lutein and neoxanthin makes an angle of 47° and 38° with theQ y transition moment of their adjacent Chla molecules, for the Chls absorbing at 670 and 677 nm, respectively. TheT z triplet magnetic transition moment of lutein is parallel to the lutein singlet and triplet absorptions, whereas theT x axis of neoxanthin makes an angle of about 20 degrees with the optical transition moments of the carotenoid molecule. The major Chla absorption bands of the optical absorption spectrum and the ADMR-detected T—S spectrum is best explained by assuming that all Chla is present in dimers. It is proposed that a free Chl dimer absorbs at 664 and 670 nm, whereas a Chl dimer bound to a carotenoid absorbs at 670 and 677 nm.