Microwave and Optical Spectroscopy of Carotenoid Triplets in Light-Harvesting Complex LHC II of Spinach by Absorbance-Detected Magnetic Resonance

Absorbance-detected magnetic resonance (ADMR) of the light-harvesting complex LHC II of spinach revealed two triplet contributions, having different D values, but equal E value (vertical bar E vertical bar = 0.00379 cm(-1)). The two triplets are assigned to two of the three carotenoids present in LHC II: lutein (vertical bar D vertical bar = 0.03853 cm(-1)) and neoxanthin (vertical bar D vertical bar = 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. the chlorophyll (Chl) absorption region due to a change in interaction between lutein and Chl a at 670 nm, and neoxanthin and Chl a at 670 and 677 nm. From Linear Dichroic (LD-)ADMR-detected LD-(T-S) spectra we have determined that the triplet z-axis (which corresponds roughly to the polyenal axis) of lutein and neoxanthin makes angle of 47 degrees and 38 degrees with the Q(y) transition moment of their adjacent Chl a molecules, for the Chls absorbing at 670 and 677 nm, respectively. The T-z triplet magnetic transition moment of lutein is parallel to the lutein singlet and triplet absorptions, whereas the T-z axis of neoxanthin makes angle of about 20 degrees with the optical transition moments of the carotenoid molecule. The major Chl a absorption bands of the optic. absorption spectrum and the ADMR-detected T-S spectrum is best explained by assuming that all Chl a 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.