Triplet-Triplet Absorption Spectra and Extinction Coefficients of Lutein, Neoxanthin And Violaxanthin

Abstract

A carotenoid (Car) molecule, when in van der Waals contact with a chlorophyll (Chl) or CM-like partner molecule, acts as an efficient quencher of the lowest excited triplet state of the partner (Par) by participating in the process Car⁰:Part^†→ Car^†:Par⁰, where the colon signifies the contiguity of the two chromophores, and the superscripts 0 and dagger denote, respectively, the occupation of So, the ground singlet state and T1 the lowest excited triplet state (1,2). Nanosecond laser kinetic spectroscopy of isolated light-harvesting complexes has revealed that (bacterio)Chlt decays rapidly (that is, within ca. 200 ns or sooner), transferring its triplet excitation to a neighbouring Car⁰ (3–9) This means that the yield of the formation of Cart equals the corresponding yield of Part; to determine the latter yield, which is a quantity of much interest, one must know ε⁰ and ε^†, the extinction coefficients (respectively) for the Sn←S0 and Tn←T1 transitions in Car. The values of ε⁰ are known for a great many Car’s (10), but the determination of et has lagged behind. The present study was undertaken with a view to filling this gap; here we present some preliminary results pertaining to the triplet-triplet spectra of lutein (Lut), neoxanthin (Neo), and violaxanthin (Vio), the three xanthophylls bound to LHCII (the Chla/b light-harvesting complex associated with photosystem II of green plants).