The conventional organic chemistry concept of chirality relates to single molecules. This article deals with cases in which exciton chirality is generated by the interaction of associated carotenoids. The handed property responsible for exciton signals in these systems is due to the alignment of neighboring molecules held together by secondary chemical forces. Their mutual positions are characterized by the overlay angle. Experimental manifestation is obtained by spectroscopic studies on carotenoid aggregates. Compared to molecular spectra, both UV/visible and circular dichroism spectroscopic observations reveal modified absorption bands and induced Cotton effects of opposite sign (exciton couplets), respectively. A new term, "supramolecular exciton chirality," is suggested for these phenomena, allowing the detection of weak chemical interactions not readily accessible for experimental studies, although highly important in the mechanism of biological processes.

The changes in the carotenoid pigments of the Capsicum annuum var. lycopersiciforme rubrum during maturation have been investigated quantitatively by means of a HPLC technique. In all of the chromatograms, 40 peaks were detected; 34 carotenoids were identified. The total carotenoid content of the ripe fruits was about 1.3 g/100 g of dry weight, of which capsanthin constituted 37%, zeaxanthin was 8%, cucurbitaxanthin A was 7%, capsorubin constituted 3.2%, and beta-carotene accounted for 9%. The remainder was composed of capsanthin 5,6-epoxide, capsanthin 3,6-epoxide, 5,6-diepikarpoxanthin, violaxanthin, antheraxanthin, beta-cryptoxanthin, and several cis isomers and furanoid oxides. The possible biosynthetic routes for the formation of minor carotenoids containing 3,5,6-trihydroxy-beta-, 3,6-epoxy-beta-, and 6-hydroxy-gamma-end groups are described.

Spontaneously forming H- and J-type chiral self-assemblies of lutein and lutein diacetate were studied by means of ultraviolet−visible and circular dichroism spectroscopy (CD). Spectroscopic properties of their thin films has been determined for the first time. Existence of chiral superstructures in both solution and solid phases with opposite and same handedness was demonstrated. The molecular exciton theory and the exciton chirality rule were applied to define both qualitatively and quantitatively the spectral shifts, band shape changes, and the highly intense but quite different excitonic CD activity in the two forms of aggregates. Atomic force microscopy images revealed definite threadlike morphology in lutein diacetate films referring to the formation of a nematic liquid crystal phase which probably exists in the aqueous solution either. On this basis, the vibronically coupled CD bands of lutein diacetate are attributed to an excitonic interaction arised from the twisted nematic layers.

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Pulsed electron nuclear double resonance (ENDOR) and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) studies in combination with density functional theory (DFT) calculations revealed that photo-oxidation of natural zeaxanthin (ex Lycium halimifolium) and violaxanthin (ex Viola tricolor) on silica-alumina produces the carotenoid radical cations (Car*+) and also the neutral carotenoid radicals (#Car*) as a result of proton loss (indicated by #) from the C4(4') methylene position or one of the methyl groups at position C5(5'), C9(9'), or C13(13'), except for violaxanthin where the epoxide at positions C5(5')-C6(6') raises the energy barrier for proton loss, and the neutral radicals #Car*(4) and #Car*(5) are not observed. DFT calculations predict the largest isotropic beta-methyl proton hyperfine couplings to be 8 to 10 MHz for Car*+, in agreement with previously reported hyperfine couplings for carotenoid pi-conjugated radicals with unpaired spin density delocalized over the whole molecule. Anisotropic alpha-proton hyperfine coupling tensors determined from the HYSCORE analysis were assigned on the basis of DFT calculations with the B3LYP exchange-correlation functional and found to arise not only from the carotenoid radical cation but also from carotenoid neutral radicals, in agreement with the analysis of the pulsed ENDOR data. The formation of the neutral radical of zeaxanthin should provide another effective nonphotochemical quencher of the excited state of chlorophyll for photoprotection in the presence of excess light.

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The later steps of carotenoid biosynthesis involve the formation of cyclic carotenoids. The reaction is catalyzed by lycopene b-cyclase (LCY-B), which converts lycopene into b-carotene, and by capsanthin-capsorubin synthase (CCS), which is mainly dedicated to the synthesis of k-cyclic carotenoids (capsanthin and capsorubin) but also has LCY-B activity. Although the peptide sequences of plant LCY-Bs and CCS contain a putative dinucleotide-binding motif, it is believed that these two carotenoid cyclases proceed via protic activation and stabilization of resulting carbocation intermediates. Using pepper (Capsicum annuum) CCS as a prototypic carotenoid cyclase, we show that the monomeric protein contains one noncovalently bound flavin adenine dinucleotide (FAD) that is essential for enzyme activity only in the presence of NADPH, which functions as the FAD reductant. The reaction proceeds without transfer of hydrogen from the dinucleotide cofactors to b-carotene or capsanthin. Using site-directed mutagenesis, amino acids potentially involved in the protic activation were identified. Substitutions of alanine, lysine, and arginine for glutamate-295 in the conserved 293-FLEET-297 motif of pepper CCS or LCY-B abolish the formation of b-carotene and k-cyclic carotenoids. We also found that mutations of the equivalent glutamate-196 located in the 194-LIEDT-198 domain of structurally divergent bacterial LCY-B abolish the formation of b-carotene. The data herein reveal plant carotenoid cyclases to be novel enzymes that combine characteristics of non-metal-assisted terpene cyclases with those attributes typically found in flavoenzymes that catalyze reactions, with no net redox, such as type 2 isopentenyl diphosphate isomerase. Thus, FAD in its reduced form could be implicated in the stabilization of the carbocation intermediate.

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