Carotenoid molecules

The extinction coefficients of carotenoids have been listed completely bnt solvent effects can shift the absorption patterns. If a colorant molecnle is transferred into a more polar environment, then the absorption will be snbjected to a bathochro-mic (red) shift. If the colorant molecnle is transferred into a more apolar enviromnent, the absorption will be subjected to a hypsochromic (blue) shift. If a carotenoid molecule is transferred from a hexane or ethanol solution into a chloroform solution, the bathochromic shift will be 10 to 20 nm. 

Oxidation — Oxidizing radicals with high redox potential can remove one electron from the carotenoid molecule to yield a radical cation CAR - e- CAR+ (e.g. CAR H- R CAR + R). 

Reduction — The addition of one electron to the carotenoid molecule would give the radical anion CAR -H e- — CAR". 

Caco-2 cells and ezetimibe, a potent inhibitor of chloresterol absorption in humans, it was reported that (1) carotenoid transport was inhibited by ezetimibe up to 50% and the extent of that inhibition diminished with increasing polarity of the carotenoid molecule, (2) the inhibitory effects of ezetimibe and the antibody against SR-BI on P-carotene transport were additive, and (3) ezetimibe may interact physically with cholesterol transporters as previously suggested - and also down-regulate the gene expression of three surface receptors, SR-BI, NPCILI, and ABCAl. 

In order to exhibit provitamin A activity, the carotenoid molecule must have at least one unsubstituted p-ionone ring and the correct number and position of methyl groups in the polyene chain. Compared to aU-trans P-carotene (100% provitamin A activity), a-carotene, P-cryptoxanthin, and y-carotene show 30 to 50% activity and cis isomers of P-carotene less than 10%. Vitamin A equivalence values of carotenoids from foods have been recently revised to higher ratio numbers (see Table 3.2.2) due to poorer bioavailability of provitamin A carotenoids from foods than previously thought when assessed with more recent and appropriate methods. 

The characteristic system of conjugated double bonds in the carotenoid molecule in which the Tt electrons are delocahzed over the whole polyene chain is responsible for both absorption of visible light and chemical reactivity, resulting in unstable and easily destroyed compounds. Based on this information, the following precautions must be taken during analysis ... 

Detailed information about carotenoids found in food or extracted from food and evaluated for their potential as food colorants appeared in Sections 4.2 and 6.2. We would like to mention some new data about the utilization of pure carotenoid molecules or extracts as allowed food additives. Looking to the list of E-coded natural colorants (Table 7.2.1), we can identify standardized colorants E160a through f, E 161a, and E161b as natural or semi-synthetic derivatives of carotenoids provided from carrots, annatto, tomatoes, paprika, and marigold. In addition, the extracts (powders or oleoresins) of saffron, - paprika, and marigold are considered more economical variants in the United States and European Union. 

Carotenoid molecules incorporated into the lipid membranes considerably interfere with both the structural and the dynamic membrane properties. Both effects are directly related to the chemical structure of carotenoid molecules. Importantly, it is the rigid, rod-like backbone of the carotenoids,... 

The use of selective isotope replacement of carbon and hydrogen atoms in the structure of xanthophylls in combination with LHCII reconstitution should greatly aid the assignment of multiple v4 twisting bands. This assignment would help localize the areas of distortion within the carotenoid molecule and understand the possible causes of this distortion. 

FIGURE 8.2 (a) Simplified energy-level scheme of a carotenoid molecule. The solid arrow represents the... 

The aggregation-induced changes of absorption spectra result from intermolecular interactions between closely spaced carotenoid molecules. For two molecules whose transition dipole moment vectors, p, are located at places characterized by position vectors /- and r2, with the relative position vector defined as R=r1-r2, Figure 8.3, the interaction energy is expressed as (van Amerongen et al. 2000)... 

A large set of results obtained in recent years for various carotenoids (see, e.g., Simonyi et al. (2003) for review) suggests that planarity of the carotenoid molecule is crucial for aggregation. This hypothesis is supported by the observation that zeaxanthin and astaxanthin, both fairly planar molecules, form aggregates more readily than other carotenoids. Moreover, zeaxanthin and astaxanthin are the only two carotenoids studied so far that can, depending on preparation conditions, form exclusively either H- or J-aggregates (Billsten et al. 2005, Kopsel et al. 2005, Avital... 

Besides the main band, H-aggregates also exhibit weaker bands in the red part of the absorption spectrum (marked by in Figure 8.5). Although in some cases the position of these bands coincides with the vibrational bands of the monomeric carotenoid and can be therefore assigned to nonaggregated carotenoid molecules, certain spectral features do not match the vibrational bands... 

Adsorption of carotenoids on activated silica-alumina results in their chemical oxidation and carotenoid radical formation. Tumbling of carotenoid molecules adsorbed on solid support is restricted, but the methyl groups can rotate. This rotation is the only type of dynamic processes which is evident in the CW ENDOR spectrum. 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

Photo-oxidation of carotenoids in Ni-MCM-41 produces an intense EPR signal (Figure 9.11) with -value 2.0027 due to the carotenoid radical another, less intense EPR signal, with =2.09 is attributed to an isolated Ni(I) species produced as a result of electron transfer from the carotenoid molecule to Ni(II). It has been reported that Ni(I) ions prepared upon reduction of Ni(II)-MCM-41 by heating in a vacuum or in dry hydrogen exhibits an EPR spectrum with , =2.09 and N=2.5... 

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