Carotenoids isomerisation

Nguyen, M., Erancis, D., and Schwartz, S., Thermal isomerisation susceptibility of carotenoids in different tomato varieties, J. Sci. Food Agric., 81, 910, 2001. 

Due to their antioxidant status, carotenoids are extremely sensitive to UV light, as well as air and temperature [16]. In nature, each carotenoid occurs with several geometrical ZIE stereoisomers, which can isomerise or oxidise easily [17]. These carotenoid stereoisomers can differ considerably in their biological effectiveness, for instance in bioavailability [18,19], in quenching free radicals, or in the prevention of diseases [20]. Therefore, the unambiguous and quantitative analysis of the pattern of carotenoid stereoisomers from biological matrices is indispensable, especially with regard to dietary supplementation. 

Matrix solid-phase dispersion (MSPD) is the extraction method of choice for the analysis of solid samples, such as plant material, foodstuffs or tissue samples [26]. This method has been developed especially for solid or viscous matrices. MSPD is preferable to other extraction techniques, because the solid or viscous sample can be directly mixed with the sorbent material of the stationary phase [27]. As the carotenoid stereoisomers stay bound in their biological matrix until the elution step, they are protected against isomerisation and oxidation [28]. The extraction scheme of MSPD is shown in Figure 5.2.1. 

Therefore, a mild and quick extraction technique is necessary to exclude the preparation of artifacts. The carotenoid stereoisomers can be quantitatively analysed, employing MSPD extraction, from plant material, as well as from serum samples, using on-line SPE without any isomerisation or oxidation of the carotenoids. The extraction step is coupled to the separation and identification steps. Here, LC-NMR hyphenation, employing C30 stationary phases, is suitable for unambiguous distinction between all of these stereoisomers. 

The syntheses of methyl bixin (24) and other natural carotenoids were mentioned above. A biogenetically inspired synthesis of e-carotene used the titanium tetrachloride complex of lycopene (16). Dehydrolycopene was also isolated. Two more syntheses of ) -carotene (2) have been reported which use intermediates in the synthesis of vitamin A (see Scheme 2). Although ll-cis-j5-carotene was produced it is rapidly isomerised to the all-frons form. The cross-conjugated system (72) has a previously unknown chromophore. Another example of this system, but with an additional 4 -oxo-group, was synthesised by Surmatis et al. in a study of the synthesis of keto carotenoids. They prepared echinenone (73) and the two protected 3,3 -dioxo-j8-carotene compounds (74) and (75). Treatment of the ketal with sulphuric acid gave mainly 3,3 -dioxo- -carotene (76) while hydrochloric acid gave 3,3 -dioxo-c-carotene (77). Under both conditions the enol ether gave the latter product. 

It is well established that unsatnrated fatty acids undergo oxidation, via a radical reaction mechanism. Carotenoids undergo similar reactions and indeed do this so readily they can act as antioxidants in food materials. This antioxidant ability of carotenoids derives from their ability to form a resonance stabilised free radical. In certain controlled conditions chemical oxidation of carotenoids can give rise to epoxide formation and isomerisation of this to a furanoxide (Wong,... 

Fig. 8.6 A proposed mechanism for the degradation and isomerisation of carotenoids in processed and stored vegetables.

As described above, different cA-isomers are formed during oxidation. The carotenoid stractnre makes a c/x-tranx-isomerisation possible around the double bonds (Fig. 8.2). In vegetables, the carotenoids are predominantly the aS -trans isomers, which may be converted to the cix-isomers. Cis-trans isomerisation resnlts in colonr changes in vegetable products as the spectral properties of the cA-carotenoids are different from the corresponding frawx-carotenoids. Insertion of one or more cA-double bonds in an aH-trans conjugated system results in a... 

Illumination promotes isomerisation of carotenoids. During iodine-catalysed photoisomerisation of dOA -trans-a- and all-/rans -/3-carotenes the major isomer was 13,15-di-cA-carotenes (Chen eta/., 1994). Different chlorophyll compounds may act as photosentisers and when added to a solution of /3-carotene, photoisomerisation may occur when exposed to light (O Neil and Schwartz, 1995). 

NGUYEN M, FRANCIS D and SCHWARTZ s (2001), Thermal isomerisation snsceptibihty of carotenoids in different tomato varieties , J Sci Food Agric, 81, 910-917. 

In recent years the universal presence of various 15-cw carotenoids in the reaction centres of phototrophic organisms has been established and associated with photoprotective functions in PS2. It has been demonstrated that the 15-c/s configuration has a unique property of efficient isomerisation to the all-trans configuration upon triplet excitation [43],... 

An intermediate role of cation radicals in cis-trans isomerisation of carotenoids has been considered [138]. AMI molecular orbital calculations show that the energy barrier of cis-trans isomerisation are much lower in cation radicals ( 20 kcal/mol) and dications ( 0 kcal/mol) than in neutral carotenoids ( 55 kcal/mol). HPLC analyses of the product mixture after bulk electrolysis of [3-carotene (1), canthaxanthin (16) and apocarotenoids showed the presence of 5-cis, 13-cis, 9-cis, 9,13-dicz s and all-trans isomers [138]. 

It was observed that electrochemical oxidation of all-trans P-carotene (1) and canthaxanthin (16) in CH2CI2 leads to significant trans-cis isomerisation [105]. It was suggested that the isomerisation mechanism involved cation radicals and/or dications which could easily undergo geometrical isomerisation. This proposal was supported by AMI molecular orbital calculations, which showed that the energy barrier from trans to cis is much lower in the cation radical and dication species than in the neutral carotenoid [105]. 

An alternative method to gain information on the structure of carotenoid dications is by studying their reactions with suitable nucleophiles [11]. We have recently investigated the products obtained when reacting the p-carotene dication (11) with H2O as a nucleophile in acetone providing isomerised isocryptoxanthin (49), mutatochrome (50) and strongly isomerised isocarotene (51), according to HPLC, Vis, MS and H NMR data. The formation of these products are rationalised in Fig. 

Carotenoids, Retenoids, Pheromones and Polyenes.-A convenient route to exclusively ( )-allylic phosphonates (e.g. 165), compounds which are useful in polyene synthesis, is provided by the base-catalysed isomerisation of the corresponding vinylphosphonates. " The bis(trifluoromethyl) analogue (167) of a known, potent squalene synthase inhibitor has been synthesised using the Wittig reaction of (166) with hexafluoroacetone as a key step. ... 

Isomerisation of the carotenoid skeleton is not aimed at the thermodynamic equilibrium, but at a selective isomerisation of the doubly-substituted double bonds. 

Starting from their experience in manufacturing /3-ionone, Hoffmann-La Roche initially favoured acetylene as the universal building block for further syntheses. The reaction of methyl vinyl ketone with lithium acetylide in ammonia gives a tertiary alcohol, which is isomerised with sulfuric acid into a mixture of the (Eland (Z)-isomers of 3-methylpent-2-en-4-ynol. The isomers can be separated by distillation. Whereas the main component, the (Z)-isomer, is used for the production of Vitamin A, the (E)-isomer finds application in carotenoid synthesis. 

Joachim Buddrus from the Technical University of Berlin developed a variant of the Wittig reaction, by using an epoxide as HBr scavenger this method produces carotenoids ofhigh purity. [71] Thermal (Z/ )-isomerisation and crystallisation follow simultaneously under solvent-exchange (chloroform/ethanol). The yield of 93 % is the best that has been achieved in Wittig reactions of this type. 

Ketoisophorone (KIP) is a key intermediate in the production of nutritional products (e.g. vitamins and carotenoids) and in the flavours and fragrances industries. One option for a technical access to KIP is the catal5rtic oxidation of isophorone (Fig. 16.13). For good selectivity and yield in the oxidation step a thermal isomerisation of a-isophorone to /1-isophorone is necessary. However, in order to avoid this additional step and because the isomerisation equilibrium is strongly in favour of the a-isomer, a direct oxidation of a-isophorone to KIP would clearly be preferred. 

All-trans-isomers of carotenoids in fresh and thermally processed materials are accompanied by small amounts of ds-isomers, called neocarotenoids. fS-Carotene is accompanied mainly by geometric isomers 9-cis, 13-cis- and 15,15 -cis-P-carotene. Lutein is accompanied mainly by 9-ds- and 9 -cis, 13-cis- and 13 -cis isomers less common are 15-cis and 15 -cis isomers. Neoxanthin is accompanied by 9-cis-, 9 -cis-, 13-cis- and 13 -cis isomers. Thermal processing can induce carotenoid trans to cis isomerisation. 

The concentration of carotenoids in crude vegetable oils is about 0.03-0.25%. Their content in refined oils is lower and depends on the conditions during refining. Pigments that are found in refined oils differ in their structure from natural pigments as they contain products of isomerisation and degradation of natural pigments. 

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