Carotenoids geometrical isomerism

The characteristic absorption spectrum of each carotenoid is determined by a series of conjugated double bounds, the so-called chromophore. Usually the spectrum shows three absorption bands, which are affected by the length of the chromophore, the nature of the double bound, and the taking out of conjunction of one double bond. Several absorption spectra of some common carotenoids are shown in Fig. 2. A change of solvent may, however, cause a shift of the absorption bands. Owing to the extensive double-bond system, carotenoids exist in many geometrical isomeric forms (Z or E isomers). In nature most carotenoids occur in the all-trans form (E isomers) cis isomers (Z isomers) are frequently present in small amounts (6). Cis isomers can be distinguished from trans isomers by a characteristic absorption band ( cis peak ) that appears at 300-360 nm (7). 

The bacterial RCs contain a non-covalently bound carotenoid molecule that is located at the B branch. The lacking of a symmetrically related cofactor in the A branch introduces an element of asymmetry into the RC s architecture. The structural effect of this asymmetry on the Co chains of the L- and M-subunits is not drastic as can be seen in Fig. 5. The carotenoid is spheroidene in the RC of Rb. sphaeroides and 1,2-dihy oneurosporene in the RC of Rp. viridis. In both RCs, the carotenoids interact with helices A, B, C (see below) and cd of the M-subunit and the plane spanned by the carotenoid molecule is oriented parallel to the plane of the membrane. The carotenoid shows a c 5-geometric isomeric form and its binding pocket is mainly formed by hydrophobic residues, especially in the Rb. sphaeroides RC where six phenylalanines and five tryptophans are found within a radius of 5 A around the spheroidene molecule. In... 

Jeevarajan AS, Kispert LD and Piekara-Sady L (1993b) An ENDOR study of carotenoid cation radicals on silica-alumina solid supports. Chem Phys Lett 209 269-274 Jeevarajan AS, Wei CC and Kispert LD (1994a) Geometrical isomerization of carotenoids in dichloromethane. J Chem Soc Perkin Trans 2 861-869... 

The spectroscopic and photochemical properties of the synthetic carotenoid, locked-15,15 -cA-spheroidene, were studied by absorption, fluorescence, CD, fast transient absorption and EPR spectroscopies in solution and after incorporation into the RC of Rb. sphaeroides R-26.1. High performance liquid chromatography (HPLC) purification of the synthetic molecule reveal the presence of several Ai-cis geometric isomers in addition to the mono-c/x isomer of locked-15,15 -c/x-spheroidene. In solution, the absorption spectrum of the purified mono-cA sample was red-shifted and showed a large c/x-peak at 351 nm compared to unlocked all-spheroidene. Spectroscopic studies of the purified locked-15,15 -mono-c/x molecule in solution revealed a more stable manifold of excited states compared to the unlocked spheroidene. Molecular modeling and semi-empirical calculations revealed that geometric isomerization and structural factors affect the room temperature spectra. RCs of Rb. sphaeroides R-26.1 in which the locked-15,15 -c/x-spheroidene was incorporated showed no difference in either the spectroscopic properties or photochemistry compared to RCs in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 RCs which naturally contain spheroidene. The data indicate that the natural selection of a c/x-isomer of spheroidene for incorporation into native RCs of Rb. sphaeroides wild type strain 2.4.1 was probably more determined by the structure or assembly of the RC protein than by any special quality of the c/x-isomer of the carotenoid that would affect its ability to accept triplet energy from the primary donor or to carry out photoprotection. 

Particular aspects of carotenoids have been reviewed before in this series Geometrical isomerism was treated by Zechmeister (181) in 1960 and the application of spectroscopic methods in structural elucidation by Weedon (172) in 1969. 

Since much activity has been centered around determination of absolute configuration of carotenoids during the last decade, this review will be devoted mainly to this particular aspect of carotenoid research, but will also include recent advances concerning geometrical isomerism. Literature available through July 1978 has been evaluated for this review. 

The literature on geometrical isomerism of carotenoids from 1923 to 1970 has been discussed in detail by Zechmeister 181,182) and Weedon 174). Problems related to conformation have also been considered 174). 

Chemical shift considerations have also proved adequate for assignment of geometrical isomerism in the phytoene (21) series 115), for configurational assignment of the hydroxylated isopropylidene end group in decaprenoxanthin (8) and lycoxanthin (22) after allylic oxidation to the corresponding aldehydes 133, 156), as well as for analysis of 13-cw and 3-trans mixtures of 20-substituted carotenoids related to reniera-purpurin-20-al (23) 49). 

According to the new lUPAC nomenclature for carotenoids (98) the cis-trans convention is still used to denote geometrical isomerism of the polyene chain. However, the EjZ designations may also be used, especially when the prefixes cis and tram might lead to ambiquity (98). 

Geometrical isomerism in the carotenoid series is well covered in the earlier literature (174, 181, 182) through 1970. It has been known... 

Retinal (retinaldehyde) occurs in significant quantities only in ocular tissue of mammals. Inasmuch as ll-cw and all-Iran isomers of retinal and retinol play important roles in the visual cycle, the separation of geometric isomers of both retinoids is discussed here. All retinoids (and carotenoids) are isomerized by heat and light, so samples should be handled with extreme precautions if information on the original isomer distribution is desired. Because the m-isomers generally have lower molecular extinction coefficients than the -trans forms (Table 1), the presence of large amounts of c -isomers may result in underestimation of retinoid (or carotenoid) quantities. 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

Lycopene is the most predominant carotenoid in human plasma with a half-life of about 2-3 days (Stahl and Sies, 1996). Although the most prominent geometric isomers of lycopene in plants are the all-trans, in human plasma, lycopene is present as an isomeric mixture containing 50% of the... 

IMS is a relatively new technique in which ions are separated based on size and shape using an electric field. IMS was utilized by Dong et ah (2010) to separate all-trans -lycopene from cw-lycopene and all-irans -(3-carotene from cw-(3-carotene. Unfortunately, the various cis isomers could not be separated from each other using IMS alone. The authors provided evidence to suggest that cis/trans isomerization of carotenoids occur in-source (ESI positive mode was used in these experiments). Because of this isomerization, it does not appear likely that IMS will replace HPLC as a means of separating geometrical isomers of carotenoids in the near future (Dong et ah, 2010). 

While only two geometrical isomers exist in simple unsaturated molecules, a number of cis-trans isomers can exist in polyenes (e.g. carotenoids ). Normally, the alI-/ra/w-isomer is preponderant in carotenoids. Isomerization of the all-ftms-carotenoids by acid- or iodine-catalysis yield the so-called neo-carotenoids. The neo-carotenoids are believed to possess one c/s-ethylenic bond in the central position of the conjugated chain9. The neo-carotenoids exhibit not only a weaker long wavelength band 470 nyx) than the all... 

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