Polar carotenoids

The most common mobile phase is a gradient of petroleum ether or hexane with increasing concentrations of acetone or diethyl ether. Development of the column should be optimized for each sample to afford a quick and effective separation to avoid band broadening. The separation can be followed visually. The most non-polar a- and 3-carotenes are eluted first as a yellow band followed by the chlorophylls and other more polar carotenoids like cryptoxanthin, lutein, and zeaxanthin that frequently fuse together and appear as a single band. ... 

Owing to their chemical structure, carotenes as polyterpenoids are hydrophobic in nature (Britton et al., 2004). Therefore, as it might be expected, the carotenes are bound within the hydrophobic core of the lipid membranes. Polar carotenoids, with the molecules terminated on one or two sides with the oxygen-bearing substitutes, also bind to the lipid bilayer in such a way that the chromophore, constituted by the polyene backbone is embedded in the hydrophobic core of the membrane. There are several lines of evidence for such a localization of carotenoids with respect to the lipid bilayers. 

With apolar carotenoids With polar carotenoids... 

FIGURE 2.2 Model representation of organization of the lipid membrane containing apolar and polar carotenoid pigments. 

Subczynski, W.K., E. Markowska, W.I. Gruszecki, and J. Sielewiesiuk. 1992. Effects of polar carotenoids on dimyristoylphosphatidylcholine membranes A spin-label study. Biochim. Biophys. Acta 1105 97-108. 

Wisniewska, A. and W.K. Subczynski. 1998. Effects of polar carotenoids on the shape of the hydrophobic barrier of phospholipid bilayers. Biochim. Biophys. Acta 1368 235-246. 

Figure 4.7 shows the structures of important carotenoids (all-E) lutein, (all-E) zeaxanthin, (all-E) canthaxanthin, (all-E) p-carotene, and (all-E) lycopene. Employing a self-packed C30 capillary column, the carotenoids can be separated with a solvent gradient of acetone water=80 20 (v/v) to 99 1 (v/v) and a flow rate of 5 pL min, as shown in Figure 4.8 (Putzbach et al. 2005). The more polar carotenoids (all-E) lutein, (all-E) zeaxanthin, and (all-E) canthaxanthin elute first followed by the less polar (all-E) p-carotene and the nonpolar (all-E) lycopene. Figure 4.9 shows the stopped-flow II NMR spectra of these five carotenoids. The chromatographic run was stopped when the peak maximum of the compound of interest reached the NMR probe detection volume.

Barriers of Lipid Bilayers Formed by Polar Carotenoids.203... 

Accumulation of Polar Carotenoids in Unsaturated Membrane Domains.205... 

The phase-transition temperature, 7 , and the width of transition, A7j/2, were operationally defined based on EPR data, as shown in Figure 10.6a. As a rule, in the presence of polar carotenoids the phase transition broadens and shifts to lower temperatures (Subczynski et al. 1993, Wisniewska et al. 2006). The effects on Tm are the strongest for dipolar carotenoids, significantly weaker for monopolar carotenoids, and negligible for nonpolar carotenoids. The effects decrease with the increase of membrane thickness. Additionally, the difference between dipolar and monopolar carotenoids decreases for thicker membranes (Subczynski and Wisniewska 1998, Wisniewska et al. 2006). These effects for lutein, P-cryptoxanthin, and P-carotene are illustrated in Figure 10.6b... 

The hypothesis that polar carotenoids regulate membrane fluidity of prokaryotes (performing a function similar to cholesterol in eukaryotes) was postulated by Rohmer et al. (1979). Thus, the effects of polar carotenoids on membrane properties should be similar in many ways to the effects caused by cholesterol. These similarities were demonstrated using different EPR spin-labeling approaches in which the effects of dipolar, terminally dihydroxylated carotenoids such as lutein,... 

Membranes of extreme halophilic (Kushwaha et al. 1975, Anwar et al. 1977, Anton et al. 2002, Lutnaes et al. 2002, Oren 2002) and thermophilic bacteria (Alfredsson et al. 1988, Yokoyama et al. 1995) contain a large concentration of polar carotenoids. Membranes of these bacteria, which live in extreme conditions, should provide a high barrier to block nonspecific permeation of polar and nonpolar molecules. Incorporation of dipolar carotenoids into these membranes at a high concentration serves this purpose well because dipolar carotenoids increase the hydrophobic barrier for polar molecules (Wisniewska and Subczynski 1998, Wisniewska et al. 2006) and increase the rigidity barrier... 

Subczynski, W. K. and A. Wisniewska. 1998. Effects of P-carotene on physical properties of lipid membranes-comparison with effects of polar carotenoids. Curr. Top. Biophys. 22 44—51. 

Chemical properties of carotenoids play an important role in carotenoid micellarization and, therefore, bioavailability. Apolar carotenoids (carotenes) are generally incorporated in the central region, which is highly hydrophobic, of the oil droplets, whereas polar carotenoids (xanthophylls) are localized on the surface, and therefore xanthophylls are more easily micellarized and absorbed than carotenes (Borel and others 1996). van het Hof and others (2000) found in humans that lutein is five times more bioavailable than (3-carotene. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

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