Procyanidin hexamers

The finding that water-soluble flavonoids could exert their beneficial properties at the hydrophobic portion of the membrane was also observed in in vivo studies and in cells in culture. For example, erythrocytes obtained from animals fed a flavanol- and procyanidin-rich meal showed reduced susceptibility to free-radical-mediated hemolysis [Zhu et al., 2002]. Consistently, we demonstrated that procyanidin hexamers, which interact with membranes but would not be internalized, protected Caco-2 cells from AMVN- and bile-induced oxidation [Erlejman et al., 2006]. When liposomes were preincubated with a series of flavonoids with diverse hydrophobicity, not only hydrophobic flavonoids prevented AMVN-mediated lipid oxidation but also the more hydrophilic ones [Erlejman et al., 2004]. Similarly to what was previously found in liposomes, the protective effects of flavonoids against AMVN-supported oxidation was strongly associated with their capacity to prevent membrane disruption by detergents, supporting the hypothesis of a physical protection of membranes by preventing oxidants to reach fatty acids. 

The profile (%) of polyphenolics in tamarind pericarp was dominated by proan-thcyanidins in various forms, such as (+)-catechin, procyanidin B2, (-)-epicatechin (9.4), procyanidin trimer (11.3), procyanidin tetramer (22.2), procyanidin pentamer (11.6) and procyanidin hexamer (12.8), along with taxifolin (7.4), apigenin (2.0), eriodictyol (6.9), luteolin (5.0) and naringenin (1.4) of total phenols, respectively. Tamarind seeds comprised procyanidins only, represented (%) mainly by oligomeric procyanidin tetramer (30.2), procyanidin hexamer (23.8), procyanidin trimer (18.1), procyanidin pen-tamer (17.6), with lower amounts of procyanidin B2 (5.5) and (-)-epicatechin (Sudjaroen et al., 2005). 

Fig. 2.115. Mass chromatograms of catechin monomers (m/z 289) and procyanidin oligomers (dimer through hexamers m/z 577 to 1729). C, ( + )-catechin, EC, ( — )-epicatechin, and PB2, procyanidin B2 as identified by the retention times of authentic standard. Reprinted with permission from J. Wollgast et al. [256].

Another membrane property physically affected by flavonoids is the surface potential. Working with liposomes composed of PC and PS, we found that (—)-epicatechin and certain procyanidins (dimer to hexamer) decreased liposome surface potential. This effect relied on both, procyanidin concentration and number of monomeric units [Verstraeten et al., 2003]. On the other hand, when liposomes were composed exclusively of PC, it was found that (—)-epi-catechin dimers Al and B2, and the trimers A and C2 increased liposome surface potential in a concentration-dependent manner [Verstraeten et al.,... 

Figure 2. Relative IC50 values for the antioxidant effect of dimer, tetramer, and hexamer. Antioxidant effect was evaluated in liposomes incubated at 37°C during 60 min in the presence of different oxidant systems 1) AAPH 10 mM II) AMVN 10 mM III) 25 pM ferrous iron/25 pM ascorbate IV) UV irradiation. Black bars are relative values for IC50 calculated considering the concentration of the procyanidins based on moles of monomer equivalent and gray bars for relative IC50 calculated considering the concentration of procyanidins...

In order to correlate these pharmaceutical activities with their degree of polymerization, it is necessary to establish an efficient, reliable separation method. However, the application of liquid chromatography to the separation of these procyanidin oligomers is difficult because the oligomers above hexamers tend to cause irreversible adsorption onto the column packing materials. 

Moreover, other studies indicated that UHPLC was also characterized by better efficiency and sensitivity. For example, procyanidin oligomers up to hexamers could be detected by HPLC, whereas the UHPLC method allowed the detection of oligomers up to nonamers [97], as can be observed in Figure 16.2. 

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