A range of fatty acids differing in
chain length (C8 to C22), degree and location of unsaturation (saturated vs
unsaturated and terminal vs nonterminal olefins) and the presence of hydroxyl
moiety were conjugated with coniferyl alcohol. Antioxidant activity of phenolic
compounds depends on the polarity of their microenvironment (McClements and Decker 2000; Foti et al. 2004; Figueroa-Espinoza and Villeneuve 2005). Thus, the antioxidant activity of
the synthesized capsiconinoid analogues were studied in different experimental
models, such as 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging
activity in polar methanolic medium, inhibition of lipid (fish oil) oxidation
by thiobarbituric acid reactive substances (TBARS) assay, and autoxidation of
linoleic acid in Tween 20 micellar medium. The aim of the work is to evaluate
the influence of the chain length and type of fatty acids of the prepared coniferyl
esters of fatty acids on their antioxidant activities.
Phenolipids are diversified group of
compounds present abundantly in nature and are known for many potent biological
activities (Stasiuk and Kozubek 2010). They have been used for decades as
industrial antioxidants in cosmetic and food industries (Shahidi and Zhong
2010). Capsicum annum L. is found to
be source of two well known natural phenolipids, capsinoids and capsaicinoids.
Other than these two phenolipids, capsiconinoids have also been isolated from
the fruits of pepper, Capsicum baccatum L.
(Kobata et al. 2008). First synthesis of capsiconiate and dihydrocapsiconiate was
carried out by lipase-catalyzed transesterification of coniferyl alcohol with
methyl esters of 8-methylnon-6-enoic and 8-methylnonanoic acids (Kobata et al.
2008). But, the synthesis of natural capsiconinoid is complicated and costly
due to the specific nature of the alkyl side chain. However, linear analogues
of capsiconinoids such as coniferyl stearate, coniferyl palmitate and coniferyl
oleate have been isolated, synthesized and evaluated for antioxidant activities
(Lee et al. 2004; Appendino et al. 2004).
The present work was taken up to
synthesize a series of linear analogues of capsiconinoids differing in alkyl
chain length and functional moieties. The objective is to study the effect of
hydrophobicity as well as the presence of functional moiety in the hydrophobic
chain on antioxidant activity of the linear capsiconinoid analogues. Coniferyl
alcohol and fatty acids were esterified using immobilized lipase Candida antarctica (Novozyme 435) in t-butanol at 55 ?C for 4 h. The
structures of the compounds were confirmed by NMR (1H and 13C)
and high resolution mass spectrometry. Altogether 10 compounds were
synthesized by varying the alkyl chain length as well as functionality in the
alkyl chain. Out of them three compounds (e-g)
were reported earlier whereas seven compounds are reported in this work (a to d & h to j) (Fig. 2).
The antioxidant activity of the synthesized compounds was
determined by using three in vitro assays: DPPH radical scavenging assay (polar medium), thiobarbituric acid reactive
substances (TBARS) assay (nonpolar medium) and autoxidation of linoleic acid (micellar
DPPH free radical scavenging activity
The DPPH radical scavenging capacity of all the synthesized compounds (Table 1) indicated
good radical scavenging activity similar with BHT, but significantly
better than ?-tocopherol (p