Isolation and molecular structure of an oligomer produced enzymatically from the cuticle of lime fruit

Abstract

The selective enzymatic cleavage of primary ester linkages with porcine pancreatic lipase has been used to investigate the structural framework of Citrus aurantifolia (lime) cutin, a cross-linked protective biopolymer obtained from the outer fruit skins. Among the soluble oligomeric products isolated from this reaction is a novel 1307-amu pentamer based on C 16 fatty acids and hydroxyfatty acids and assembled solely with linkages between the esters of secondary alcohols. This compound was isolated by high-performance preparative thin-layer chromatography, and a structure was proposed using evidence from electron-impact mass spectrometry supported by solution-state NMR spectroscopy. The building blocks of the pentamer include 10,16-dihydroxyhexadecanoic acid, hexadecanoic acid, 10-hydroxyhexadecanoic acid, and 1,7,16-trihydroxyhexadecanol, which can participate in polymer chain elongation as well as the formation of cross-link structures. Three of the four building blocks of the pentamer are among the major monomeric constituents of citrus cutins, but ω-hydroxyoxo acid monomers are absent from the soluble oligomers produced by enzymatic reaction. The resistance of fruit cutins to these breakdown procedures suggests that despite a predominance of primary ester linkages, a support network of secondary ester cross-links may limit the accessibility of interior ester bonds to the lipase enzyme. The selective enzymatic cleavage of primary ester linkages with porcine pancreatic lipase has been used to investigate the structural framework of Citrus aurantifolia (lime) cutin, a cross-linked protective biopolymer obtained from the outer fruit skins. Among the soluble oligomeric products isolated from this reaction is a novel 1307-amu pentamer based on C 16 fatty acids and hydroxyfatty acids and assembled solely with linkages between the esters of secondary alcohols.