Linear and Branched Poly(ω-hydroxyacid) Esters in Plant Cutins

Poly(omega-hydroxyacid) ester oligomers were obtained from the cutin of four plant cuticles by partial depolymerization methanolysis. The structure of these oligomers was determined by electrospray ionization coupled to tandem mass spectrometry (ESI-MS/MS) analysis of their lithium-adducts, showing which monomers were present and their relative position. Dimers up to heptamers were identified, in a total yield of oligomers of up to 50% of the total cutin content. Two main structural types of poly(omega-hydroxyacid) ester oligomers were found. Poly(10,16-dihydroxyhexadecanoic acid) esters were the main oligomers in the cutin of Lycopersicum esculentum fruit, with a predominantly branched structure. The 1D and 2D NMR analysis of these oligomers showed that inter-monomer linkages were mostly in secondary midchain hydroxyls, in a ratio of 4.5 to 1, compared with esterification in the primary omega-hydroxyls. In Hedera helix leaves cutin, poly(9-epoxy-18-hydroxyoctadecanoic acid) esters were the dominant oligomers, composed of monomers linearly linked head-to-tail. It is proposed that the cutin polyesters have different poly(omega-hydroxyacid)ester domains, either built of linear chains, where the C-18-epoxy omega-hydroxyacids are dominant, or forming a highly branched network, where the C-16-dihydroxy omega-hydroxyacids predominate. It is hypothesized that the C-18 linear polyester can account for the ordered lamellae seen at ultrastructural level in H. helix cuticle, and that the C-16 mostly branched polyester will be the basis of the reticulate structure seen in L. esculentum cuticle.