Stereochemistry of C18 Monounsaturated Cork Suberin Acids Determined by Spectroscopic Techniques Including 1H-NMR Multiplet Analysis of Olefinic Protons

Introduction Suberin is a biopolyester responsible for the protection of secondary plant tissues, and yet its molecular structure remains unknown. The C-18:1 omega-hydroxyacid and the C-18:1 alpha,omega-diacid are major monomers in the suberin structure, but the configuration of the double bond remains to be elucidated. Objective To unequivocally define the configuration of the C-18:1 suberin acids. Methods Pure C-18:1 omega-hydroxyacid and C-18:1 alpha,omega-diacid, isolated from cork suberin, and two structurally very close C-18:1 model compounds of known stereochemistry, methyl oleate and methyl elaidate, were analysed by NMR spectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopy, and GC-MS. Results The GC-MS analysis showed that both acids were present in cork suberin as only one geometric isomer. The analysis of dimethyloxazoline (DMOX) and picolinyl derivatives proved the double bond position to be at C-9. The FTIR spectra were concordant with a cis-configuration for both suberin acids, but their unambiguous stereochemical assignment came from the NMR analysis: (i) the chemical shifts of the allylic C-13 carbons were shielded comparatively to the trans model compound, and (ii) the complex multiplets of the olefinic protons could be simulated only with (3)J(HH) and long-range (4)J(HH) coupling constants typical of a cis geometry. Conclusion The two C-18:1 suberin acids in cork are (Z)-18-hydroxyoctadec-9-enoic acid and (Z)-octadec-9-enedoic acid. Copyright (c) 2013 John Wiley & Sons, Ltd.