Stereochemistry of C18 monounsaturated cork suberin acids determined by spectroscopic techniques including (1) H-NMR multiplet analysis of olefinic protons

Abstract

Introduction: Suberin is a biopolyester responsible for the protection of secondary plant tissues, and yet its molecular structure remains unknown. The C18:1 ω-hydroxyacid and the C18:1 α,ω-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 C18:1 suberin acids.

Methods: Pure C18:1 ω-hydroxyacid and C18:1 α,ω-diacid, isolated from cork suberin, and two structurally very close C18:1 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 (13) C carbons were shielded comparatively to the trans model compound, and (ii) the complex multiplets of the olefinic protons could be simulated only with (3) JHH and long-range (4) JHH coupling constants typical of a cis geometry.

Conclusion: The two C18:1 suberin acids in cork are (Z)-18-hydroxyoctadec-9-enoic acid and (Z)-octadec-9-enedoic acid.

Keywords: (Z)-18-hydroxyoctadec-9-enoic acid; (Z)-octadec-9-enedoic acid; 1H-NMR multiplet analysis; Cis/trans configuration; cork suberin.