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

Article abstract of DOI:10.1002/pca.2491

Introduction Suberin is a biopolyester responsible for the protection of secondary plant tissues, and yet its molecular structure remains unknown. The C_18:1 ω-hydroxyacid and the C_18:1 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 C_18:1 suberin acids. Methods Pure C_18:1 ω-hydroxyacid and C_18:1 α,ω-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 ¹³C carbons were shielded comparatively to the trans model compound, and (ii) the complex multiplets of the olefinic protons could be simulated only with ³J_HH and long-range ⁴J_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 2013 John Wiley & Sons, Ltd. The two C_18:1 cork suberin ω-hydroxyacid and α,ω-diacid were proved to have a cis configuration: (Z)-18-hydroxyoctadec-9-enoic acid and (Z)-octadec-9-enedoic acid. Their double bond stereochemistry was elucidated by spectroscopic techniques, namely ¹H-NMR and ¹³C-NMR. The chemical shifts of the olefinic and allylic protons and of the allylic carbons were diagnostic, and the ¹H-NMR multiplets of the olefinic protons could only be simulated using ³J_HH and long-range ⁴J _HH coupling constants typical of the cis configuration. The revealed stereochemistry of this two major suberin monomers brings a new insight into the mostly unknown suberin macromolecular structure. Copyright