Comparison of flash pyrolysis, differential scanning calorimetry, 13C NMR and IR spectroscopy in the analysis of a highly aliphatic biopolymer from plant cuticles

Abstract

Recently we described the occurrence of a highly aliphatic and resistant biopolymer in leaf cuticles of some modern and fossil higher plants. The highly aliphatic nature of this biopolymer was deduced from flash pyrolysis-gas chromatographic and flash pyrolysis-gas chromatographic-mass spectrometric data. In an attempt to further identify its molecular structure, a “purified” fraction of the biopolymer was subjected to solid state 13C NMR and IR spectroscopy, differential scanning calorimetry and elemental analysis. Surprisingly the 13C NMR data indicated that ca. 50% of the sample consists of carbohydrate moieties and that the remainder can be ascribed to the expected polyethylene moieties. The IR and differential scanning calorimetry data were in accordance with these 13C NMR data as well as the elemental analysis. The cross-polarization behaviour during the 13C NMR experiments and the percentage of crystallinity in both the polyethylene and the saccharide moiety (ca. 50%) are indicative of covalent bonds between the two. This was further supported by additional density scanning calorimetric and flash pyrolysis-gas chromatographic analyses. Obviously analysis of the aliphatic biopolymer by flash pyrolysis alone was highly misleading since the pyrolysate only reflected a limited and specific part of the macromolecular structure. This investigation very clearly demonstrates that analytical pyrolysis should be used only in combination with other analytical methods, especially solid state 13C NMR and IR spectroscopy, to elucidate the molecular substructures of insoluble macromolecular or polymeric compounds.