The Chemistry of Sporopollenin Ektexine and Endexine Layers Isolated from Sunflower Pollen through an Ionic Liquid-Mediated Process

Sporopollenin is a plant polymer present in the exine of the pollen grains that comprises two layers: the endexine and the ektexine. It possesses remarkable mechanical, thermal, and chemical stability and is also highly recalcitrant to hydrolysis. The chemical backbone of sporopollenin mostly consists of a polyhydroxylated aliphatic component and polyketide-derived aliphatic α-pyrone elements. Recent works have provided important insights into its molecular structure, yet due to the extreme inertness of the polymer, outstanding questions still exist. In this work, we produced and characterized sporopollenin enriched materials obtained from dewaxed sunflower pollen using conventional acetolysis and two ionic liquid solvents or combinations of both. Microscopic (SEM) and spectroscopic analyses (mostly NMR) showed that either method alone could render sporopollenin enriched fractions. Only the acetolyzed materials showed an increase in acetate content. Ionic liquids used alone led to the isolation of naked spore capsules containing only the endexine layer, suggesting that the ektexine layer could be solubilized by the ionic liquid. On the contrary, the acetolyzed sporopollenin capsules could not be further modified by the ionic liquid treatment, preserving the two exine layers and an echinate surface. Our results suggest that the acetolysis altered the surface hydrophobicity of sporopollenin due to the introduction of acetate. The ionic liquid process led to the isolation of either exine layer, with both showing virtually the same chemistry.