Sporopollenin Surface Characterization with Inverse Liquid Chromatography

Pollen grains are microscopic particles with unique surface chemistries tuned by evolution to protect delicate genetic material. The exine is composed of a cross-linked material, sporopollenin, which is known to have a high modulus and to be highly resistant to chemical and oxidative degradation. Developing a quantitative understanding of sporopollenin surface chemistry and its interactions with other molecules is relevant to understanding sporopollenin's natural function as well as applications as an engineering material. By utilizing ragweed (<i>A. artemisiifolia</i>) pollen spores as a fixed phase, we describe a novel use of inverse liquid chromatography to probe the interactions of sporopollenin with organic compounds of varying polarity. By measuring capacity factors of probes at various temperatures, heats of interaction were calculated. Retention behavior of defatted (D) and acid-base treated (AB) pollen were compared. D pollen displayed low capacity factors that were nearly the same for most probes, except for ethanol, the smallest and most polar probe. On the other hand AB pollen displayed capacity factors that depended strongly on the polarity of the chemical probe. Chemical probes with alcohol and amine groups were highly retained on AB-treated pollen. Notably, the capacity factor, peak asymmetry, and enthalpy of interaction passed through a maximum as a function of probe polarity, and enthalpy ranged from -9 kJ/mol (benzene) to -41 kJ/mol (pyridine). These results suggest that the surface of the AB pollen is polar and likely protic with strong Lewis acid characteristics.