Solid-state NMR characterization of pyrene-cuticular matter interactions

One- and two-dimensional nuclear magnetic resonance (NMR) experiments were performed on Agave americana cutan and tomato cutin to examine the interactions between a hydrophobic pollutant, pyrene, and cuticular material. Variable-temperature NMR experiments show that cutan, an acid- and base-resistant cuticular biopolymer, undergoes the characteristic melting behavior of "poly-ethylene-like" crystallites, while the tomato cutin does not. The melting point of A. americana cutan was found to be approximately 360 K, which is consistent with the thickness of the polyethylene crystallites of 30-40 methylene units. Sorption models predict that the sorption behavior of hydrophobic pollutants should depend on the phase of the cuticular material. ¹³C NMR experiments on labeled pyrene were performed. The ¹³C T₁ of pyrene decreases significantly from that of crystalline pyrene upon sorption to both tomato fruit cutin and A. americana cutan, indicating that the pyrene is mobile upon sorption. Magic angle spinning experiments at low spinning frequencies (2-4 kHz) provided the chemical shift anisotropy (CSA) parameters δ, the anisotropy, and η, the asymmetry parameter, for crystalline and sorbed pyrene. For crystalline pyrene, two types of crystallographically distinctive pyrenes were observed. The first had δ = -97.4 ± 0.5 ppm and η = 0.934 ± 0.006, while the second had δ = -98.1 ± 0.5 ppm and η = 0.823 ± 0.008. After sorption to cutan, these CSA parameters were found to be δ = -78.9 ± 5.3 ppm and η < 0.70 independent of the length of time since completion of the sorption procedure. In tomato cutin, the CSA parameters were found to be dependent upon the time since completion of the sorption procedure. One and one-half months after sorption, δ was found to have a value of -30.4 ppm < δ < 0.0 ppm and η was undeterminable, while after 22 months these values become δ = -80.0 ± 3.3 ppm and η 0.42. These changes in the CSA parameters demonstrate that upon sorption of pyrene to cutan, the pyrene undergoes anisotropic motion, while in cutin pyrene initially can tumble isotropically, but after 22 months this motion also becomes anisotropic. 20 heteronuclear correlation experiments indicate that pyrene is in close proximity to aliphatic cuticular materials after sorption. This work is directly relevant toward understanding the physical and chemical mechanisms of pollutant sorption to soil organic matter and, thus, help develop improved sorption models and pollution remediation techniques.