The Characterization of Phospholipid Functional Group Probe Species on Respirable Silicon-Containing Dusts by Solid-State 13C and 31P Nuclear Magnetic Resonance Spectroscopy

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopic studies are reported for the interactions of probe molecules with respirable silicon-containing dusts as experimental evidence complementing computational studies reported by Snyder and Madura recently in J. Phys. Chem. B 112, 7095 (2008). The selected probe molecules represent the individual functional groups of a model lung surfactant dipalmitoylphosphatidyl choline (DPPC) deposited on a respirable silica and kaolin from water solution. (13)C and (31)P solid-state NMR spectroscopies were employed to detect chemical shift, line width, and chemical shift anisotropy, providing experimental evidence of mobility and relaxation changes describing the site and orientation of surface-associated species. NMR results confirm that only the phosphate and adjacent carbons are immobilized by surface hydroxyls on kaolin, while these and the carbons of the cationic head group are likewise immobilized by surface silanols on Miu-U-Sil 5. The phosphates in phosphoryl- and phosphatidyl-cholines were the primary interaction sites, with additional weak coordination with the trimethylammonium cation species. Covalent Al-O-P formation is not likely a factor in in vivo or in vitro toxicity mechanisms of respirable silicon-containing materials, but is rather the result of dehydration or demethoxylation reactions occurring over time or during heating or reduced pressure used in preparing materials for NMR spectroscopic study. Hydration is a critical factor in the formation and preparation for spectroscopic observation of coated dusts. Care must be taken to ensure that products formed and studied correspond to species formed in vivo under suitable concentration and hydration conditions.