Interaction of the phospholipid head group with representative quartz and aluminosilicate structures: An ab initio study

Silica dust particles in the form of quartz (but not kaolin) have been hypothesized to promote pulmonary diseases such as silicosis. The hypothesis is that quartz and kaolin have a comparable membranolytic potential on a specific surface area basis, and they have a comparable cytotoxic potential for lavaged pulmonary macrophages. Suppression of the cytotoxic activity occurs when these dust particles are treated with dipalmitoylphosphatidylcholine (DPPC), a common phospholipid component of the lung pulmonary surfactant. However, the enzyme phospholipase A2 is known to digest the phospholipid component more readily in the presence of quartz than kaolin. Since surface silanol (Si-OH) and aluminol (Al-OH) groups may interact differently with the phospholipid, an understanding of the selective removal of phospholipid by PLA2 may explain in vivo differences in cytotoxicity between quartz and kaolin. To develop some insight into this phenomenon, the interaction between a phospholipid and silica particles was examined by performing ab initio DFT calculations on clusters constructed with small (one or two silica tetrahedral units) representative parts of the silicate surface and the phospholipid head group. The clusters consisted of a phospholipid head group or functional groups from the head group complexed with Si(OSiH3)(3)OH, Al(OSiH3)(3)OH- or Al(OSiH3)(3)OH2. Fully optimized geometries of the complexes were used to determine binding energies, -OH vibrational frequency shifts, and NMR chemical shieldings. Results indicate that interaction of the protonated aluminol group (Al-OH2+) with the phosphate portion of the head group is strongest, while interaction of the -OH2+ group with the trimethyl-choline moiety of the head group is weakest. The presence of the choline moiety increased the magnitude of the -OH vibrational frequency shifts, and the shifts were significantly larger in complexes with protonated aluminol groups relative to silanol complexes. Analysis of ChelpG atomic charges shows that a net transfer of charge occurs from the silica unit to the head group within the complexes.