DNA-BINDING TO CRYSTALLINE SILICA CHARACTERIZED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

The interaction of DNA with crystalline silica in buffered aqueous solutions at physiologic pH has been investigated by Fourier-transform infrared spectroscopy (FT-IR). In aqueous buffer, significant changes occur in the spectra of DNA and silica upon coincubation, suggesting that a DNA-silica complex forms as silica interacts with DNA. As compared to the spectrum of silica alone, the changes in the FT-IR spectrum of silica in the DNA-silica complex are consistent with an Si-O bond perturbation on the surface of the silica crystal. DNA remains in a B-form conformation in the DNA-silica complex. The most prominent changes in the DNA spectrum occur in the 1225 to 1000 cm(-1) region. Upon binding, the PO2- asymmetric stretch at 1225 cm(-1) is increased in intensity and slightly shifted to lower frequencies; the PO2- symmetric stretch at 1086 cm(-1) is markedly increased in intensity; and the band at 1053 cm(-1), representing either the phosphodiester or the C-O stretch of DNA backbone, is significantly reduced in intensity. In D2O buffer, the DNA spectrum reveals a marked increase in intensity of the peak at 1086 cm(-1) and a progressive decrease in intensity of the peak at 1053 cm(-1) when DNA is exposed to increasing concentrations of silica. The carbonyl band at 1688 cm(-1) diminishes and shifts to slightly lower frequencies with increasing concentrations of silica. The present study demonstrates that crystalline silica binds to the phosphate-sugar backbone of DNA. The close proximity of the silica surface to the DNA molecule, brought about by this binding, may contribute to DNA strand breakage produced by silica-derived free radicals. The ability of silica to form stable complexes with DNA may play an important role in the mechanisms of silica-induced toxicity and carcinogenesis.