STUDY OF THE INTERACTION OF DNA WITH CALCIUM-IONS USING VIBRATIONAL-SPECTRA

Infrared spectroscopy was used to study DNA complexes with Ca2+ ions in films with different relative humidity at [Ca2+]/[P] molar ratios ranging from 0.4 to 20. It was shown that in complex with Ca2+ the DNA passes into the B form at a larger average number of water molecules bound per nucleotide (n = 18 at [Ca2+]/[P] = 10, n = 24 at [Ca2+]/[P] = 20, and n = 12 in the absence of Ca2+). Calcium ions cause a low-frequency shift of the absorption band corresponding to the valence vibrations of water sorbed on DNA. This evidenced stronger binding of water molecules in the hydration shell of the DNA-Ca2+ complex as compared with free DNA. The results obtained confirm the previously proposed model of the complex. The Ca2+ ions bind to theoxygen of the DNA phosphate groups and to the N7 atom of guanine, forming chelate complexes of the N7-Ca2+-O6 type.