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

Infrared spectroscopy was used to study DNA complexes with Ca2+ ions i n films with different relative humidity at [Ca2+]/[P] molar ratios ra nging from 0.4 to 20. It was shown that in complex with Ca2+ the DNA p asses into the B form at a larger average number of water molecules bo und 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-frequ ency shift of the absorption band corresponding to the valence vibrati ons of water sorbed on DNA. This evidenced stronger binding of water m olecules in the hydration shell of the DNA-Ca2+ complex as compared wi th free DNA. The results obtained confirm the previously proposed mode l 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 th e N7-Ca2+-O6 type.