EXAFS studies of the calcium salts of natural DNA and polydA:polydT

Preliminary EXAFS experiments were carried out on film of the Ca salts of the synthetic polynucleotide polydA:polydT at 95%, 81%, and 76% relative humidity (r.h.) and for the Ca salt of chicken erythrocyte DNA at 81% r.h. (approximately 43% GC pairs). Detailed analysis of EXAFS data shows that the Ca2+ ion is in fairly close proximity (within 4 A) to a number of phosphorous atoms. This is in contradiction with the recently proposed model, which assumes a close coordination between the cations and the purine and pyrimidine bases deep inside the polynucleotide molecule, so that the distance to the nearest phosphorous atoms must not be less than 5 A. Instead, the EXAFS results suggest that the Ca2+ ions are, for the most part, located at the periphery of individual polydA:polydT (or DNA) molecules, possibly serving as intermolecular links.