title: Role of divalent cations on DNA polymorphism under low ionic strength conditions.
abstract: We have examined the conformational properties of poly(dG-m5dC) under a variety of low salt conditions and sample preparations. Extensive dialysis against 0.5 mM Na-cacodylate resulted in a left-handed polynucleotide conformation as determined by circular dichroism, in agreement with recently reported results. Similarly, extensive dialysis against Tris-EGTA also led to a left-handed conformation. Dilution of these samples led to a transition to the right-handed conformation. More stringent treatments such as dialysis followed by passage over an ion exchange column also resulted in a right-handed conformation. When these various solutions were examined using atomic absorption spectroscopy, significant levels of Mg+2 were observed (greater than or equal to 190 per 1000 nucleotides) in all samples showing a left-handed form, while much lower levels (less than or equal to 45 per 1000 nucleotides) were found in the low salt samples displaying a right-handed conformation. Addition of MgCl2 to samples in which divalent cations had been almost completely removed led to the reformation of the left-handed form. These results indicate that the left-handed form seen under certain low salt conditions is due to the presence of Mg+2 ions that remain bound to the polynucleotide, even in the presence of EDTA.
mesh_terms: D002413:Cations, Divalent; D002942:Circular Dichroism; D004247:DNA; D009690:Nucleic Acid Conformation; D009994:Osmolar Concentration; D011089:Polydeoxyribonucleotides; D012965:Sodium Chloride; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature
chemical_list: D002413:Cations, Divalent; D011089:Polydeoxyribonucleotides; D012965:Sodium Chloride; C029988:poly(dG-m(5)dC).poly(dG-m(5)dC); D004247:DNA