Nucleic acid interactions with metal ions and amino acids

Conductiometric titrations and equilibrium dialysis measurements have been used to measure (I) the binding of Na +, Mg ++ and Cu ++ by deoxyribose nucleic acid (DNA) and (2) the binding of amino acids by DNA and RNA all in the presence of 0.2 M NaCl. It is concluded that about half the Na + counterions are held near the DNA by the electrostatic field. Mg ++ are bound to specific sites but very weakly. However, upon thermal denaturation of the DNA the binding becomes much stronger; the number of sites is about 70% of the nucleotides. Cu ++ ions are tightly bound to undenatured DNA and cause marked aggregation. Arginine is bound to DNA to the extent of about 6 molecules per 100 nucleotides at 0.01 M arginine. However, non-basic amino acids, serine and glutamic acid, are not significantly bound by either undenatured or denatured DNA. The binding of glutamic acid, threonine and proline by RNA was also found to be negligible, i.e., less than one amino acid per 100 nucleotides.