Hexaamminecobalt(III) binding environments on double-helical DNA

Previous cation nmr evidence suggests that univalent cations such as Na+ bind to DNA in a diffuse, nonspecific manner, whereas di- and trivalent cations show distinct binding heterogeneity. Here are reported 59Co- and 23Na-nmr measurements of the %GC dependence of the DNA binding behavior of the trivalent hexaamminecobalt(III) cation. When Co(NH3)6Cl3 titrations are performed on one mammalian and three bacterial DNAs, evidence is found for at least three distinct classes of bound Co(NH3)6(3+). A comparison of titration curves for all four DNAs demonstrates that an increase in GC content correlates with an increase in the fraction of specific Co(NH3)6(3+). binding sites. For M. lysodeikticus DNA (72% GC), a slowly exchanging class of bound 59Co(NH3)6(3+) is apparent. This class of sites is saturated at very low binding densities (between 0.02 and 0.03 cobalt cations per DNA phosphate). At higher binding densities (greater than 0.03), the signal due to slowly exchanging 59Co(NH3)6(3+) disappears into the noise, and a single 59Co(NH3)6(3+) signal is observed. Within the sensitivity limitations of these measurements, no evidence for slowly exchanging bound 59Co(NH3)6(3+) could be found for any of the other DNAs, for which a single, rapidly exchanging 59Co(NH3)6(3+) signal is observed at all binding densities. For this rapidly exchanging signal, for all four DNAs, the measured 59Co(NH3)6(3+) nmr parameters depend significantly on (a) binding density and (b) GC content of the DNA.(ABSTRACT TRUNCATED AT 250 WORDS)