Thermic melting and optical rotatory dispersion of DNA-Hg2+ complexes

Abstract

The effect of Hg2+ on the secondary structure of various DNA's with different (G + C) content is investigated by optical rotatory dispersion and melting temperature measurements.Marked deformations of the optical rotatory dispersion spectra can be observed already at 1 Hg2+ per 500 DNA‐P. The changes in specific rotations beyond 250 nm are more effective for AT‐rich than for GC‐rich DNA. Comparison to corresponding DNA‐Cu2+‐complexes indicates the greater tendency of Hg2+ to produce conformational changes in the DNA double‐helical structure due to its higher affinity to the bases. DNA from Proteus mirabilis (42 mole‐% G + C) shows an increase in its melting temperature around 0.02 and at 0.09 Hg2+/DNA‐P. At 6 nM Na+ the Tm increases up to 0.03 Hg2+/DNA‐P. Between 0.03 and 0.07 Hg2+/DNA‐P Tm decreases whereas awt higher Hg2+ concentrations melting tempeature increases agains. Similar behaviour was found with DNA's of varying GC content. The decrease of Tm is more effective for GC‐rich than for AT‐rich DNA's.The observed changes in the optical ratotory dispersion and melting profiles of DNA‐Hg2+ complexes are explained in terms of a specific binding of Hg2+ to adjacent thymine bases and AT‐pairs in accordance with previous results obtained by other authors. At 6 nM Na+ Hg2+ may combine with phosphate sites up to 0.03 Hg2+/DNA‐P. Within the decreasing Tm range the destabilization is attributed to a preferred formation of complexes of the type thymine‐Hg2+‐thymine whereas the stabilization effect at higher metal ion concentrations is presumably due to cross‐link formation between AT paris of opposite stands.