Calorimetric and spectroscopic studies on the poly[d(GA).d(CT)] structural polymorphism induced by zinc

The interaction of zinc (II) with poly[d(GA).d(CT)] and salmon testes DNA has been investigated by Differential Scanning Calorimetry (DSC) and Circular Dichroism (CD). We have detected and energetically characterized the existence of two different structural forms in poly[d(GA).d(CT)] which behave differently during a DSC experiment. The overall melting of DNA shows two calorimetric transitions at different temperatures. Moreover, the presence of zinc, at an input ratio of ion to nucleotide (r) above two, renders a complex DSC profile which is characterized by a negative enthalpy transition. Besides, the low-temperature transition observed in the presence of zinc is practically reversible after re-cooling/re-heating cycles. Nevertheless, the high-temperature transition characterized by a negative delta H degree cal does not appear in re-heating experiments, and remains stable below 100 degrees C. A calorimetric negative enthalpy transition is also found using salmon DNA in the presence of zinc ions. It seems that the combination of a temperature effect and zinc binding might induce the production of a stable metal-DNA complex, which can also be detected by changes in some bands in the CD profiles. The experimental results show that the presence of DNA structures and binding processes involving a negative calorimetric enthalpy contribution might be more widespread than previously reckoned.