Unoriented films of Na-DNA were prepared by dessicating a gel with different amounts of NaCl. For low salt concentrations, the resulting films were smooth and patternless. For high salt concentrations, the resulting films had visible patterns that were very similar to those recently reported by Sclavi et al. [(1994) Biopolymers, Vol. 34, pp. 1105-1113]. Scanning electron and tunneling microscopies and electron dispersive spectroscopy studies were used to study the spatial distribution of NaCl throughout these films. Two main conclusions were reached about the NaCl: (1) some NaCl is distributed throughout every film, and (2) the visible patterns observed in films with a large amount of NaCl were caused by the growth of NaCl crystals during the dehydration process.
The binding of bipyridyl-͑ethylenediamine͒platinum͑II͒ to calf-thymus DNA has been studied in solution via isothermal titration calorimetry and in unoriented films via midinfrared spectroscopy. The calorimetric data reveal that the ligand binds to DNA at two different sites with the first binding site being filled by about one ligand for every five base pairs. The binding is entropically driven: ϩ25 cal/mol K for the first site and ϩ22 cal/mol K for the second site. Midinfrared absorption data (400-1800 cm Ϫ1 ) from unoriented films show no dependence on the ligand content at relative humidities of 23% and 80%. ͓S1063-651X͑98͒09912-7͔
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