STRUCTURE OF STRAND-SEPARATED DNA IN DIFFERENT ENVIRONMENTS STUDIED BY LINEAR DICHROISM

The denaturation of DNA was studied by linear dichroism (LD) techniques in the following systems: in aqueous solution at low pH, in 90% wt/wt ethylene glycol and in 90% glycerol solutions, and in a matrix of polyvinyl alcohol (PVA). Denatured DNA at low ionic strength can be oriented by shear forces in all these systems and its exhibits positive LD with markedly varying LD/A over the absorption band centered at 260 nm (A being the ordinary absorbance) as compared to the negative LD with approximately constant LD/A or ordinary native DNA. These results are interpreted in terms of a large tilt of the planes of the DNA bases. DNA adopts denatured forms in aqueous solutions at pH < 3.5, ethylene glycol and glycerol at low Na+ concentrations and PVA after heating. If the denaturation is done in dry PVA, a complete re-formation of the double helix is obtained after humidifying (60% wt/wt) the matrix. This shows that the matrix can keep the 2 strands in a position allowing a perfect fitting on reassociation. Previous attempts to determine an intrinsic LD of denatured DNA have failed, probably due to the fact that, unless a very low ionic strength is used, the flexibility of the single strands prevents the orientation required to give a detectable LD signal even with recently developed high-sensitivity measurement techniques.