Alcohol induced B-A transition of DNAs with different base compositions studied by circular dichroism

The circular dichroic (CD) spectra of natural DNAs (from Cl. perfringens, T2 phage, calf thymus, E. coli, and M. lysodeikticus) as well as duplexes of synthetic DNAs (poly(dA) X poly(dT), poly(dA-dT), and poly(dG-dC] were measured in water-ethanol mixtures with 0.3 mM NaCl. A conformational change from the B to the A form was observed for the natural DNAs on adding ethanol. The ethanol concentration that induces the transition and the extent of the change in the CD spectrum are different for the five natural DNAs depending on their GC contents. The higher the GC content is, the more easily the transition to the A form takes place. The results indicate that the GC content of a DNA is an important factor for induction of the B-A transition. The results for the synthetic DNAs show that their properties cannot be inferred by simple extrapolation of those of natural DNAs. Coexisting ions and the molecular weight of a DNA were also found to affect the induction of the B-A transition.