TRANSITIONS OF POLY(DI-DC), POLY(DI-METHYL5DC) AND POLY(DI-BROMO5DC) AMONG AND WITHIN THE B-DNA, Z-DNA, A-DNA AND X-DNA FAMILIES OF CONFORMATIONS

It is shown, using circular dichroism spectroscopy, that poly(dl-dC) is capable to isomerize into both Z-DNA and A-DNA in concentrated NaCl + NiCl2 and trifluoroethanol solutions, respectively. This polynucleotide also undergoes a cooperative, two-state transition in ethanol into a structure which most probably is a canonical B-DNA. This implies that the conformation of poly(dl-dC) is unusual in low-salt aqueous solution. The canonical B-DNA is also adopted by poly(dl-methyl5dC) in trifluoroethanol while this polynucleotide adopts Z-DNA not only in NaCl + NiCl2 but also in the presence of MgCl2. Poly(dl-methyl5dC) partially adopts X-DNA in concentrated CsF and mainly ethanolic solutions. Poly(dl-bromo5dC) isomerizes into Z-DNA not only in concentrated NaCl even in the absence of NiCl2 but also in concentrated MgCl2. This polynucleotide transforms between two distinct variants of Z-DNA in ethanol or trifluoroethanol solutions.