A kinetic model for the B-Z transition of poly[d(G-C]center dot poly[d(G-C)] and poly[d(G-m(5)C)]center dot poly[d(G-m(5)C)]

The analysis of the kinetic data of the B-Z conformational changes induced by salt in sized double-stranded poly[d(G-C)].poly[d(G-C)] and poly[d(G-m(5 )C)].poly[d(G-m(5)C)] polymers indicated that there exists a salt threshold which reveals some largely, as yet, unrecognised characteristics of the tr ansition. It was observed that there is a direct correlation between the le ngth of the polymer and the rate of the B-Z transition when the salt concen tration in the polymer solution is lower than the salt threshold. The corre lation is inverse when the salt concentration is higher than the salt thres hold. Thus, the molecular mechanism of the B- to Z-DNA transition varies de pending on whether the salt concentration is higher or lower than the thres hold. In this context, we have found that the contrasting results reported in the literature describing the rate of the B-Z transition are not contrad ictory but complementary. The finding of a salt threshold leads to the esta blishment of a relationship between the cooperativity index of the B-Z tran sition and the polymer chain length. That relationship is dependent on the chemical structure of the polymer but is temperature independent.