A kinetic model for the B–Z transition of poly[d(G-C)]·poly[d(G-C)] and poly[d(G-m5C)]·poly[d(G-m5C)]

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-m5C)]·poly[d(G-m5C)] polymers indicated that there exists a salt threshold which reveals some largely, as yet, unrecognised characteristics of the transition. It was observed that there is a direct correlation between the length of the polymer and the rate of the B–Z transition when the salt concentration in the polymer solution is lower than the salt threshold. The correlation is inverse when the salt concentration is higher than the salt threshold. Thus, the molecular mechanism of the B- to Z-DNA transition varies depending on whether the salt concentration is higher or lower than the threshold. In this context, we have found that the contrasting results reported in the literature describing the rate of the B–Z transition are not contradictory but complementary. The finding of a salt threshold leads to the establishment of a relationship between the cooperativity index of the B–Z transition and the polymer chain length. That relationship is dependent on the chemical structure of the polymer but is temperature independent.