[Reversal of B to A transition in DNA by bis-(20guanidoethyl)disulfide]

Transition from B to A form of DNA in water/ethanol solutions is hindered by stoichiometric quantities of a bifunctional cation, bis-(2-guanidoethyl)disulfide (GED). Using the data on the shift and widening of the B--A transition curves in the presence of GED, a value of the order of 10 base pairs has been estimated for the cooperativity length of the B--A transition which agrees well with the values earlier obtained.