The Bn substate of the biologically active B-DNA family is characterized by a different phosphate backbone geometry. In contrast to the current belief, the Bn substate of the d(CGCGAATTCGCG)(2) dodecamer is shown-both experimentally and by molecular dynamics simulations-to be significantly populated at water activities in the range of those in the cell nucleus. Three systems with different water contents show an increase in B-I reversible arrow B-II transitions as the water activity decreases. In addition, the transition mechanism is investigated and the sequence dependence of these interconversion processes in the form of pyrimidine-purine interconversion dominance is demonstrated.