  Clarification of the detailed mechanisms involved in the DNA polymorphism is
an important challenge for computational molecular biophysics. This paper
reports about reversible A/B transitions in DNA observed in silico in a
simulated titration experiment by smooth variation of the size of a water drop
around a double helical solute. The estimated range of hydration numbers
corresponding to the B/A transition roughly agrees with experimental data. The
chain length dependence was studied and it appeared that the transition to
A-form is hindered when the fragment becomes shorter than one helical turn.
Dynamics of the A/B transition at low hydration is cooperative and is driven
mainly by medium range electrostatic interactions of counterions sandwiched
between phosphate strands in the major groove. The correspondence of these
computational observations to common experimental conditions of A/B transitions
is discussed.
