  Due to its polyionic character the DNA double helix is stable and
biologically active only in salty aqueous media where its charge is compensated
by solvent counterions. Monovalent metal ions are ubiquitous in DNA environment
and they are usually considered as the possible driving force of
sequence-dependent modulations of DNA structure that make it recognizable by
proteins. In an effort to directly examine this hypothesis, MD simulations of
DNA in a water drop surrounded by vacuum were carried out, which relieves the
requirement of charge neutrality. Surprisingly, with zero concentration of
counterions a dodecamer DNA duplex appears metastable and its structure remains
similar to that observed in experiments.
