  We present Molecular Dynamics simulations of a single stranded unprotonated
DNA i-motif in explicit solvent. Our results indicate that the native structure
in non-acidic solution at 300 K is unstable and completely vanishes on a time
scale up to 10 ns. Two unfolding mechanisms with decreasing connectivity
between the initially interacting nucleobases can be identified where one
pathway is characterized as entropically more favorable. The entropic
preference can be mainly explained by strong water ordering effects due to
hydrogen bonds for several occurring structures along the pathways. Finally we
are able to indicate via free energy calculations the most stable
configurations belonging to distinct hairpin structures in good agreement to
experimental results.
