  Nucleic acids are negatively charged macromolecules and their structure
properties are strongly coupled to metal ions in solutions. In this paper, the
salt effects on the flexibility of single stranded (ss) nucleic acid chain
ranging from 12 to 120 nucleotides are investigated systematically by the
coarse grained Monte Carlo simulations where the salt ions are considered
explicitly and the ss chain is modeled with the virtual bond structural model.
Our calculations show that, the increase of ion concentration causes the
structural collapse of ss chain and multivalent ions are much more efficient in
causing such collapse, and trivalent and small divalent ions can both induce
more compact state than a random relaxation state. We found that monovalent,
divalent and trivalent ions can all overcharge ss chain, and the dominating
source for such overcharging changes from ion exclusion volume effect to ion
Coulomb correlations. In addition, the predicted Na and Mg dependent
persistence length lp of ss nucleic acid are in accordance with the available
experimental data, and through systematic calculations, we obtained the
empirical formulas for lp as a function of Na, Mg and chain length.
