Persistence length of short homopolymeric single-stranded DNA sequences in polyvalent cations

We used simulations of short single stranded DNA (ssDNA) homopolymers, based on the sequence dependent Three Interaction Site (TIS) model, to calculate the persistence length (lp) in polyvalent cations. The TIS model accounts for stacking interactions and electrostatic interactions are treated using the Coulomb potential. We find that lp for dT30 (T is thymine) and dA30 (A is adenine) is quantitatively fit using lp=lp0 + {lambda}{kappa}-1 ( lp0 is the bare persistence length, {lambda} is a dimensionless constant, and {kappa} is the inverse Debye length) in the divalent cations Mg2+ and Mg2+. The dependence of lp on {kappa} is surprising because it was derived for long flexible polyelectrolytes in which the charges interact via the Debye-Hückel potential. The lp0 values are 0.4 nm and 1.1 nm for polyT and polyA, respectively. Strikingly, lp is almost independent of the tetravalent spermine concentration. There is no clear theoretical explanation although simulations suggest that the number of spermine molecules that bind to the ssDNA saturates at a small value. A qualitative picture, based on the restrictions of access to the phosphate groups due to volume exclusion of the anisotropic structure of Spm4+, rationalizes the simulation results. The predicted dependence of lp in spermine awaits experimental test.