Interaction Between Ions And Poly Electrolytes:from Poisson-boltzmann Theory To Tightly Bound Ion Theory

RNAs are negatively charged biomolecules,and metal ions in solutions are important to their folding structures and thermodynamics,especially multivalent ions.However,it has been suggested that the binding of multivalent ions to RNAs cannot be quantitatively described by the well-established nonlinear Poisson-Boltzmann(PB)theory which ignores ion-ion correlations.In this thesis,we investigated the ion distribution around charged spherical macroions and RNAs by the nonlinear PB theory and Monte Carlo(MC)simulations.In addition,we developed an extended tightly bound ion(TBI)model to investigate the potential of mean force(PMF)between like-charged macroions.The main research contents of the thesis are as follows:1.We made extensive calculations on ion distributions around various spherical macroions in divalent and trivalent salt solutions by the PB theory and MC simulations.The spherical macroions radii included 10A,15A,20A,25A and 30A.The extensive comparisons between the PB theory and MC simulations indicate that when a spherical macroion gets an ion neutralization beyond a"critical" value,the multivalent ion distribution around the spherical macroion can be approximately described by the PB theory.Furthermore,an empirical formula was obtained to approximately quantify the "critical" ion neutralization for various spherical macroions in multivalent salt solutions.It is interesting that such empirical formula was shown to work well for various realistic RNAs.2.With the extended tightly bound ion model,we first calculated the ion distribution around charged colloid in divalent salt solution and made comparison with MC simulations.In addition,we calculated the PMF between like-charged colloids in divalent salt solution by the extended TBI model and MC simulations.The results obtained from the extended TBI model are in good agreement with those from MC simulations.Finally,we calculated the PMF between like-charged colloids which are highly charged in divalent salt solutions.