Monte Carlo description of oligoelectrolyte properties of DNA oligomers: range of the end effect and the approach of molecular and thermodynamic properties to the polyelectrolyte limits.

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Abstract
Applications of the grand canonical Monte Carlo method demonstrate the importance of end effects on fundamental molecular and thermodynamic properties of oligoelectrolyte solutions. Simulations are carried out for a series of solutions containing double-helical DNA oligomers of varying numbers of phosphate charges N (8 .ltoreq. N .ltoreq. 100) and univalent electrolyte at fixed activity (a.+-. = 1.76 mmol/dm3). These results are used to evaluate as follows: CN+(a), the local concentration of cations at various axial positions along the oligomer surface; .hivin.CN+(a), the axial average of these concentrations; .GAMMA.N, the preferential interaction coefficient expressed per oligomer charge, which is directly related to the fractional thermodynamic extent of association of counterions. A sufficiently long oligomer (N .gtoreq. 48 under the conditions simulated) is characterized by an interior region over which CN+(a) is uniform and equal to .hivin.C.infin.+(a), the polyion limit. This interior region is flanked by two symmetric terminal regions, in which CN+(a) varies linearly with axial position from the end of the oligomer to a distance .apprxeq. 18 monomer units (.apprxeq. 3.1 nm) from that end. For long oligomers, the characteristics of the terminal regions [length and axial profile of CN+(a)] do not vary with N and, by inference, also pertain to the polyion under the same conditions. Both .hivin.CN+(a) and .GAMMA.N approach their polyelectrolyte limits as linear functions of 1/N. These linear dependences can be attributed to the increasing predominance of the contribution due to the polyion-like interior of the oligomer as N increases.