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Counterion condensation in DNA systems: The cylindrical Poisson–Boltzmann model revisited
Author(s) -
Rajasekaran E.,
Jayaram B.
Publication year - 1994
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360340315
Subject(s) - new delhi , chemistry , library science , humanities , polymer science , philosophy , computer science , medicine , pathology , metropolitan area
Nucleic acids are highly charged systems and counterions condense on the negatively charged phosphates, imparting electrostatic stability to the system.' This counterion condensation (CC ) hypothesis enjoys wide support from numerous experimental and theoretical studies?-? Earlier efforts to capture this phenomenon with Poisson-Boltzmann ( P B ) methodology as applied to led to the conclusion that a fraction of the counterions lie within the vicinity of the cylinder. These are called condensed counterions; their fraction depends on the linear charge density of the polyion and they do not contribute to colligative properties to any significant extent.'.".'2 The extent of counterion condensation, however, depended upon the added salt concentration, at variance with the constancy of the condensed fraction as predicted by the CC theory' and inferred from 23Na nmr exper iment~ . '~ More precisely, the PB equation predicted that the counterion fraction enclosed in a coaxial cylinder of fixed radius around the polyion varied with added salt, or conversely, the radius of the coaxial cylinder containing the fixed counterion fraction varied with added salt. Extensive comparisons between cylindrical PB results and those emerging from computer simulations have shown that the neglect of small ion correlations in the former method leads to an underestimation of the condensed fraction, but the results in both approaches are qualitatively similar, and in particular, the condensed fraction varies with added sa1t.'3-15*20 The constancy as implied by the CC theory and experiment is restored in simulations upon introduction of a dielectric saturation function for the polyion-small ion interactions?.20.21 Given the success of the PB approach in characterizing several biophysical p h e n ~ m e n a , ~ ~ , ~ ~ the disagreement between the PB predictions and experiment with regard to counterion condensation is quite intriguing. We were thus led to a reexamination of the cylindrical model of DNA with a very specific question: Are there any spatial scales that are invariant to added salt concentration in the cylindrical PB model system?