The competition of monovalent and divalent cations for proximity to negatively charged DNA is of biological importance and can provide strong constraints for theoretical treatments of polyelectrolytes. Resonant x-ray scattering experiments have allowed us to monitor the number and distribution of each cation in a mixed ion cloud around DNA. These measurements provide experimental evidence to support a general theoretical prediction: the normalized distribution of each ion around polyelectrolytes remains constant when ions are mixed at different ratios. In addition, the amplitudes of the scattering signals throughout the competition provide a measurement of the surface concentration parameter that predicts the competition behavior of these cations. The data suggest that ion size needs to be taken into account in applying Poisson-Boltzmann treatments to polyelectrolytes such as DNA.
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Andresen, K., Das, R., Park, H.Y., Smith, H., Kwok, L.W., Lamb, J.S., Kirkland, E.J., Herschlag, D., Finkelstein, K.D., & Pollack, L. (2004). Spatial distribution of competing ions around DNA in solution. Physical Review Letters, 93(248103). http://dx.doi.org/10.1103/PhysRevLett.93.248103
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Copyright 2004, The American Physical Society. First published as K Andresen et al, Spatial Distribution of Competing Ions around DNA in Solution, Physical Review Letters, 93, 248103, doi: 10.1103/PhysRevLett.93.248103.