Mono- and Trivalent Ions around DNA: A Small-Angle Scattering Study of Competition and Interactions

Kurt Andresen, Gettysburg College
Xiangyun Qiu, Cornell University
Suzette A. Pabit, Cornell University
Jessica S. Lamb, Cornell University
Hye Yoon Park, Cornell University
Lisa W. Kwok, Cornell University
Lois Pollack, Cornell University

Copyright 2008, The Biophysical Society


The presence of small numbers of multivalent ions in DNA-containing solutions results in strong attractive forces between DNA strands. Despite the biological importance of this interaction, e.g., DNA condensation, its physical origin remains elusive.Wecarried out a series of experiments to probe interactions between short DNA strands as small numbers of trivalent ions are included in a solution containing DNA and monovalent ions. Using resonant (anomalous) and nonresonant small angle x-ray scattering, we coordinated measurements of the number and distribution of each ion species around the DNA with the onset of attractive forces between DNA strands. DNA-DNA interactions occur as the number of trivalent ions increases. Surprisingly good agreement is found between data and size-corrected numerical Poisson-Boltzmann predictions of ion competition for non- and weakly interacting DNAs. We also obtained an estimate for the minimum number of trivalent ions needed to initiate DNA-DNA attraction.