Biophys J, October 1999, p. 1858-1870, Vol. 77, No. 4

Brownian Dynamics Simulation of DNA Condensation

Laboratoire d'Analyse Ultrastructurale, Bâtiment de Biologie, Université de Lausanne, CH-1015 Lausanne, Switzerland

DNA condensation observed in vitro with the addition of polyvalent counterions is due to intermolecular attractive forces. We introduce a quantitative model of these forces in a Brownian dynamics simulation in addition to a standard mean-field Poisson-Boltzmann repulsion. The comparison of a theoretical value of the effective diameter calculated from the second virial coefficient in cylindrical geometry with some experimental results allows a quantitative evaluation of the one-parameter attractive potential. We show afterward that with a sufficient concentration of divalent salt (typically ~20 mM MgCl₂), supercoiled DNA adopts a collapsed form where opposing segments of interwound regions present zones of lateral contact. However, under the same conditions the same plasmid without torsional stress does not collapse. The condensed molecules present coexisting open and collapsed plectonemic regions. Furthermore, simulations show that circular DNA in 50% methanol solutions with 20 mM MgCl₂ aggregates without the requirement of torsional energy. This confirms known experimental results. Finally, a simulated DNA molecule confined in a box of variable size also presents some local collapsed zones in 20 mM MgCl₂ above a critical concentration of the DNA. Conformational entropy reduction obtained either by supercoiling or by confinement seems thus to play a crucial role in all forms of condensation of DNA.

Biophys J, October 1999, p. 1858-1870, Vol. 77, No. 4
© 1999 by the Biophysical Society   0006-3495/99/10/1858/13  $2.00

This article has been cited by other articles:

				H. M. Harreis, C. N. Likos, and H. Lowen Azimuthal Frustration and Bundling in Columnar DNA Aggregates Biophys. J., June 1, 2003; 84(6): 3607 - 3623. [Abstract] [Full Text] [PDF]