PARAMETRIZATION OF DIRECT AND SOFT STERIC-UNDULATORY FORCES BETWEEN DNA DOUBLE-HELICAL POLYELECTROLYTES IN SOLUTIONS OF SEVERAL DIFFERENT ANIONS AND CATIONS

Directly measured forces between DNA helices in ordered arrays have be en reduced to simple force coefficients and mathematical expressions f or the interactions between pairs of molecules. The tabulated force pa rameters and mathematical expressions can be applied to parallel molec ules or, by transformation, to skewed molecules of variable separation and mutual angle. This ''toolbox'' of intermolecular forces is intend ed for use in modelling molecular interactions, assembly, and conforma tion. The coefficients characterizing both the exponential hydration a nd the electrostatic interactions depend strongly on the univalent cou nterion species in solution, but are only weakly sensitive to anion ty pe and temperature (from 5 to 50 degrees C). Interaction coefficients for the exponentially varying hydration force seen at spacings less th an 10 to 15 Angstrom between surfaces are extracted directly from pres sure versus interaxial distance curves. Electrostatic interactions are only observed at larger spacings and are always coupled with configur ational fluctuation forces that result in observed exponential decay l engths that are twice the expected Debye-Huckel length. The extraction of electrostatic force parameters relies on a theoretical expression describing steric forces of molecules ''colliding'' through soft expon entially varying direct interactions.