ELECTROSTATIC INTERACTION BETWEEN HELICAL MACROMOLECULES IN DENSE AGGREGATES - AN IMPETUS FOR DNA POLYMORPHISM AND MESOMORPHISM

DNA exhibits a surprising multiplicity of structures when it is packed into dense aggregates. It undergoes various polymorphous transitions (e.g., from the B to A form) and mesomorphous transformations (from he xagonal to orthorhombic or monoclinic packing, changes in the mutual a lignment of nearest neighbors, etc). In this report we show that such phenomena may have their origin in the specific helical symmetry of th e charge distribution on DNA surface. Electrostatic interaction betwee n neighboring DNA molecules exhibits strong dependence on the patterns of molecular surface groups and adsorbed counter-ions. As a result, i t is affected by such structural parameters as the helical pitch, groo ve width, the number of base pairs per helical turn, etc, We derive ex pressions which relate the energy of electrostatic interaction with th ese parameters and with the packing variables characterizing the axial and azimuthal alignment between neighboring macromolecules. We show, in particular, that the structural changes upon the B-to-A transition reduce the electrostatic energy by approximate to kcal/mol per base pa ir, at a random adsorption of counter ions. Ion binding into the narro w groove weakens or inverts this effect, stabilizing B-DNA, as it is p resumably the case in Li+-DNA assemblies. The packing symmetry and mol ecular alignment in DNA aggregates are shown to be affected by the pat terns of ion binding.