Structure of DNA toroids and electrostatic attraction of DNA duplexes

DNA-DNA electrostatic attraction is considered as the driving force for the formation of DNA toroids in the presence of DNA condensing cations. This attraction comes from the DNA helical charge distribution and favours hexagonal toroidal cross-sections. The latter is in agreement with recent cryo,electron microscopy studies on DNA condensed with cobalt hexammine. We treat the DNA-DNA interactions within the modem theory of electrostatic interaction between helical macromolecules. The size and thickness of the toroids is calculated within a simple model; other models of stability of DNA toroids are-discussed and compared.