Counterion specificity of DNA condensation is rationalized from a theory of
electrostatic interaction between helical molecules that accounts for diff
erent charge distribution patterns. an axial charge separation due to ion b
inding in helical grooves allows close approach of opposite charges along t
he DNA-DNA contact and forms an electrostatic "zipper" that "fastens" the m
olecules together. Predictions of the theory are in agreement with experime
ntal data.