Behaviour of inorganic and organic cations in the Debye-Hückel layer of DNA

Inorganic, monovalent cations (Li, Na, K, Rb, Cs), when present in the Debye-Hückel layer of DNA, are found to bind to the negatively charged groups of the helix solely on the basis of their charge/mass ratio. Thus, when an electric field is applied, the free mobility of the DNA is seen to increase from Li- to Cs-equilibrated DNAs, since the latter cation, having a weaker surface charge distribution and a larger physical size (in the non-hydrated state), is more loosely bound to the DNA helix, thus providing less screening of its negative charges. On the contrary, organic amines (Tris and a number of Good's buffers) are found to bind not only via electrostatic interactions, but by additional bonds, notably H-bonds. In particular, Tris can form two H-bonds, with a purine and pyrimidine, respectively, and a third H-bond shared between the -OH groups of two adjacent Tris. Hence, these buffer components may be unwitting participants in reactions carried out in in vitro systems.