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.