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

Inorganic, monovalent cations (Li, Na, K, Rb, Cs), when present in the Deby e-Huckel layer of DNA, are found to bind to the negatively charged groups o f the helix solely on the basis of their charge/mass ratio. Thus, when an e lectric 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 tin the non-hydrated state), is more loosely bound to the DNA helix, thus providing less screen ing of its negative charges. On the contrary, organic amines (Tris and a nu mber of Good's buffers) are found to bind not only via electrostatic intera ctions, but by additional bonds, notably H-bonds. In particular, Tris can f orm 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 buffe r components may be unwitting participants in reactions carried out in in v itro systems. (C) 2001 Elsevier Science B;V. Ail rights reserved.