[Binding of polyamines by the double-helical DNA molecule in unfolded and compact forms]

The influence of the natural polyamines [spermine (Sp) and spermidine (Spd)] on the conformation of thymus DNA molecule (M = 4 and 15 MDa) was studied by means of the viscometric method over the range of low supporting electrolyte concentrations (CNaCl = 0.6 divided by 8.4 mM). In was shown that at sufficiently low degrees of ligand binding (theta PA) which satisfy the condition ZPA theta PA less than 0.76 (where ZPA is the valence of PA), the PA addition results in a slight decrease of the volume effects in the DNA molecule due to the increase of the ionic strength (mu) of solution conditioned by the presence of polyvalent PA cations and also to the supplanting of "bound" Na+ cations. The further increase in CPA induces the corresponding increase in theta PA up to the value which provides ZPA theta PA greater than 0.76, and is accompanied by drastic decrease of macromolecule's dimensions and partial DNA condensation as well. At larger degrees of binding (ZPA theta PA approximately 0.90) a transition from the expanded to compact form of the DNA molecule is observed. The calculations of theta 1 = theta Na + theta PA and rcr = theta 1 + ZPA theta PA as a function of mu were carried out according to Manning's two-variables theory on the basis of the obtained dependences of the critical PA concentrations (Ccr PA), which correspond to the midpoint of the condensational transition, versus CNaCl. The increase of rcr and/or decrease of the effective site dimensions (ZPA) was shown to be necessary for the DNA molecule collapse over the range of low mu less than 0.01. The binding constants of the PA association to the compact DNA form were evaluated by the simplest model of McGhee and von Hippel. On the basis of the results obtained for Spd it was hypothesized that the non-electrostatic interactions are significant by the binding of the PA to the compact form of the DNA molecule.