Equilibrium dialysis studies of polyamine binding to DNA.

Abstract

AbstractBinding constants and binding site sizes for the interactions of the polyamines spermine (+4), spermidine (+3), and putrecine (+2) with helical DNA have been determined as a function of ionic conditions and temperature by equilibrium dialysis using 14C‐labeled polyamines. In addition, competition equilibrium dialysis has been used to determine binding parameters for the divalent cations putrescine and Mg2+ from the competitive effect of these ions on the binding of spermine or spermidine. In all cases, the logarithm of the binding constant (log Kobs) varies linearly with the logarithm of the monovalent salt concentration; the slopes d log Kobs/d log[NaCl] are proportional to the valence of the ligand, and values of the extrapolated binding constants at 1M NaCl obtained from the intercepts are small (of order 1–10M−1). In those cases examined, Kobs is insensitive to temperature; the free energy of binding is predominantly entropic. Consequently, polymines as DNA‐binding ligands behave analogously to the oligolysìnes investigated previously [cf. Record, Lohman & de Haseth (1976) J. Mol. Biol. 107, 145–158; Lohman, de Haseth & Record (1980) Biochemistry 19, 3522–3530]. The interactions of these oligocations with DNA are predominantly electrostatic and are driven by the release of thermodynamically bound electrolyte ions from the vicinity of the DNA. The extent to which these oligocations are localized at individual phosphate binding sites or delocalized on the DNA molecule is currently not known.