Interaction of an N-methylated polyamine analogue, hexamethonium(2+), with sodium-DNA: quantitative nitrogen-14 and sodium-23 NMR relaxation rate studies of the cation-exchange process

Abstract

The interactions of the divalent hexamethonium (Hex2+) cation with double-helical calf thymus DNA are investigated by means of 14N NMR and, indirectly, by means of 23Na NMR. During a titration of NaDNA with HexBr2, the displacement of Na+ from DNA by Hex2+ is monitored by concurrent measurements of the Lorentzian 14N signals and the bi-Lorentzian 23Na signals. the variations in the quadrupolar relaxation rates of 14N and 23Na are analyzed according to a simple two-state model for the competition between Hex2+ and Na+ associated with DNA. From this analysis parameters characterizing the exchange process are evaluated, and the following conclusions are drawn: (1) The association of one Hex2+ displaces 1.7-2.0 sodium ions from the vicinity of the DNA. (2) Cation accumulation near DNA neutralizes approximately half of the phosphate charge at all points in the titration. (3) The exchange coefficient characterizing the displacement of Na+ by Hex2+ is of the same order of magnitude as the exchange coefficients determined by NMR for other divalent cations such as Mg2+ and putrescine. These findings imply that the interaction of Hex2+ with DNA is primarily electrostatic in character. The transverse and longitudinal relaxation rates observed for 14N are analyzed under the assumption that the quadrupolar relaxation processes of 14N in Hex2+ associated with DNA can be characterized by a single-exponential correlation function with correlation time .tau.NB. The resulting value of .tau.NB, 7.8 .+-. 0.8 ns, is 3 orders of magnitude greater than that estimated for Hex2+ in the absence of DNA and is only 3-4 times greater than correlation times reported for 23Na and other quadrupolar cations near DNA. These comparisons indicate that the observed enhancements in the relaxation rates of 14N are due mainly to slowing of the motions that modulate its quadrupolar interactions in Hex2+ near DNA. The magnitudes of .tau.NB and of the quadrupolar coupling constant of Hex2+ associated with DNA are consistent with the conclusion that this association is primarily electrostatic.