Magnetic relaxation dispersion of lithium ion in solutions of DNA

The magnetic field dependence of the nuclear spin-lattice relaxation rate constant defines the magnetic relaxation dispersion (MRD) and provides a direct characterization of the molecular dynamics that cause fluctuations in the magnetic couplings in the system and may also indicate the dimensional constraints on the motion. The counterion cloud surrounding a linear polyelectrolyte ion, such as DNA in solution, provides an interesting opportunity for ion confinement that helps in understanding the thermodynamics and the dynamics of the interactions between the polyion and other solutes. The MRD profiles of lithium ion and tetramethylammonium ion were recorded in dilute aqueous solutions of native calf thymus DNA, which provides a long, charged rod that reorients slowly. The ⁷Li ion relaxes through the nuclear electric quadrupole coupling and the proton-lithium dipole-dipole coupling; the protons of the tetramethylammonium ion relax by dipole-dipole coupling. MRD profiles of the ⁷Li⁺ ion are dominated by transient interactions with the DNA that yield a linear dependence of the spin-lattice relaxation rate constant on the logarithm of the Larmor frequency. This magnetic field dependence is consistent with diffusive ion motions that modulate two spatial coordinates that characterize the relaxation couplings in the vicinity of the polyion. Motions around the rod and fluctuations in the ion distance from the rod are consistent with these constraints for lithium. The magnetic field dependence of the tetramethylammonium ion proton relaxation rate constant is weak, but also approximately a linear function of the logarithm of the Larmor frequency, which implies that the field dependence is caused in part by local order in the DNA solution.