Proton Magnetic Relaxation Study of the Manganese–Transfer-RNA Complex

At 20°C, transfer ribonucleic acid (tRNA) binds manganese ions cooperatively. The binding sites are studied by spin relaxation of the water protons in a solution of the Mn²⁺–tRNA complex, as a function of temperature and proton NMR frequency. Below room temperature the frequency dependence of the proton relaxation shows that the correlation time ${\tau }_c$ of the ion–proton dipolar interaction is limited by electron spin relaxation. Hence ${\tau }_c$ is not in these conditions an indicator of molecular motion. This explains why the relaxation effects of Mn²⁺ bound to different polynucleotides were previously observed to be quite similar. This also shows that the rotational correlation time is long (> 2 × 10⁻⁹ sec) and hence there is rigidity in the binding of the ion to tRNA. The “effective” number, $\mathrm{pw}$ , of water molecules in the first hydration shell, taking into account possible anisotropies of the molecular movements, is equal to two. This may indicate that the first hydration shell contains only two water molecules, the other ligands being groups from tRNA. PolyA also has $\text{pw\hspace{0.17em}=\hspace{0.17em}2}$ , whereas polyU has five water molecules in the first hydration shell. Above room temperature, the rotational correlation time decreases with an enthalpy of ∼ 15 kcal/mol, which is similar to that observed in optical absorption. Therefore the large‐scale movements observed by proton relaxation correlate with the near‐neighbor geometry which governs hypochromism. There seems to remain some structure up to 100°C. These results reinforce the proposition that strongly bound divalent cations play a role in the macromolecule's tertiary structure. They are discussed with reference to recent models of tRNA.