We have monitored the exchange of imino and amino protons by NMR after quick transfer of yeast tRNAAsp in 2H2O solvent. When the concentration of exchange-catalyzing buffer is not too high, one imino proton exchanges considerably more slowly than any other (e.g., 100 hr versus 4 hr for the second-slowest imino proton at 18°C in 15 mM Mg). This provides excellent conditions for identification, by the nuclear Overhauser effect, of the slowest exchanging proton, which we show to be the imino proton of the U-8. A-14 reverse Hoogsteen tertiary-structure base pair; other slowly exchanging protons are identified as imino protons from A.U-11 and G.ψ-13. In preliminary experiments, we find that the exchange of these protons is catalyzed by cacodylate or Tris buffer. The lifetimes of two other imino protons, ca. 10 min at 28°C, are buffer independent. Slowly exchanging amino protons have also been observed. Correlation with the exchange of the uracil-8 imino proton suggests that they may be from adenine-14
