NMR study of slowly exchanging imino protons in yeast tRNAasp

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 degrees 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 . psi-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 degrees 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.