Results directed primarily toward spectral assignment and nuclear spin dynamics are described for yeast tRNAPhe in 0.1 M NaCl, pH 7. Magnesium titrations were performed. Changes in the spectrum occur for Mg2+/tRNA ratios of about 2 and above 10. Difference spectroscopy between 43 and 29 degrees C in zero Mg2+ concentration, together with prior identification of the GU4 acceptor stem base pair, indicates early acceptor melting and is used to identify acceptor resonances. Transport of spin energy (spin diffusion) is described in tRNA together with a summary of relevant experiments. A survey of nuclear Overhauser effects (NOE's) between imino and aromatic and amino protons is included, together with some recent conclusions based on methyl NOE's and experiments with tRNAs deuterated at the purine C8 position. Assignment of the imino NMR spectrum on the basis of these and previous data is reviewed and discussed in detail. Preliminary distance estimates based on the NOE for AU and GU4 base pairs are in reasonable agreement with the expected distances.

Solvent exchange rates of all the protons of yeast tRNAphe resonating in the lowfield NMR region (-11 to-15 ppm from DSS) have been measured by saturation-recovery long-pulse Fourier transform NMR. All these protons in yeast tRNAphe are in the fast exchange limit with H2O relative to their intrinsic longitudinal relaxation processes. Most rates show very little temperature dependence; however, tertiary base pair protons are preferentially destabilized in the absence of Mg++ at higher temperatures. The measured exchange rates are between 2 and 125 sec-1 for a temperature range from 10 degrees C to 45 degrees C and MgCl2 concentrations between 0 and 15 mM.

Nuclear magnetic resonance (NMR) measurements of proton exchange were performed on yeast tRNAPhe, and in much less detail on Escherichia coli tRNAfMet, over a range of Mg2+ concentrations and temperatures, at neutral pH and 0.1 M NaCl. The resonances studied were those of ring nitrogen protons, resonating between 10 and 15 ppm downfield from sodium 3-(trimethylsilyl)-1-propanesulfonate, which partake in hydrogen bonding between bases of secondary and tertiary pairs. Methods include saturation--recovery, line width, and real-time observation after a change to deuterated solvent. The relevant theory is briefly reviewed. We believe that most of the higher temperature rates reflect major unfolding of the molecule. For E. coli tRNAfMet, the temperature dependence of the rate for the U8--A14 resonance maps well onto previous optical T-jump studies for a transition assigned to tertiary melting. For yeast tRNAPhe, exchange rates of several resolved protons could be studied from 30 to 45 degrees C in zero Mg2+ concentration and had activation energies on the order of 40 kcal/mol. Initially, the tertiary structure melts, followed shortly by the acceptor stem. At high Mg2+ concentration, relatively few exchange rates are measurable below the general cooperative melt at about 60 degrees C; these are attributed to tertiary changes. Real-time observations suggest a change in the exchange mechanism at room temperature with a lower activation energy. The results are compared with those obtained by other methods directed toward assaying ribonucleic acid dynamics.

We demonstrate a fairly general method for identification of NMR absorption lines of macromolecules extracted from microorganisms, based on nuclear Overhauser effects (NOE). Several

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