Protein NMR resonance assignment by isotropic mixing experiments on random fractionally deuterated samples

The 108-residue protein E. coli thioredoxin has been uniformly enriched to 50% with deuterium at all carbon-bound hydrogen positions. Isotropic mixing (i.e. TOCSY) experiments have been conducted for both the deuterated and natural-abundance samples. Using a 54 ms mixing time correlation peaks can be seen for all four protons on the benzenoid ring of tryptophan in both samples. The deuteration results in an average decrease in cross-sectional area of a factor of 2-3 for the TOCSY cross-peaks. The cross-peak intensities for the deuterated sample systematically decrease as a function of the number of protons involved in the transfer process thus overcoming a common ambiguity in the TOCSY experiment.