CPMG relaxation dispersion NMR experiments measuring glycine 1H alpha and 13C alpha chemical shifts in the 'invisible' excited states of proteins.

Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion NMR experiments are extremely powerful for characterizing millisecond time-scale conformational exchange processes in biomolecules. A large number of such CPMG experiments have now emerged for measuring protein backbone chemical shifts of sparsely populated (>0.5%), excited state conformers that cannot be directly detected in NMR spectra and that are invisible to most other biophysical methods as well. A notable deficiency is, however, the absence of CPMG experiments for measurement of (1)H(alpha) and (13)C(alpha) chemical shifts of glycine residues in the excited state that reflects the fact that in this case the (1)H(alpha), (13)C(alpha) spins form a three-spin system that is more complex than the AX (1)H(alpha)-(13)C(alpha) spin systems in the other amino acids. Here pulse sequences for recording (1)H(alpha) and (13)C(alpha) CPMG relaxation dispersion profiles derived from glycine residues are presented that provide information from which (1)H(alpha), (13)C(alpha) chemical shifts can be obtained. The utility of these experiments is demonstrated by an application to a mutant of T4 lysozyme that undergoes a millisecond time-scale exchange process facilitating the binding of hydrophobic ligands to an internal cavity in the protein.