A methyl H double quantum CPMG experiment to study protein conformational exchange.

Protein conformational changes play crucial roles in enabling function. The Carr-Purcell-Meiboom-Gill (CPMG) experiment forms the basis for studying such dynamics when they involve the interconversion between highly populated and sparsely formed states, the latter having lifetimes ranging from ~ 0.5 to ~ 5 ms. Among the suite of experiments that have been developed are those that exploit methyl group probes by recording methyl H single quantum (Tugarinov and Kay in J Am Chem Soc 129:9514-9521, 2007) and triple quantum (Yuwen et al. in Angew Chem Int Ed Engl 55:11490-11494, 2016) relaxation dispersion profiles. Here we build upon these by developing a third experiment in which methyl H double quantum coherences evolve during a CPMG relaxation element. By fitting single, double, and triple quantum datasets, akin to recording the single quantum dataset at static magnetic fields of B, 2B and 3B, we show that accurate exchange values can be obtained even in cases where exchange rates exceed 10,000 s. The utility of the double quantum experiment is demonstrated with a pair of cavity mutants of T4 lysozyme (T4L) with ground and excited states interchanged and with exchange rates differing by fourfold (~ 900 s and ~ 3600 s), as well as with a fast-folding domain where the unfolded state lifetime is ~ 80 µs.