Figure 4.

Schematic representations of NMR experiments used to probe fast (nanosecond-to-picosecond) time scale dynamics in side-chain methyl groups. The ranges of protein molecular weights and time scales where each experiment is sensitive are shown at the bottom of each panel. (A) Schematic of the relaxation-violated methyl ¹H “forbidden” experiment.^65,79 The raw peak intensities for the “forbidden” (DQ or TQ) coherences (I_forbid) and the allowed (SQ) coherences (I_allow) are shown at five different delay times (left). The ratio of “forbidden” and allowed coherences (I_forbid/I_allow) produces a buildup curve (right), which can be fit to obtain methyl group order parameters reporting on the amplitude of fast-time-scale dynamics. (B) Representative example of deuterium relaxation for a partially deuterated side-chain methyl group (i.e., ¹³CHD₂ or ¹³CH₂D).⁷⁴ The decreasing peak intensities at different delay times (left) produce a decay curve (right), which reports on fast-time-scale dynamics.