An enhanced sensitivity methyl 1H triple-quantum pulse scheme for measuring diffusion constants of macromolecules

We present a pulse scheme that exploits methyl H-1 triple-quantum (TQ) coherences for the measurement of diffusion rates of slowly diffusing molecules in solution. It is based on the well-known stimulated echo experiment, with encoding and decoding of TQ coherences. The size of quantifiable diffusion coefficients is thus lowered by an order of magnitude with respect to single-quantum (SQ) approaches. Notably, the sensitivity of the scheme is high, approximately A 3/4 that of the corresponding single quantum experiment, neglecting relaxation losses, and on the order of a factor of 4 more sensitive than a previously published sequence for AX(3) spin systems (Zheng et al. in JMR 198:271-274, 2009) for molecules that are only C-13 labeled at the methyl carbon position. Diffusion coefficients measured from TQ- and SQ-based experiments recorded on a range of protein samples are in excellent agreement. We present an application of this technique to the study of phase-separated proteins where protein concentrations in the condensed phase can exceed 400 mg/mL, diffusion coefficients can be as low as similar to 10(-9) cm(2)s(-1) and traditional SQ experiments fail.