We present a pulse scheme that exploits methyl H 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 ? 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 spin systems (Zheng et al. in JMR 198:271-274， 2009) for molecules... More
We present a pulse scheme that exploits methyl H 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 ? 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 spin systems (Zheng et al. in JMR 198:271-274， 2009) for molecules that are only C 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 ~10 cms and traditional SQ experiments fail.
