Development of an NMR Method to Determine the Activation Barrier of Chemical Exchange
The ability to probe the amplitude and timescale of molecular motion is important; as such processes influence chemical transformations. Nuclear magnetic resonance (NMR) spectroscopy is particularly well-suited for the characterization of molecular motion at a wide range of time scales. While many NMR methods exist for the characterization of molecular motions that occur on the μs-ms timescale, such techniques are quite laborious and expensive to perform. This is particularly true for slow tumbling molecules with low sensitivity, as is the case with proteins. Here we introduce a simplistic approach to quantify the energy barrier to chemical exchange by the analysis of inverse peak heights. As a proof of concept, 1H NMR spectra of N,N-dimethylacetamide (NNDMA) were acquired at various temperatures. The activation barrier describing the rotational isomerization of NNDMA was determined with our technique and compared well to previously reported values in the temperature range used. We extended this routine to the analysis of conformational exchange processes in the protein ubiquitin, revealing comparable activation energies for regions in ubiquitin that have been previously analyzed. Thus, this new method provides a simplistic and inexpensive technique to study molecular motions, circumventing some of the limitations that other approaches face.