Normal carbon acid referencing for protein amide hydrogen exchange

Measurement of the exchange kinetics for amide hydrogens along the protein backbone continues to offer valuable insights into structural stability and conformational dynamics. Since such studies routinely compare samples that differ in solution conditions or mechanical handling, normalization of the relative exchange rates can present a potentially significant source of experimental uncertainty. The carbon acids 1,3-dimethylimidazolium cation and thiomethylacetonitrile exhibit base catalyzed exchange rates similar to those of the slowly exchanging amides, under conditions typical for protein studies. With 13C enrichment at the acidic carbon position to facilitate selective observation, such carbon acids offer practical internal calibration of exchange.