Exchangeable deuterons introduce artifacts in amide 15N CEST experiments used to study protein conformational exchange
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Protein molecules sample different conformations in solution and characterizing these conformations is crucial to understanding protein function. 15N CEST experiments are now routinely used to study slow conformational exchange of protein molecules between a ‘visible’ major state and ‘invisible’ minor states. These experiments have also been adapted to measure the solvent exchange rates of amide protons by exploiting the one bond deuterium isotope effect on the amide 15N chemical shifts. However at moderately high temperatures (~ 50 °C) that are sometimes required to populate protein minor conformers to levels (~ 1%) that can be detected by CEST experiments solvent H/D exchange can lead to ‘dips’ in low B115N CEST profiles that can be wrongly assigned to the conformational exchange process being characterized. This is demonstrated in the case of ~ 18 kDa T4 Lysozyme (T4L) at 50 °C and the ~ 11 kDa E. coli hibernation promoting factor (HPF) at 52 °C. This problem is trivially solved by eliminating the exchangeable deuterons in the solvent by using either an external D2O lock or by using a small amount (~ 1–3%) of a molecule like d6-DMSO that does not contain exchangeable deuterons to lock the spectrometer.
KeywordsChemical exchange 15N CEST H/D exchange Protein dynamics
We would like to thank Dr. Tairan Yuwen and Prof. Lewis Kay (University of Toronto) for providing the Bruker 15N CEST pulse sequence, Dr. G Bouvignies for making ChemEx along with the source code freely available, the national NMR facility at TIFR, Hyderabad and Dr. Krishna Rao for spectrometer time. The work was supported by generous funds from TCIS/TIFRH and Grant No. ECR/2016/001088 from SERB awarded to PV.
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