Exchangeable deuterons introduce artifacts in amide 15 N CEST experiments used to study protein conformational exchange

Abstract

Protein molecules sample different conformations in solution and characterizing these conformations is crucial to understanding protein function. 15 N 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 15 N 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 B 115 N 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 D 2 O 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.