Measurement of Proton Chemical Shifts in Invisible States of Slowly Exchanging Protein Systems by Chemical Exchange Saturation Transfer

Chemical exchange saturation transfer (CEST) NMR spectroscopy has emerged as a powerful technique for studies of transiently formed, sparsely populated (excited) conformational states of protein molecules in slow exchange with a dominant structure. The most popular form of the experiment, and the version originally developed, uses a weak H-1 radio frequency field to perturb longitudinal magnetization of one state with the effect transferred to magnetization in the second conformation via chemical exchange. A significant limitation of the method for protein applications emerges from H-1 magnetization transfer via dipolar relaxation (NOE effect) that can severely complicate analysis of the resulting CEST profile. This is particularly an issue since the H-1 chemical shifts of the excited state, critical for structural studies of these elusive conformers, become difficult to extract. Here we present a method for measurement of these shifts via CEST experiments in which the NOE effect is not an issue. The methodology is illustrated through applications to a pair of exchanging systems where the results are cross-validated.