Separating Dipolar and Chemical Exchange Magnetization Transfer Processes in 1H-CEST

An amide H-1-Chemical Exchange Saturation Transfer (CEST) experiment is presented for studies of conformational exchange in proteins. The approach, exploiting spin-state-selective magnetization transfer, completely suppresses undesired NOE-based dips in CEST profiles so that chemical exchange processes can be studied. The methodology is demonstrated with applications involving proteins that interconvert on the millisecond timescale between major and invisible minor states, and accurate amide H-1 chemical shifts of the minor conformer are obtained in each case. The spin-state-selective magnetization transfer approach offers unique possibilities for quantitative studies of protein exchange through H-1-CEST.