Frequency-switched single-transition cross-polarization: a tool for selective experiments in biomolecular NMR.

Frequency-switched single-transition cross-polarization (FS-ST-CP) provides a versatile tool for selective coherence transfer in heteronuclear NMR of biomolecules such as proteins and nucleic acids. This type of coherence transfer is spin-state-selective and can therefore benefit from the extension of the life-times of selected coherences due to partial cancellation of interfering relaxation mechanisms. The limits of the selectivity of the transfer are discussed by theory and illustrated by experiment. The methods are particularly efficient to obtain quantitative structural and dynamic information for selected residues in medium-sized nitrogen-15 or carbon-13 labeled macromolecules.