Guidelines for the use of band-selective radiofrequency pulses in hetero-nuclear NMR: Example of longitudinal-relaxation-enhanced BEST-type 1H-15N correlation experiments

Band-selective radiofrequency (rf) pulses provide powerful spectroscopic tools for many biomolecular NMR applications. Band-selectivity is commonly achieved by pulse shaping where the rf amplitude and phase are modulated according to a numerically optimized function. This results in complex spin evolution trajectories during the pulse duration. Here we introduce simplified representations of a number of shaped pulses, commonly used in biomolecular NMR spectroscopy. These simple schemes, consisting in a suite of free evolution delays and ideal rf pulses, reproduce astonishingly well the effect on a scalar-coupled hetero-nuclear two-spin system. As a consequence, optimal use of such pulse shapes in complex multi-pulse sequences becomes straightforward, as demonstrated here for the example of longitudinal-relaxation-enhanced BEST-HSQC and BEST-TROSY experiments. Applications of these optimized pulse sequences to several proteins in the size range of 8-21 kDa are shown.