Cross-Polarization Schemes for Improved Heteronuclear Transfers Involving Labile Protons in Biomolecular Solution NMR

INEPT-based experiments are widely used for ¹ H→¹⁵ N transfers, but often fail when involving labile protons due to solvent exchanges. J-based cross polarization (CP) strategies offer a more efficient alternative to perform such transfers, particularly when leveraging the H^water $$ H^N exchange process to boost the ¹ H→¹⁵ N transfer process. This leveraging, however, demands the simultaneous spin-locking of both H^water and H^N protons by a strong ¹ H RF field, while fulfilling the γ_H B_1,H =γ_N B_1,N Hartmann-Hahn matching condition. Given the low value of γ_N /γ_H , however, these demands are often incompatible-particularly when experiments are executed by the power-limited cryogenic probes used in contemporary high field NMR. The present manuscript discusses CP alternatives that can alleviate this limitation, and evaluates their performance on urea, amino acids, and intrinsically disordered proteins. These alternatives include new CP variants based on frequency-swept and phase-modulated pulses, designed to simultaneously fulfill the aforementioned conflicting conditions. Their performances vis-à-vis current options are theoretically analyzed with Liouville-space simulations, and experimentally tested with double and triple resonance transfer experiments.