t1-Noise Suppression by γ-Free Recoupling Sequences in Solid-State NMR for Structural Characterization of Fully Protonated Molecules at Fast MAS

H-1 multi-quantum (MQ)/single-quantum (SQ) correlation spectroscopy at fast magic angle spinning (MAS) is widely used to probe spatial proximity between protons. However, MQ/SQ spectra are always associated with strong t(1)-noise along the indirect dimension. Most of the t(1)-noise in these recoupling experiments originates from MAS fluctuation through initial rotor-phase dependence (gamma-angle) of the recoupled Hamiltonian. Here, we introduce a novel strategy to suppress t(1)-noise by recoupling a dipolar Hamiltonian independent of the initial rotor-phase called gamma-free Hamiltonian. The proper choice of recoupling terms enables eliminating the initial rotor-phase dependence in the second-order average Hamiltonian. gamma-free recoupling sequence achieves significantly reduced t(1)-noise in the H-1 triple-quantum/single-quantum (TQ/SQ) correlation spectrum together with a TQ filtering efficiency higher than 30%. The sequence is further applied to record a 3D H-1 triple-quantum/double-quantum/single-quantum (TO/DO/SQ) correlation spectrum of the tripeptide, N-formyl-L-methionyl-Lleucyl-L-phenylalanine (f-MLF), to characterize three spin connectivity with high sensitivity owing to the cumulative effect of very high TQ filtering efficiency and reduced t(1)-noise. We envisage that the gamma-free concept could be employed to recouple other anisotropic spin interactions for minimizing t(1) noise in multidimensional NMR experiments.