<i>t</i><sub>1</sub>-Noise Suppression by γ-Free Recoupling Sequences in Solid-State NMR for Structural Characterization of Fully Protonated Molecules at Fast MAS
Yusuke Nishiyama, Vipin Agarwal, Rongchun Zhang
Abstract
1H 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 t1-noise along the indirect dimension. Most of the t1-noise in these recoupling experiments originates from MAS fluctuation through initial rotor-phase dependence (γ-angle) of the recoupled Hamiltonian. Here, we introduce a novel strategy to suppress t1-noise by recoupling a dipolar Hamiltonian independent of the initial rotor-phase called γ-free Hamiltonian. The proper choice of recoupling terms enables eliminating the initial rotor-phase dependence in the second-order average Hamiltonian. γ-free recoupling sequence achieves significantly reduced t1-noise in the 1H 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 1H triple-quantum/double-quantum/single-quantum (TQ/DQ/SQ) correlation spectrum of the tripeptide, N-formyl-l-methionyl-l-leucyl-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 t1-noise. We envisage that the γ-free concept could be employed to recouple other anisotropic spin interactions for minimizing t1-noise in multidimensional NMR experiments.