Revealing the Local Proton Network through Three-Dimensional 13C/1H Double-Quantum/1H Single-Quantum and 1H Double-Quantum/13C/1H Single-Quantum Correlation Fast Magic-Angle Spinning Solid-State NMR Spectroscopy at Natural Abundance.

Abstract

1H double quantum (DQ)/1H single quantum (SQ) correlation solid-state NMR spectroscopy is widely used to obtain internuclear 1H-1H proximities, especially at fast magic-angle spinning (MAS) rate (>60 kHz). However, to date, 1H signals are not well-resolved because of intense 1H-1H homonuclear dipolar interactions even at the attainable maximum MAS frequencies of ∼100 kHz and/or under 1H-1H homonuclear dipolar decoupling irradiations. Here we introduce novel three-dimensional (3D) experiments to resolve the 1H DQ/1H SQ correlation peaks using the additional 13C dimension. Although the low natural abundance of 13C (1.1%) significantly reduces the sensitivities, the 1H indirect measurements alleviate this issue and make this experiment possible even in naturally abundant samples. The two different implementations of 13C/1H DQ/1H SQ correlations and 1H DQ/13C/1H SQ correlations are discussed and demonstrated using l-histidine·HCl·H2O at natural abundance to reveal the local 1H-1H networks near each 13C. In addition, the complete 1H resonance assignments are achieved from a single 3D 13C/1H DQ/1H SQ experiment. We have also demonstrated the applicability of our proposed method on a biologically relevant molecule, capsaicin.