Multiple Quantum ¹³C NMR Spectroscopy in Solids under High-Speed Magic-Angle Spinning

We demonstrate an approach to efficient excitation and detection of high-order multiple quantum (MQ) 13 C NMR signals in solids under high-speed magic angle spinning (MAS). This approach combines homonuclear dipolar recoupling by the finite-pulse radio-frequency-driven recoupling (fpRFDR) pulse sequence with multiple quantum excitation pulse sequences developed for static solids based on time reversal. MQ 13 C NMR spectroscopy is demonstrated on the model compounds 1- 13 C - L -alanine, 13 C ε - L -methionine, and U- 13 C, 15 N - L -valine, at an MAS frequency of 20.0 kHz. MQ signals resulting from coherences of 8−10 spins are observed experimentally in the singly labeled model compounds. The fpRFDR-based MQ NMR technique is also applied to both singly labeled and multiply labeled peptides in the form of amyloid fibrils. Additionally, the competition between the dipolar recoupling mechanisms of fpRFDR and delta-pulse RFDR is explored by numerical effective Hamiltonian calculations.