Proton-detected solid-state NMR spectroscopy of fully protonated proteins at slow to moderate magic-angle spinning frequencies

Abstract

(1)H-detection offers a substitute to the sensitivity-starved experiments often used to characterize biomolecular samples using magic-angle spinning solid-state NMR spectroscopy (MAS-ssNMR). To mitigate the effects of the strong (1)H-(1)H dipolar coupled network that would otherwise severely broaden resonances, high MAS frequencies (>40kHz) are often employed. Here, we have explored the alternative of stroboscopic (1)H-detection at moderate MAS frequencies of 5-30kHz using windowed version of supercycled-phase-modulated Lee-Goldburg homonuclear decoupling. We show that improved resolution in the (1)H dimension, comparable to that obtainable at high spinning frequencies of 40-60kHz without homonuclear decoupling, can be obtained in these experiments for fully protonated proteins. Along with detailed analysis of the performance of the method on the standard tri-peptide f-MLF, experiments on micro-crystalline GB1 and amyloid-β aggregates are used to demonstrate the applicability of these pulse-sequences to challenging biomolecular systems. With only two parameters to optimize, broadbanded performance of the homonuclear decoupling sequence, linear dependence of the chemical-shift scaling factor on resonance offset and a straightforward implementation under experimental conditions currently used for many biomolecular studies (viz. spinning frequencies and radio-frequency amplitudes), we expect these experiments to complement the current (13)C-detection based methods in assignments and characterization through chemical-shift mapping.

Keywords: Homonuclear decoupling; Phase-modulated Lee Goldburg; Proton-detection; Solid-state magic-angle-spinning NMR; Windowed-detection.