Abstract

Determination of Molecular Structure by Fast Magic-Angle Spinning Solid-state NMR,

X. Zhao, A. Brinkmann, M. Edén, J. L. Sudmeier, W. W. Bachovchin, A. Sebald, and M. H. Levitt,
43rd Experimental NMR Conference, Asilomar, California, USA (Apr. 2002), poster.

Solid-state NMR is an increasingly important tool for molecular structure determination. Molecular local environments, geometry and reaction mechanisms can be examined through measurements of inter-nuclear distances, torsional angles and CSA tensors. However, many existing methods require rather low sample spinning frequencies, which impedes the simultaneous mapping of many spin interactions in multiply-labelled systems. We describe some new experimental methods, using symmetry-based C and R-sequences, which allow the simultaneous estimation of many heteronuclear distances, many torsional angles and many CSA tensors on multiply labelled samples at high MAS frequencies.

(1) C–H...O H-bonding in histidine squarate
Based on our recent work [1], a modified sequence has been developed to quantify CH-bond elongation caused by C–H...O hydrogen bonding. The new method has enhanced suppression of unwanted contributions from proton chemical shifts, proton chemical shift anisotropies, and rf field inhomogeneity. Small H-bond elongations for the C(ε1)–H and C(δ2)–H bonds in unlabelled histidine squarate have been detected at a spinning frequency of 20 kHz.

(2) Torsional angle measurements
A new method has been developed for mapping many torsion angles in multiply-labelled systems under fast magic-angle spinning. The new pulse sequences exploit the evolution of 13C2 double-quantum coherences under rotor-synchronized heteronuclear recoupling sequences based on C and R symmetries. The method has been demonstrated on [2,3-13C2]-diammonium fumarate and [2,3-13C2]-ammonium hydrogen maleate at 20 kHz sample spinning frequency. The new method is highly sensitive to the molecular geometry.

(3) Chemical Shift Anisotropy (CSA) recoupling
Recoupling of 13C and 15N CSA interactions using R sequences has been demonstrated at 10 kHz sample spinning frequency. The relationships between molecular conformation, H-bonding, and CSA principal values are discussed. The pulse sequences achieve homonuclear dipolar decoupling at the same time as CSA recoupling and are applicable to multiply-labelled systems. The methods are demonstrated on [U-13C, 15N]-L-histidine hydrochloride monohydrate.

  1. X. Zhao, J. E. Sudmeier, W. W. Bachovchin, and M. H. Levitt, Measurement of N–H Bond Lengths by Fast Magic-Angle Spinning Solid-State NMR Spectroscopy: A New Method for the Quantification of Hydrogen Bonds, J. Am. Chem. Soc. 123, 11097-11098, (2001).