Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments.1–3 Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated,4–6 which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield–Rhim–Elleman–Vaughn (T-MREV) and Lee–Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled ¹³C or ¹H nuclei, as well as due to B₁ inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B₁ field is highly homogeneous. The T-MREV experiment is far better compensated for B₁ inhomogeneity, and it also performs better in situations with limited signal. Copyright © 2006 John Wiley & Sons, Ltd.

Hydrogen-bonded pairs of the 2-methylimidazolium cation and 2-methylimidazole are expected to have matched pK_a values, owing to chemical symmetry. These structures serve as models for enzyme active sites. Several crystalline salts of this system were prepared and characterized by crystallography and solid-state NMR. Short N···N distances are observed (∼2.65 Å), and fast (>10⁵ s⁻¹) proton transfer through the N—H···N bridge was suggested by NMR lineshape measurements at temperatures from 200 to 320 K. The equilibrium constants for these transfer processes were found to differ from unity, and to be strongly temperature and counter-ion dependent. For the perchlorate salt, the proton is observed to be mainly attached to one of the bridge nitrogens across this temperature range, for the iodide salt, the proton is substantially delocalized on to both bridging nitrogens across the temperature range. Interestingly, for the chloride and bromide salts, a temperature-dependent equilibrium constant is observed, with equal populations of the two isomers in rapid exchange at room temperature and an effectively trapped proton (or very strong population of one isomer) at 200 K. This temperature-dependent equilibrium constant indicates that the proton transfer is associated with an enthalpy of the order the 20 kJ mol⁻¹. This study underscores the power of NMR spectroscopy to account for protons, and the important influence of remote ionic interactions on proton transfer coordinates. Copyright © 2001 John Wiley & Sons, Ltd.

N—H bond lengths in imidazolium–carboxylate pairs were studied as models for enzyme active site motifs. The bond lengths were measured using ‘2D 2ϕ-DipShift,’ a solid-state NMR method wherein the N,H dipolar coupling is determined using heteronuclear dipolar spinning sideband patterns. Like the situation for compressed O—H···O systems, some complexes exhibit very short N···O distances and weaker N,H dipolar coupling. The weaker time-average N,H dipolar coupling for systems with short N···O hydrogen bonds can be most likely associated with an altered potential well for the proton, or a ‘stretched’ covalent bond, although other interpretations involving a double well potential are discussed. The range or variation in average bond lengths seen in this study is much narrower than that previously reported for O—H···O systems (1.01–1.07 Å for N—H vs 1.0–1.3 Å for O—H bonds). Copyright © 2001 John Wiley & Sons, Ltd.