NMR crystallography on paramagnetic systems: solved and open issues

Abstract

The idea of NMR crystallography was conceived as soon as NMR spectroscopy was invented. Over the years, several efforts have been devoted to the development of NMR tools to complement X-ray diffraction results. Many of the NMR-related observables are short-ranged in nature, but the paramagnetic ones are not. Therefore, paramagnetism-based restraints are in principle suitable to assess spatial relationships among molecules. We will here review some of the underlying concepts and see how they apply to the problem of obtaining structural information on molecules in the lattice, and on the lattice itself. We will provide positive examples and discuss the negative issues that still haunt paramagnetism-based NMR crystallography.

NMR of sedimented, fibrillized, silica-entrapped and microcrystalline (metallo)proteins

“…Indeed, in parallel to what happens for diamagnetic proteins, Curie-spin relaxation, that is formally equivalent to CSA relaxation, is absent [122] or largely abolished [123], thus the resolution is independent of the molecular weight. Fig.…”

Two-dimensional NMR measurement and point dipole model prediction of paramagnetic shift tensors in solids

“…This class of 2D NMR methods, correlating the anisotropic paramagnetic shift interaction to interactions that are independent of magnetic susceptibility effects, such as the first-order quadrupolar coupling, should prove useful in structural studies and NMR crystallography of paramagnetic samples [16][17][18]. Furthermore, our molecular orbital guided refinements to the point dipole model provide a simple approach for a more accurate prediction of the full paramagnetic shift tensor, i.e., principal component values and tensor orientation.…”

“…This is particularly important for the future development of NMR crystallography in paramagnetic systems [16][17][18], and it is our belief that our experimental approaches, correlating the paramagnetic shift to other NMR interactions, will play an important role in validating future computational approaches. results in fractional delocalization of 13.3% onto the two closest chloride and 1.6% onto the two closest oxygen ligands that is more consistent with electron magnetic resonance [84] and magnetic neutron scattering experiments [87,91].…”

“…The paramagnetic shift contribution [18,60,61] to the NMR frequency can be written using symmetry pathway notation as the sum of two components…”

“…As MAS spinning speeds increased it became possible to develop solid-state NMR analogs of many solution-state paramagnetic NMR (pNMR) experiments [15]. The use of both PCS and PRE in solid-state NMR increased dramatically over the last decade and has proven powerful in the emerging field of NMR crystallography [16][17][18].…”