Structure Solution of Nano-Crystalline Small Molecules Using MicroED and Solid-State NMR Dipolar-Based Experiments

Abstract

Three-dimensional electron diffraction crystallography (microED) can solve structures of sub-micrometer crystals, which are too small for single crystal X-ray crystallography. However, R factors for the microED-based structures are generally high because of dynamic scattering. That means R factor may not be reliable provided that kinetic analysis is used. Consequently, there remains ambiguity to locate hydrogens and to assign nuclei with close atomic numbers, like carbon, nitrogen, and oxygen. Herein, we employed microED and ssNMR dipolar-based experiments together with spin dynamics numerical simulations. The NMR dipolar-based experiments were 1H-14N phase-modulated rotational-echo saturation-pulse double-resonance (PM-S-RESPDOR) and 1H-1H selective recoupling of proton (SERP) experiments. The former examined the dephasing effect of a specific 1H resonance under multiple 1H-14N dipolar couplings. The latter examined the selective polarization transfer between a 1H-1H pair. The structure was solved by microED and then validated by evaluating the agreement between experimental and calculated dipolar-based NMR results. As the measurements were performed on 1H and 14N, the method can be employed for natural abundance samples. Furthermore, the whole validation procedure was conducted at 293 K unlike widely used chemical shift calculation at 0 K using the GIPAW method. This combined method was demonstrated on monoclinic l-histidine.

Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods

“… 36 , 38 Alternatively, selective dipolar recoupling specific to one spin pair may be employed. 115 − 117 …”

“…(ii) The (large) spin system was approximated by a smaller counterpart of 2–4 spins, ,,, with the internuclear distances varied to fit the NMR-signal buildup to the experimental counterpart. Such internuclear-distance extraction procedures have been applied extensively to “isolated” spin pairs, such as 13 C– 13 C pairs in organic/biological samples, ,,, where specific 13 C-site labeling in conjunction with diluting the 13 C– 13 C pairs by co-crystallization with a natural abundance material is often straightforward. , Alternatively, selective dipolar recoupling specific to one spin pair may be employed. − …”

Deuteron-proton isotope correlation spectroscopy of molecular solids

“…For L-histidine.HCl.H2O deuterated from water-d at room temperature (295 ±3 K), the H/D exchange yields are in the range of 20-36% for different labile sites such as -NH in the heterocycle, NH3 and water protons, however, a high degree of H/D exchange of up to 90% can be obtained by recrystallizing the same compound at elevated temperatures. [19] Relatively high yields of deuteration and shorter relaxation delays associated 2 H sites allow us to rapidly detect a 2 H MAS spectrum (Figure 1b), in which the water-D, ND3 and the D7/D5 peaks (10)(11)(12)(13)(14)(15)(16)(17)(18) ppm are observed.…”

“…[2] Additionally, ssNMR in conjunction with diffraction-based techniques and modelling approaches (for example, NMR crystallography) is increasingly applied to determine the three-dimensional structures of (semi)crystalline and amorphous solids. [11][12][13][14][15][16][17] A large number of elements in the periodic table exhibit more than one NMR active isotope such as 1 H/ 2 H, 6 Li/ 7 Li, 10 B/ 11 B, 14 N/ 15 N, 35 Cl/ 37 Cl, 47 Ti/ 48 Ti and 85 Rb/ 87 Rb, to name a few. However, only a handful of studies have reported the solid-state 2D correlation NMR spectra of isotopes of the same nuclei.…”

Combining heteronuclear correlation NMR with spin-diffusion to detect relayed Cl–H–H and N–H–H proximities in molecular solids

“…Applications have been presented in the areas of optoelectronics, catalysis, biomacromolecules, pharmaceutical formulations, energy harvesting and storage materials [1][2][3][4][5][6][7]. In addition, ssNMR, in conjunction with diffraction-based techniques and modelling approaches (also referred to as NMR crystallography), is increasingly applied to characterize the intermolecular interactions and to determine the three-dimensional structures of (semi)crystalline and amorphous solids [8][9][10][11][12][13][14][15][16][17][18].…”