Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations.

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

High and fast: NMR protein-proton side-chain assignments at 160 kHz and 1.2 GHz.

The NMR spectra of side-chain protons in proteins provide important information, not only about their structure and dynamics, but also about the mechanisms that regulate interactions between macromolecules. However, in the solid-state, these resonances are particularly difficult to resolve, even in relatively small proteins. We show that magic-angle-spinning (MAS) frequencies of 160 kHz, combined with a high magnetic field of 1200 MHz proton Larmor frequency, significantly improve their spectral resolution. We investigate in detail the gain for MAS frequencies between 110 and 160 kHz MAS for a model sample as well as for the hepatitis B viral capsid assembled from 120 core-protein (Cp) dimers. For both systems, we found a significantly improved spectral resolution of the side-chain region in the 1H-13C 2D spectra. The combination of 160 kHz MAS frequency with a magnetic field of 1200 MHz, allowed us to assign 61% of the aliphatic protons of Cp. The side-chain proton assignment opens up new possibilities for structural studies and further characterization of protein-protein or protein-nucleic acid interactions.

The effect of methyl group rotation on 1H-1H solid-state NMR spin-diffusion spectra.

Fast magic-angle spinning (MAS) NMR experiments open the way for proton-detected NMR studies and have been explored in the past years for a broad range of materials, comprising biomolecules and pharmaceuticals. Proton-spin diffusion (SD) is a versatile polarization-transfer mechanism and plays an important role in resonance assignment and structure determination. Recently, the occurrence of negative cross peaks in 2D 1H-1H SD-based spectra has been reported and explained with higher-order SD effects, in which the chemical shifts of the involved quadruple of nuclei need to compensate each other. We herein report negative cross peaks in SD-based spectra observed for a variety of small organic molecules involving methyl groups. We combine experimental observations with numerical and analytical simulations to demonstrate that the methyl groups can give rise to coherent (SD) as well as incoherent (Nuclear Overhauser Enhancement, NOE) effects, both in principle manifesting themselves as negative cross peaks in the 2D spectra. Analytical calculations and simulations however show that higher-order coherent contributions dominate the experimentally observed negative peaks in our systems. Methyl groups are prone to the observation of such higher order coherent effects. Due to their low-frequency shifted 1H resonances, the chemical-shift separation relative to for instance aromatic protons in spatial proximity is substantial (>4.7 ppm in the studied examples) preventing any sizeable second-order spin-diffusion processes, which would mask the negative contribution to the peaks.