Selective recoupling of protons (SERP) is a method to selectively and quantitatively measure magnetic dipole–dipole interaction between protons and, in turn, the proton–proton distance in solid-state samples at fast magic-angle spinning. We present a bimodal operator-based Floquet approach to describe the numerically optimized SERP recoupling sequence. The description calculates the allowed terms in the first-order effective Hamiltonian, explains the origin of selectivity during recoupling, and shows how different terms are modulated as a function of the radio frequency amplitude and the phase of the sequence. Analytical and numerical simulations have been used to evaluate the effect of higher-order terms and offsets on the polarization transfer efficiency and quantitative distance measurement. The experimentally measured 1H–1H distances on a fully protonated thymol sample are ∼10%–15% shorter than those reported from diffraction studies. A semi-quantitative model combined with extensive numerical simulations is used to rationalize the effect of the third-spin and the role of different parameters in the experimentally observed shorter distances. Measurements at high magnetic fields improve the match between experimental and diffraction distances. The measurement of 1H–1H couplings at offsets different from the SERP-offset has also been explored. Experiments were also performed on a perdeuterated ubiquitin sample to demonstrate the feasibility of simultaneously measuring multiple quantitative distances and to evaluate the accuracy of the measured distance in the absence of multispin effects. The estimation of proton–proton distances provides a boost to structural characterization of small pharmaceuticals and biomolecules, given that the positions of protons are generally not well defined in x-ray structures.
Hydrophobic interactions
are often explored in solution-state aggregation
of molecules. However, an experimental electron density description
about these interactions is still lacking. Here, we report a systematic
study on the electronic nature of methyl···methyl hydrophobic
interactions in a series of multicomponent crystals of biologically
active molecules. Charge density models based on high-resolution X-ray
diffraction allow the visualization of subtle details of electron
density features in the interaction region. Our study classifies these
interactions as atypical group···group interactions
in contrast to σ-hole interactions, which are stabilized by
the minimized electrostatic repulsion and maximized dispersion forces.
For the first time, we quantified the solid-state entropic contribution
from the torsional mode of the methyl groups in stabilizing these
interactions by thermal motion analysis based on neutron diffraction
as well as variable-temperature crystallography. The carbon atoms
in methyl···methyl interactions show a unique upfield
chemical shift in the 13C solid-state NMR signal.
We employed a reductionist
approach in designing the first heterochiral
tripeptide that forms a robust heterogeneous short peptide catalyst
similar to the “histidine brace” active site of lytic
polysaccharide monooxygenases. The histidine brace is a conserved
divalent copper ion-binding motif that comprises two histidine side
chains and an amino group to create the T-shaped 3N geometry at the
reaction center. The geometry parameters, including a large twist
angle (73°) between the two imidazole rings of the model complex,
are identical to those of native lytic polysaccharide monooxygenases
(72.61°). The complex was synthesized and characterized as a
structural and functional mimic of the histidine brace. UV–vis,
vis-circular dichroism, Raman, and electron paramagnetic resonance
spectroscopic analyses suggest a distorted square-pyramidal geometry
with a 3N coordination at pH 7. Solution- and solid-state NMR results
further confirm the 3N coordination in the copper center of the complex.
The complex is pH-dependent and could catalyze the oxidation of benzyl
alcohol in water to benzaldehyde with yields up to 82% in 3 h at pH
7 and above at 40 °C. The catalyst achieved 100% selectivity
for benzaldehyde compared to conventional copper catalysis. The design
of such a minimalist building block for functional soft materials
with a pH switch can be a stepping stone in addressing needs for a
cleaner and sustainable future catalyst.
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