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Small molecules that bind to oligomeric protein species such as membrane proteins and fibrils are of clinical interest for development of therapeutics and diagnostics. Definition of the binding site at atomic resolution via NMR is often challenging due to low binding stoichiometry of the small molecule. For fibrils and aggregation intermediates grown in the presence of lipids, we report atomic-resolution contacts to the small molecule at sub nm distance via solid-state NMR using dynamic nuclear polarization (DNP) and orthogonally labelled samples of the protein and the small molecule. We apply this approach to α-synuclein (αS) aggregates in complex with the small molecule anle138b, which is a clinical drug candidate for disease modifying therapy. The small central pyrazole moiety of anle138b is detected in close proximity to the protein backbone and differences in the contacts between fibrils and early intermediates are observed. For intermediate species, the 100 K condition for DNP helps to preserve the aggregation state, while for both fibrils and oligomers, the DNP enhancement is essential to obtain sufficient sensitivity.

Chemistry - a European journal 29(12), e202203466 (2023). doi:10.1002/chem.202203466, Published by Wiley-VCH, Weinheim

Fluorophores emitting in the near-infrared (NIR) wavelength region present optimal characteristics for photonics and especially bioimaging. Unfortunately, only few NIR fluorescent materials are known, and even fewer are biocompatible. For this reason, the scientific interest in designing NIR fluorophores is very high. Egyptian Blue (CaCuSi4O10, EB) is an NIR fluorescent layered silicate that can be exfoliated into fluorescent nanosheets (EB-NS). So far, its surface chemistry has not been tailored, but this is crucial for colloidal stability and biological targeting. Here, we demonstrate covalent surface functionalization of EB nanosheets (EBfunc) via Si–H activation using hydrosilanes with variable functionalities. In the first part of this work, EB-NS are grafted with the visible fluorescent pyrene (Pyr) moieties to demonstrate conjugation by colocalization of the Vis/NIR fluorescence on the (single) EB-NS level. Next, the same grafting procedure was repeated and validated with carboxyl group (COOH)-containing hydrosilanes. These groups serve as a generic handle for further (bio)functionalization of the EB-NS surface. In this way, folic acid (FA) could be conjugated to EB-NS, allowing the targeting of folic acid receptor-expressing cancer cells. These results highlight the potential of this surface chemistry approach to modify EB-NS, enabling targeted NIR imaging for biomedical applications.

Mechanical forces, including compressive stresses as one of the most important consequences, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. In this vein, we studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) NMR was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample-of-interest. Ex-situ 13C-detected NMR spectra of ball-milling products showed the formation of a rather crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in-situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

A structural characterization of the hydrated form of the brownmillerite-type phase Ba2In2O5, Ba2In2O4(OH)2, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H2O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. (Solid State Ionics2004, 170, 25−32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba2In2O4(OH)2 yield a set of low energy structures; GIPAW NMR calculations on these configurations yield 1H and 17O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct 1H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. 1H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The 17O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by 1H–17O double resonance experiments.