Internuclear distances measured using NMR provide crucial constraints of three-dimensional structures but are often restricted to about 5 Å due to the weakness of nuclear-spin dipolar couplings. For studying macromolecular assemblies in biology and materials science, distance constraints beyond 1 nm will be extremely valuable. Here we present an extensive and quantitative analysis of the feasibility of F spin exchange NMR for precise and robust measurements of interatomic distances up to 1.6 nm at a magnetic field of 14.1 T, under 20-40 kHz magic-angle spinning (MAS). The measured distances are comparable to those achievable from paramagnetic relaxation enhancement but have higher precision, which is better than ±1 Å for short distances and ±2 Å for long distances. ForF spins with the same isotropic chemical shift but different anisotropic chemical shifts, intermediate MAS frequencies of 15-25 kHz without H irradiation accelerate spin exchange. For spectrally resolvedF-F spin exchange, H-F dipolar recoupling significantly speeds up F-F spin exchange. On the basis of data from five fluorinated synthetic, pharmaceutical, and biological compounds, we obtained two general curves for spin exchange between CF groups and between CF and CF groups. These curves allow F-F distances to be extracted from the measured spin exchange rates after taking into account F chemical shifts. These results demonstrate the robustness ofF spin exchange NMR for distance measurements in a wide range of biological and chemical systems.
High degrees of delithiation of layered transition metal oxide cathode active materials (NCMs and HE-NCM) for lithium-ion batteries (LIBs) was shown to lead to the release of singlet oxygen, which is accompanied by enhanced electrolyte decomposition. Here, we study the reactivity of chemically produced singlet oxygen with the commonly used cyclic and linear carbonate solvents for LIB electrolytes. On-line gassing analysis of the decomposition of ethylene carbonate (EC) and dimethyl carbonate (DMC) reveals different stability toward the chemical attack of singlet oxygen, which is produced in situ by photoexcitation of the Rose Bengal dye. Ab initio calculations and on-the-fly simulations reveal a possible reaction mechanism, confirming the experimental findings. In the case of EC, hydrogen peroxide and vinylene carbonate (VC) are found to be the products of the first reaction step of EC with singlet oxygen in the reaction cascade of the EC chemical decomposition. In contrast to EC, simulations suggested DMC to be stable in the presence of singlet oxygen, which was also confirmed experimentally. Hydrogen peroxide is detrimental for cycling of a battery. For all known cathode active materials, the potential where singlet oxygen is released is found to be already high enough to electrochemically oxidize hydrogen peroxide. The formed protons and/or water both react with the typically used LiPF6 salt to HF that then leads to transition metal dissolution from the cathode active materials. This study shows how important the chemical stability toward singlet oxygen is for today’s battery systems and that a trade-off will have to be found between chemical and electrochemical stability of the solvent to be used.
Molecular motion of biopolymers in vivo is known to be strongly influenced by the high concentration of organic matter inside cells, usually referred to as crowding conditions. To elucidate the effect of intermolecular interactions on Brownian motion of proteins, we performed (1)H pulsed-field gradient NMR and fluorescence correlation spectroscopy (FCS) experiments combined with small-angle X-ray scattering (SAXS) and viscosity measurements for three proteins, αB-crystalline (αBc), bovine serum albumin, and hen egg-white lysozyme (HEWL) in aqueous solution. Our results demonstrate that long-time translational diffusion quantitatively follows the expected increase of macro-viscosity upon increasing the protein concentration in all cases, while rotational diffusion as assessed by polarized FCS and previous multi-frequency (1)H NMR relaxometry experiments reveals protein-specific behavior spanning the full range between the limiting cases of full decoupling from (αBc) and full coupling to (HEWL) the macro-viscosity. SAXS was used to study the interactions between the proteins in solution, whereby it is shown that the three cases cover the range between a weakly interacting hard-sphere system (αBc) and screened Coulomb repulsion combined with short-range attraction (HEWL). Our results, as well as insights from the recent literature, suggest that the unusual rotational-translational coupling may be due to anisotropic interactions originating from hydrodynamic shape effects combined with high charge and possibly a patchy charge distribution.
A wide variety of organic dyes form, under certain conditions, clusters know as J- and H-aggregates. Cyanine dyes are such a class of molecules where the spatial proximity of several dyes leads to overlapping electron orbitals and thus to the creation of a new energy landscape compared to that of the individual units. In this work, we create artificial H-aggregates of exactly two Cyanine 3 (Cy3) dyes by covalently linking them to a DNA molecule with controlled subnanometer distances. The absorption spectra of these coupled systems exhibit a blue-shifted peak, whose intensity varies depending on the distance between the dyes and the rigidity of the DNA template. Simulated vibrational resolved spectra, based on molecular orbital theory, excellently reproduce the experimentally observed features. Circular dichroism spectroscopy additionally reveals distinct signals, which indicates a chiral arrangement of the dye molecules. Molecular dynamic simulations of a Cy3-Cy3 construct including a 14-base pair DNA sequence verified chiral stacking of the dye molecules.
Nanostructured multiphase polymers exhibiting a mobile and a rigid phase also contain a phase of intermediate mobility that is usually assumed to be a continuous, uninterrupted interphase layer. This assumption is contrary to recent molecular-resolution micrographs and contradicts results from NMR spin diffusion experiments, all of which suggest a nontrivial interface structure. In this contribution, we reconsider our previous 1 H NMR spin diffusion data sets (Roos et al. Colloid. Polym. Sci. 2014, 292, 1825 and perform optimized 2D and 3D numerical spin diffusion calculations to characterize the basic intermediate-phase morphological pattern, thus overcoming previous inconsistencies in data fitting. For the diblock copolymer poly(butadiene)-poly(styrene), PS-b-PB, we demonstrate that the interphase region comprises nanometersize intermixed immobile, intermediate and mobile subregions. In contrast, for the semicrystalline polymer poly(ε-caprolactone), PCL, the spin diffusion data are best reproduced by an intermediate phase that is fully embedded within the rigid phase, which is attributed to an imperfect crystalline structure. For both samples, the new findings reveal a complex discontinuous, dynamically inhomogeneous structure of the intermediate phase.
Nanometer-range distances are important for restraining the three-dimensional structure and oligomeric assembly of proteins and other biological molecules. Solid-state NMR determination of protein structures typically utilizes C-C and C-N distance restraints, which can only be measured up to ∼7 Å because of the low gyromagnetic ratios of these nuclear spins. To extend the distance reach of NMR, one can harvest the power of F, whose large gyromagnetic ratio in principle allows distances up to 2 nm to be measured. However,F possesses large chemical shift anisotropies (CSAs) as well as large isotropic chemical shift dispersions, which pose challenges to dipolar coupling measurements. Here, we demonstrate F-F distance measurements at high magnetic fields under fast magic-angle spinning (MAS) using radiofrequency-driven dipolar recoupling (RFDR). We show that F-F cross-peaks for distances up to 1 nm can be readily observed in two-dimensional F-F correlation spectra using less than 5 ms of RFDR mixing. This efficient F-F dipolar recoupling is achieved using practically accessible MAS frequencies of 15-55 kHz, moderate F radio frequency field strengths, and noH decoupling. Experiments and simulations show that the fastest polarization transfer for aromatic fluorines with the highest distance accuracy is achieved using either fast MAS (e.g., 60 kHz) with large pulse duty cycles (>50%) or slow MAS with strong F pulses. Fast MAS considerably reduces relaxation losses during the RFDR π-pulse train, making finite-pulse RFDR under fast-MAS the method of choice. Under intermediate MAS frequencies (25-40 kHz) and intermediate pulse duty cycles (15-30%), theF CSA tensor orientation has a quantifiable effect on the polarization transfer rate; thus, the RFDR buildup curves encode both distance and orientation information. At fast MAS, the impact of CSA orientation is minimized, allowing pure distance restraints to be extracted. We further investigate how relayed transfer and dipolar truncation in multifluorine environments affect polarization transfer. This fast-MAS F RFDR approach is complementary toF spin diffusion for distance measurements and will be the method of choice under high-field fast-MAS conditions that are increasingly important for protein structure determination by solid-state NMR.
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