Surface Structure of Lecithin-Capped Cesium Lead Halide Perovskite Nanocrystals Using Solid-State and Dynamic Nuclear Polarization NMR Spectroscopy.

Inorganic colloidal cesium lead halide perovskite nanocrystals (NCs) encapsulated by surface capping ligands exhibit tremendous potential in optoelectronic applications, with their surface structure playing a pivotal role in enhancing their photophysical properties. Soy lecithin, a tightly binding zwitterionic surface-capping ligand, has recently facilitated the high-yield synthesis of stable ultraconcentrated and ultradilute colloids of CsPbX3 NCs, unlocking a myriad of potential device applications. However, the atomic-level understanding of the ligand-terminated surface structure remains uncertain. Herein, we use a versatile solid-state nuclear magnetic resonance (NMR) spectroscopic approach, in combination with dynamic nuclear polarization (DNP) and atomistic molecular dynamics (MD) simulations, to explore the effect of lecithin on the core-to-surface structures of CsPbX3 (X = Cl or Br) perovskites, sized from micron to nanoscale. Surface-selective (cross-polarization, CP) solid-state and DNP NMR (133Cs and 207Pb) methods were used to differentiate the unique surface and core chemical environments, while the head-groups {trimethylammonium [-N(CH3)3+] and phosphate (-PO4-)} of lecithin were assigned via 1H, 13C, and 31P NMR spectroscopy. A direct approach to determining the surface structure by capitalizing on the unique heteronuclear dipolar couplings between the lecithin ligand (1H and 31P) and the surface of the CsPbCl3 NCs (133Cs and 207Pb) is demonstrated. The 1H-133Cs heteronuclear correlation (HETCOR) DNP NMR indicates an abundance of Cs on the NC surface and an intimate proximity of the -N(CH3)3+ groups to the surface and subsurface 133Cs atoms, supported by 1H{133Cs} rotational-echo double-resonance (REDOR) NMR spectroscopy. Moreover, the 1H-31P{207Pb} CP REDOR dephasing curve provides average internuclear distance information that allows assessment of -PO4- groups binding to the subsurface Pb atoms. Atomistic MD simulations of ligand-capped CsPbCl3 surfaces aid in the interpretation of this information and suggest that ligand -N(CH3)3+ and -PO4- head-groups substitute Cs+ and Cl- ions, respectively, at the CsCl-terminated surface of the NCs. These detailed atomistic insights into surface structures can further guide the engineering of various relevant surface-capping zwitterionic ligands for diverse metal halide perovskite NCs.

Designer phospholipid capping ligands for soft metal halide nanocrystals.

The success of colloidal semiconductor nanocrystals (NCs) in science and optoelectronics is inextricable from their surfaces. The functionalization of lead halide perovskite NCs1-5 poses a formidable challenge because of their structural lability, unlike the well-established covalent ligand capping of conventional semiconductor NCs6,7. We posited that the vast and facile molecular engineering of phospholipids as zwitterionic surfactants can deliver highly customized surface chemistries for metal halide NCs. Molecular dynamics simulations implied that ligand-NC surface affinity is primarily governed by the structure of the zwitterionic head group, particularly by the geometric fitness of the anionic and cationic moieties into the surface lattice sites, as corroborated by the nuclear magnetic resonance and Fourier-transform infrared spectroscopy data. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic-inorganic lead halide perovskites (FAPbBr3 and MAPbBr3 (FA, formamidinium; MA, methylammonium)) and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to acetone and alcohols. These NCs exhibit photoluminescence quantum yield of more than 96% in solution and solids and minimal photoluminescence intermittency at the single particle level with an average ON fraction as high as 94%, as well as bright and high-purity (about 95%) single-photon emission.

On the Mechanism of Alkylammonium Ligands Binding to the Surface of CsPbBr3 Nanocrystals.

CsPbBr3 nanocrystals (NCs) suffer from instabilities caused by the dynamic and labile nature of both the inorganic core and the organic-inorganic interface. Surface ligand engineering thus remains an imminent research topic. In this study, classical molecular dynamics simulations with an explicit solvent are used to gain insights into the inherent binding properties of three different alkylammonium ligands-primary dodecylammonium (DA), secondary didodecylammonium (DDA), and quaternary dimethyldi- dodecylammonium (DMDDA). Our simulations uncover three main factors that govern the effective ligand-substrate interactions: (i) the ability of the head-group to penetrate into the binding pocket, (ii) the strength of head-group interactions with the polar solvent, and (iii) the higher barrier for ligand adsorption/desorption in the case of multiple alkyl chains. The interplay between these factors causes the following order of the binding free energies: DDA < DA ≈ DMDDA, while surface capping with DDA and DMDDA ligands is additionally stabilized by the kinetic barrier. These findings are in agreement with previous experimental observations and with the results of presented ligand-exchange experiments, wherein DDA is found to loosely bind to the CsPbBr3 surface, while DMDDA capping is more stable than capping with the primary oleylammonium ligand. The presented mechanistic understanding of the ligand-NC interactions will aid in the design of cationic ligands that make perovskite NC surfaces more robust.

Highly purified calf hemodialysate (Actovegin®) may improve endothelial function by activation of proteasomes: A hypothesis explaining the possible mechanisms of action.

Highly purified calf hemodialysate (HPCH) known as Actovegin® or Solcoseryl® is one of the most controversial drugs currently marketed worldwide. It is not registered as drug in some countries and therefore its medical use there is illegal, while in others it is often among the top 10 of the best-selling medications. It could be also found in the list of the "most useless drugs" and was banned for short time by World Anti-Doping Agency as performance enhancer. However, the degree of its usefulness or uselessness remains unclear and there is not enough convincing data to make reliable conclusions. HPCH is claimed to have wound/muscular injuries healing, neuroprotective and antioxidant properties, to enhance glucose uptake and oxygen consumption, and possibly to improve performance of athletes. Since HPCH consists of over 200 naturally occurring substances which potentially may exert some pharmacological effects, it is extremely difficult to perform pharmacokinetic and pharmacodynamical studies. In this paper we have analyzed the available literature concerning clinical evidence, in vitro, ex vivo and in vivo effects of HPCH. Based on these data we suggest that the main target of the drug may be endothelium and improvement of endothelial function may be responsible for numerous largely nonspecific effects. We also propose the improvement of protein quality control by the means of activation of ubiquitin-proteasomal system as the most important biochemical mechanism responsible for its effects. The role of sphingolipids as potential proteasome-activators is extensively discussed. The effects of HPCH may also include direct or indirect ones on NF-kB-, Nrf2- and FOXO-mediated regulation of metabolic processes in the cells, which result in improved protein quality control, enhanced energy metabolism and increased resistance to oxidative stress.

Tandem intramolecular Michael-aldol reaction as a tool for the construction of the C-ring of hexacyclinic acid.

[reaction: see text] The tandem intramolecular Michael-aldol reaction was studied as a tool for the construction of the C-ring of hexacyclinic acid. By changing the reaction conditions it was possible to selectively obtain either the kinetic or the thermodynamic product. Retro-aldol reaction and subsequent epimerization provides four individual cyclopentane derivatives that can be incorporated as building blocks in natural product syntheses.