Synthesis of Cationic Dumbbell-shaped Fullerene Nanostructures as Potential Photodynamic Sensitizers

A design of novel hydrophilic tetracationic dumbbell-shaped [60]fullerene nanostructures was made by balancing the hydrophilicity and hydrophobicity characteristics of the fullerene adduct for their potential application as photodynamic sensitizers in the PDT treatment. A sequential protection-deprotection reaction pathway was applied for the functional differentiation between primary and secondary amine moieties of pentaethylene hexamine. Synthesis of the target molecule involves two key steps of unsymmetrical esterification and amidation of malonic acid and subsequent fullerenation. The synthetic strategy was accomplished using mild reaction conditions in the intermediate molecule preparation and led a moderate overall product yield.     

Sol–gel synthesis and optical behavior of Mg–Ce–O nano-crystallites

The Mg–Ce–O powder are shown to contain periclase-type MgO and/or fluoride-type cerium oxide (CeO₂) depending upon the composition (x) defined by Ce/(Ce + Mg) atomic ratio. Lattice contraction of pariclase phase of MgO (average crystallite size ~8.8 nm) at Ce content of ‘x’ = 0.20 in comparison to pure MgO (crystallite size ~9.5 nm) has been realized due to oxygen vacancy formation. The optical band gap values of CeO₂ varies (3.0–3.2 eV) due to oxygen vacancy formation in CeO₂ phase, crystallite size and/or Ce³⁺/Ce⁴⁺ ratio. Further, the addition of Ce has shown to reduce the physisorption and chemisorption of water significantly as reflected by (1) suppression of related absorption peaks and (2) absence of magnesium hydroxide, Mg(OH)₂, bands in Fourier transform infrared spectra.     

The effect of dendritic pendants on the folding of amphiphilic copolymers via supramolecular interactions

The supramolecular folding of amphiphilic heterograft copolymers equipped with dendritic pendants is investigated using a combination of proton nuclear magnetic resonance (¹H NMR) spectroscopy, small‐angle X‐ray scattering, and circular dichroism spectroscopy. Hereto, the linear poly(ethylene glycol) pendants normally used to convey water compatibility are partially substituted with branched analogues. For one set of copolymers, second‐generation polyglycerol dendrons are directly attached to the polymer backbone, while for the other a hydrophilic linker is placed in between. The results show that the branching of the hydrophilic pendants affects the local structure of the folded copolymer but does not influence the overall conformation and single‐chain character of the folded copolymers in solution. All copolymers fold into 4–5 nm single‐chain polymeric nanoparticles with a very compact spherical morphology, independent of the dendritic content of the copolymer. Intriguingly, the incorporation of the dendritic pendants affects the formation of a structured interior even at low incorporation ratios. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 411–421     

Design, synthesis, and biochemical evaluation of 1,5,6,7-tetrahydro-6,7-dioxo-9-D-ribitylaminolumazines bearing alkyl phosphate substituents as inhibitors of lumazine synthase and riboflavin synthase

The last two steps in the biosynthesis of riboflavin, an essential metabolite that is involved in electron transport, are catalyzed by lumazine synthase and riboflavin synthase. To obtain structural probes and inhibitors of these two enzymes, two ribityllumazinediones bearing alkyl phosphate substituents were synthesized. The synthesis involved the generation of the ribityl side chain, the phosphate side chain, and the lumazine system in protected form, followed by the simultaneous removal of three different types of protecting groups. The products were designed as intermediate analogue inhibitors of lumazine synthase that would bind to its phosphate-binding site as well as its lumazine binding site. Both compounds were found to be effective inhibitors of Bacillus subtilislumazine synthase as well as Escherichia coli riboflavin synthase. Molecular modeling of the binding of one of the two compounds provided a structural explanation for how these compounds are able to effectively inhibit both enzymes. In phosphate-free buffer, the phosphate moieties of the inhibitors were found to contribute positively to their binding to Mycobacterium tuberculosis lumazine synthase, resulting in very potent inhibitors with Ki values in the low nanomolar range. The additional carbonyl in the dioxolumazine system versus the purinetrione system was found to make a positive contribution to its binding to E. coli riboflavin synthase.     

Asymmetric synthesis of the putative structure of (-)-oryzoxymycin

[reaction: see text] A short asymmetric synthesis of (2'S,5R,6R)-2'-[6-amino-5-hydroxy-1,3-cyclohexadiene-1-carbonyloxy]propionic acid, the enantiomer of the reported structure of (+)-oryzoxymycin is described. The reported spectral data does not match that obtained for synthetic "oryzoxymycin".     

Photocleavage of lysozyme by cobalt(III) complexes

Photochemical reagents that cleave proteins at specific sites (photoproteases) are useful for studying protein structure and protein-ligand interactions. PolyammineCo(III) complexes are tested here as photochemical probes to cleave proteins. Irradiation of a mixture of lysozyme, a model protein, and polyammineCo(III) complexes resulted in the facile cleavage of the peptide backbone. Photocleavage yielded two fragments of molecular weights 10.6 and 3.7 kDa, and these masses sum to the molecular mass of lysozyme (14.3 kDa). No cleavage was detected in the absence of the metal complex, in the dark, or upon irradiation at wavelengths of >420 nm. The photocleavage yield increased with irradiation time and with the concentrations of the metal complex and the protein. N-terminal sequencing of the 10.6 kDa fragment indicated residues that are identical to the N-terminus of lysozyme, and sequencing of the 3.7 kDa fragment indicated Val-Ala-Trp-Arg, an internal sequence of lysozyme. From the known primary sequence of lysozyme and the sequencing data, the cleavage site was assigned to Trp108-Val109. Molecular modeling indicates that the observed cleavage site is within few angstroms from the proposed metal binding site at Glu35-Asp52. This is the first report of the successful photocleavage of proteins, with high selectivity, by transition metal complexes. This novel observation can facilitate the rational design of transition metal complexes for the photochemical footprinting of metal binding sites on proteins.     

Interaction of torsion and lateral bending in aircraft nose landing gear shimmy

In this paper we consider the onset of shimmy oscillations of an aircraft nose landing gear. To this end we develop and study a mathematical model with torsional and lateral bending modes that are coupled through a wheel-mounted elastic tyre. The geometric effects of a positive rake angle are fully incorporated into the resulting five-dimensional ordinary differential equation model. A bifurcation analysis in terms of the forward velocity and the vertical force on the gear reveals routes to different types of shimmy oscillations. In particular, we find regions of stable torsional and stable lateral shimmy oscillations, as well as transient quasiperiodic shimmy where both modes are excited.