Iterative tandem catalysis of racemic AB monomers

Racemic AB monomers encompassing a secondary hydroxy group and a methyl ester moiety were synthesized and converted to chiral polyesters by iterative tandem catalysis (ITC). The concurrent action of an enantioselective acylation catalyst (Novozym 435) and a racemization catalyst (Ru(Shvo)) results in the high conversion of the racemic monomers to enantio‐enriched polymers. Several factors are important for attaining high ee's and high molecular weights. The enantioselectivities observed for the novel AB monomers by Novozym 435 are high enough at 70 °C (E ratio ≥ 200) for the monomers to be useful for ITC. ITC of methyl 6‐hydroxyheptanoate showed that a catalyst loading of ～1.4 mol % Ru(Shvo), 25 mg Novozym 435/mmol AB monomer, and 0.5 mmol DMP/mmol monomer employing a monomer concentration of 1 mol/L gave a monomer conversion of 94%, an ee of 91%, and an M_p of 6.0 kg/mol. Application of these conditions to the other AB monomers revealed the sensitivity of the system. Reduced enantioselectivities were observed when longer reaction times were required for attaining high conversions. These long reaction times were necessary due to the slow (or absent) racemization activity of the Ru(Shvo) catalyst as a result of catalyst deactivation. Since quantitative conversions are crucial to attain high molecular weight polymers in polycondensation reactions, we could significantly improve the system by switching to isopropyl esters of the AB monomers and/or by strict exclusion of oxygen during the ITC. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2721–2733, 2008     

Au(I)- and Pt(II)-catalyzed cycloetherification of omega-hydroxy propargylic esters

Depending on the nature of the metal catalyst, omega-hydroxy propargylic acetates choose between alternative cycloetherification manifolds to produce functionalized heterocycles in high yields. AuCl catalyzes the formation of oxacyclic enol acetates, whereas [Cl(2)Pt(CH(2)CH(2))]2 (Zeise's dimer) will induce a propargylic substitution via an unprecedented S(N)2'-type allenic substitution from within a chelated square planar cationic Pt(II) complex.     

Double cation adduction in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of electron deficient anthraquinone derivatives

Six anthraquinone derivatives were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Clear (pseudo) molecular ions were observed for all the compounds. Interestingly, for some derivatives, strong ions with double cation adduction were also recorded in the positive mode. It is remarkable that all these ions are singly charged. In this work, possible mechanisms for the double cation adduction were investigated and discussed. It appears that the double cation adduction was due to the electron deficient nature of the derivatives, and formed by taking up two singly charged cations and one electron. Substituents on the anthraquinone ring were found to have a significant effect on the double cation adduction. In contrast, no considerable influence of the acidity of MALDI matrix/solution was observed, even on the double proton adduction. Furthermore, it was demonstrated that double cation adduction might occur in the MALDI gas-phase plume. In addition to the anthraquinones, three more electron deficient compounds of different types, i.e. a perylene bisimide derivative (PB), 3,7-decanoylamino-4,8-dihydrobenzo[1,2-b:4,5-b']dithiophene-4,8-dione (TQ) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), were also analyzed with MALDI TOF MS. The results indicate that the 'abnormal' double cation adduction might be a 'normal' phenomenon in the MALDI TOF MS analysis of many electron deficient compounds.     

Recent progress in the LC–MS/MS analysis of oxidative stress biomarkers

The presence of a dynamic and balanced equilibrium between the production of reactive oxygen (ROS) and nitrogen (RNS) species and the in-house antioxidant defense mechanisms is characteristic for a healthy body. During oxidative stress (OS), this balance is switched to increased production of ROS and RNS, exceeding the capacity of physiological antioxidant systems. This can cause damage to biological molecules, leading to loss of function and even cell death. Nowadays, there is increasing scientific and clinical interest in OS and the associated parameters to measure the degree of OS in biofluids. An increasing number of reports using LC–MS/MS methods for the analysis of OS biomarkers can be found. Since bioanalysis is usually complicated by matrix effects, various types of cleanup procedures are used to effectively separate the biomarkers from the matrix. This is an essential part of the analysis to prepare a reproducible and homogenous solution suitable for injection onto the column. The present review gives a summary of the chromatographic methods used for the determination of OS biomarkers in both urine and plasma, serum, and whole blood samples. The first part mainly describes the biological background of the different OS biomarkers, while the second part reports examples of chromatographic methods for the analysis of different metabolites connected with OS in biofluids, covering a period from 2015 till early 2020. The selected examples mainly include LC–MS/MS methods for isoprostanes, oxidized proteins, oxidized lipoproteins, and DNA/RNA biomarkers. The last part explains the clinical relevance of this review.     

Promysalin, a salicylate-containing Pseudomonas putida antibiotic, promotes surface colonization and selectively targets other Pseudomonas

Under control of the Gac regulatory system, Pseudomonas putida RW10S1 produces promysalin to promote its own swarming and biofilm formation, and to selectively inhibit many other pseudomonads, including the opportunistic pathogen Pseudomonas aeruginosa. This amphipathic antibiotic is composed of salicylic acid and 2,8-dihydroxymyristamide bridged by a unique 2-pyrroline-5-carboxyl moiety. In addition to enzymes for salicylic acid synthesis and activation, the biosynthetic gene cluster encodes divergent type II fatty acid biosynthesis components, unusual fatty acid-tailoring enzymes (two Rieske-type oxygenases and an amidotransferase), an enzyme resembling a proline-loading module of nonribosomal peptide synthetases, and the first prokaryotic member of the BAHD family of plant acyltransferases. Identification of biosynthetic intermediates enabled to propose a pathway for synthesis of this bacterial colonization factor.     

Controlled supramolecular oligomerization of C3-symmetrical molecules in water: the impact of hydrophobic shielding

The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions.     

Strong magnetic field effects on solid-liquid and particle-particle interactions during the processing of a conducting liquid containing non-conducting particles

The behavior of micrometer-sized weak magnetic insulating particles migrating in a conductive liquid metal is of broad interest during strong magnetic field processing of materials. In the present paper, we develop a numerical method to investigate the solid-liquid and particle-particle interactions by using a computational fluid dynamics (CFDs) modeling. By applying a strong magnetic field, for example, 10 Tesla, the drag forces of a single spherical particle can be increased up to around 15% at a creeping flow limit. However, magnetic field effects are reduced when the Reynolds number becomes higher. For two identical particles migrating along their centerline in a conductive liquid, both the drag forces and the magnetic interaction will be influenced. Factors such as interparticle distance, Reynolds number and magnetic flux density are investigated. Shielding effects are found from the leading particle, which will subsequently induce a hydrodynamic interaction between two particles. Strong magnetic fields however do not appear to have a significant influence on the shielding effects. In addition, the magnetic interaction forces of magnetic dipole-dipole interaction and induced magneto-hydrodynamic interaction are considered. It can be found that the induced magneto-hydrodynamic interaction force highly depends on the flow field and magnetic flux density. Therefore, the interaction between insulating particles can be controlled by applying a strong magnetic field and modifying the flow field. The present research provides a better understanding of the magnetic field induced interaction during liquid metal processing, and a method of non-metallic particles manipulation for metal/ceramic based materials preparation may be proposed.     

Amplification of Chirality in Helical Supramolecular Columns

Strong amplification of chirality occurs in dynamic, but highly ordered, helical columns in n-butanol, for which one chiral seed molecule suffices to render a column of 400 molecules to become homochiral. The chiral columns are formed in a thermally dependent stepwise process. The transition from achiral stacks to helical columns is highly cooperative owing to well-defined intermolecular interactions. `Sergeant and Soldiers' measurements allow for the calculation of the association constant and cooperativity length of the homochiral segments. The `Sergeant and Soldiers' data on the number of molecules within a column show a strikingly good match with data obtained from a theoretical model describing the self-assembly of the discotic molecules as a function of temperature and concentration.