Structural investigations of magnesium silicate glasses by Si 2D Magic-Angle Flipping NMR

The two-dimensional Magic Angle Flipping Nuclear Magnetic Resonance (2D MAF NMR) experiment on ²⁹Si nuclei is used to determine the distribution of Q⁽ⁿ⁾ sites in two ²⁹Si-enriched magnesium silicate glasses with compositions 2MgO·SiO₂ and MgO·SiO₂. A significant degree of polymerization is observed in the 2MgO·SiO₂ glass, supporting previous studies using Raman and ²⁹Si NMR spectroscopy. Relative abundances of 0.629 ± 0.001 for Q⁽⁰⁾ and 0.371 ± 0.001 for Q⁽¹⁾ were obtained from spectral fits of the 2D MAF spectrum of the 2MgO·SiO₂ glass. Mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species were calculated and used in a thermodynamic model of Q⁽ⁿ⁾ disproportionation to calculate an equilibrium constant of k₀ = 0.04 ± 0.02 in 2MgO·SiO₂. In the MgO·SiO₂ glass relative abundance of 0.014 ± 0.001 for Q⁽⁰⁾, 0.191 ± 0.003 for Q⁽¹⁾, 0.530 ± 0.004 for Q⁽²⁾, 0.252 ± 0.003 for Q⁽³⁾, and 0.014 ± 0.001 for Q⁽⁴⁾ were measured. The mole fractions for the free oxygen anion and each Q⁽ⁿ⁾-species in MgO·SiO₂ were used to calculate corresponding disproportionation equilibrium constants of k₁ = 0.19 ± 0.02, k₂ = 0.174 ± 0.009, and k₃ = 0.11 ± 0.01. A comparison of k₃ values from previous MAF studies of various alkali and alkaline earth silicate glasses indicate an exponential increase in k₃ with the increasing modifying cation potential. Using the van't Hoff relation, we show that differences in both thermal history and modifier cation potential contribute to this spread in k₃ values. Nuclear shielding tensor anisotropy, ζ, and asymmetry, η, values of ζ = 0.0 ppm and η = 0.0 for Q⁽⁰⁾ and ζ = 33.0 ± 0.1 ppm, and η = 0.4 ± 0.1 for Q⁽¹⁾ in 2MgO·SiO₂ glass were determined from its 2D MAF spectrum. These values were used in obtaining the remaining values of ζ = − 36.0 ± 0.5 ppm and η = 0.99 ± 0.01 for Q⁽²⁾, and ζ = − 27.5 ± 0.5 ppm and η = 0.45 ± 0.11 for Q⁽³⁾, ζ = 0.0 ppm and η = 0.0 for Q⁽⁴⁾ in the MgO·SiO₂ glass from its 2D MAF spectrum. The magnitude of ζ values observed are lower than those reported in previous MAF studies of alkali and alkaline earth silicate glasses containing different modifier cations, consistent with previously reported trends in ζ versus modifying cation potential.     

Direct observation of surface-mediated thioacetyl deprotection: covalent tethering of a thiol-terminated porphyrin to the Ag(100) surface

Covalent tethering of macromolecules such as porphyrins to metal surfaces underpins bottom-up fabrication of systems intended for a variety of applications. Thiol linkages are especially useful but often need protection during macromolecule synthesis. By means of scanning tunneling microscopy, we directly observe the spontaneous deprotection of an acetyl-protected thioporphyrin upon contact with a silver surface, without the intervention of any solution-mediated chemistry.     

The Dirac delta function in two settings of Reverse Mathematics

The program of Reverse Mathematics (Simpson 2009) has provided us with the insight that most theorems of ordinary mathematics are either equivalent to one of a select few logical principles, or provable in a weak base theory. In this paper, we study the properties of the Dirac delta function (Dirac 1927; Schwartz 1951) in two settings of Reverse Mathematics. In particular, we consider the Dirac Delta Theorem, which formalizes the well-known property ${\int_\mathbb{R}f(x)\delta(x)\,dx=f(0)}$ of the Dirac delta function. We show that the Dirac Delta Theorem is equivalent to weak K?nig’s Lemma (see Yu and Simpson in Arch Math Log 30(3):171–180, 1990) in classical Reverse Mathematics. This further validates the status of WWKL₀ as one of the ‘Big’ systems of Reverse Mathematics. In the context of ERNA’s Reverse Mathematics (Sanders in J Symb Log 76(2):637–664, 2011), we show that the Dirac Delta Theorem is equivalent to the Universal Transfer Principle. Since the Universal Transfer Principle corresponds to WKL, it seems that, in ERNA’s Reverse Mathematics, the principles corresponding to WKL and WWKL coincide. Hence, ERNA’s Reverse Mathematics is actually coarser than classical Reverse Mathematics, although the base theory has lower first-order strength.     

Differentiation of Monofloral Honey Using Volatile Organic Compounds by HS-GCxIMS

Honey is a natural product and can be described by its botanical origin, determined by the plants from which the bees collect nectar. It significantly influences the taste of honey and is often used as a quality criterion. Unfortunately, this opens up the possibility of food fraud. Currently, various methods are used to check the authenticity of monofloral honey. The laborious, manual melissopalynology is considered an essential tool in the verification process. In this work, the volatile organic compounds obtained from the headspace of honey are used to prove their authenticity. The headspace of 58 honey samples was analyzed using a commercial easy-to-use gas chromatography-coupled ion mobility spectrometer with a headspace sampler (HS-GCxIMS). The honey samples were successfully differentiated by their six different botanical origins using specific markers with principal component analysis in combination with linear discriminant analysis. In addition, 15 honey-typical compounds were identified using measurements of reference compounds. Taking a previously published strategy, retention times of marker compounds were correlated with GC-coupled mass spectrometry (GC-MS) measurements to assist in the identification process.     

Enzymatic Benzofuranoindoline Formation in the Biosynthesis of the Strained Bridgehead Bicyclic Dipeptide (+)-Azonazine A

We uncovered and reconstituted a concise biosynthetic pathway of the strained dipeptide (+)-azonazine A from marine-derived Aspergillus insulicola. Formation of the hexacyclic benzofuranoindoline ring system from cyclo-(l -Trp-N-methyl-l -Tyr) is catalyzed by a P450 enzyme through an oxidative cyclization. Supplementing the producing strain with various indole-substituted tryptophan derivatives resulted in the generation of a series of azonazine A analogs.