Biosynthetic Studies on Antibiotics

The intricacies of the synthetic chemistry executed by living cells in the assembly of complex organic natural products can be probed in great detail by stable-isotope labeling in conjunction with sophisticated 2D and multiple-quantum NMR methods of analysis. Mechanistic details are revealed by unraveling the “cryptic” stereochemistry of biosynthetic reactions with the aid of stereospecific isotope labeling. Analysis of blocked mutants and cloning of biosynthetic genes provides new tools for the study of biosynthetic pathways and for the exploitation of the chemical capabilities of living organisms.     

Unraveling the crystallization mechanism of CoAPO-5 molecular sieves under hydrothermal conditions

The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.     

A computation of the Witt index for rational quadratic forms

Summary This paper adds the finishing touches to an algorithmic treatment of quadratic forms over the rational numbers. The Witt index of a rational quadratic form is explicitly computed. When combined with a recent adjustment in the Haase invariants, this gives a complete set of invariants for rational quadratic forms, a set which can be computed and which respects all of the standard natural operations (including the tensor product) for quadratic forms. The overall approach does not use (at least explicitly) anyp-adic methods, but it does give the Witt ring of thep-adics as well as the Witt ring of the rationals.     

Shubnikov-de Haas effects in Bi2Se3 with high carrier concentrations

Shubnikov-de Haas frequencies were measured in highly degenerate n-type Bi₂Se₃ having a higher carrier density (～9 × 10²⁵m^-3) than previously reported. The Fermi surface was found to be elongated along the trigonal axis, fitting a spheroidal model with an axial ratio of 5.0 for angles up to θ = 45°. Comparison of the number of carriers obtained from Hall measurements with that obtained from the Shubnikov-de Haas measurement supports the contention that the lowest conduction band minimum is a single surface located in the center of the Brillouin zone. The higher effective mass (0.25 m₀) found for these carrier concentrations indicates that the band is non-parabolic.     

Beitr?ge zur Harnanalyse

Ohne Zusammenfassung