Simultaneous determination of a spectrum of trichothecene toxins out of residuals of biogas production

A sensitive and selective method for the simultaneous determination of a spectrum of trichothecenes in residuals of biogas production has been developed. It comprises sample clean-up by liquid/liquid partition for digested manure and solid phase extraction for digested solid phase. Quantification of A- and B-type trichothecenes as their trifluoroacetyl derivatives is performed by gas chromatography mass spectrometry (GC/MS), that of B-type trichothecenes alternatively by high-performance liquid chromatography (HPLC). Fluorescence detection (FLD) after post-column derivatisation using methylacetoacetate and ammonium acetate after alkaline decomposition of toxins was applied. Detection limits in digestates were between 1 and 30 microg/l and 20 and 50 microg/l for GC/MS and HPLC/FLD, respectively. Recovery rates were between 52 and 129% for GC/MS detection with the exception of T-2 tetraol with 22%, and between 56 and 123% for HPLC/FLD.     

Evolution of natural product scaffolds by acyl- and arylnitroso hetero-diels-alder reactions: new chemistry on piperine

Piperine, a natural product containing a conjugated diene, was reacted with polymer-supported acyl- and arylnitroso dienophiles. The reactions with arylnitroso dienophiles were also carried out in solution. The oxazine rings formed by the corresponding hetero-Diels-Alder reactions were further transformed and novel acyclic as well as heterocyclic derivatives including pyrroles and quinoxalinones were prepared.     

Accurate determination of the structure of cyclohexane by femtosecond rotational coherence spectroscopy and ab initio calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar molecules cyclohexane (C(6)H(12)) and cyclohexane-d(12) (C(6)D(12)). We measured the rotational B(0) and centrifugal distortion constants D(J), D(JK) of the v = 0 states of C(6)H(12) and C(6)D(12) to high accuracy, for example, B(0)(C(6)H(12)) = 4306.08(5) MHz, as well as B(v) for the vibrationally excited states ν(32), ν(6), ν(16) and ν(24) of C(6)H(12) and additionally ν(15) for C(6)D(12). To successfully reproduce the experimental RCS transient, the overtone and combination levels 2ν(32), 3ν(32), ν(32) + ν(6), and ν(32) + ν(16) had to be included in the RCS model calculations. The experimental rotational constants are compared to those obtained at the second-order M?ller-Plesset (MP2) level. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows determination of accurate semiexperimental equilibrium structure parameters, for example, r(e)(C-C) = 1.526 ± 0.001 ?, r(e)(C-H(axial)) = 1.098 ± 0.001 ?, and r(e)(C-H(equatorial)) = 1.093 ± 0.001 ?. The equilibrium C-C bond length of C(6)H(12) is only 0.004 ? longer than that of ethane. The effect of ring strain due to the unfavorable gauche interactions is mainly manifested as small deviations from the C-C-C, C-C-H(axial), and C-C-H(equatorial) angles from the tetrahedral value.     

High resolution NMR study of T(1) magnetic relaxation dispersion. III. Influence of spin 1∕2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons

Effects of spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of protons were studied in a situation where spin [fraction one-half] hetero-nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 129, 234513 (2008); S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, ibid. 133, 194502 (2010)], spin-spin interactions have a pronounced effect on the relaxivity tending to equalize the longitudinal relaxation times once the spins become strongly coupled at a sufficiently low magnetic field. In addition, we have found influence of (19)F nuclei on the proton NMRD, although in the whole field range, studied protons and fluorine spins were only weakly coupled. In particular, pronounced features in the proton NMRD were found; but each feature was predominantly observed only for particular spin states of the hetero-nuclei. The features are explained theoretically; it is shown that hetero-nuclei can affect the proton NMRD even in the limit of weak coupling when (i) protons are coupled strongly and (ii) have spin-spin interactions of different strengths with the hetero-nuclei. We also show that by choosing the proper magnetic field strength, one can selectively transfer proton spin magnetization between spectral components of choice.     

Synthesis of Linear Oligomers of (R)-3-Hydroxybutyrate and Solid-State Structural Investigations by electron microscopy and X-ray scattering

Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 10⁵ Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at ?70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15-, 30-, and 45mer. Discrete fragments are also isolated when P(3-HB) is heated under reflux (89°) in neat Et₃N. Linear oligo(3-HB) derivatives ( 3-7 ) containing up to 96 3-HB units are synthesized using an exponential segment-coupling strategy. These oligomers are used to calibrate size-exclusion chromatography columns for the analysis of oligo(3-HB) samples from the different sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron microscopy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair-pin’-type conformation. High-molecular-weight P(3-HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Å packing within the tight folds of P(3-HB) is discussed in view of its possible function in ion transport through cell membranes.     

Single particle dynamics in an anharmonic potential with translation-rotation coupling: crossover from weak localisation via chaos to free rotation

The single particle dynamics of a rigid NH₃-molecule in an anharmonic mean crystal potential is analysed. The potential parameters used have been derived earlier from experimental neutron diffraction data on single crystals of Ni(NH₃)₆X₂ (X = I, NO ₃, PF ₆): in all these compounds the ammonia molecules show dynamical orientational disorder. The mean crystal potential which is experienced by the three protons of one ammonia molecule is given by a two-dimensional anharmonic four-well potential, which leads to a coupling of the rotation of the molecule around its threefold axis to the translational motion of the molecular center of mass. Thus the dynamical problem is restricted to three degrees of freedom. The corresponding Hamiltonian equations of motion are solved numerically. Fourier analysis, reconstruction of trajectories in the six dimensional phase space and next-amplitude-maps from the simulated time series reveal either multiple periodic or chaotic solutions, depending on the potential parameters and the energy of the system. The anharmonic potential produces, as a generic property, three different kinds of proton orbits. At low energy, i. e. low temperatures, closed orbits related to hypocycloid functions occur. At intermediate temperatures the orbits are chaotic. High temperature simulations show circular orbits with a week high frequency jitter superimposed. Thus a crossover from weak localization via chaos to nearly free rotation is obtained by a variation of the energy in the simulation.     

Infrared depletion spectra of 2-aminopyridine2-pyridone, a Watson-Crick mimic of adenine.uracil

The 2-aminopyridine2-pyridone (2AP2PY) dimer is linked by N-H...O=C and N-H...N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine.thymine and adenine.uracil nucleobase pairs. Mass-specific infrared spectra of 2AP2PY and its seven N-H deuterated isotopomers have been measured between 2550 and 3650 cm(-1) by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N-H stretch is a very intense band spread over the range 2700-3000 cm(-1) due to large anharmonic couplings. It is shifted to lower frequency by 710 cm(-1) or approximately 20% upon H bonding to 2AP. On the 2AP moiety, the "bound" amino N-H stretch gives rise to a sharp band at 3140 cm(-1), which is downshifted by 354 cm(-1) or approximately 10% upon H bonding to 2PY. The amino group "free" N-H stretch and the H-N-H bend overtone are sharp bands at approximately 3530 cm(-1) and 3320 cm(-1). Ab initio structures and harmonic vibrations were calculated at the Hartree-Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.