Philippus L. Wessels

Configuration assignments of the amino acid residues and the presence of N-methyldehydroalanine in toxins from the blue-green alga, Microcystis aeruginosa

The configuration assignment of the alpha-carbon atom of amino acid residues in four toxin variants from Microcystis aeruginosa have been made by stereospecific enzymic transformations. The relative conformation assignment of the beta-carbon atom of beta-CH3-aspartic acid could be made by comparison of the electrophoretic mobility with literature values reported for the authentic compound. The presence of an N-methyldehydroalanine residue, which, due to elimination of methylamine under hydrolytic conditions, previously escaped detection by conventional means, has been confirmed by identification of N-methylalanine in the hydrolysate after reduction of toxin with sodium borohydride.

Rotational isomerism: X. A nuclear magnetic resonance study of 2-fluoro-ethanol and ethylene glycol

Abstract The proton magnetic resonance spectra of 2-fluoro-ethanol and ethylene glycol in various solvents have been recorded and analysed. The results of previous investigations of 2-fluoro-ethanol, suggesting the gauche isomer to be strongly favoured, have been confirmed (⩾ 95% gauche ). The solvent and temperature dependence of the vicinal coupling constants in ethylene glycol indicates, however, that this compound does not - as frequently stated - exist as gauche conformer only, but contains up to 20 % of the trans isomer in solvents of low dielectric constant.

Sensitivity gain in a progressive-saturation selective population inversion NMR experiment

Abstract A pulsed NMR experiment is described which combines the principles of progressive-saturation and selective population inversion. A considerable sensitivity gain is obtained for the X resonances of an AX spin system with T1,A

Intramolecular non-bonded interactions and β- and γ-hydroxy substituent 13C NMR chemical shifts

Abstract A systematic study of β- and γ-substituent 13 C chemical shifts (SCS) in terms of intramolecular non-bonded interactions has been carried out. It is demonstrated that for a variety of molecules substituted with an OH substituent (cf. some conformationally non-rigid multifunctional alcohols ( I–IX in Table 1), 1-hydroxy ( 1a ) and 2-hydroxy ( 1b ) substituted adamantanes, exo 2-hydroxy ( 2a ), endo 2-hydroxy ( 2b ), 1-hydroxy ( 2c ), and 7-hydroxy ( 2d ) substituted norbornanes, and the hydroxy derivative of caged compound ( 3 )), β- and γ-SCS for primary, secondary and tertiary carbon atoms can be compared to the total local van der Waals and electrostatic non-bonded interaction energies in the same correlation. Furthermore, it is shown that the relationship can be extended to molecules bearing two such substituents, e.g. in the dihydroxy derivatives of certain caged compounds ( 4a ) to ( 4c ) and ( 5 ). The local electrostatic potentials are mostly responsible for the variations of β- and γ-SCS in the series I–IX , and ( 1 )–( 5 ) substituted molecules. On the other hand, β- and γ-SCS in the strained ( 6 ) and ( 7 ) molecules are mainly influenced by changes in the local van der Waals interaction energies. This approach presents a more consistent treatment of the influences of geometric distortions and electronic interactions on chemical shifts of sp 3 hybridized carbon atoms.

13C-chemical shifts in series of adamantane and norbornane derivatives. INDO calculations

Abstract 13 C α-substituent chemical shift (α-SCS) calculations were performed with GIAOs and at the INDO-FPT and London levels of approximation for 1- and 2-substituted adamantanes, and 1-, 7-, exo 2- and endo 2-substituted norbornanes. A good overall agreement between calculated and observed α-SCS was revealed when a modified set of INDO parameters for the fluorine, oxygen and nitrogen electronegative atoms is used. α-SCS are shown to correlate with the local calculated charge, and in addition, to be determined almost entirely by the local paramagnetic term, which in turn depends on the atomic charge. 13 C substituent chemical shifts as well as the corresponding Δ q atomic charges are also observed to be directly related to the group electronegativity of the α-substituent, hence emphasizing the importance of the σ inductive mechanism in the transmission of the α-substituent effect. SCS values at positions other than α in the series of substituted adamantanes and norbornanes are generally not well predicted theoretically.

One-dimensional homonuclear polarization transfer. Studies of parameters which affect intensities in the SPI NMR technique

Abstract A study is presented of the effects of strong scalar coupling and of the angle of the nonselective observing pulse, as well as of the effects of selective pulses, in one-dimensional selective population inversion (SPI) experiments of homonuclear spin systems. Density matrix computer simulations, which also take into account relaxation effects, of {A}-B SPI experiments, and for comparison also of {A}-X, reproduce different aspects of the experimental spectra. An insight is therefore offered into the parameters which affect intensities in the SPI one-dimensional NMR technique for homonuclear polarization transfer.

The determination of long-range (C,H) coupling constants by one- and two-dimensional NMR experiments

Abstract A comparative study is presented between the NMR techniques of two-dimensional heteronuclear correlation (HETCOR) with optimal delay times, HETCOR with the inclusion of a BIRD pulse sequence in the middle of t 1 , and of one-dimensional selective population inversion (SPI), all of which are utilized in establishing connectivities between carbon and proton nuclei over more than one bond for molecular-structure determinations. Some of the connectivities observed in salicylaldehyde with the SPI experiment could not be detected with long-range HETCOR. Various aspects of the most appropriate values to be used for the delay times in long-range HETCOR are discussed, and the absence of any long-range (C,H) connectivities is reasoned. SPI is the preferred technique for the estimation of the magnitudes of >1 J (C,H).