Carbon-13 NMR spectra of rifamycins

The natural abundance ¹³C Fourier transform magnetic resonance spectra of rifamycin S and some of its derivatives have been studied. A combination of five different approaches has made unambiguous assignments for most of the resonances possible: (1) comparative study of the non- decoupled and noise-decoupled spectra; (2) ¹³C spectral characteristics; (3) spectral comparison between derivatives; (4) selective proton decoupling; (5) biogenetic evidence. Pulse and Fourier transform ¹³C NMR spectroscopy provides a more complete picture of these complex molecules than was previously obtained by ¹H NMR spectroscopy.     

Provitamin D and vitamin D isomerizations in the mass spectrometer. Translational energy release and collision-induced dissociation studies

Collision-induced dissociation mass-analysed ion kinetic energy spectrometry and translational energy release studies have established that provitamin D₃ (7-dehydrocholesterol), provitamin D₂ (ergosterol), vitamin D₃ and vitamin D₂ isomerizations in the mass spectrometer occur at the fragment ion level leading to [M? H₂O? CH₃]⁺ ions of identical structure. It was found that the reaction, [M]⁺˙→[M? H₂O]⁺˙, plays a central role in these isomerizations.     

The dependence of collision induced fragmentation pattern on the internal energy of the precursor ion

A Method is presented whereby product organic ions formed as the result of fragmentation of metastable ions can be selected on the basis of their internal energies. The method requires requires angular collimation of the beam of reactant ions issuing from the ion source and that the fragmentation of the metastable ions is studied in the first field free region of a reversed geometry double focusing mass spectrometer. The product ions that make up the metastable peak are allowed to fall on the intermediate resolving slit and, by adjusting the magnet current over a small range, ions contained in different regions of the peak can be allowed into the second field free region. It is shown that the position of an ion within the metastable peak correlates with its internal energy, ions near the edges of the peak being the least excited. The ions entering the second field free region can be investigated by collisional activation. This has been done for molecular ions of p-chlorophenol, methylbenzoate, benzaldehyde, m-chlorotoluene and n-butane. The in which the collision induced fragmentation pattern varies with internal energy of the ions is illustrated.     

Structure in metastable peaks arising from reactions in which[C3H3]+ ions are produced

The metastable peak resulting from the fragmentation of [C₇H₇]²⁺? ions fram a variety of sources shows structure due to the presence of two reactions releasing different amounts of translational energy. The translational energy differences has been measured as 0.52 ± 0.04eV and is thought to be due to the formation of product ions of different structure via competing reactions from a single transition state. The possible structures of these ions are discussed, and it is proposed that the effect observed is due to the formation of [C₃H₃]⁺ ions in two forms, cyclopropenyl and proparg1. The metastable singly charged ions which also lead to product ions of formula [C₃H, ₃]⁺.     

Oxygen rearrangement of molecular ions of 3-phenylpropionates

The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH₃ ? CH₂CO]⁺ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration.     

On the formation of artefact peaks in linked scans of the magnet and electric sector fields in a mass spectrometer

The apperrance of peaks at non-integral masses in linked scans, where the ratio B/E is kept constant, is shown to be caused by fragmentations of metastable ions occuring in the region between the magnetic and electric sectors. A formula is given for calculating the mass of the parent metastable ion and that of its daughter ion.