Fast‐atom bombardment mass spectrometric study of some N‐ and S‐glycosides of acetylated hexose isomers

Abstract

Various N-aryl, S-aryl and S-alkyl glycosides of acetylated D -~~U C O S~,o-galactose and o-mannose have been studied by fast-atom bombardment mass spectrometry. It was found that the matrix used has a considerable effect on the relative peak intensities in the molecular mass region. Glycerol apparently promotes the formation of protonated molecules (MH') whereas in 3-nitrobenzyl alcohol both MH+ ions and M+' radical cations are abundant. The unimolecular decomposition spectra of the MH' ions exhibit two significant peaks corresponding to losses of acetic acid and of the aglycon. The intensity ratios of these fragment ions are dependent on the nature of the aglycon and the hexose unit and can be used to identify the monosaccharides. It has also been established that acetic acid elimination occurs mainly from the C-3 position.The mass spectrometric behaviour of peracetylated monosaccharides and related compounds has been the subject of several studies. The earliest electronionization (EI) studies on the fragmentation behaviour of (per)acetylated saccharides were based on data from metastable-ion transitions and deuterium-labelledThe molecular ion could not be detected under EI conditions and its decomposition was found to involve sequential losses of acetic acid and ketene molecules. On the basis of measurements on the methylated derivatives2 it was established that the C-1 substituent is lost preferentially in the first step, resulting in an oxonium ion of considerable stability. The elimination of the C-3 substituent is the most probable second step. Smale and Waight3 studied various aryl tetra-0-acetyl-D-ghcosides and proposed a fragmentation pathway. By using different labelled derivatives Hogg and Nagabhushan4 determined the sequence of acetic acid losses from the protonated D-glucose pentaacetate but it was later established5 that the decomposition of the oxonium ion formed in the first step can proceed via two parallel pathways. If C-1 is occupied by an alkyl or aryl group, the corresponding alcohol molecule is eliminated. This process predominates over other fragmentations, as was found in the case of methyl 2,3,4-tri-Oacetyl-P-D-xylopyranoside.6 In the mass-analysed ion kinetic energy (MIKE) spectrum of the MH' ion of this compound, generated by methane chemical ionization (CI), one single peak due to methanol loss can be detected. Guevremont and Wright7-9 studied the tandem mass spectra (MS/MS) of [MH -AcOH]' ions generated from peracetylated hexose (glucose and galactose) and 2-deoxy-2-amino-hexose (glucosamine, galactosamine and mannosamine) isomers and found characteristic differences in the intensity ratios of daughter-ion peaks, which were explained as resulting from differences in the three-dimensional structures of the precursor ions.?Paper delivered at the 11th Informal Meeting on Mass Spectrometry, Budapest, 1993. Author to whom correspondence should be addressed at the Research Group for Antibiotics of the Hungarian Academy of Sciences, Lajos Kossuth University, H-4010 Debrecen, PO Box 70, Hu...

Characterization of 2‐methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry

“…9(b)) shows a unique ion at m / z 420, which is explained by loss of acetic acid from the M +• ion (Scheme ). This favorable elimination of acetic acid involves a hydrogen at a 3‐position relative to the acetate group23 and does not occur in the other isomer (RT 52.3 min) in which only hydrogen atoms at the 2‐position are available.…”