Stereoisomer differentiation of 2-acetamido-2-deoxyhexose by mass spectrometric measurement of relative gas-phase alkali ion affinities

Abstract

Catlonlzed dlmers [hexosamlne-cat-matrlx]'of 2-acetamldo-2-deoxy-~glucose, 2-acetamldo-2-deoxy-~-mannose, and 2-acetamldo-2deoxy~alactose were generated by fast atom bombardment (FAB) uslng different alkall catlons and glycerol, S-valerolactame, and diethanolamine matrlces. From thelr unimolecular decomposltlon (metastable Ions) malnly two klnds of Ions are observed resultlng from the retention of the cation either by the amino sugar (Ion a) orland by the matrlx (Ion b). For a given hexosamlne their relatlve abundance depends on the alkali cation and the matrlx used.When the matrlx Is diethanolamine and the alkall catlon Is ltthium or sodlum, the N-acetylglucosamine stereolsomers can be dlfferentlated by the relatlve abundances of ions a and b of the catlonlzed dimer.Amino sugars are important constituents of antibiotics such as aminoglycosides ( I ) , biological polymers such as glycoproteins (2), and lipopolysaccharides (3) which can present immunological activities.Among these amino sugars the N-acetylhexosamines have been extensively studied by mass spectrometry. Their identification requires great mass spectrum selectivity due to their isomeric forms. Structural investigation by EI/MS and CI/MS involves their derivatization. E1 fragmentation pathways of MeaSi ethers of 2-acetamido-2-deoxyhexose have already been established. Differences between mass spectra are insufficient to allow anomer or epimer identification ( 4 ) . However it has been shown that EI-MS of permethylated N-acetylhexosamines allows the localization of the acetamido group in the C-1, (2-2, (2-3, or C-4 positions. Moreover, CI of MesSi derivatives of some 2-N-acetyl hexosamine stereoisomers using methane as reactant gas gives mass spectra in which it is the relative intensities of the fragment ions that allows their differentiation (5).Recently, the use of surface ionization modes such as FD, laser, and FAB has permitted the structural analysis of underivatized oligosaccharides (6). Among these different ionization modes FAB seems the most suitable since molecular weight and sequencing information have been obtaind from their mass spectra in both the positive and negative mode (7, 8). FAB ionization requires the use of a matrix, usually glycerol, in which the sample is dissolved (9). We have shown by MIKE analyses that one way to generate protonated or cationized disaccharide molecular ions is to desolvatate protonated or cationized dimers (see Scheme I) (10, 11).Similar cationized dimers [S-cat-M]+ are generated by FAB from aldohexoses and methyl glycosides (12-14). Moreover as described in Scheme I it has been observed that their unimolecular dissociation gives two kinds of ions: a and b, whose relative abundances allow epimer and anomer differentiation of certain glycosides (12,14). Using a similar method Cooks et al. (15) ordered the relative gas phase (Ag+) affinities

Gas-phase nitronium ion affinities.

“…In most cases, equilibrium 1 could be attained from both sides, and the AG' changes reported in Table 1 The Kinetic Method. The limitations of the equilibrium method have suggested extension to nitronium ion-bound dimers of the kinetic method developed by Cooks and coworkers (35,36) for proton-bound dimers and subsequently applied to alkali cation-bound dimers (37) and other metal cation-bound clusters (38)(39)(40)(41). It is a limiting procedure, based on several nonrigorous assumptions that nevertheless has proved highly successful in a var-iety of applications, especially in those systems where weakly bound dimers undergo simple dissociation kinetics, which is the case of many nitronium ion-bound dimers (see below).…”

Proton affinity differences among three N‐acetylhexosamines studied by the kinetic method

“…Carbohydrates constitute one of the most important classes of biomolecules and N ‐acetylhexosamines in particular form important constituents of antibiotics such as aminoglycosides,1 biological polymers such as glycoproteins,2 and lipopolysaccharides,3 which possess immunological activities. Several groups have reported structural elucidation of isomeric monosaccharides by mass spectrometry 4–18. During our earlier study of hydrogen‐bonding interactions of lysine with mono‐ and disaccharides under ESI conditions19 we observed that lysine forms abundant proton‐bound heterodimers with N ‐acetylhexosamines under electrospray ionization (ESI) conditions.…”

Thermochemical determinations by the kinetic method

“…In response to this possibility, special measures have sometimes been taken; for example, Chen and Miller have used a high-pressure FAB source to generate thermalized cluster ions by collisional cooling (45,46). FAB has also proven extremely useful in generating dimer ions of nonvolatile species, and alkali metal ions as well as protons have been used as the binding species (47)(48)(49); (also see Section VB). Desorption chemical ionization (DCI) is another particularly useful method for ionizing less volatile or thermally labile compounds in kinetic method studies.…”