Distinguishing Hexose Isomers by Lithiated Ion Adduction to Water (2016)

Undergraduates: Dazhe Chen, Matthew T. Campbell

Faculty Advisor: Gary Glish
Department: Chemistry

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Identification of carbohydrates (monosaccharides) is difficult because of the similarity in their structures and the range of stereochemical diversity. Aldohexoses in particular are monosaccharides with four chiral centers, allowing for 16 diastereomers and other structural isomers. Since collision induced dissociation does not provide sufficient information to distinguish those isomers, current methods rely on chromatography or addition of derivatizing reagents. Here we present a simpler method for distinguishing sugar isomers by the abilities of lithiated sugars to adduct to water in a quadrupole ion trap. Solutions of monosaccharides (glucose, galactose, mannose, or fructose) were analyzed on a Bruker Equire 3000 ion trap mass spectrometer via electrospray ionization. The lithiated monosaccharide was isolated, and a delay was applied before scanning ions to allow for controlled reaction times. Signal intensities of the unreacted peak at m/z 187 and the water adducted peak at m/z 205 were used to plot a linear graph of ln(unreacted/total) vs. time, the slope of which was used to successfully distinguish the isomers. Also, after a sufficiently long delay time only a fraction of the lithiated monosaccharides will adduct to water. The final ratio of the I187 to I205 for each sugar was found to be statistically different such that the four monomers could be identified. This work thus provides two separate methods for distinguishing glucose, galactose, mannose, and fructose.