Mass spectrometric characterization of amino sugar aldononitrile acetate derivatives used for isotope enrichment assessment of microbial residues

Identifying the transformation of amino sugars in soils is essential in understanding microbial contribution to soil organic matter turnover and sequestration. Using a recently developed method, combining gas chromatograph/mass spectrometry (GC/MS) with laboratory incubation of substrates containing ¹³C or ¹⁵N isotopes, we were able to trace isotopic changes in amino sugar compounds. This allows us to quantitatively evaluate C or N enrichment in amino sugars during transformation in soils using the fragment (F) abundance ratio of m/z F + n to F (n is original skeleton atom number in each fragment). However, there is still lack of detail structural and substitutional information for each ion fragment. In order to improve the interpretation and increase our ability to study amino sugar turnover, we grew labeled amino sugars in lab-cultured organisms. We spectrometrically investigated the ion structures and original skeleton C number (mass shift n) in major ion fragments based on applying multiple representative isotope labels. Our results categorically confirm that previously made assumptions were correct regarding the substitutional number “n” of the glucosamine (He et al., 2006). Our study also added valuable structural information for aldononitrile acetate derivatized glucosamine and muramic acid upon electron impact ionization in MS.