The isomeric complexity
of glycans make their analysis by traditional
techniques particularly challenging. While the recent combination
of ion mobility spectrometry (IMS) with cryogenic IR spectroscopy
has demonstrated promise as a new technique for glycan analysis, this
approach has been limited by the modest resolution of the ion mobility
stage. In this work we report results from a newly developed instrument
that combines ultrahigh-resolution IMS with cryogenic IR spectroscopy
for glycan analysis. This apparatus makes use of the recent development
in traveling-wave IMS called structures for lossless ion manipulation.
The IMS stage allows the selection of glycan isomers that differ in
collisional cross section by as little as 0.2% before injecting them
into a cryogenic ion trap for IR spectral analysis. We compare our
results to those using drift-tube IMS and highlight the advantages
of the substantial increase in resolution. Application of this approach
to glycan mixtures demonstrates our ability to isolate individual
components, measure a cryogenic IR spectrum, and identify them using
a spectroscopic database.
The analysis of carbohydrates, or glycans, is challenging for established structure-sensitive gas-phase methods. The multitude of possible stereo-, regio-and structural isomers make them substantially more complex to analyze than DNA or proteins, and no one method is currently able to fully resolve them. While the combination of tandem mass spectrometry (MS) and ion mobility spectrometry (IMS) have made important inroads in glycan analysis, in many cases this approach is still not able to identify the precise isomeric form. To advance the techniques available for glycan analysis we employ two important innovations. First, we perform ultrahigh-resolution mobility separation using structures for lossless ion manipulations (SLIM) for isomer separation and pre-selection. We then complement this IMS-MS stage with a cryogenic IR spectroscopic dimension, since a glycan's vibrational spectrum provides a fingerprint that is extremely sensitive to the precise isomeric form. Using this unique approach in conjunction with oxygen-18 isotopic labelling, we show on a range of disaccharides how the two a and b anomers that every reducing glycan adopts in solution can be readily separated by mobility and identified based on their IR spectra. In addition to highlighting the power of our technique to detect minute differences in the structure of isomeric carbohydrates, these results provide the means to determine if and when anomericity is retained during collision-induced dissociation (CID) of larger glycans.
The
isomeric heterogeneity of glycans poses a great challenge for
their analysis. While combining ion mobility spectrometry (IMS) with
tandem mass spectrometry is a powerful means for identifying and characterizing
glycans, it has difficulty distinguishing the subtlest differences
between isomers. Cryogenic infrared spectroscopy provides an additional
dimension for glycan identification that is extremely sensitive to
their structure. Our approach to glycan analysis combines ultrahigh-resolution
IMS-IMS using structures for lossless ion manipulation (SLIM) with
cryogenic infrared spectroscopy. We present here the design of a SLIM
board containing a series of on-board traps in which we perform collision-induced
dissociation (CID) at pressures in the millibar range. We characterize
the on-board CID process by comparing the fragments generated from
a pentapeptide to those obtained on a commercial tandem mass spectrometer.
We then apply our new technique to study the mobility and vibrational
spectra of CID fragments from two human milk oligosaccharides. Comparison
of both the fragment drift times and IR spectra with those of suitable
reference compounds allows us to identify their specific isomeric
form, including the anomericity of the glycosidic linkage, demonstrating
the power of this tool for glycan analysis.
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