  Glycans play a central role as mediators in most biological processes, but
their structures are complicated by isomerism. Epimers and anomers,
regioisomers, and branched sequences contribute to a structural variability
that dwarfs those of nucleic acids and proteins, challenging even the most
sophisticated analytical tools, such as NMR and mass spectrometry. Here, we
introduce an electron tunneling technique that is label-free and can identify
carbohydrates at the single-molecule level, offering significant benefits over
existing technology. It is capable of analyzing sub-picomole quantities of
sample, counting the number of individual molecules in each subset in a
population of coexisting isomers, and is quantitative over more than four
orders of magnitude of concentration. It resolves epimers not well separated by
ion-mobility and can be implemented on a silicon chip. It also provides a
readout mechanism for direct single-molecule sequencing of linear
oligosaccharides.
