Tracking hydrogen/deuterium exchange at glycan sites in glycoproteins by mass spectrometry

Hydrogen/deuterium exchange coupled to mass spectrometry (HDX-MS) has emerged as a technique for studying glycoproteins, which are often refractory to classical methods. Glycan chains are generally assumed to exchange protons very rapidly, making them invisible to this technique. Here, we show that under conditions commonly used for HDX-MS, acetamido groups within glycan chains retain a significant amount of deuterium. Using mono- and polysaccharide standards along with glycopeptides from a panel of glycoproteins, we demonstrate that N-acetyl hexosamines, along with modified Asn side chains, are responsible for this effect. Model compounds for sialic acid also displayed similar exchange kinetics, but terminal sialic acids in the context of an entire glycan chain did not contribute to deuterium retention. Furthermore, the presence of sialic acid appears to enhance the exchange rate of the nearby N-acetyl glucosamines. The ability to detect deuterium exchange at the glycan level opens the possibility of applying HDX-MS to monitor glycan interactions and dynamics.