Anomalous hydration of glycolipid ohmline revealed by surface-specific vibrational spectroscopy

Glycolipids are key structural and functional components of biological membranes, yet their interfacial hydration behavior remains poorly understood. Here, we use vibrational heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy to probe the molecular structure of the air-water interface formed by monolayers of ohmline, a glycolipid bearing a lactose headgroup and carrying no formal charge. Upon electrolyte addition, we observe a striking reorientation of interfacial water and a reversal of the HD-SFG signal, indicative of apparent surface charging by an otherwise neutral headgroup. This charge-like behavior stems from ion-specific interactions: cations exhibit strong affinity for the glycolipid interface, while anion effects are comparatively weak. Quantitative analysis reveals that approximately one cation is trapped per ∼16 ohmline molecules. These results uncover an unconventional hydration mechanism for glycolipids, distinguishing them from classical phospholipids and shedding light on salt-modulated self-assembly processes such as the reversible formation of lipid nanotubes (LNTs). Our findings establish a framework for understanding ion-mediated phenomena in glycolipid-rich interfacial systems, with implications for membrane biophysics and soft materials design.