Fig. 1

SFG spectra in the O−D (2,100–2,750 cm⁻¹) and C–H (2,800–3,000 cm⁻¹) stretch region of heavy water D₂O (a, b; top) and isotopically diluted HDO (c, d; bottom) underneath a lipid (left) and protein (right) monolayer. In both cases several peaks appearing in the hydrogen-bonded region for the pure D₂O spectrum collapse onto one peak upon isotopic dilution. For pure D₂O the different peaks have traditionally been ascribed to different (‘ice-like’ or ‘liquid-like’) types of interfacial water. We show here that that interpretation is not correct. The red lines illustrate fits to the spectra, and the thin blue lines represent the different contributing fit components of the O−D stretch vibrations