Attenuated Total Reflection IR Spectroscopy, ATR-IR ATR-IR spectra were recorded on a Bio-Rad Excalibur spectrometer in wavenumber region of 800 cm -1 to 4000 cm -1 equipped with a mercury-cadmium-telluride (MCT) detector. A multi reflection ATR unit (Spectra-Tech, Inc.) was used. Double side polished, N-type doped Ge(100) crystals (Nano Quartz Wafer GmbH, Germany) of size 52 mm × 20 mm × 0.5 mm were used as internal reflection elements. The Ge substrates were polished into a trapezoidal shape giving an incident angle of 60 • . IR spectra were recorded in p-polarization with 256 scans and spectral resolution of 4 cm -1 . In a home build liquid cell mounted on the Ge crystal, first several IR spectra of the 650 µL tris-HCl buffer were collected as background and for use of the water vapor subtraction. The buffer was removed and IR spectra in 650 µL of S protein (0.015 ng/mL, 0.015 µM) in tris-HCl were recorded. Spectra were corrected for water vapour by factored subtraction of a respective spectrum and averaged.