The IR spectra were recorded with a Nicolet 670 FT-IR (ThermoFisherScientific GmbH) in attenuated total reflection (ATR), with which "Golden Gate" sample holder was used. After each measurement, the sample holder was cleaned with ethanol and acetone. The background spectrum was measured in air. 32 scans with a resolution of 4 cm -1 were recorded per measurement. The spectra were recorded in absorption mode in a spectral range of 4000 -600 cm -1 . The instrument was controlled with OMNIC software. All spectra were mathematically processed in the same way. An asymmetric least squares baseline correction and a subsequent normalisation were performed.