FTIR spectroscopy study of the pressure-dependent behaviour of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-palmitoyl-2-oleolyl-sn-glycero-3-phosphocholine (POPC) at low degrees of hydration

•Water vapour hydration enables high pressure investigations using FTIR spectroscopy.•There are hydration ranges where red shift of the CH2-vibrations is observed.•The onset pressure of the correlation field splitting is enhanced during dehydration. High-pressure investigations of phospholipids at low degrees of hydration are highly important due to the continuous interest in hydration related phenomena. This is for instance the case when discussing differences between the molecular behaviour under hydrostatic or hydration pressure, e.g. to characterise the structural and thermodynamic conditions that hold for the different theoretical approaches proposed in the literature. The stability of phospholipid aggregates in aqueous solutions is determined by a balance of attractive end repulsive forces as well as entropic and energetic interactions. The pressure-dependent variation of the frequency of molecular vibrations determined by FTIR spectroscopy might be an appropriate tool to study the nature of these interactions as well as of conformational changes for various degrees of hydrations. The paper reports preliminary results for the stretching and bending vibration of the CH2 groups that are mainly situated in the hydrocarbon chains. In a hydration range of about 2 water molecules per lipid in the case of DOPC and 6 water molecules for POPC, the pressure-dependent vibrations in the liquid crystalline phase change between red shift and blue shift. A further interesting parameter is the onset pressure for the correlation field splitting. It increases with dehydration, and it is assumed that the correlation field splitting does not only depend on the disorder related to the gauche conformer population in the hydrocarbon chains but also on the chain tilt in the gel phase.