FTIR study of temperature- and pressure-induced phase transitions and intermolecular interaction changes in DMPC bilayers in H2O and D2O

The thermodynamic and microscopic characteristics of membrane states accompanying temperature-induced phase transitions in phospholipid bilayer membranes forming liposomes have been well elucidated in previous studies. However, microscopic insights into membrane states during pressure-induced phase transitions—particularly changes in intermolecular interactions between phospholipid molecules—remain limited. In this study, we analyzed such interactions in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer membranes using temperature-variable and pressure-variable infrared (IR) spectroscopy in both H2O and D2O environments. We focused on symmetric and antisymmetric stretching modes of methylene (νs(CH2), νas(CH2)) and methyl groups (νs(CH3), νas(CH3)), which effectively reflect molecular interactions. Frequency shifts in these vibrational modes correlated well with membrane curvature and solvent-induced changes in gel phase packing. Moreover, temperature and pressure derivatives of the vibrational frequencies provided insight into bilayer stability across different phases. In particular, pressure derivative of the frequency for νs(CH2) was about half as large in the interdigitated gel (LβI) phase as in the lamellar gel (Lβ′) phase, suggesting reduced repulsive interactions between methylene chains in the LβI phase.