Recent advances in infrared spectroscopy have made it possible to study structural and dynamic properties of biomembranes and model systems at molecular level by spectral analysis. Using pressure tuning infrared spectroscopy we studied intermolecular interactions in 1,2 dioleoyl phosphatidylcholine (DOPC), 1,2 dipalmitoyl phosphatidylcholine (DPPC). Infrared spectra from model biomembranes and abnormal tissues were measured and compared to their normal species under 0 to 45 kbar pressure. Discontinuities in the pressure dependencies of various infrared spectral parameters indicate that structural phase transitions occur at 4.2, 5 and 15 kbar. More than two correlation field components of CH2 scissoring modes are observed in spectra of the high-pressure phases, suggesting that more than two acyl chains are present in the unit cell of these phases.