Investigation of the temperature behavior of the bands due to the methylene stretching vibrations of phospholipid acyl chains by two-dimensional infrared correlation spectroscopy

The temperature-induced gel-to-liquid crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) is characterized by a shift towards high frequencies and an increase of the width of the bands due to the methylene stretching vibrations. These spectral modifications are frequently used to measure the conformational order of lipid acyl chains. However, it is not clear whether these bands contain two spectral components due to trans and gauche conformers or whether they gradually shift with temperature. The temperature-induced gel-to-liquid crystalline phase transition of DPPC has been investigated in the present study by two-dimensional infrared (2D-IR) correlation spectroscopy. Our results show that each methylene stretching band in both the synchronous and the asynchronous maps is characterized by two peaks. The same pattern is also observed when the temperature range is restricted to the gel phase. These results were compared to those obtained by spectral simulations using either a single band that shifts in frequency and gets broader with the increase of temperature (shifting-band model), simulating a continuously evolving one-phase system, or a band made of two components (two-band model), simulating the trans and gauche spectral contributions of a two-phase system. The results obtained for the asynchronous maps of the simulated spectra indicate clearly that the experimental results cannot be modeled by a pure two-phase system and are best simulated by the shifting-band model.