Study of the structure and phase behavior of dipalmitoylphosphatidylcholine by infrared spectroscopy: characterization of the pretransition and subtransition

Abstract

Despite the fact that fully hydrated multilayers of phosphatidylcholines have been extensively studied in the past, phase transitions such as the pretransition and the subtransition are still the object of different interpretations and contradictory observations. In order to obtain more information on the structural changes associated to these transitions for dipalmitoyl phosphatidylcholine (DPPC), a study has been done using both transmission and polarized attenuated total reflection infrared spectroscopy. At the pretransition, we observe that the extended hydrocarbon chains become less tilted with respect to the bilayer normal. The pretransition is also characterized by an increase in the hydration of the DPPC bilayer and the appeareance of the ripple structure. As opposed to previous findings, we do not observe a major variation of the acyl chain packing mode at the pretransition. These observations are supported by the investigation of DPPC with 5 mol% cholesterol, since this sterol is known to be a strong inhibitor of the pretransition. On the other hand, a drastic change in chain packing, characterized by infrared bands due to the methylene scissoring mode, is observed at the subtransition. Also, below the subtransition, we observe an increase of the hydrocarbon chain rotational disorder. Some correlations have also been made between the spectral parameters normally used to characterize the degree of order of the acyl chains. These correlations have allowed us to evaluate more precisely at the molecular level the subtle structural changes that occur during phase transitions of DPPC.