Revealing cooperativity in mixed lipid membranes from their FTIR spectra

The localization of different types of lipids in the plasma membrane leads to the formation of domains that are crucial for the processes of cell division and substance transport. Closely related to the concept of lipid domain in the membranes is the concept of cooperativity unit size (CUS), which encompasses a certain number of lipids that exert a synchronized response to a corresponding stimulus. The CUS of an individual domain is usually determined in (mixed) lipid membranes using the DSC technique, whereby the (individual) building blocks of the membrane undergo a phase transition in a certain temperature range. This work introduced the application of temperature-dependent FTIR spectroscopy in characterizing the cooperativity in ternary lipid mixtures in terms other than the size of CU, i.e., on the number of lipids. By analyzing the vibrational bands originating from phosphates and methylene groups of hydrocarbon chains using a chemometric tool, we obtained different melting temperatures for the polar and non-polar parts of the lipid molecule. Besides reconstructing the chronology of the melting process, the broadness of the temperature range in which melting appears is perfectly correlated with the CUS obtained from DSC measurements. The methodology outlined in this work sheds new light on understanding the cooperativity in mixed lipid bilayers.