Separable contributions of ordered and disordered lipid fatty acyl chain segments to nuCH2 bands in model and biological membranes: a Fourier transform infrared spectroscopic study.

In this article, the assignment of the nu(C-H) stretching region of lipid molecules is revisited. This region is extensively used to follow lipid phase transitions, and especially the frequency shifts and bandwidth alterations in the nu(sym)CH2 band have been utilized in this respect. Here, we propose and prove that behind these phenomena there are pairs of component bands in the cases of both the nu(sym)CH2 and the nu(as)CH2 bands. The lower-frequency components of the pairs are assigned to the vibrations of CH2 groups on trans segments of the fatty acyl chains, while the higher-frequency components of the pairs are assigned to CH2 groups on gauche segments. To prove these assignments, we have shown that the nuCH2 frequencies are characteristic of the conformation of the lipid fatty acyl chain itself, and not the state of the whole lipid matrix. Curve fitting in fact revealed the conformer-specific components. With the use of singular value decomposition analysis we have demonstrated that the relative intensity changes in the components, and not the shifts in the whole bands, cause the observed shifts in the nuCH2 bands upon lipid phase transition. The results of this approach are presented for deuterium-saturated dioleoyl-phosphatidylcholine mixtures, for the gel --> liquid-crystalline phase transition of dipalmitoyl-phosphatidylcholine multilayers, and for a biological membrane, barley thylakoid. This refined assignment offers physically plausible reasoning for the observed phenomena and is able to explain frequency shifts and bandwidth changes observed previously upon lipid phase transitions, including their nonconcerted temperature dependences. In biological membranes, this interpretation allows the separation of protein- and membrane-dynamics-induced lipid conformational changes.