Thermotropic and structural effects of poly(malic acid) on fully hydrated multilamellar DPPC–water systems

Abstract

The thermotropic and structural effects of low molecular weight poly(malic acid) (PMLA) on fully hydrated multilamellar dipalmitoylphosphatidylcholine (DPPC)-water systems were investigated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and freeze-fracture transmission electron microscopy (FFTEM). Systems of 20 wt% DPPC concentration and 1 and 5 wt% PMLA to lipid ratios were studied. The PMLA derivatives changed the thermal behavior of DPPC significantly and caused a drastic loss in correlation between lamellae in the three characteristic thermotropic states (i.e., in the gel, rippled gel and liquid crystalline phases). In the presence of PBS or NaCl, the perturbation was more moderate. The structural behavior on the atomic level was revealed by FTIR spectroscopy. The molecular interactions between DPPC and PMLA were simulated via modeling its measured infrared spectra, and their peculiar spectral features were interpreted. Through this interpretation, the poly(malic acid) is inferred to attach to the headgroups of the phospholipids through hydrogen bonds between the free hydroxil groups of PMLA and the phosphodiester groups of DPPC