Infrared spectroscopic characterization of the interaction of ascorbyl palmitate with phospholipid bilayers

Abstract

The interaction of ascorbyl palmitate (AP) with phospholipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE), and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) has been studied by variable temperature infrared spectroscopy. The incorporation of AP into these lipid bilayers induces a considerable increase in the gel to liquid-crystal phase transition temperature of the lipid. The increase in this temperature is dependent on the nature of the lipid head group, on the relative concentration of AP, and on its degree of ionization; i.e., smaller temperature increases are observed when AP is ionized, forming salts with cations (NH4+, Li+, K+, and Ca2+). The stabilizing effect of the long chains in the bilayers plays an important role in inducing temperature increases. However, in the case of AP the present data also demonstrate that specific interactions at the polar head group serve to further stabilize the incorporation of AP into lipid bilayers. The infrared spectra demonstrate that in the case of DPPC:AP mixtures there are changes compatible with the formation of hydrogen bonds between the OH group at position 3 in AP and the ester carbonyl group at the sn-2 chain in DPPC. in the case of PE:AP mixtures the spectra are compatible with AP intercalated between PE head groups acting as a hydrogen bond bridge linking the phosphate and ammonium groups of two different PE molecules.