NMR and EPR studies of lipid membranes in the presence of ATP

Adenosine triphosphate (ATP) is an organic molecule with a net negative charge and is a source of metabolic energy. We investigated the effect of ATP on lipid membranes made of phospholipids with charged and neutral headgroups, using solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy, complemented by zeta potential measurements. Order parameter profiles for lipid hydrocarbon chains were obtained from deuterium NMR spectra and interpreted in terms of a mean-field approximation of lipid-lipid interactions. We find that ATP affects the alignment field of electrically charged lipids more than that of neutral lipids, indicating an essential role of ATP-headgroup interactions. To investigate this aspect, EPR spectra were recorded with 3 different spin-labeled probes that examined the bilayer interior and interfacial regions. The EPR results quantified in terms of order parameters and rotational correlation times are in agreement with NMR results. Furthermore, zeta potential measurements show that ATP makes the membrane potential more positive consistent with both NMR and EPR results. Our results interpreted in terms of an alignment field of lipid chains can guide the development of biomaterials with useful electro-mechanical properties.