Total external reflection X-ray absorption spectroscopy reveals a zinc coordination shell in phospholipid Langmuir-Blodgett films

We have studied the local environment of zinc in phospholipid Langmuir-Blodgett films with extended X-ray absorption fine structure spectroscopy in total reflection conditions (reflEXAFS). Mono- and multilayer films made of dilauroyl-phosphatidic acid (DLPA) and monolayer films made of dimyristoyl-phosphatidylcholine (DMPC) were deposited on quartz slides from ZnCl2 solutions, and their molecular order was checked by means of X-ray reflectivity measurements. A very good signal-to-noise ratio in the reflEXAFS spectra of both multi- and monomolecular films was obtained. The theoretical fits to the experimental data show that zinc coordinates four atoms about 0.20 nm distant in both acidic DLPA and neutral DMPC films, while in water solutions, zinc coordinates six atoms about 0.21 nm distant.