Structural models of lipid surface monolayers from X-ray and neutron reflectivity measurements

Structural investigations of phospholipid monolayers on aqueous subphases on the submolecular level using X-ray and neutron reflectivity measurements are reviewed. While such investigations have been limited in the past by a relatively restricted accessible momentum transfer range, recent developments in synchrotron technology — almost doubling this range — have considerably improved the capabilities of the technique. Until recently, data interpretation has entirely relied on ‘box models’ which describe the structures as molecularly homogeneous slabs — one hydrophobic and one hydrophilic. It is shown that box models of phospholipid monolayers are rather inadequate to model data at the high momentum transfer available nowadays in X-ray measurements. As an alternative, a hybrid data inversion strategy is proposed that treats the hydrophobic alkane phase as a homogeneous slab and describes the position of submolecular fragments of the lipid headgroups by means of distribution functions along the interface. Within this approach, composition-space refinement — enabling the coupling of data sets from various X-ray and neutron contrasts — in connection with volumetric constraints enables structural characterization of lipid monolayers in unprecedented detail. Extending a recent characterization of dimyristoylphosphatidic acid (DMPA) monolayers on pure water [Schalke et al., Biochim. Biophys. Acta 1464 (2000) 113–126] it is shown that stoichiometric binding of the divalent cations — DMPA−:Cat2+=2:1 — occurs only at exceedingly low areas per molecule, Alipid. At low surface pressure π, both cations and anions are incorporated into the headgroup in significant amounts, ∼0.68 Ba2+ and ∼0.35 Cl– per PA molecule at π=2 mN m−1. They are continuously squeezed out upon compression, until upon approaching Alipid=41 Å2, the stoichiometric ratio between bound cations and acidic headgroups is observed. The average inclination angle α of the headgroups as well as their water content is constant along the whole isotherm. The intrinsic contribution to the distribution width — i.e. the spread that is due to a distribution of the fragments within the headgroup without the action of capillary waves — increases with compression up to π∼30 mN m−1 and drops sharply thereafter in a regime of the isotherm where Alipid approaches its limiting value. The same general picture is observed for DMPA on subphases with 10 mM Ca2+, although the lower electron density of that cation limits the precision of the results.