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

Structural investigations of phospholipid monolayers on aqueous subphases o n 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 developmen ts in synchrotron technology - almost doubling this range - have considerab ly improved the capabilities of the technique. Until recently, data interpr etation has entirely relied on 'box models' which describe the structures a s molecularly homogeneous slabs - one hydrophobic and one hydrophilic. It i s shown that box models of phospholipid monolayers are rather inadequate to model data at the high momentum transfer available nowadays in X-ray measu rements. As an alternative, a hybrid data inversion strategy is proposed th at 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, compositi on-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. Ex tending a recent characterization of dimyristoylphosphatidic acid (DMPA) mo nolayers on pure water [Schalke et al., Biochim. Biophys. Acta 1464 (2000) 113-126] it is shown that stoichiometric binding of the divalent cations - DMPA(-):Cat(2+)= 2:1 - occurs only at exceedingly low areas per molecule, A (lipid). At low surface pressure pi, both cations and anions are incorporat ed into the headgroup in significant amounts, similar to 0.68 Ba2+ and simi lar to 0.35 Cl- per PA molecule at pi = 2 mN m(-1). They are continuously s queezed out upon compression, until upon approaching A(lipid) = 41 Angstrom (2) the stoichiometric ratio between bound cations and acidic beadgroups i s observed. The average inclination angle oc of the headgroups as well as t heir water content is constant along the whole isotherm. The intrinsic cont ribution to the distribution width - i.e. the spread that is due to a distr ibution of the fragments within the headgroup without the action of capilla ry waves - increases with compression up to pi similar to 30 mN m(-1) and d rops sharply thereafter in a regime of the isotherm where A(lipid) approach es its limiting value. The same general picture is observed for DMPA on sub phases with 10 mM Ca2+ although the lower electron density of that cation l imits the precision of the results. (C) 2000 Elsevier Science B.V. All righ ts reserved.