The effect of incorporation of phosphonic acid dibutyl ester in POPC model membranes on the hydration force

Two closely related {4-[(N-substituted amino)(diethoxyphosphoryl)methyl]phenyl}boronic acids

Organic phosphonic acids and organic phosphonic acid esters have been of much interest due to their applications in the fields of medicine, agriculture and industrial chemistry. Boronic acids can act as synthetic intermediates and building blocks and are used in sensing, protein manipulation, therapeutics, biological labelling and separation. The additional introduction of an aminophosphonic acid group into a boronic acid may give new opportunities for application. To study the structure of such multifunctional compounds, we prepared two new derivatives which can be easily converted to the corresponding phosphonic acids. In the title compounds, {4-[(butylamino)(diethoxyphosphoryl)methyl]phenyl}boronic acid monohydrate, C₁₅H₂₇BNO₅P·H₂O, (I), and {4-[(diethoxyphosphoryl)(4-nitroanilino)methyl]phenyl}boronic acid, C₁₇H₂₂BN₂O₇P, (II), three different substituents are attached to a central C-H group, namely 4-boronophenyl, diethoxyphosphoryl and amine. Compound (I) crystallizes as a monohydrate and O_B-H...N hydrogen bonds link neighbouring molecules into chains along the [001] direction. The solvent water molecule connects two such chains running in opposite directions. Compound (II) crystallizes as an ansolvate and classical hydrogen bonds result in a layer structure in the (001) plane.

The structure of ions and zwitterionic lipids regulates the charge of dipolar membranes

In pure water, zwitterionic lipids form lamellar phases with an equilibrium water gap on the order of 2 to 3 nm as a result of the dominating van der Waals attraction between dipolar bilayers. Monovalent ions can swell those neutral lamellae by a small amount. Divalent ions can adsorb onto dipolar membranes and charge them. Using solution X-ray scattering, we studied how the structure of ions and zwitterionic lipids regulates the charge of dipolar membranes. We found that unlike monovalent ions that weakly interact with all of the examined dipolar membranes, divalent and trivalent ions adsorb onto membranes containing lipids with saturated tails, with an association constant on the order of ∼10 M(-1). One double bond in the lipid tail is sufficient to prevent divalent ion adsorption. We suggest that this behavior is due to the relatively loose packing of lipids with unsaturated tails that increases the area per lipid headgroup, enabling their free rotation. Divalent ion adsorption links two lipids and limits their free rotation. The ion-dipole interaction gained by the adsorption of the ions onto unsaturated membranes is insufficient to compensate for the loss of headgroup free-rotational entropy. The ion-dipole interaction is stronger for cations with a higher valence. Nevertheless, polyamines behave as monovalent ions near dipolar interfaces in the sense that they interact weakly with the membrane surface, whereas in the bulk their behavior is similar to that of multivalent cations. Advanced data analysis and comparison with theory provide insight into the structure and interactions between ion-induced regulated charged interfaces. This study models biologically relevant interactions between cell membranes and various ions and the manner in which the lipid structure governs those interactions. The ability to monitor these interactions creates a tool for probing systems that are more complex and forms the basis for controlling the interactions between dipolar membranes and charged proteins or biopolymers for encapsulation and delivery applications.

The water adsorption characteristics of charged phospholipids

The hydration isotherms of the negatively charged phospholipids, egg phosphatidic acid, bovine heart cardiolipin and two phosphatidylserines as well as one positively charged phospholipid, 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine, have been obtained gravimetrically. The presence of an electrical charge on these phospholipids does not, in itself, determine whether the water binding to the phospholipids is 'strong' or 'weak'. Interestingly, hysteresis effects were present for certain charged phospholipids suggesting some rearrangement of the lipid molecular organization upon hydration and dehydration, perhaps due to the presence of the ionizable moiety. The hydration isotherms of the charged phospholipids have been analyzed by BET theory, although, of the charged lipids studied, all may not be amenable to the application of BET theory. The hydration isotherms and the resulting parameters obtained from the BET analysis are compared to those found previously for zwitterionic phospholipids, especially egg phosphatidylcholine. The water adsorption characteristics of phospholipids are found to depend mainly on the total head-group structure including the presence of hydrophobic groups as well as electrical charge on the head group.