Influence of ibuprofen on phospholipid membranes

A basic understanding of biological membranes is of paramount importance as these membranes comprise the very building blocks of life itself. Cells depend in their function on a range of properties of the membrane, which are important for the stability and function of the cell, information and nutrient transport, waste disposal, and finally the admission of drugs into the cell and also the deflection of bacteria and viruses. We have investigated the influence of ibuprofen on the structure and dynamics of L-$\alpha$-phosphatidylcholine (SoyPC) membranes by means of grazing incidence small-angle neutron scattering, neutron reflectometry, and grazing incidence neutron spin echo spectroscopy. From the results of these experiments, we were able to determine that ibuprofen induces a two-step structuring behavior in the SoyPC films, where the structure evolves from the purely lamellar phase for pure SoyPC over a superposition of two hexagonal phases to a purely hexagonal phase at high concentrations. A relaxation, which is visible when no ibuprofen is present in the membrane, vanishes upon addition of ibuprofen. This we attribute to a stiffening of the membrane. This behavior may be instrumental in explaining the toxic behavior of ibuprofen in long-term application.

Figure 1

Structures of (a) SoyPC [12] and (b) ibuprofen [13].

Figure 2

Sketch of the sample cell used to mount the silicon block. The two black stripes on the sample are cadmium spacers to keep the cover glass from touching the sample. Additional o-rings (not shown) ensure that the sample cell does not leak. Various drill holes (not shown) allow for attaching the sample cell to a water thermostat, as well as filling the space between sample and cover glass with ${\text{D}}_2\text{O}$.

Figure 3

Geometry of a GISANS experiment. In the case of reflectometry, only intensity for ${\alpha }_i={\alpha }_f$ is evaluated. The path of the neutrons is in green.

Figure 4

GISANS images at an incident angle of $0.2^{\circ }$. Concentrations of ibuprofen are (a) 0 mol %, (b) 13.6 mol %, (c) 25.0 mol %, (d) 34.5 mol %, (e) 43.1 mol %, (f) 50.2 mol %, and (g) 53.3 mol %. Scattering with a hexagonal symmetry is shown by yellow and red lines; colored labels show the angles between the respective lines. Indexed peaks are shown by red (hexagonal lattice with parallel axis to the surface), yellow (hexagonal lattice with perpendicular axis to the surface), and black(primitive tetragonal lattice) circles, where labels show the indexes.

Figure 5

Relative scattering contributions from lamellae parallel to the surface (red solid circles), disordered lamellae (open red circles), primitive tetragonal lattice (black open squares), hexagonal lattice with a parallel axis to the surface (open blue hexagons), and hexagonal lattice standing on an edge on the surface (blue solid hexagons). The vertical dashed lines mark the boundaries of the different regimes where lamellar scattering, scattering from two superimposed hexagonal lattices, and scattering from a hexagonal lattice with an axis parallel to the substrate dominate.

Figure 6

(a) Representative reflectometry data with fits. Data sets are shifted for better visibility. Concentrations of ibuprofen (bottom to top) are 0, 13.6, and 25.0 mol %. (b) Depiction of the layer model used by the Parratt algorithm. For better visibility, the hydrophobic parts of SoyPC are in red, and those of ibuprofen are in black.

Figure 7

(a) SLD profiles from fits with the Parratt algorithm to reflectometry data for all concentrations. Concentrations are 0 mol % (red), 13.6 mol % (green), 43.1 mol % (blue), and 53.3 mol % (black). Dashed lines indicate the SLD from Table 2 for decane, SoyPC, ibuprofen, and ${\text{D}}_2\text{O}$ (from bottom to top). (b) Results from modeling the reflectometry curves with the model shown in Fig. 6: Thickness of the ${\text{D}}_2\text{O}$ layer (red circles), thickness of the SoyPC layer (green upright triangles), roughness of the layers (blue squares), and thickness of the ibuprofen layer (black downright triangles).

Figure 8

Dynamic structure factors for 0 (red) and 34.5 mol % (black) of ibuprofen in a SoyPC layer. For the pure SoyPC, a distinct relaxation in the investigated time regime is visible, while for 34.5 mol % the sample does not show any relaxations.

Figure 9

Sketch of the structural evolution of the sample. (a) For low concentrations of ibuprofen, the system is dominated by a lamellar structure while the introduction of more ibuprofen (b) induces disordering of lamellar areas and thus powder scattering of lamellar areas. In an intermediate concentration area (c), there are two different hexagonal lattices that are stabilized against each other by a primitive tetragonal lattice. At very high concentrations (d) only the hexagonal structure with an axis parallel to the substrate is retained. Color coding of the hexagonal structure corresponds to the color coding used in Fig. 4. The lattice forming units in (c) and (d) could be similar to those found by Aeffner et al. [1], even though the process of formation is different.

Figure 10

Sketches of packing for (a) pure SoyPC layers and (b) SoyPC and ibuprofen. The ibuprofen with the small hydrophilic unit increases the curvature of the multilayer forming SoyPC.