Thomas Gutberlet

Layer-by-layer deposition of polyelectrolytes. Dipping versus spraying.

We studied the properties of polyelectrolyte multilayer films prepared using the technique of polyelectrolyte deposition from solution (dipping) or supplying the solutions to the surface by spraying. The quality of films obtained by those two techniques was compared to find out whether the well-established dipping procedure can be replaced with the spraying technique. Neutron and X-ray reflectometric studies were performed on the samples of interest. We found that multilayers prepared by dipping are thicker, denser and less rough than films having the same number of layers, i.e., having the same number of deposition cycles, obtained by spraying.

Investigation of Phospholipid Area Compression Induced by Calcium-Mediated Dextran Sulfate Interaction

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca(2+) bridges results in a perturbation of lipid packing at physiologically relevant Ca(2+) concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by (2)H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca(2+) results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca(2+) concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of DeltaA = 2.7 A(2) per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca(2+)/DS-complexes. Lipid area compression after addition of DS and Ca(2+) to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca(2+) bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca(2+) concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Hydration dependent studies of highly aligned multilayer lipid membranes by neutron scattering

We investigated molecular motions on a picosecond timescale of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes as a function of hydration by using elastic and quasielastic neutron scattering. Two different hydrations corresponding to approximately nine and twelve water molecules per lipid were studied, the latter being the fully hydrated state. In our study, we focused on head group motions by using chain deuterated lipids. Information on in-plane and out-of-plane motions could be extracted by using solid supported DMPC multilayers. Our studies confirm and complete former investigations by König et al. [J. Phys. II (France) 2, 1589 (1992)] and Rheinstädter et al. [Phys. Rev. Lett. 101, 248106 (2008)] who described the dynamics of lipid membranes, but did not explore the influence of hydration on the head group dynamics as presented here. From the elastic data, a clear shift of the main phase transition from the P(β) ripple phase to the L(α) liquid phase was observed. Decreasing water content moves the transition temperature to higher temperatures. The quasielastic data permit a closer investigation of the different types of head group motion of the two samples. Two different models are needed to fit the elastic incoherent structure factor and corresponding radii were calculated. The presented data show the strong influence hydration has on the head group mobility of DMPC.

Hydration of the dienic lipid dioctadecadienoylphosphatidylcholine in the lamellar phase--an infrared linear dichroism and x-ray study on headgroup orientation, water ordering, and bilayer dimensions.

In the phospholipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inserted in a position near the polar/apolar boundary that is exceptionally sensitive for membrane stability. DODPC transforms upon gradual dehydration from the liquid-crystalline to a metastable gel state, which rearranges into two subgel phases at low and intermediate degrees of hydration. The molecular dimensions of the respective bilayers were determined by means of x-ray diffraction. Infrared linear dichroism of selected vibrations of the phosphate and trimethylammonium groups and of the nu13(OH) band of water adsorbed onto the lipid was used to study the molecular order in the polar part of the bilayers in macroscopically oriented samples. The dense packing of the tilted acyl chains in the subgel causes the in-plane orientation of the phosphatidylcholine headgroups with direct interactions between the phosphate and trimethylammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bilayer increases and the lateral interactions between the lipid headgroups weaken. The higher order in the headgroup region of the subgels correlates with shorter decay lengths of the repulsive forces acting between opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the work to break up the lipid-water interactions without compensation by adequate lipid-lipid contacts. Almost similar area compressibility moduli are found in the liquid-crystalline and solid phases. Obviously, the lipid avoids lateral stress by the structural rearrangement.

Interaction of the MARCKS peptide with PIP2 in phospholipid monolayers

In this present work we have studied the effect of MARCKS (151-175) peptide on a mixed DPPC/PIP2 monolayer. By means of film balance, fluorescence microscopy, x-ray reflection/diffraction and neutron reflection measurements we detected changes in the lateral organization of the monolayer and changes in the perpendicular orientation of the PIP2 molecules depending on the presence of MARCKS (151-175) peptide in the subphase. In the mixed monolayer, the PIP2 molecules are distributed uniformly in the disordered phase of the monolayer, whereas the PI(4,5) groups elongate up to 10 A below the phosphodiester groups. This elongation forms the precondition for the electrostatic interaction of the MARCKS peptide with the PIP2 molecules. Due to the enrichment of PIP2 in the disordered phase, the interaction with the peptide occurs primarily in this phase, causing the PI(4,5) groups to tilt toward the monolayer interface.

Phospholipid bilayer formation at a bare Si surface: a time-resolved neutron reflectivity study

Neutron reflectivity was applied to monitor in situ the adsorption of small unilamellar phospholipid vesicles on a solid bare hydrophilic Si interface. The obtained reflectivity curves are consistent with the rupture and fusion model for the adsorption of phosphatidylcholine vesicles to solid interfaces. The results show details of the adsorbed bilayer system at angstrom resolution and indicate the presence of a thin thick water leaflet that separates the bilayer from the Si surface. The resolved structural details provide the basis for further investigation of processes such as adsorption and penetration of peptides and proteins towards the supported bilayer at high resolution.

Unusual penetration of phospholipid mono- and bilayers by Quillaja bark saponin biosurfactant.

The interactions between a model phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a biosurfactant Quillaja Bark Saponin (QBS) obtained from the bark of Quillaja saponaria Molina were studied using simple models of biological membranes. QBS is known to interact strongly with the latter, exerting a number of haemolytic, cytotoxic and anti-microbial actions. The interaction of QBS dissolved in the subphase with DPPC monolayers and silicon-supported bilayers was studied above the cmc (10(-3)M). Surface pressure relaxation and surface dilatational rheology combined with quartz crystal microbalance (QCM) and neutron reflectivity (NR) were employed for this purpose. The DPPC-penetrating abilities of QBS are compared with those of typical synthetic surfactants (SDS, CTAB and Triton X-100). We show that the penetration studies using high surface activity (bio)surfactants should be performed by a subphase exchange, not by spreading onto the surfactant solution. In contrast to the synthetic surfactants of similar surface activity, QBS does not collapse DPPC mono- and bilayers, but penetrates them, improving their surface dilatational elastic properties even in the highly compressed solid state. The dilatational viscoelasticity modulus increases from 204 mN/m for pure DPPC up to 310 mN/m for the QBS-penetrated layers, while it drops to near zero values in the case of the synthetic surfactants. The estimated maximum insertion pressure of QBS into DPPC monolayers exceeds the maximum surface pressure achievable in our setup, in agreement with the surface rheological response of the penetrated layers.

Ordering of Fe3O4 Nanoparticles in Polyelectrolyte Multilayer Films

In our work we have focused on the incorporation of magnetite nanoparticles (NPs) into poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) polyelectrolyte multilayers (PEMs). The main goal of presented studies was to control the two-dimentional ordering of NPs within polyelectrolyte films. The ordering of NPs depended on the treatment of the underlying polyelectrolyte films. The NPs were uniformly distributed in freshly prepared samples leading to an interfacial mixture of polyelectrolytes and particles, while a highly concentrated layer of NP was formed only when the PEMs were exposed to elevated temperature after their preparation. The observed effect was correlated to glass-melt phase transitions of the PEMs. Such ordering of functionalized species in a polymer matrix may enhance the response from the studied nanocomposites.

Control of Protein Interfacial Affinity by Nonionic Cosolvents

In a biological cell, proteins perform their functions in a highly complex environment comprising crowding and confinement effects as well as interactions with interfaces, cosolvents, and other biomolecules. Cosolvents can stabilize or destabilize the native folded structure of proteins in solution. In this study, we show that nonionic cosolvents also affect the interfacial affinity of proteins. We use bovine ribonuclease A and a planar silica-water interface as model system and apply neutron and optical reflectometry to analyze this system. The degree of protein adsorption and the density profile of adsorbed protein molecules were determined in the absence and the presence of cosolvents. It has been found that both the protein stabilizing glycerol and the protein destabilizing urea cause a distinct reduction in protein interfacial affinity, which may represent a rather unexpected result. However, it is suggested that different mechanisms are underlying the similar effects of glycerol and urea.

High hydrostatic pressure effects investigated by neutron scattering on lipid multilamellar vesicles

The effects of high hydrostatic pressure on the structure and dynamics of model membrane systems were investigated using neutron scattering. Diffraction experiments show shifts of the pre- and main-phase transitions of multilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) to higher temperatures with increased pressure which are close to results observed previously by other techniques, namely (10.4 ± 1.0) K kbar(-1) and (20.0 ± 0.5) K kbar(-1) for the two transitions. Backscattering spectroscopy reveals that the mean square displacements in the liquid phase are about 10% smaller at 300 bar and about 20% smaller at 600 bar compared to atmospheric pressure, whereas in the gel phase below the main phase transition the mean square displacements show a smaller difference in the dynamics of the three pressure values within the studied pressure range.