Microscopic Analysis of the Water/Glycerol/EO30PS System in Bulk and on a Solid Substrate.

Surfactant systems are often employed in cosmetic formulations where they dry on skin as a surface, thereby becoming increasingly concentrated systems. To better understand this drying process, we focused on the difference of self-assembled structures of the water/glycerol/polyoxyethylene (30) phytosteryl ether (EO30PS) system in bulk and on a solid substrate because the interaction between the substrate and the surfactant may have a substantial effect on the self-assembly, which may be related to the bulk structure but in detail may also differ strongly from the bulk situation. In bulk, small-angle neutron scattering (SANS) experiments showed that with increasing loss of water, the degree of ordering increases but changes of the aggregate structure are rather small. The results indicate that ellipsoidal micelles of EO30PS are densely packed and simply become more ordered in bulk during the drying process. On the other hand, neutron reflectometry revealed that EO30PS molecules adsorb onto a Si surface in the form of bilayers and analysis indicates that at a high concentration (c = 20 wt %), there are on average two bilayers (a double bilayer) on the Si substrate. The adsorbed membrane structure of EO30PS is rather thin with respect to its hydrophobic part, indicating tilted molecules, containing only some solvent, and being not highly ordered. These experimental results then allow for a much deeper understanding of the structural properties of practical formulations as they are applied, for instance, in cosmetic lotions.

Location of Polyelectrolytes in Swollen Lipid Oligobilayers.

Osteoarthritis is caused by degeneration of the cartilage, which covers the bone ends of the joints and is decorated with an oligolamellar phospholipid (PL) bilayer. The gap between the bone ends is filled with synovial fluid mainly containing hyaluronic acid (HA). HA and PLs are supposed to reduce friction and protect the cartilage from wear in joint movement. However, a detailed understanding of the molecular mechanisms of joint lubrication is still missing. Previously, we found that aqueous solutions of HA and poly(allylamine hydrochloride) (PAH), the latter serving as a polymeric analogue to HA, adsorb onto the headgroups of surface-bound 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) oligobilayers and significantly enhance their stability with respect to shear forces, typically occurring in joint movement. We now investigated the precise location of PAH chains across the lipid films in neutron reflectivity measurements, as bridging of the oligobilayers by polyelectrolytes (PEs) might be the cause for their improved mechanical stability. In a first set of experiments, we used hydrogenated PAH and chain-deuterated DMPC (DMPC-d54) to improve the contrast between the lipids and potentially intruding PAH. However, due to difficulties in distinguishing between incorporation of water and PAH, penetration into the lipid chain region could hardly be proven quantitatively. Therefore, we designed a more elaborate experiment based on mixed films of DMPC-d54 and hydrogenated DMPC, which is insensitive to water penetration into the films. Beside facilitating a detailed structural characterization of the oligolamellar system, this elaborate approach showed that PAH adsorbs to the DMPC heads and penetrates the lipid tail strata. No PAH was found in the lipid head strata, which excludes bridging of several lipid bilayers by the PE chains. The data are consistent with the assumption that PAH bridges are formed between the headgroups of two adjacent bilayers and contribute to the enhanced mechanical stability.

Drastic Swelling of Lipid Oligobilayers by Polyelectrolytes: A Potential Molecular Model for the Internal Structure of Lubricating Films in Mammalian Joints.

Osteoarthritis is the most common arthropathy in western civilization. It is primarily caused by the degeneration of lipid-coated cartilage, leading to increased friction in joints. Hyaluronic acid (HA), a negatively charged polysaccharide and the main component of the synovial fluid, is held responsible for joint lubrication. It is believed that HA, adsorbed to the lipid-coated cartilage, forms a protective layer against wear. Studies have shown that the concentration and molecular weight (MW) of HA are reduced in joints suffering from osteoarthritis. On the basis of these observations, local joint injections of HA or mixtures of HA and surface-active phospholipids (SAPLs) have been applied as medical cures to restore the functionality of the joints in a procedure called viscosupplementation. However, this cure is still disputed, and no consensus has been reached with respect to optimum HA concentration and MW. To provide detailed insight in the structural rearrangement of lipid films upon contact with HA or polymeric analogues, we studied the interaction of the polyelectrolyte poly(allylamine hydrochloride) (PAH) with surface-bound oligobilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by neutron reflectivity (NR) and ellipsometry. Using this model system, we found a drastic swelling of the lipid films as a function of PAH concentration, whose strength compares to that in previous studies on HA incubation. In contrast, no significant dependence of film thickness on PAH MW was observed. A detailed picture of the film architecture was developed which inter alia shows that charged PAH is adsorbed to the lipid headgroups, leading to electrostatic repulsion. The swelling behavior is well explained by the equilibrium of Coulomb and van der Waals interactions in a DLVO-based model. Our detailed structural analysis of the PAH/lipid interfacial layer may help to elucidate the mechanisms of viscosupplementation and derive a structure-function relationship for the lubricating interface in mammalian joints.

Preparation of a New Oligolamellar Stratum Corneum Lipid Model.

In this study, we present a preparation method for a new stratum corneum (SC) model system, which is closer to natural SC than the commonly used multilayer models. The complex setup of the native SC lipid matrix was mimicked by a ternary lipid mixture of ceramide [AP], cholesterol, and stearic acid. A spin coating procedure was applied to realize oligo-layered samples. The influence of lipid concentration, rotation speed, polyethylenimine, methanol content, cholesterol fraction, and annealing on the molecular arrangement of the new SC model was investigated by X-ray reflectivity measurements. The new oligo-SC model is closer to native SC in the total number of lipid membranes found between corneocytes. The reduction in thickness provides the opportunity to study the effects of drugs and/or hydrophilic penetration enhancers on the structure of SC in full detail by X-ray or neutron reflectivity. In addition, the oligo-lamellar systems allows one to infer not only the lamellar spacing, but also the total thickness of the oligo-SC model and changes thereof can be monitored. This improvement is most helpful for the understanding of transdermal drug administration on the nanoscale. The results are compared to the commonly used multilamellar lipid model systems and advantages and disadvantages of both models are discussed.

Alzheimer's peptide amyloid-ß, fragment 22-40, perturbs lipid dynamics.

The peptide amyloid-ß (Aß) interacts with membranes of cells in the human brain and is associated with Alzheimer's disease (AD). The intercalation of Aß in membranes alters membrane properties, including the structure and lipid dynamics. Any change in the membrane lipid dynamics will affect essential membrane processes, such as energy conversion, signal transduction and amyloid precursor protein (APP) processing, and may result in the observed neurotoxicity associated with the disease. The influence of this peptide on membrane dynamics was studied with quasi-elastic neutron scattering, a technique which allows a wide range of observation times from picoseconds to nanoseconds, over nanometer length scales. The effect of the membrane integral neurotoxic peptide amyloid-ß, residues 22-40, on the in- and out-of-plane lipid dynamics was observed in an oriented DMPC/DMPS bilayer at 15 °C, in its gel phase, and at 30 °C, near the phase transition temperature of the lipids. Near the phase-transition temperature, a 1.5 mol% of peptide causes up to a twofold decrease in the lipid diffusion coefficients. In the gel-phase, this effect is reversed, with amyloid-ß(22-40) increasing the lipid diffusion coefficients. The observed changes in lipid diffusion are relevant to protein-protein interactions, which are strongly influenced by the diffusion of membrane components. The effect of the amyloid-ß peptide fragment on the diffusion of membrane lipids will provide insight into the membrane's role in AD.

Direct comparison of elastic incoherent neutron scattering experiments with molecular dynamics simulations of DMPC phase transitions.

Neutron scattering techniques have been employed to investigate 1,2-dimyristoyl-sn -glycero-3-phosphocholine (DMPC) membranes in the form of multilamellar vesicles (MLVs) and deposited, stacked multilamellar-bilayers (MLBs), covering transitions from the gel to the liquid phase. Neutron diffraction was used to characterise the samples in terms of transition temperatures, whereas elastic incoherent neutron scattering (EINS) demonstrates that the dynamics on the sub-macromolecular length-scale and pico- to nano-second time-scale are correlated with the structural transitions through a discontinuity in the observed elastic intensities and the derived mean square displacements. Molecular dynamics simulations have been performed in parallel focussing on the length-, time- and temperature-scales of the neutron experiments. They correctly reproduce the structural features of the main gel-liquid phase transition. Particular emphasis is placed on the dynamical amplitudes derived from experiment and simulations. Two methods are used to analyse the experimental data and mean square displacements. They agree within a factor of 2 irrespective of the probed time-scale, i.e. the instrument utilized. Mean square displacements computed from simulations show a comparable level of agreement with the experimental values, albeit, the best match with the two methods varies for the two instruments. Consequently, experiments and simulations together give a consistent picture of the structural and dynamical aspects of the main lipid transition and provide a basis for future, theoretical modelling of dynamics and phase behaviour in membranes. The need for more detailed analytical models is pointed out by the remaining variation of the dynamical amplitudes derived in two different ways from experiments on the one hand and simulations on the other.

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.

Surface-Active Lipid Linings under Shear Load--A Combined in-Situ Neutron Reflectivity and ATR-FTIR Study.

We study shear effects in solid-supported lipid membrane stacks by simultaneous combined in-situ neutron reflectivity (NR) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The stacks mimic the terminal surface-active phospholipid (SAPL) coatings on cartilage in mammalian joints. Piles of 11 bilayer membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are immobilized at the interface of the solid silicon support and the liquid D2O backing phase. We replace the natural hyaluronic acid (HA) component of synovial fluid by a synthetic substitute, namely, poly(allylamine hydrochloride) (PAH), at identical concentration. We find the oligolamellar DMPC bilayer films strongly interacting with PAH resulting in a drastic increase of the membranes d spacing (by a factor of ∼5). Onset of shear causes a buckling-like deformation of the DMPC bilayers perpendicular to the applied shear field. With increasing shear rate we observe substantially enhanced water fractions in the membrane slabs which we attribute to increasing fragmentation caused by Kelvin-Helmholtz-like instabilities parallel to the applied shear field. Both effects are in line with recent theoretical predictions on shear-induced instabilities of lipid bilayer membranes in water (Hanasaki, I.; Walther, J. H.; Kawano, S.; Koumoutsakos, P. Phys. Rev. E 2010, 82, 051602). With the applied shear the interfacial lipid linings transform from their gel state Pß' to their fluid state Lα. Although in chain-molten state with reduced bending rigidity the lipid layers do not detach from their solid support. We hold steric bridging of the fragmented lipid bilayer membranes by PAH molecules responsible for the unexpected mechanical stability of the DMPC linings.

Temperature-dependent vibrational and conformational dynamics of photosystem II membrane fragments from spinach investigated by elastic and inelastic neutron scattering.

Vibrational and conformational protein dynamics of photosystem II (PS II) membrane fragments from spinach were investigated by elastic and inelastic incoherent neutron scattering (EINS and IINS). As to the EINS experiments, the average atomic mean square displacement values of PS II membrane fragments hydrated at a relative humidity of 57% exhibit a dynamical transition at ~230K. In contrast, the dynamical transition was absent at a relative humidity of 44%. These findings are in agreement with previous studies which reported a "freezing" of protein mobility due to dehydration (Pieper et al. (2008) Eur. Biophys. J. 37: 657-663) and its correlation with an inhibition of electron transfer from Q(A)(-) to Q(B) (Kaminskaya et al. (2003) Biochemistry 42, 8119-8132). IINS spectra of a sample hydrated at a relative humidity of 57% show a distinct Boson peak at ~7.5meV at 20K, which shifts towards lower energy values upon temperature increase to 250K. This unexpected effect is interpreted in terms of a "softening" of the protein matrix along with the onset of conformational protein dynamics as revealed by the EINS experiments. Information on the density of vibrational states of pigment-protein complexes is important for a realistic calculation of excitation energy transfer kinetics and spectral lineshapes and is often routinely obtained by optical line-narrowing spectroscopy at liquid helium temperature. The data presented here demonstrate that IINS is a valuable experimental tool in determining the density of vibrational states not only at cryogenic, but also at nearly physiological temperatures up to 250K. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

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.

Dynamic properties of photosystem II membranes at physiological temperatures characterized by elastic incoherent neutron scattering. Increased flexibility associated with the inactivation of the oxygen evolving complex.

Elastic incoherent neutron scattering (EINS), a non-invasive technique which is capable of measuring the mean square displacement of atoms in the sample, has been widely used in biology for exploring the dynamics of proteins and lipid membranes but studies on photosynthetic systems are scarce. In this study we investigated the dynamic characteristics of Photosystem II (PSII) membrane fragments between 280 and 340 K, i.e., in the physiological temperature range and in the range of thermal denaturation of some of the protein complexes. The mean square displacement values revealed the presence of a hydration-sensitive transition in the sample between 310 and 320 K, suggesting that the oxygen evolving complex (OEC) plays an important role in the transition. Indeed, in samples in which the OEC had been removed by TRIS- or heat-treatments (323 and 333 K) no such transition was found. Further support on the main role of OEC in these reorganizations is provided by data obtained from differential scanning calorimetry experiments, showing marked differences between the untreated and TRIS-treated samples. In contrast, circular dichroism spectra exhibited only minor changes in the excitonic interactions below 323 K, showing that the molecular organization of the pigment-protein complexes remains essentially unaffected. Our data, along with earlier incoherent neutron scattering data on PSII membranes at cryogenic temperatures (Pieper et al., Biochemistry 46:11398-11409, 2007), demonstrate that this technique can be applied to characterize the dynamic features of PSII membranes, and can be used to investigate photosynthetic membranes under physiologically relevant experimental conditions.

Activity and molecular dynamics relationship within the family of human cholinesterases.

The temperature dependence of the dynamics of recombinant human acetylcholinesterase (hAChE) and plasma human butyrylcholinesterase (hBChE) is examined using elastic incoherent neutron scattering. These two enzymes belong to the same family and present 50% amino acid sequence identity. However, significantly higher flexibility and catalytic activity of hAChE when compared to the ones of hBChE are measured. At the same time, the average height of the potential barrier to the motions is increased in the hBChE, e.g. more thermal energy is needed to cross it in the latter case, which might be the origin of the increase in activation energy and the reduction in the catalytic rate of hBChE observed experimentally. These results suggest that the motions on the picosecond timescale may act as a lubricant for those associated with activity occurring on a slower millisecond timescale.

Carbohydrate-carbohydrate interaction drives the preferential insertion of dirhamnolipid into glycosphingolipid enriched membranes.

Rhamnolipids (RLs) are among the most important biosurfactants produced by microorganisms, and have been widely investigated because of their multiple biological activities. Their action appears to depend on their structural interference with lipid membranes, therefore several studies have been performed to investigate this aspect. We studied by X-ray scattering, neutron reflectometry and molecular dynamic simulations the insertion of dirhamnolipid (diRL), the most abundant RL, in model cellular membranes made of phospholipids and glycosphingolipids. In our model systems the affinity of diRL to the membrane is highly promoted by the presence of the glycosphingolipids and molecular dynamics simulations unveil that this evidence is related to sugar-sugar attractive interactions at the membrane surface. Our results improve the understanding of the plethora of activities associated with RLs, also opening new perspectives in their selective use for pharmaceutical and cosmetics formulations. Additionally, they shed light on the still debated role of carbohydrate-carbohydrate interactions as driving force for molecular contacts at membrane surface.

Softness of atherogenic lipoproteins: a comparison of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) using elastic incoherent neutron scattering (EINS).

Apolipoprotein B100 (apoB100)-containing plasma lipoproteins (LDL and VLDL) supply tissues and cells with cholesterol and fat. During lipolytic conversion from VLDL to LDL the size and chemical composition of the particles change, but the apoB100 molecule remains bound to the lipids and regulates the receptor mediated uptake. The molecular physical parameters which control lipoprotein remodeling and enable particle stabilization by apoB100 are largely unknown. Here, we have compared the molecular dynamics and elasticities of VLDL and LDL derived by elastic neutron scattering temperature scans. We have determined thermal motions, dynamical transitions, and molecular fluctuations, which reflect the temperature-dependent motional coupling between lipid and protein. Our results revealed that lipoprotein particles are extremely soft and flexible. We found substantial differences in the molecular resiliences of lipoproteins, especially at higher temperatures. These discrepancies not only can be explained in terms of lipid composition and mobility but also suggest that apoB100 displays different dynamics dependent on the lipoprotein it is bound to. Hence, we suppose that the inherent conformational flexibility of apoB100 permits particle stabilization upon lipid exchange, whereas the dynamic coupling between protein and lipids might be a key determinant for lipoprotein conversion and atherogenicity.

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(ß) 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.

In Situ Studies of Solid Electrolyte Interphase (SEI) Formation on Crystalline Carbon Surfaces by Neutron Reflectometry and Atomic Force Microscopy.

The solid electrolyte interphase (SEI) is a complex and fragile passivation layer with crucial importance for the functionality of lithium-ion batteries. Due to its fragility and reactivity, the use of in situ techniques is preferable for the determination of the SEI's true structure and morphology during its formation. In this study, we use in situ neutron reflectometry (NR) and in situ atomic force microscopy (AFM) to investigate the SEI formation on a carbon surface. It was found that a lithium-rich adsorption layer is already present at the open circuit voltage on the carbon sample surface and that the first decomposition products start to deposit close to this potential. During the negative potential sweep, the growth of the SEI can be observed in detail by AFM and NR. This allows precise monitoring of the morphology evolution and the resulting heterogeneities of individual SEI features. NR measurements show a maximum SEI thickness of 192 Å at the lower cutoff potential (0.02 V vs Li/Li+), which slightly decreases during the positive potential scan. The scattering length density (SLD) obtained by NR provides additional information on the SEI's chemical nature and structural evolution.

Lithiation of Crystalline Silicon As Analyzed by Operando Neutron Reflectivity.

We present an operando neutron reflectometry study on the electrochemical incorporation of lithium into crystalline silicon for battery applications. Neutron reflectivity is measured from the ⟨100⟩ surface of a silicon single crystal which is used as a negative electrode in an electrochemical cell. The strong scattering contrast between Si and Li due to the negative scattering length of Li leads to a precise depth profile of Li within the Si anode as a function of time. The operando cell can be used to study the uptake and the release of Li over several cycles. Lithiation starts with the formation of a lithium enrichment zone during the first charge step. The uptake of Li can be divided into a highly lithiated zone at the surface (skin region) (x ∼ 2.5 in LixSi) and a much less lithiated zone deep into the crystal (growth region) (x ∼ 0.1 in LixSi). The total depth of penetration was less than 100 nm in all experiments. The thickness of the highly lithiated zone is the same for the first and second cycle, whereas the thickness of the less lithiated zone is larger for the second lithiation. A surface layer of lithium (x ∼ 1.1) remains in the silicon electrode after delithiation. Moreover, a solid electrolyte interface is formed and dissolved during the entire cycling. The operando analysis presented here demonstrates that neutron reflectivity allows the tracking of the kinetics of lithiation and delithiation of silicon with high spatial and temporal resolution.

Polymer-Induced Swelling of Solid-Supported Lipid Membranes.

In this paper, we study the interaction of charged polymers with solid-supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes by in-situ neutron reflectivity. We observe an enormous swelling of the oligolamellar lipid bilayer stacks after incubation in solutions of poly(allylamine hydrochloride) (PAH) in D2O. The positively charged polyelectrolyte molecules interact with the lipid bilayers and induce a drastic increase in their d-spacing by a factor of ~4. Temperature, time, and pH influence the swollen interfacial lipid linings. From our study, we conclude that electrostatic interactions introduced by the adsorbed PAH are the main cause for the drastic swelling of the lipid coatings. The DMPC membrane stacks do not detach from their solid support at T > Tm. Steric interactions, also introduced by the PAH molecules, are held responsible for the stabilizing effect. We believe that this novel system offers great potential for fundamental studies of biomembrane properties, keeping the membrane's natural fluidity and freedom, decoupled from a solid support at physiological conditions.

Effect of hydration of sugar groups on adsorption of Quillaja bark saponin at air/water and Si/water interfaces.

Adsorption of a natural glycoside surfactant Quillaja bark saponin ("QBS", Sigma Aldrich 84510) was studied at the air/water and Si/water interfaces using a combination of surface pressure (SP), surface dilatational rheology, neutron reflectivity (NR), Infra-Red Attenuated Total Reflection Spectroscopy (IR ATR) and Quartz Crystal Microbalance (QCM). The adsorbed layers formed at the air/water interface are predominantly elastic, with the dilatational surface storage modulus reaching the maximum value of E'=184 mN/m. The NR results point to a strong hydration of the adsorbed layers (about 65% hydration, corresponding to about 60 molecules of water per one QBS molecule), most likely related to the presence of multiple sugar groups constituting the glycone part of the QBS molecules. With a layer thickness of 19 Å, the adsorbed amount obtained from NR seems largely underestimated in comparison to the value obtained from the surface tension isotherm. While this high extent of hydration does not prevent formation of dense and highly elastic layers at the air-water surface, QBS adsorption at the Si/water interface is much weaker. The adsorption isotherm of QBS on Si obtained from the QCM study reflects much lower affinity of highly hydrated and negatively charged saponin molecules to the Si/water interface. We postulate that at the air/water interface, QBS adsorbs through the triterpene aglycone moiety. In contrast, weak hydrogen bonding between the glycone part and the surface silanol groups of Si is responsible for QBS adsorption on more polar Si/water interface.

Relation between dynamics, activity and thermal stability within the cholinesterase family.

Incoherent neutron scattering is one of the most powerful tools for studying dynamics in biological matter. Using the cold neutron backscattering spectrometer IN16 at the Institut Laue Langevin (ILL, Grenoble, France), temperature dependence of cholinesterases' dynamics (human butyrylcholinesterase from plasma: hBChE; recombinant human acetylcholinesterase: hAChE and recombinant mouse acetylcholinesterase: mAChE) was examined using elastic incoherent neutron scattering (EINS). The dynamics was characterized by the averaged atomic mean square displacement (MSD), associated with the sample flexibility at a given temperature. We found MSD values of hAChE above the dynamical transition temperature (around 200K) larger than for mAChE and hBChE, implying that hAChE is more flexible than the other ChEs. Activation energies for thermodynamical transition were extracted through the frequency window model (FWM) (Becker et al. 2004) [1] and turned out to increase from hBChE to mAChE and finally to hAChE, inversely to the MSDs relations. Between 280 and 316K, catalytic studies of these enzymes were carried out using thiocholine esters: at the same temperature, the hAChE activity was systematically higher than the mAChE or hBChE ones. Our results thus suggest a strong correlation between dynamics and activity within the ChE family. We also studied and compared the ChEs thermal inactivation kinetics. Here, no direct correlation with the dynamics was observed, thus suggesting that relations between enzyme dynamics and catalytic stability are more complex. Finally, the possible relation between flexibility and protein ability to grow in crystals is discussed.