Effects of lipid heterogeneity on model human brain lipid membranes.

Cell membranes naturally contain a heterogeneous lipid distribution. However, homogeneous bilayers are commonly preferred and utilised in computer simulations due to their relative simplicity, and the availability of lipid force field parameters. Recently, experimental lipidomics data for the human brain cell membranes under healthy and Alzheimer's disease (AD) conditions were investigated, since disruption to the lipid composition has been implicated in neurodegenerative disorders, including AD [R. B. Chan et al., J. Biol. Chem., 2012, 287, 2678-2688]. In order to observe the effects of lipid complexity on the various bilayer properties, molecular dynamics simulations were used to study four membranes with increasing heterogeneity: a pure POPC membrane, a POPC and cholesterol membrane in a 1 : 1 ratio (POPC-CHOL), and to our knowledge, the first realistic models of a healthy brain membrane and an Alzheimer's diseased brain membrane. Numerous structural, interfacial, and dynamical properties, including the area per lipid, interdigitation, dipole potential, and lateral diffusion of the two simple models, POPC and POPC-CHOL, were analysed and compared to those of the complex brain models consisting of 27 lipid components. As the membranes gain heterogeneity, a number of alterations were found in the structural and dynamical properties, and more significant differences were observed in the lateral diffusion. Additionally, we observed snorkeling behaviour of the lipid tails that may play a role in the permeation of small molecules across biological membranes. In this work, atomistic description of realistic brain membrane models is provided, which can add insight towards the permeability and transport pathways of small molecules across these membrane barriers.

On the microscopic origin of the cryoprotective effect in lysine solutions.

The amino acid lysine has been shown to prevent water crystallization at low temperatures in saturated aqueous solutions [S. Cerveny and J. Swenson, Phys. Chem. Chem. Phys., 2014, 16, 22382-22390]. Here, we investigate two ratios of water and lysine (5.4 water molecules per lysine (saturated) and 11 water molecules per lysine) by means of the complementary use of computer simulations and neutron diffraction. By performing a detailed structural analysis we have been able to explain the anti-freeze properties of lysine by the strong hydrogen bond interactions of interstitial water molecules with lysine that prevent them from forming crystalline seeds. Additional water molecules beyond the 1 : 5.4 proportion are no longer tightly bonded to lysine and therefore are free to form crystals.

On the hydration of DOPE in solution.

The atomic-scale hydration structure around the 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) headgroup in a chloroform/water solution has been investigated using neutron diffraction enhanced by isotopic substitution and NMR, coupled with empirical potential structure refinement and molecular dynamics simulations. The results obtained show the preferential binding sites for water molecules on the DOPE headgroups, with the most predominant interactions being with the ammonium and phosphate groups. Interestingly, the level of hydration, as well as the association of DOPE molecules, varies according to the simulation method used. The results here suggest the presence of a tight water network around these lipid headgroups that could affect the permeability of the membrane for lipid-mediated diffusion.

Role of Water in Sucrose, Lactose, and Sucralose Taste: The Sweeter, The Wetter?

Natural sugars combine energy supply and, except a few cases, a pleasant taste. On the other hand, exaggerated consumption may impact population health. This has busted the research for the synthesis of increasingly cheaper artificial sweeteners, with low energy content and intense taste. Here, we suggest that studies of the hydration properties of three disaccharides, namely, the natural sucrose and lactose and the artificial sucralose, may explain the difference by orders of magnitude among their sweetness. This is done by analyzing via Monte Carlo simulations the neutron diffraction differential cross sections of aqueous solutions of the three sugars and their isotopes. Our results show that the strength of the sugar-water hydrogen bond interaction is one of the factors influencing sweetness, another being the number of water molecules within the first neighboring shell of the sugar whether bonded or not.

Trehalose in Water Revisited.

Trehalose, commonly found in living organisms, is believed to help them survive severe environmental conditions, such as drought or extreme temperatures. With the aim of trying to understand these properties, two recent neutron scattering studies investigate the structure of trehalose water solutions but come to seemingly opposite conclusions. In the first study, which looks at two concentrations of trehalose-water mole ratios of 1:100 and 1:25, the conclusion is that trehalose hydrogen-bonds to water rather weakly and has a relatively minor impact on the structure of water in solution compared to bulk water. On the other hand, for the other, using a mole ratio of 1:38, the conclusion is that the water structure is rather substantially modified by the presence of trehalose and that the hydrogen bonding between water and trehalose hydroxyl groups is significant. In an attempt to try to understand the origin of these divergent views, which arise from similar but independent analyses of different neutron diffraction data, we have performed additional X-ray scattering experiments, which are highly sensitive to water structure, at the same trehalose-water concentrations used in the first study, and combined these with empirical potential structure refinement on the previously collected neutron data. The new analysis unequivocally confirms that trehalose does indeed have only a minor impact on the structure of water, at all three concentrations, and forms relatively weak hydrogen bonds with water. Far from being discrepant with the existing literature, our new analysis of the different datasets suggests a natural explanation for the increased glass-transition temperature of trehalose compared to other sugars and hence its enhanced effectiveness as a protectant against drought stress.

Proline and Water Stabilization of a Universal Two-Step Folding Mechanism for ß-Turn Formation in Solution.

The atomic scale process by which proteins fold into their functional forms in aqueous solutions is still not well understood. While there is clearly an interplay between the sequence of the protein and the surrounding water solvent that leads to highly specific and reproducible folding in nature, there is still an ongoing debate concerning how water molecules aid in driving the folding process. By using a combination of techniques that provide information at the atomic level-neutron and X-ray diffraction and computer simulations-the mechanism of folding in a series of peptides that only vary with respect to the central side-chain residue has been determined. Specifically, ß-turn formation for the KGXGK peptide (where X = P, G, S or L) occurs via a two-step water-driven attraction between specific sites on the peptide backbone. This proposed mechanism suggests that the site-specific hydration of the backbone facilitates the initial stages of protein folding and that this hydration interaction in combination with the presence of proline in the i + 1 position helps to stabilize the folded and intermediate folding state of the peptide in solution, leading to a greater propensity for PG containing sequences to occur in ß-turns in proteins.

On the solvation of the phosphocholine headgroup in an aqueous propylene glycol solution.

The atomic-scale structure of the phosphocholine (PC) headgroup in 30 mol. % propylene glycol (PG) in an aqueous solution has been investigated using a combination of neutron diffraction with isotopic substitution experiments and computer simulation techniques-molecular dynamics and empirical potential structure refinement. Here, the hydration of the PC headgroup remains largely intact compared with the hydration of this group in a bilayer and in a bulk water solution, with the PG molecules showing limited interactions with the headgroup. When direct PG interactions with PC do occur, they are most likely to coordinate to the N(CH3)3+ motifs. Further, PG does not affect the bulk water structure and the addition of PC does not perturb the PG-solvent interactions. This suggests that the reason why PG is able to penetrate into membranes easily is that it does not form strong-hydrogen bonding or electrostatic interactions with the headgroup allowing it to easily move across the membrane barrier.

On the structure of prilocaine in aqueous and amphiphilic solutions.

The solvation of prilocaine has been investigated in pure water and in an amphiphilic methanol/water solution using a combination of neutron diffraction with isotopic substitution augmented by Empirical Potential Structure Refinement (EPSR) simulations. This combination of techniques allows for details of the solvation structure on the atomic scale to be unravelled. The hydration of prilocaine is significantly altered relative to when this molecule is in pure water (as a hydrochloride salt) or in an amphiphilic environment (as a freebase compound). Interestingly, there is not a significant change in hydration around the amine group on prilocaine hydrochloride compared with prilocaine as a freebase. Despite this group being an ammonium group in water and an amine group in methanol/water solutions, the hydration of this motif remains largely intact. The changes in hydration between prilocaine as a free base and prilocaine·HCl instead appears to arise from a change in hydration around the aromatic ring and the amide group in the prilocaine molecule.

Salt Interactions in Solution Prevent Direct Association of Urea with a Peptide Backbone.

There is an ongoing debate as to how urea denatures proteins in solution. Using a combination of neutron scattering and computer simulation of a model peptide, KGPGK, it was found that the ionic strength and pH have a significant impact on the urea-peptide interaction. From the work presented here, it appears that urea first and foremost decreases the charge-based interactions in solution, such as the TFA-TFA association, before interacting with the peptide backbone via hydrogen bonds. This gives insight into the pH and salt concentration dependency of urea-caused protein denaturation and might unify direct and indirect theories of urea-induced protein denaturation. The observed differences between MD and neutron and X-ray diffraction data might show that MD, in this particular case, underestimates the influence of charged fluorinated solutes.

Structure-activity relationships in carbohydrates revealed by their hydration.

One of the more intriguing aspects of carbohydrate chemistry is that despite having very similar molecular structures, sugars have very different properties. For instance, there is a sensible difference in sweet taste between glucose and trehalose, even though trehalose is a disaccharide that comprised two glucose units, suggesting a different ability of these two carbohydrates to bind to sweet receptors. Here we have looked at the hydration of specific sites and at the three-dimensional configuration of water molecules around three carbohydrates (glucose, cellobiose, and trehalose), combining neutron diffraction data with computer modelling. Results indicate that identical chemical groups can have radically different hydration patterns depending on their location on a given molecule. These differences can be linked with the specific activity of glucose, cellobiose, and trehalose as a sweet substance, as building block of cellulose fiber, and as a bioprotective agent, respectively. This article is part of a Special Issue entitled "Recent Advances in Bionanomaterials" Guest Editors: Dr. Marie-Louise Saboungi and Dr. Samuel D. Bader.

On the structure of an aqueous propylene glycol solution.

Using a combination of neutron diffraction and empirical potential structure refinement computational modelling, the interactions in a 30 mol. % aqueous solution of propylene glycol (PG), which govern both the hydration and association of this molecule in solution, have been assessed. From this work it appears that PG is readily hydrated, where the most prevalent hydration interactions were found to be through both the PG hydroxyl groups but also alkyl groups typically considered hydrophobic. Hydration interactions of PG dominate the solution over PG self-self interactions and there is no evidence of more extensive association. This hydration behavior for PG in solutions suggests that the preference of PG to be hydrated rather than to be self-associated may translate into a preference for PG to bind to lipids rather than itself, providing a potential explanation for how PG is able to enhance the apparent solubility of drug molecules in vivo.

On the positional and orientational order of water and methanol around indole: a study on the microscopic origin of solubility.

Although they are both highly polar liquids, there are a number of compounds, such as many pharmaceuticals, which show vastly different solubilities in methanol compared with water. From theories of the hydrophobic effect, it might be predicted that this enhanced solubility is due to association between drugs and the less polar -CH3 groups on methanol. In this work, detailed analysis on the atomic structural interactions between water, methanol and the small molecule indole - which is a precursor to many drugs and is sparingly soluble in water yet highly soluble in methanol - reveal that indole preferentially interacts with both water and methanol via electrostatic interactions rather than with direction interactions to the -CH3 groups. The presence of methanol hydrogen bonds with π electrons of the benzene ring of indole can explain the increase in solubility of indole in methanol relative to water. In addition, the excess entropy calculations performed here suggest that this solvation is enthalpically rather than entropically driven.

The solvation structure of alprazolam.

Alprazolam is a benzodiazepine that is commonly prescribed for the treatment of anxiety and other related disorders. Like other benzodiazepines, it is thought to exert its effect through interaction with GABAA receptors. However, it has also been described as a potent and selective protein interaction inhibitor of bromodomain and extra-terminal (BET) proteins. Indeed, the only crystal structure of alprazolam bound to a protein is a complex between alprazolam and the BRD4 bromodomain. The structure shows that the complex also involves many water interactions that mediate contacts between the drug and the protein, a scenario that exists in many drug-protein complexes. How such waters relate to solvation patterns of small molecules may improve our understanding of what dictates their appearance or absence in bridging positions within complexes and thus will be important in terms of future rational drug-design. Here, we use neutron diffraction in conjunction with molecular dynamics simulations to provide a detailed analysis of how water molecules interact with alprazolam in methanol/water mixtures. The agreement between the neutron diffraction and the molecular dynamics is extremely good. We discuss the results in the context of drug design.

Comparative atomic-scale hydration of the ceramide and phosphocholine headgroup in solution and bilayer environments.

Previous studies have used neutron diffraction to elucidate the hydration of the ceramide and the phosphatidylcholine headgroup in solution. These solution studies provide bond-length resolution information on the system, but are limited to liquid samples. The work presented here investigates how the hydration of ceramide and phosphatidylcholine headgroups in a solution compares with that found in a lipid bilayer. This work shows that the hydration patterns seen in the solution samples provide valuable insight into the preferential location of hydrating water molecules in the bilayer. There are certain subtle differences in the distribution, which result from a combination of the lipid conformation and the lipid-lipid interactions within the bilayer environment. The lipid-lipid interactions in the bilayer will be dependent on the composition of the bilayer, whereas the restricted exploration of conformational space is likely to be applicable in all membrane environments. The generalized description of hydration gathered from the neutron diffraction studies thus provides good initial estimation for the hydration pattern, but this can be further refined for specific systems.

Atomic scale insights into urea-peptide interactions in solution.

The mechanism by which proteins are denatured by urea is still not well understood, especially on the atomic scale where these interactions occur in vivo. In this study, the structure of the peptide GPG has been investigated in aqueous urea solutions in order to understand the combination of roles that both urea and water play in protein unfolding. Using a combination of neutron diffraction enhanced by isotopic substitution and computer simulations, it was found, in opposition with previous simulations studies, that urea is preferred over water around polar and charged portions of the peptides. Further, it appears that while urea directly replaces water around the nitrogen groups on GPG that urea and water occupy different positions around the peptide bond carbonyl groups. This suggests that urea may in fact weaken the peptide bond, disrupting the peptide backbone, thus ultimately causing denaturation.

On the atomic structure of cocaine in solution.

Cocaine is an amphiphilic drug which has the ability to cross the blood-brain barrier (BBB). Here, a combination of neutron diffraction and computation has been used to investigate the atomic scale structure of cocaine in aqueous solutions. Both the observed conformation and hydration of cocaine appear to contribute to its ability to cross hydrophobic layers afforded by the BBB, as the average conformation yields a structure which might allow cocaine to shield its hydrophilic regions from a lipophilic environment. Specifically, the carbonyl oxygens and amine group on cocaine, on average, form ∼5 bonds with the water molecules in the surrounding solvent, and the top 30% of water molecules within 4 Šof cocaine are localized in the cavity formed by an internal hydrogen bond within the cocaine molecule. This water mediated internal hydrogen bonding suggests a mechanism of interaction between cocaine and the BBB that negates the need for deprotonation prior to interaction with the lipophilic portions of this barrier. This finding also has important implications for understanding how neurologically active molecules are able to interact with both the blood stream and BBB and emphasizes the use of structural measurements in solution in order to understand important biological function.

Amphipathic solvation of indole: implications for the role of tryptophan in membrane proteins.

The microscopic structure of the tryptophan side chain, indole, in an amphiphilic environment has been investigated using a combination of neutron diffraction measurements and simulations in solution. The results show that indole is preferentially solvated by hydrogen bonding interactions between water and alcohol -OH groups rather than the interaction being dominated by indole-methyl interactions. This has implications for understanding how tryptophan interacts with the amphipathic membrane environment to anchor proteins into membranes, where the results here suggest that the benzene ring of tryptophan interacts directly with the interfacial water at the membrane surface rather than being buried into the hydrophobic regions of the membrane bilayer.

Conformation and interactions of dopamine hydrochloride in solution.

The aqueous solution of dopamine hydrochloride has been investigated using neutron and X-ray total scattering data together with Monte-Carlo based modelling using Empirical Potential Structure Refinement. The conformation of the protonated dopamine molecule is presented and the results compared to the conformations found in crystal structures, dopamine-complexed protein crystal structures and predicted from theoretical calculations and pharmacophoric models. It is found that protonated dopamine adopts a range of conformations in solution, highlighting the low rotational energy barrier between different conformations, with the preferred conformation being trans-perpendicular. The interactions between each of the species present (protonated dopamine molecules, water molecules, and chloride anions) have been determined and are discussed with reference to interactions observed in similar systems both in the liquid and crystalline state, and predicted from theoretical calculations. The expected strong hydrogen bonds between the strong hydrogen bond donors and acceptors are observed, together with evidence of weaker CH hydrogen bonds and π interactions also playing a significant role in determining the arrangement of adjacent molecules.

Solvation and hydration of the ceramide headgroup in a non-polar solution.

The microscopic hydration of the ceramide headgroup has been determined using a combination of experimental-both NMR and neutron diffraction techniques and computational techniques-empirical potential structure refinement (EPSR) and molecular dynamics (MD). The addition of water to ceramide in chloroform solutions disrupts the chloroform solvation of the ceramide headgroup, and the water forms distinct pockets of density. Specifically, water is observed to preferentially hydrate the two hydroxyl groups and the carbonyl oxygen over the amide NH motif. Further assessment of the location and orientation of the water molecules bound to the ceramide headgroup makes it clear that the strongly solvated carbonyl moiety of the amide bond creates an anchor from which water molecules can bridge via hydrogen bonding interactions to the hydroxyl groups. Moreover, a significant difference in the hydration of the two hydroxyl groups indicates that water molecules are associated with the headgroup in such a way that they bridge between the carbonyl motif and the nearest neighbor hydroxyl group.