Dioctadecyldimethylammonium bromide, a surfactant model for the cell membrane: Importance of microscopic dynamics

Enhancing the Antimicrobial Properties of Experimental Resin-Based Dental Composites through the Addition of Quaternary Ammonium Salts

Secondary caries is one of the main reasons for dental filling replacement. There is a need to obtain dental restorative material that is able to act against caries-inducing microorganisms. This study explores the antimicrobial properties of cetyltrimethylammonium bromide (CTAB) or dimethyldioctadecylammonium bromide (DODAB)-modified photo-cured experimental dental composites against Escherichia coli, Streptococcus mutans, and Candida albicans. The antimicrobial activity against Escherichia coli, Streptococcus mutans, and Candida albicans was assessed by using an Accuri C6 flow cytofluorimeter, and then analyzed using BD CSampler software (1.0.264). Bacterial/yeast surface colonization was carried out by using an GX71 inverted-optics fluorescence microscope equipped with a DP 73 digital camera. For bactericidal surface analysis of each sample type, simultaneous standardization was performed using a positive control (live cells) and a negative control (dead cells). A positive correlation between the increasing concentration of CTAB or DODAB and the dead cell ratio of Escherichia coli, Streptococcus mutans, and Candida albicans was revealed. In particular, CTAB at a 2.0 wt% concentration exhibits superior efficiency against pathogens (65.0% dead cells of Escherichia coli, 73.9% dead cells of Streptococcus mutans, and 23.9% dead cells of Candida albicans after 60 min). However, Candida albicans is more resistant to used salts than bacteria. A CTAB- or DODAB-modified experimental dental composite exhibits antimicrobial potential against Escherichia coli, Streptococcus mutans, and Candida albicans after 10 and 60 min of incubation, and the antimicrobial efficiency increases with the wt% of QAS in the tested material.

Anion Competition at Positively Charged Surfactant Monolayers

Interactions of anions with hydrophobic surfaces of proteins and water-soluble polymers depend on the ability of the ions to shed their hydration shells. At positively charged surfactant monolayers, the interactions of anions are less well understood. Due to the interplay of electrostatic surface forces, hydration effects, and ion-ion interactions in the electrostatic double layer, a comprehensive microscopic picture remains elusive. Herein, we study the interactions of chloride, bromide, and a mixture of these two anions at the aqueous interface of dihexadecyldimethylammonium (DHDA+) and dioctadecyldimethylammonium (DODA+) cationic monolayers. Using molecular dynamics simulations and three surface-sensitive X-ray scattering techniques, we demonstrate that bromide interacts preferentially over chloride with both monolayers. The structure of the two monolayers and their interfacial electron density profiles obtained from the simulations quantitatively reproduce the experimental data. We observe that chloride and bromide form contact ion pairs with the quaternary ammonium groups on both monolayers. However, ion pairing with bromide leads to a greater reduction in the number of water molecules hydrating the anion, resulting in more energetically stable ion pairs. This leads to long-range (>3 nm) lateral correlations between bromide ions on the structured DODA+ monolayer. These observations indicate that ion hydration is the dominant factor determining the interfacial electrolyte structure.

Modulation of Phase Behavior and Microscopic Dynamics in Cationic Vesicles by 1-Decyl-3-methylimidazolium Bromide

Synthetic cationic lipids have garnered significant attention as promising candidates for gene/DNA transfection in therapeutic applications. The phase behavior of the vesicles formed by these lipids is intriguing, revealing intricate connections to the structure and dynamics of the membrane. These phenomena emerge from the complex interplay between hydrophobic and electrostatic interactions of the lipids. In this study, we explore the impact of an ionic liquid-based surfactant, 1-decyl-3-methylimidazolium bromide (DMIM[Br]), on the structural, dynamical, and phase behavior of cationic dihexadecyldimethylammonium bromide (DHDAB) vesicles. Our investigations indicate that the addition of DMIM[Br] increases the vesicle size while thinning the membrane. Further, DMIM[Br] also induces substantial changes in the membrane phase behavior. At 10 and 25 mol %, DMIM[Br] eliminates the pre-transition from coagel to intermediate crystalline (IC) phase and decreases the onset temperature of the main phase transition to the fluid phase. In the cooling cycle, the addition of DMIM[Br] further induces the formation of an intermediate gel phase. This behavior is reminiscent of the non-synchronous ordering observed in the DODAB membrane, a longer-chain counterpart of DHDAB. Interestingly, at 40 mol % of DMIM[Br], the formation of the intermediate gel phase is largely suppressed. Neutron scattering data provide evidence that the addition of DMIM[Br] enhances lipid mobility in coagel and fluid phases, suggesting that DMIM[Br] acts as a plasticizer, enhancing membrane fluidity across all of the phases. Our findings infer that DMIM[Br] modulates the membrane's phase behavior and fluidity, two essential ingredients for the efficient transport of cargo, by controlling the balance of electrostatic and hydrophobic interactions.

Aspirin-Induced Ordering and Faster Dynamics of a Cationic Bilayer for Drug Encapsulation

The lipid dynamics and phase play decisive roles in drug encapsulation and delivery to the intracellular target. Thus, understanding the dynamic and structural alterations of membranes induced by drugs is essential for targeted delivery. To this end, united-atom molecular dynamics simulations of a model bilayer, dioctadecyldimethylammonium bromide (DODAB), are performed in the absence and presence of the usual nonsteroidal anti-inflammatory drug (NSAID), aspirin, at 298, 310, and 345 K. At 298 and 310 K, the bilayers are in the interdigitated two-dimensional square phases, which become rugged in the presence of aspirin, as evident from height fluctuations. At 345 K, the bilayer is in the fluid phase in both the absence and presence of aspirin. Aspirin is preferentially located near the oppositely charged headgroup and creates void space, which leads to an increase in the interdigitation and order parameters. Although the center of mass of lipids experiences structural arrest, they reach the diffusive regime faster and have higher lateral diffusion constants in the presence of aspirin. Results are found to be consistent with recent quasi-elastic neutron scattering studies that reveal that aspirin acts as a plasticizer and enhances lateral diffusion of lipids in both ordered and fluid phases. Different relaxation time scales of the bonds along the alkyl tails of DODAB due to the multitude of lipid motions become faster upon the addition of aspirin. Our results show that aspirin insertion is most favorable at physiological temperature. Thus, the ordered, more stable, and faster DODAB bilayer can be a potential drug carrier for the protected encapsulation of aspirin, followed by targeted and controlled drug release with antibacterial activity in the future.

Effects of ionic liquids on biomembranes: A review on recent biophysical studies

Ionic liquids (ILs) have been emerged as a versatile class of compounds that can be easily tuned to achieve desirable properties for various applications. The ability of ILs to interact with biomembranes has attracted significant interest, as they have been shown to modulate membrane properties in ways that may have implications for various biological processes. This review provides an overview of recent studies that have investigated the interaction between ILs and biomembranes. We discuss the effects of ILs on the physical and chemical properties of biomembranes, including changes in membrane fluidity, permeability, and stability. We also explore the mechanisms underlying the interaction of ILs with biomembranes, such as electrostatic interactions, hydrogen bonding, and van der Waals forces. Additionally, we discuss the future prospects of this field.

Curcumin Accelerates the Lateral Motion of DPPC Membranes

Curcumin, the main ingredient in turmeric, has attracted attention due to its potential anti-inflammatory, anticancer, wound-healing, and antioxidant properties. Though curcumin efficacy is related to its interaction with biomembranes, there are few reports on the effects of curcumin on the lateral motion of lipids, a fundamental process in the cell membrane. Employing the quasielastic neutron scattering technique, we explore the effects of curcumin on the lateral diffusion of the dipalmotylphosphatidylcholine (DPPC) membrane. Our investigation is also supported by Fourier transform infrared spectroscopy, dynamic light scattering, and calorimetry to understand the interaction between curcumin and the DPPC membrane. It is found that curcumin significantly modulates the packing arrangement and conformations of DPPC lipid, leading to enhanced membrane dynamics. In particular, we find that the presence of curcumin substantially accelerates the DPPC lateral motion in both ordered and fluid phases. The effects are more pronounced in the ordered phase where the lateral diffusion coefficient increases by 23% in comparison to 9% in the fluid phase. Our measurements provide critical insights into molecular mechanisms underlying increased lateral diffusion. In contrast, the localized internal motions of DPPC are barely altered, except for a marginal enhancement observed in the ordered phase. In essence, these findings indicate that curcumin is favorably located at the membrane interface rather than in a transbilayer configuration. Further, the unambiguous evidence that curcumin modulates the membrane dynamics at a molecular level supports a possible action mechanism in which curcumin can act as an allosteric regulator of membrane functionality.

Multiscale lipid membrane dynamics as revealed by neutron spectroscopy

The plasma membrane is one of the principal structural components of the cell and, therefore, one of the key components of the cellular life. Because the membrane's dynamics links the membrane's structure and function, the complexity and the broad range of the membrane's motions are essential for the enormously diverse functionality of the cell membrane. Even for the main membrane component, the lipid bilayer, considered alone, the range and complexity of the lipid motions are remarkable. Spanning the time scale from sub-picosecond to minutes and hours, the lipid motion in a bilayer is challenging to study even when a broad array of dynamic measurement techniques is employed. Neutron scattering plays a special role among such dynamic measurement techniques, particularly, because it involves the energy transfers commensurate with the typical intra- and inter- molecular dynamics and the momentum transfers commensurate with intra- and inter-molecular distances. Thus, using neutron scattering-based techniques, the spatial and temporal information on the lipid motion can be obtained and analysed simultaneously. Protium vs. deuterium sensitivity and non-destructive character of the neutron probe add to the remarkable prowess of neutron scattering for elucidating the lipid dynamics. Herein we present an overview of the neutron scattering-based studies of lipid dynamics in model membranes, with a discussion of the direct relevance and implications to the real-life cell membranes. The latter are much more complex systems than simple model membranes, consisting of heterogeneous non-stationary domains composed of lipids, proteins, and other small molecules, such as carbohydrates. Yet many fundamental aspects of the membrane behavior and membrane interactions with other molecules can be understood from neutron scattering measurements of the model membranes. For example, such studies can provide a great deal of information on the interactions of antimicrobial compounds with the lipid matrix of a pathogen membrane, or the interactions of drug molecules with the plasma membrane. Finally, we briefly discuss the recently emerging field of neutron scattering membrane studies with a reach far beyond the model membrane systems.

Melittin exerts opposing effects on short- and long-range dynamics in bicontinuous microemulsions

Bicontinuous microemulsions (BμEs) are a promising biomembrane mimetic system for investigating the behavior of antimicrobial peptides (AMPs) and their delivery to open wounds to combat antibiotic-resistant microorganisms. The properties of the BμE host are in turn affected by the guest AMP and can deviate from those of the unperturbed BμEs, especially at higher AMP concentrations. Here we report the effect of an archetypal AMP, melittin, over a wide range of concentrations, on the nanoscopic dynamics of BμEs formed by water/sodium dodecyl sulfate (SDS)/1-pentanol/dodecane, investigated using quasi-elastic neutron scattering (QENS). Two distinct motions are observed, namely, (i) the lateral motion of the surfactant on the surface of the oil channels and (ii) the internal motion of the surfactants. It is found that melittin restricts both the lateral and the internal motion, thereby acting as a stiffening agent. The lateral motion is more strongly affected, at low concentration of melittin. The lateral diffusion coefficient decreased sharply, approaching a constant value at higher melittin concentration. These results are in sharp contrast with the recent dynamic light scattering and neutron spin echo results which showed that at the length and time scales longer than those probed in the current work, melittin enhanced the long-range collective and local undulation motions of BμEs. Considered together, our results indicate that incorporation of melittin modulates the dynamics differently depending on the spatial and temporal regimes, in which the dynamics are being probed. The addition of melittin at low concentrations increased the magnitude of the zeta potential, but further increase of the melittin concentration decreased it. This suggests that addition of melittin at low concentrations led to increase in the surfactant concentration, but did not affect the negative charge per surfactant molecule, while further addition of melittin led to ion pairing of melittin with the oppositely charged surfactant. This study therefore demonstrates how the addition of melittin hinders the lateral motion of surfactants as a result of the strong association between melittin and SDS, suggesting that the release of AMPs from BμE-based delivery vehicles may be hindered.

Quantum Calculations on Ion Channels: Why Are They More Useful Than Classical Calculations, and for Which Processes Are They Essential?

There are reasons to consider quantum calculations to be necessary for ion channels, for two types of reasons. The calculations must account for charge transfer, and the possible switching of hydrogen bonds, which are very difficult with classical force fields. Without understanding charge transfer and hydrogen bonding in detail, the channel cannot be understood. Thus, although classical approximations to the correct force fields are possible, they are unable to reproduce at least some details of the behavior of a system that has atomic scale. However, there is a second class of effects that is essentially quantum mechanical. There are two types of such phenomena: exchange and correlation energies, which have no classical analogues, and tunneling. Tunneling, an intrinsically quantum phenomenon, may well play a critical role in initiating a proton cascade critical to gating. As there is no classical analogue of tunneling, this cannot be approximated classically. Finally, there are energy terms, exchange and correlation energy, whose values can be approximated classically, but these approximations must be subsumed within classical terms, and as a result, will not have the correct dependence on interatomic distances. Charge transfer, and tunneling, require quantum calculations for ion channels. Some results of quantum calculations are shown.

A Comparative Study on DMSO-Induced Modulation of the Structural and Dynamical Properties of Model Bilayer Membranes

Modulating the structures and properties of biomembranes via permeation of small amphiphilic molecules is immensely important, having diverse applications in cell biology, biotechnology, and pharmaceuticals, because their physiochemical and biological interactions lead to new pathways for transdermal drug delivery and administration. In this work, we have elucidated the role of dimethyl sulfoxide (DMSO), broadly used as a penetration-enhancing agent and cryoprotective agent on model lipid membranes, using a combination of fluorescence microscopy and time-resolved fluorescence spectroscopy. Spatially resolved fluorescence lifetime imaging microscopy (FLIM) has been employed to unravel how the fluidity of the DMSO-induced bilayer regulates the structural alteration of the vesicles. Moreover, we have also shown that the dehydration effect of DMSO leads to weakening of the hydrogen bond between lipid headgroups and water molecules and results in faster solvation dynamics as demonstrated by femtosecond time-resolved fluorescence spectroscopy. It has been gleaned that the water dynamics becomes faster because bilayer rigidity decreases in the presence of DMSO, which is also supported by time-resolved rotational anisotropy measurements. The enhanced diffusivity and increased membrane fluidity in the presence of DMSO are further ratified at the single-molecule level through fluorescence correlation spectroscopy (FCS) measurements. Our results indicate that while the presence of DMSO significantly affects the 1,2-dimyristoyl-rac-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-rac-glycero-3-phosphatidylcholine (DPPC) bilayers, it has a weak effect on 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) vesicles, which might explain the preferential interaction of DMSO with the positively charged choline group present in DMPC and DPPC vesicles. The experimental findings have also been further verified with molecular dynamics simulation studies. Moreover, it has been observed that DMSO is likely to have a differential effect on heterogeneous bilayer membranes depending on the structure and composition of their headgroups. Our results illuminate the importance of probing the lipid structure and composition of cellular membranes in determining the effects of cryoprotective agents.