Investigating the Vapor-Phase Adsorption of Aroma Molecules on the Water-Vapor Interface using Molecular Dynamics Simulations.

Surfactants are amphiphilic additives primarily used to reduce the surface tension of water and manipulate its wettability on various surfaces. Recent reports suggest that volatile surfactants, such as aroma molecules, diffuse more quickly to the interface from the vapor-phase than conventional surfactants typically used in the aqueous phase. The ability to adsorb from the vapor phase, in addition to their use as cosurfactants, expands the potential applications of volatile surfactants, particularly in situations where adding surfactants from the liquid phase is difficult. Here, we present a molecular level understanding of the adsorption kinetics of linalool, a common aroma molecule, on the water interface using molecular dynamics simulations. We note that the value of surface tension while adsorption from vapor and liquid phases is dependent only on the surface coverage. A minimum surface tension of 32 ± 1.8 mN/m is obtained in both cases at a maximum surface coverage of 4.88 µmol/m2 at 300 K. We observe the extent of decrease of the H-bonds between linalool-water and linalool-linalool molecules at various surface coverages to explain the mechanism of surface tension reduction. We solve Gibb's adsorption equation to establish a correlation between the surface coverage of linalool and the corresponding bulk concentration in experiments. We investigate the free energy profile of linalool's adsorption behavior at different surface coverages and temperatures. Our report suggests that linalool adsorption onto the water interface is an enthalpy-driven process primarily dependent on the strength of the interaction between the hydroxyl group of linalool and water molecules. These insights are crucial for selecting a suitable aroma molecule for various applications that target the vapor-phase adsorption mechanism.

Elucidating the relationship between red fluorescence and structural dynamics of carbon dots dispersed in different solvents.

The mechanism of intrinsic fluorescence of carbon dots (CDs), the latest nanomaterial from the carbon family, was supposedly deciphered through multiple theories. However, the much sought-after persistent red emission of CDs as a foreseeable consequence of experiments remains elusive prompting the question of whether tuning of the red emission of CDs is a predictable outcome or a serendipitous coincidence. Herein, we tried to decode the same by exploring Alizarin Red S (ARS)-based red emitting CDs in different solvents with wisely chosen analytical tools. The findings are aptly supported by molecular dynamics studies through an experimental intuition-driven model-building approach. Parallel interception of the CDs with powder X-ray diffraction (pXRD) and photophysical spectroscopic studies revealed an important relationship between the solvent and CDs. Tautomerism, a well-known phenomenon with chemical entities, was found to be operative for CDs that greatly influence the Stokes shift and ultimately the fluorescence outcome. Most importantly, pXRD studies established the turbostratism of the CDs where the well-ordered graphitic structure of CDs gets disrupted with solvent molecules. The extent of such disruption is a function of solvent and CD composition that plays a formidable role in obtaining red fluorescence. Thus, for the first time, we demonstrate that the red emission of CDs is related to its structural integrity and if taken care of could be sustained, a tremendously desirable outcome for relevant applications.

How does excess phenylalanine affect the packing density and fluidity of a lipid membrane?

Phenylketonuria (PKU) is an autosomal recessive error of phenylalanine (Phe) metabolism, where untreated Phe becomes cytotoxic. Previous experiments found that excess Phe decreases the packing density and increases the fluidity and permeability of a lipid membrane. It was proposed that Phe forms cytotoxic nanoscopic amyloid-like fibrils. In another study, the Phe fibrils were not visible near the lipid membrane. So, what leads to the deleterious effect of Phe on the lipid membrane? We put forward a molecular mechanism for the observed effect of excess Phe on the lipid membrane using all-atom molecular dynamics simulation. This study suggests that Phe monomers spontaneously intercalate into the membrane and form small hydrogen-bonded clusters, some of which locally perturb the membrane. These local effects result in an overall reduction in the membrane packing density, enhancement of membrane fluidity, and an increase of water permeability, observed in experiments. The present study does not observe any effect of the nanoscopic fibrillar structure of Phe on the membrane. This study, therefore, provides alternative insights into the excess Phe cytotoxicity in PKU disease.

Translocation of a hydroxyl functionalized carbon dot across a lipid bilayer: an all-atom molecular dynamics simulation study.

The major applications of carbon dots (CDs) (e.g. bio-imaging and targeted drug delivery) necessitate the latter to permeate across the lipid bilayer membrane. Unfortunately, the mechanism of permeation is poorly understood. Between the two possible routes for permeation of a nanoparticle like CDs-an endocytic pathway and direct passive permeation-the endocytic path is known to be more common, despite the fact that the passive permeation is preferred over the endocytosis for targeted drug delivery. Here, we have focused on the direct permeation of a hydroxyl functionalized CD across the POPC lipid bilayer membrane using all-atom MD simulations. We have estimated the free energy profile for the translocation of the CD across the lipid bilayer, with a barrier height of ∼170 kJ mol-1 situated at the lipid bilayer center (z = 0 nm). Using the free energy profile, we have calculated a negligible permeability coefficient value, which strongly suggests that it is almost impossible for a CD to penetrate directly across the lipid bilayer. The possible impact on the lipid bilayer structure by the CD is also investigated. Although the CD does not affect the bilayer structure up to a certain degree of penetration, the impact increases substantially when entered into the bilayer interior.

Fickian Yet Non-Gaussian Nanoscopic Lipid Diffusion in the Raft-Mimetic Membrane.

Past experiments rationalized the observed dynamic heterogeneity and non-Gaussian diffusion in living cell membranes in terms of slow-active remodeling of the underlying cortical actin network. In this work, we demonstrate that the nanoscopic dynamic heterogeneity can also be explained via the lipid raft hypothesis, which postulates a phase separation between liquid-ordered (Lo) and liquid-disordered (Ld) nanodomains. Non-Gaussian displacement distribution is observed in the Lo domain for a long time, even when the mean square displacement becomes Fickian. This Fickian yet non-Gaussian diffusion is found particularly in the Lo/Ld interface consistent with the "diffusing diffusion" picture. A translational jump-diffusion model, previously employed to explain the diffusion-viscosity decoupling in supercooled water, is used here to quantitatively explain the long-term dynamic heterogeneity where a strong correlation between translational jump and non-Gaussian diffusion is observed. Therefore, this study proposes a novel approach to elucidate the dynamic heterogeneity and non-Gaussian diffusion in the cell membrane crucial for various cell membrane functionalities.

Homeoviscous Adaptation of the Lipid Membrane of a Soil Bacterium Surviving under Diurnal Temperature Variation: A Molecular Simulation Perspective.

A recent experiment has reported the lipidome remodeling of a soil-based plant-associated bacterium Methylobacterium extorquens due to diurnal temperature variations. The key adaptation strategy is the headgroup-specific remodeling of the acyl chain. To understand the idiosyncratic adaptation at the molecular level, we simulate the model membrane of the same bacterium using the reported lipidome compositions at four different experimental temperatures. We investigate the temperature-dependent packing density and fluidity of the membrane, the constancy of which is key to the homeoviscous adaptation. The results show that complex lipidome remodeling approximately preserves membrane properties under heat and cold stress. The headgroup-specific remodeling of the acyl chain serves to fine-tune the packing density and fluidity of the membrane at different temperatures. While lipids with strongly interacting headgroups are more abundant at higher temperatures, the lipidome is more dominated by lipids with weaker interacting headgroups at lower temperatures. This adaptation alleviates lipid membrane disruption caused by heat and cold stress. This study provides a molecular picture of the homeoviscous adaptation of the realistic lipid membrane of a soil-based bacterium.

Cryostabilization of the Cell Membrane of a Psychrotolerant Bacteria via Homeoviscous Adaptation.

Homeoviscous adaptation (maintenance of a critical balance between the saturated and unsaturated lipids) of the cell membrane of psychrotolerant bacteria is essential to protect them against freeze-thaw cycle. But how does the homeoviscous adaptation protect the cell membrane during cold stress? In this Letter, we answer this question using a coarse-grained molecular dynamics simulation technique. On the basis of the reported fatty acid profiles of psychrotolerant bacteria at different temperatures, multiple lipid membranes are simulated at a wide range of temperatures between 250 and 300 K. We explicate how the homeoviscous adaptation minimizes the effect of cold stress on the structure and fluidity of the membrane. Partial freezing of the saturated lipid domain occurs with the self-aggregation of saturated and unsaturated lipids near the melting temperature of the unadapted lipid membrane. The gel-like phase provides necessary local packing density that can be sensed by sensor proteins responsible for the homeoviscous adaptation.

Understanding the Origin of the Breakdown of the Stokes-Einstein Relation in Supercooled Water at Different Temperature-Pressure Conditions.

A recent experiment has measured the viscosity of water down to approximately 244 K and up to 300 MPa. The correct viscosity and translational diffusivity data at various temperature-pressure (T-P) state points allowed for checking the validity of the Stokes-Einstein (SE) relation, which accounts for the coupling between translational self-diffusion and medium viscosity. The diffusion-viscosity decoupling increases with decreasing temperature, but the increasing pressure reduces the extent of the decoupling. Earlier simulation studies explained the breakdown of the SE relation in terms of the location of the Widom line, emanating from the liquid-liquid critical point (LLCP). Although these studies made a significant contribution to the current understanding of the above phenomena, a detailed molecular picture is still lacking. Recently, our group has explained the diffusion-viscosity decoupling from a jump-diffusion perspective. The jump-diffusion coefficient, emanating from the jump translation of water molecules, is calculated using a quantitative approach for different temperatures at ambient pressure. It has been observed that jump-diffusion is the key factor for diffusion-viscosity decoupling in supercooled water. The same method is adopted in the present work to estimate the jump-diffusion coefficient for different T-P state points and, thereby, explains the role of jump-diffusion for the different extents of the SE relation breakdown at different pressures. The residual diffusion coefficient, the other component of the total diffusion that originates from small step displacement and that is calculated by subtracting the jump-diffusion coefficient from the total diffusion, is seen to be fairly coupled to the viscosity at the entire range of temperature and pressure. Furthermore, we have calculated the average number of H-bonds per water molecule and the tetrahedral order for different T-P state points and investigated an approximate correlation between the average local structure and the contribution of the jump-diffusion to the total diffusion of water. This study, therefore, puts forward a new perspective for explaining the SE relation breakdown in supercooled water under different pressure conditions.