Protein-Mediated Changes in Membrane Fluidity and Ordering: Insights into the Molecular Mechanism and Implications for Cellular Function.

Probing protein-membrane interactions is vital for understanding biological functionality for various applications such as drug development, targeted drug delivery, and creation of functional biomaterials for medical and industrial purposes. In this study, we have investigated interaction of Human Serum Albumin (HSA) with two different lipids, dipalmitoylphosphatidylglycerol (dDPPG) and dipalmitoylphosphatidylcholine (dDPPC), using Vibrational Sum Frequency Generation spectroscopy at different membrane fluidity values. In the liquid-expanded (LE) state of the lipid, HSA (at pH 3.5) deeply intercalated lipid chains through a combination of electrostatic and hydrophobic interactions, which resulted in more ordering of the lipid chains. However, in the liquid-condensed (LC) state, protein intercalation is decreased due to tighter lipid packing. Moreover, our findings revealed distinct differences in HSA's interaction with dDPPG and dDPPC lipids. The interaction with dDPPC remained relatively weak compared to dDPPG. These results shed light on the significance of protein mediated changes in lipid characteristics, which hold considerable implications for understanding membrane protein behavior, lipid-mediated cellular processes, and lipid-based biomaterial design.

Dye-induced photoluminescence quenching of quantum dots: role of excited state lifetime and confinement of charge carriers.

We investigate the role of quantum confinement and photoluminescence (PL) lifetime of photoexcited charge carriers in semiconductor core/shell quantum dots (QDs) via PL quenching due to surface modification. Surface modification is controlled by varying the number of dye molecules adsorbed onto the QD shell surface forming QD-dye nanoassemblies. We selected CuInS2/ZnS (CIS) and InP/ZnS (InP) core/shell QDs exhibiting relatively weak (664 meV) and strong (1194 meV) confinement potentials for the conduction band electron. Moreover, the difference in the emission mechanism gives rise to a long and short excited state lifetime of CIS (ca. 290 ns) and InP (ca. 37 ns) QDs. Dye molecules of different ionic characters (rhodamine 575: zwitterionic and rhodamine 560: cationic) are used as quenchers. A detailed analysis of Stern-Volmer data shows that (i) quenching is generally more pronounced in CIS-dye assemblies as compared to InP-dye assemblies, (ii) dynamic quenching is dominating in all QD-dye assemblies with only a minor contribution from static quenching and (iii) the cationic dye shows a stronger interaction with the QD shell surface than the zwitterionic dye. Observations (i) and (ii) can be explained by the differences in the amplitude of the electronic component of the exciton wavefunction near the dye binding sites in both QDs, which results in the breaking up of the electron-hole pair and favors charge trapping. Observation (iii) can be attributed to the variations in electrostatic interactions between the negatively charged QD shell surface and the cationic and zwitterionic dyes, with the former exhibiting a stronger interaction. Moreover, the long lifetime of CIS QDs facilitates us to easily probe different time scales of the trapping processes and thus differentiate the origins of static and dynamic quenching components that appear in the Stern-Volmer analysis.

Unravelling the Mechanism behind Charge Reversal at Silica Nanoparticle-Model Cell Membrane Interfaces.

Vibrational sum frequency generation spectroscopy is used to understand the interactions of silica nanoparticles (SNPs) with a model cationic membrane (1,2-dipalmitoyl-3-(trimethylammonium)propane, DPTAP) by monitoring changes in the interfacial water and lipid structure at pH ∼ 2 and pH ∼ 11. Our study reveals that, at pH ∼ 11, SNPs are attracted to DPTAP due to electrostatic forces, causing changes in the interfacial water structure and lipid membrane. At high concentrations of SNPs (≥70 pM), the interfacial charge reversed from positive to negative, inducing the formation of new hydrogen-bonded structures and reorganization of water molecules. Conversely, negligible changes are observed at pH ∼ 2 due to nearly neutral charge of the SNPs. Molecular dynamics simulations demonstrated that the interfacial potential due to model membrane and SNPs dictates the water structure at the interface. These results elucidate the fundamental mechanism governing interfacial interactions and could have implications in drug delivery, gene therapy, and biosensing.

Effect of succinonitrile on ion transport in PEO-based lithium-ion battery electrolytes.

We report the ion transport mechanisms in succinonitrile (SN) loaded solid polymer electrolytes containing polyethylene oxide (PEO) and dissolved lithium bis(trifluoromethane)sulphonamide (LiTFSI) salt using molecular dynamics simulations. We investigated the effect of temperature and loading of SN on ion transport and relaxation phenomenon in PEO-LiTFSI electrolytes. It is observed that SN increases the ionic diffusivities in PEO-based solid polymer electrolytes and makes them suitable for battery applications. Interestingly, the diffusion coefficient of TFSI ions is an order of magnitude higher than the diffusion coefficient of lithium ions across the range of temperatures and loadings investigated. By analyzing different relaxation timescales and examining the underlying transport mechanisms in SN-loaded systems, we find that the diffusivity of TFSI ions correlates excellently with the Li-TFSI ion-pair relaxation timescales. In contrast, our simulations predict distinct transport mechanisms for Li-ions in SN-loaded PEO-LiTFSI electrolytes. Explicitly, the diffusivity of lithium ions cannot be uniquely determined by the ion-pair relaxation timescales but additionally depends on the polymer segmental dynamics. On the other hand, the SN loading induced diffusion coefficient at a given temperature does not correlate with either the ion-pair relaxation timescales or the polymer segmental relaxation timescales.

Thermodynamics of translational and rotational dynamics of C9 hydrocarbons in the pores of zeolite-beta.

There has been a growing interest in the separation of aromatic hydrocarbon molecules from the petroleum stream using zeolite-based technologies. This led to numerous experimental and molecular simulation studies of the structural and dynamical properties of aromatic hydrocarbons under the confinement of microporous materials like zeolites. The understanding of the behavior of the isomers of the trimethylbenzene under confinement is crucial for their separation and purification from industrial streams. Here, we investigate the translational and rotational dynamics and associated thermodynamics of three isomers of trimethyl benzene, namely, 1,2,3-trimethyl benzene (1,2,3-TMB), 1,2,4-trimethyl benzene (1,2,4-TMB), and 1,3,5-trimethylbenzene (1,3,5-TMB) under the confinement of zeolite-beta (BEA) using molecular dynamics (MD) simulations. The trends in the diffusion coefficients of the TMB isomers calculated from our MD simulation data are in good agreement with the data already available in the literature. Analysis of dynamics and associated thermodynamic properties indicate that 1,2,4-TMB is translationally more facile than the other two isomers. The rotational motion of TMB isomers is largely anisotropic and it is relatively more significant for both 1,2,4-TMB and 1,3,5-TMB. The thermodynamic properties reveal that the distinguishability in the dynamic properties among these three isomers is essentially caused by entropy. These results are not only critical to engineer the separation process of TMB isomers across the zeolite beds but also to understand the different catalytic processes such as trans-alkylation, conversion, cracking etc.

High Interfacial Barriers at Narrow Carbon Nanotube-Water Interfaces.

Water displays anomalous fast diffusion in narrow carbon nanotubes (CNTs), a behavior that has been reproduced in both experimental and simulation studies. However, little is reported on the effect of bulk water-CNT interfaces, which is critical to exploiting the fast transport of water across narrow carbon nanotubes in actual applications. Using molecular dynamics simulations, we investigate here the effect of such interfaces on the transport of water across arm-chair CNTs of different diameters. Our results demonstrate that diffusion of water is significantly retarded in narrow CNTs due to bulk regions near the pore entrance. The slowdown of dynamics can be attributed to the presence of large energy barriers at bulk water-CNT interfaces. The presence of such intense barriers at the bulk-CNT interface arises due to the entropy contrast between the bulk and confined regions, with water molecules undergoing high translational and rotational entropy gain on entering from the bulk to the CNT interior. The intensity of such energy barriers decreases with increase in CNT diameter. These results are very important for emerging technological applications of CNTs and other nanoscale materials, such as in nanofluidics, water purification, nanofiltration, and desalination, as well as for biological transport processes.