Dynamics of Bound and Free Water in an Aqueous Micellar Solution : Analysis of the Lifetime and Vibrational Frequencies of Hydrogen Bonds at a Complex Interface

In order to understand the nature and dynamics of interfacial water molecules on the surface of complex systems, large scale molecular dynamics simulations of an aqueous micelle of cesium perfluorooctanoate surfactant molecules have been carried out. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the polar headgroups of the surfactants, are calculated. Our earlier classification of the interfacial water molecules, based on structural and energetic considerations, into bound and free type is further validated by their dynamics. Lifetime correlation functions of the water-surfactant hydrogen bonds show the long lived nature of the bound water species. The water molecules that are singly hydrogen bonded to the surfactants have longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similar to bulk water in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of few hundreds of picoseconds. The intermolecular vibrational frequencies of these interfacial water molecules shows a significant blue shift in the librational band apart from a similar shift in the near neighbor bending modes, relative to water molecules in bulk. These blue shifts suggest an increase in rigidity in the structure around interfacial water molecules. This is in good agreement with recent incoherent, inelastic neutron scattering data on macromolecular solutions. The results of the present simulations should be relevant to the understanding of dynamics of water near any hydrophilic surface.Comment: 36 Pages including 7 Figures; Submitted to Phys. Rev.

Evidence for Bound and Free Water Species in the Hydration Shell of an Aqueous Micelle

Our atomistic molecular dynamics simulations reveal the existence of bound and free water molecules in the hydration layer of an aqueous micelle. The bound water molecules can be either singly or doubly hydrogen bonded to the polar head group on the surface of the micelle. The ratio of bound to free water is found to be approximately equal to 9:1 at 300 K.Comment: 7 pages and 3 figures. Submitted to Curr. Sci.(Ind. Acad. Sci.

Ab initio molecular-dynamics study of supercritical carbon dioxide

Car-Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO2) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO2 molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO2 have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO2. The results have also been compared to classical molecular-dynamics calculations employing an empirical potential

Identity, energy, environment, and dynamics of interfacial water molecules in a micellar solution

The structure and energetics of interfacial water molecules in the aqueous micelle of cesium perfluorooctanoate have been investigated using-large scale atomistic molecular dynamics simulations, with the primary objective of classifying them. The simulations show that the water molecules at the interface fall into two broad classes, bound (IBW) and free (IFW), present in a ratio of 9:1. The bound water molecules can be further categorized based on the number of hydrogen bonds (one or two) that they form with the surfactant headgroups. The hydrogen bonds of the doubly hydrogen bonded species (IBW2) are found to be, on the average, slightly weaker than that in the singly bonded species (IBW1). The environment around interfacial water molecules is more ordered than that in the bulk. The surface water molecules have substantially lower potential energy due to interaction with the micelle. In particular, both forms of IBW have energies lower by about 2.5 to 4.0 kcal/mole. Entropy is found to play an important role in determining the relative concentration of the species. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the hydrophilic, polar headgroups (PHG) of the surfactants, are calculated. Our classification (S. Balasubramanian, S. Pal and B. Bagchi, Evidence for bound and free water species in the hydration shell of an aqueous micelle, Curr. Sci., 84, 428-430 (2003); S. Pal, S. Balasubramanian and B. Bagchi, Identity, energy, and environment of interfacial water molecules in a micellar solution, J. Phys. Chem. B, 107, 5194-5202 (2003)) of the interfacial water molecules, based on structural and energetic considerations, into IBW and IFW is further validated by their dynamics (S. Pal, S. Balasubramanian, and B. Bagchi, Dynamics of bound and free water in an aqueous micellar solution: Analysis of the lifetime and vibrational frequencies of hydrogen bonds at a complex interface, Phys. Rev. E, 67, 061502-1-061502-10 (2003)). Lifetime correlation functions of the water-surfactant hydrogen bonds show the long lived nature of the bound water species. Surprisingly, the water molecules that are singly hydrogen bonded to the surfactants have longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similar to bulk one in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of a few hundreds of picoseconds, that is, much longer than their average lifetime

Dynamics of water molecules at the surface of an aqueous micelle: atomistic molecular dynamics simulation study of a complex system

The dynamics of water molecules and ions near an aqueous micellar interface is a subject of intense current interest, as such a system serves as a prototype of more complex biological systems, in addition to being important in its own right. We study the dynamics by using an atomistic molecular dynamics simulation of a micelle of cesium pentadecafluorooctanoate (CsPFO) in water. This micellar system is stable over a range of temperature, allowing us to perform a detailed study of the microscopic dynamics of water at the surface of the micelle, at two different temperatures. The dipolar orientational correlation function of the water molecules and the polar solvation dynamics (SD) of cesium ions and tagged water molecules are calculated. Results show that the reorientational motion of water molecules near the micelle is restricted, and as a result exhibits a slow component which is slower than its bulk value by at least two orders of magnitude, in agreement with dielectric relaxation experiments. In addition, the SD of cesium ions is found to be slowed down significantly, again by more than two orders of magnitude compared to that in the bulk water. Through an analysis of partial solvation time correlation functions, we find that the cesium ions are primarily solvated by the polar headgroups, and their SD is thus intimately coupled to micellar dynamics. Both orientational dynamics and SD show strong temperature dependence

Temperature dependence of water dynamics at an aqueous micellar surface: atomistic molecular dynamics simulation studies of a complex system

In order to study the temperature dependence of water dynamics at the surface of a self-organized assembly, we perform long atomistic molecular dynamics simulations of a micelle of cesium pentadecafluorooctanoate in water at two different temperatures, 300 and 350 K. Since this micellar system is stable over a range of temperature, a detailed study of the microscopic dynamics of water at the surface of the micelle at both temperatures could be performed. The diffusion and dipolar orientational correlation function of the water molecules and the polar solvation dynamics of cesium ions at the micellar surface are calculated as a function of their location from the micellar surface. Our study reveals a strong temperature dependence. The relaxation of both the time correlation functions are highly nonexponential, and become very slow at 300 K. It is found that while the slowness in the orientational time correlation function originates partly from the formation of bridge hydrogen bonds between the polar head groups (PHG) of the micelle and the water molecules, the solvation dynamics slows down primarily due to the interaction of the positive cesium ions with the negatively charged PHGs

Unique Regulation of Enterocyte Brush Border Membrane Na-Glutamine and Na-Alanine Co-Transport by Peroxynitrite during Chronic Intestinal Inflammation

Na-amino acid co-transporters (NaAAcT) are uniquely affected in rabbit intestinal villus cell brush border membrane (BBM) during chronic intestinal inflammation. Specifically, Na-alanine co-transport (ASCT1) is inhibited secondary to a reduction in the affinity of the co-transporter for alanine, whereas Na-glutamine co-transport (B0AT1) is inhibited secondary to a reduction in BBM co-transporter numbers. During chronic intestinal inflammation, there is abundant production of the potent oxidant peroxynitrite (OONO). However, whether OONO mediates the unique alteration in NaAAcT in intestinal epithelial cells during chronic intestinal inflammation is unknown. In this study, ASCT1 and B0AT1 were inhibited by OONO in vitro. The mechanism of inhibition of ASCT1 by OONO was secondary to a reduction in the affinity of the co-transporter for alanine, and secondary to a reduction in the number of co-transporters for B0AT1, which were further confirmed by Western blot analyses. In conclusion, peroxynitrite inhibited both BBM ASCT1 and B0AT1 in intestinal epithelial cells but by different mechanisms. These alterations in the villus cells are similar to those seen in the rabbit model of chronic enteritis. Therefore, this study indicates that peroxynitrite may mediate the inhibition of ASCT1 and B0AT1 during inflammation, when OONO levels are known to be elevated in the mucosa

Dynamics of water at the interface of a small protein, enterotoxin

Fully atomistic molecular dynamics simulations have been carried out to investigate the correlation of biological activity with dynamics of water molecules in an aqueous protein solution of the toxic domain of enterotoxin (PDB ID: 1ETN). This is a small protein of 13 amino acid residues. Our study of this water soluble protein clearly reveals that water dynamics slows down in the hydration layer. Despite this general slowing down, water molecules in the vicinity of the second b turn of this protein exhibit faster dynamics than those near other regions of the protein. Since this β turn is believed to play a critical role in the receptor binding of this protein, the faster dynamics of water near the β turn may have biological significance. The collective orientational dynamics of the water molecules in the protein solution exhibits a characteristic long time component of 27 ps, which agrees well with dielectric relaxation experiments

A Fake Profile Detection Model Using Multistage Stacked Ensemble Classification

Fake profile identification on social media platforms is essential for preserving a reliable online community. Previous studies have primarily used conventional classifiers for fake account identification on social networking sites, neglecting feature selection and class balancing to enhance performance. This study introduces a novel multistage stacked ensemble classification model to enhance fake profile detection accuracy, especially in imbalanced datasets. The model comprises three phases: feature selection, base learning, and meta-learning for classification. The novelty of the work lies in utilizing chi-squared feature-class association-based feature selection, combining stacked ensemble and cost-sensitive learning. The research findings indicate that the proposed model significantly enhances fake profile detection efficiency. Employing cost-sensitive learning enhances accuracy on the Facebook, Instagram, and Twitter spam datasets with 95%, 98.20%, and 81% precision, outperforming conventional and advanced classifiers. It is demonstrated that the proposed model has the potential to enhance the security and reliability of online social networks, compared with existing models

Percutaneous nephrostomy, feasibility of mid and anterior axillary line approach and its complications: an experience

Background: Nephrostomy performed as an emergency procedure to drain the obstructed calyceal system is a lifesaving procedure. Posterolateral approach is usually done to access the avascular plane of Brodel to avoid hemorrhage. This study evaluates the feasibility and complications in percutaneous nephrostomies placed at mid and anterior axillary line entry sites as it was found to be easier approach.Methods: A prospective study with 126 percutaneous nephrostomies performed with Ultrasound guidance, using single puncture technique and 8.5F pigtail catheter.  The complications rates were analyzed according to WHO guidelines. Results: The technical success rate was 98.4%. Three hemorrhagic complications (2.38%) were recorded without any major interventions. Increased incidence (11.9%) of transient hematuria was observed with five cases (3.9%) of infection at entry site. One case (0.79%) of peritoneal breech recorded with no significant intraperitoneal collection. Conclusions: Placement of Percutaneous Nephrostomy tube at mid or the anterior axillary line appears to be a feasible and easy approach, as its complications rates are within acceptable limits with an advantage of increased patient comfort