Enhancement of quadratic nonlinearity via multiple hydrogen-bonded supramolecular complex formation

The formation and quadratic nonlinearity of a multiple hydrogen-bonded 1:1 supramolecular complex 1.2 between the 2,6-diaminopyridine-based Lambda-shaped molecule, 1, and ferrocenyl barbituric acid, 2, in solution have been investigated by the hyper-Rayleigh scattering (HRS) and NMR techniques. A 6-fold increase in the molecular hyperpolarizability (beta) of the complex 1.2 over the sum of the molecular hyperpolarizabilities of the components 1 and 2 is seen. Such a significant enhancement in beta is attributed to the alignment of the molecular dipoles of 1 and 2 in the 2D plane leading to the creation of a large dipole moment in the plane of the supramolecular complex. Depolarized HRS experiments led to the determination of the in-plane polarization components of beta of the supramolecular complex 1.2. The component of beta in the direction of the dipole moment is large. This investigation exemplifies the role of multiple hydrogen bonds in stabilizing a 2D supramolecular architecture leading to a large enhancement of molecular nonlinearity.     

Water absorption and degradation characteristics of chitosan-based polyesters and hydroxyapatite composites

Blends of chitosan and biodegradable synthetic aliphatic polyesters (polycaprolactone, poly(butylene succinate), poly[(butylene succinate)-co-adipate], poly[(butylene terephthalate)-co-adipate], and poly(lactic acid)) were injection-molded. These samples were immersed in isotonic solution at 37 degrees C for a period of 60 d. The water uptake and the degradation properties, as measured by the loss in tensile strength, were evaluated as a function of time. In this study, the rate and the equilibrium water uptake were proportional to the amount of chitosan in the blend. The addition of HA to chitosan and polyester significantly reduced the equilibrium water uptake. The water uptake did not follow the classical Fickian phenomena and could be expressed by a two-stage sorption non-Fickian diffusion model. Contact angle measurement was used to quantify the changes in surface hydrophilicity as a function of chitosan and polyester composition. The glycerol contact angle decreased with increasing synthetic components in the blend. The blends and composites also showed increased degradation, as quantified by a loss in their mechanical properties, with increase in natural content. The degradation of properties was directly related to the water uptake of the blends; the higher the water uptake, the higher the degradation. Pure polyesters, while having low water uptake, nevertheless showed significant degradation by a precipitous drop in the strain at break. Among the polyesters, poly(lactic acid) displayed maximum degradation, while polycaprolactone displayed the least.     

An object-oriented library for computational protein design

Recent advances in computational protein design have established it as a viable technique for the rational generation of stable protein sequences, novel protein folds, and even enzymatic activity. We present a new and object-oriented library of code, written specifically for protein design applications in C(++), called EGAD Library. The modular fashion in which this library is written allows developers to tailor various energy functions and minimizers for a specific purpose. It also allows for the generation of novel protein design applications with a minimal amount of code investment. It is our hope that this will permit labs that have not considered protein design to apply it to their own systems, thereby increasing its potential as a tool in biology. We also present various uses of EGAD Library: in the development of Interaction Viewer, a PyMOL plug-in for viewing interactions between protein residues; in the repacking of protein cores; and in the prediction of protein-protein complex stabilities.     

Thermotropic and hydration studies of membranes formed from gemini pseudoglyceryl lipids possessing polymethylene spacers

Membrane formation from gemini pseudoglyceryl lipids bearing n-C14H29 and n-C16H33 chains has been reported. These lipid aggregates have been characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), high sensitivity differential scanning calorimetry (DSC), and Paldan fluorescence studies. The length of the spacer between the cationic ammonium headgroups has been varied from -(CH2)3- (propandiyl) to -(CH2)12- (dodecandiyl) in these lipids. All gemini lipids were found to generate stable suspensions in aqueous media. Electron microscopic studies revealed the smaller size of the gemini lipid aggregates as compared to their monomeric lipid counterparts. DLS measurements showed that the gemini lipid suspensions with a -(CH2)8- spacer length were bigger in size than that of other analogues. DSC studies suggest the unusual behavior of the gemini lipids bearing -(CH2)3- propanediyl spacer based lipids. These observations were consistent irrespective of the hydrocarbon chain lengths of the lipids. Paldan fluorescence based hydration studies showed that the hexadecyl chain based gemini lipid aggregates bearing a -(CH2)12- spacer were the most hydrated in their gel states among all the gemini lipid series investigated herein.     

Interfacial and solution properties of tetraalkylammonium bromides and their sodium dodecyl sulfate interacted products: a detailed physicochemical study

The adsorption and solution behaviors of symmetrical tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium bromides (TMAB, TEAB, TPAB, and TBAB, respectively) were studied at the air/water interface and in the bulk aqueous environments. Their salts were prepared by reacting tetraalkylammonium bromide (TAAB) with sodium dodecyl sulfate (SDS) in a solution from which the products of the higher two homologues (tetrapropylammonium dodecyl sulfate (TPADS) and tetrabutylammonium dodecyl sulfate (TBADS)) could only be isolated as solids and for which detailed characterization has been performed. The interfacial behaviors of 1:1 molar mixtures of TAAB and SDS and the prepared TPADS and TBADS were examined. Micellization of the 1:1 mixtures along with the isolated species were studied in the presence and absence of NaBr salt. The energetics of the micellization process and the counterion binding of the micelles were evaluated. The interaction of the TAABs with SDS micelles was examined, and the results were evaluated in terms of single- and two-site binding interaction models. Of the formed tetraalkylammonium dodecyl sulfates (TAADSs), only TBADS evidenced clouding, which was investigated in detail along with 1:1 molar mixtures of TBAB and SDS in aqueous solution in the presence of additives such as NaBr, SDS, and TBAB. The solution behaviors of the TAADS and the clouding of TBADS have been rationalized in terms of a mixed micellar model.     

Effect of Surface Charge on a Microsphere on the Stability of a Nanoparticle Suspension

A stable suspension of nanoparticles is generally achieved by modifying the surface properties of the nanoparticles using a surfactant. This is to create a surface charge on the nanoparticles which prevents them from forming aggregates. This phenomenon is very sensitive to changes in the local environment. Conventional optical tweezers though capable of trapping sub-micrometer particles are not known to trap single nanoparticles. However, the stability and dynamics of a suspension of nanoparticles can be probed through the changes in the fluctuations of an optically trapped microsphere that has been added to the suspension. Adding microspheres to the nanoparticle suspension can affect the stability depending on the surface charges the microparticles themselves have. The study reports here on the variation of the dynamics of suspended nanoparticles, which have a positive surface charge, when silica microspheres, which are negatively surface charged, are added to the suspension. With the addition of silica beads, there is agglomeration of the nanoparticles. The dynamics of these agglomerated structures are then probed by measuring the Brownian fluctuations of an optically trapped silica bead. These results are in sharp contrast to those of earlier studies carried out with suspensions of identical nanoparticles but with negative surface charge.     

Odd-Stoichiometry and Secondary Phase Relations in Higher Manganese Silicides

Higher manganese silicides (denoted as MnSi_γ) are a prominent class of intermetallic compound for thermoelectric applications that crystallizes into a Nowotny Chimney Ladder (NCL) phase and constitutes a higher concentration of silicon with odd stoichiometry, typically represented by an irrational number. In this work, the genesis of Si odd stoichiometry and secondary phase relations in silicon-rich and silicon-deficient MnSi_x is presented to clarify the significance of labile (i.e., easily alterable) Si-subsystem in inheriting the modulated lattice structure with characteristic anharmonicity. The transport properties show an interrelation to the presence of a secondary phase, which is driven by the odd stoichiometry, where the thermoelectric figure of merit (zT) is found to be optimal for MnSi_1.74. The structural characterization of rapidly solidified melt-spun ribbons and spark plasma sintered bulk specimens reveal a characteristic presence of MnSi and Si as secondary phases in Si-deficient (x ≈ 1.5) and Si-excess (x ≈ 2) compositions of nominal MnSi_x, respectively, that coexists with the major MnSi_γ based NCL phase. This study provides the fundamental basis of phase evolution and the relevance of odd stoichiometry in silicon-rich manganese silicides which exhibit fascinating phonon dynamics in optimal MnSi_1.74 stoichiometry.     

The effect of precipitates on the evolution of a martensitic phase boundary

In this article, we study the role of defects in the quasistatic evolution of a martensitic phase boundary. The formulation of the model gives rise to a nonlocal free boundary problem, for which we present an implicit finite-time discretization. For an approximate model, assuming a nearly flat interface, we show that the phase boundary exhibits a sick-slip behavior in the presence of a heterogeneous environment, thus leading to a transition from viscous kinetics to rate-independent behavior. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)