Interaction between a novel gemini surfactant and cyclodextrin: NMR and surface tension studies

The interaction between cyclodextrins, hydroxypropyl-beta-cyclodextrin (HPbetaCD), and hydroxypropyl-gamma-cyclodextrin (HPgammaCD) and a novel type of nonionic surfactant synthesized from a fatty acid has been investigated. The so-called nonionic heterogemini surfactant (NIHG750) contains two hydrophobic groups and two hydrophilic groups, composed of one monomethyl ethylene glycol and one secondary OH group, CH(3)(CH(2))(7)-CH[OH]-CH[O(CH(2)CH(2)O)(16)CH(3)]-(CH(2))(7)CN. Surface tension studies indicate that micelles form in NIHG750 systems in both the presence and the absence of small quantities (molar ratio (HPbetaCD:NIHG750) approximately 2) of cyclodextrin (HPbetaCD or HPgammaCD). This gives NIHG surfactants an advantage compared to single-tailed nonionic surfactants, which generally lose their ability to micellize at much lower additions of cyclodextrins. However, the interaction between HPbetaCD and NIHG750 results in a disruption of the micellar aggregates at higher levels of cyclodextrin. In the dilute systems (C(NIHG750)<0.1% (w/w) approx) prolate-shaped mixed aggregates (HPbetaCD and NIHG750) form, with a short and a long axis of the order of 8-9 and 17-20 A, respectively. These gradually aggregate into micellar-like structures at higher concentrations. In the aqueous bulk phase HPbetaCD interacts mainly with the hydrophobic part of NIHG750, but both NMR and surface tension measurements indicate that an interaction with the hydrophilic part of NIHG750, as well, may exist. This interaction results in a better packing of NIHG750 at air-water interfaces. However, at elevated temperatures results from turbidity measurements indicate that NIHG750 and HPbetaCD interact mainly through the hydrophilic part of the surfactant; a decrease in the cloud point temperature is observed. The interaction of the larger cavity molecule, HPgammaCD, with NIHG750, on the other hand, seems to be relatively weak. The interaction, when present, most probably takes place through inclusion of the hydrophilic EO part of NIHG750. The results suggest that HPgammaCD in combination with NIHG750 is a better solubilizing system than with HPbetaCD.     

Dynamic light scattering studies of poly(ethylene oxide) adsorbed on Laponite: layer conformation and its effect on particle stability

Dynamic light scattering has been used to determine the hydrodynamic thickness of poly(ethylene oxide) (PEO) adsorbed on synthetic anisotropic clay particles (Laponite) as a function of molecular weight. The layer thicknesses, and their increase with molecular weight, indicate that the conformation of the adsorbed layer is very compact and is much smaller than those normally observed for polymer adsorption on flat interfaces. The aggregation kinetics of the polymer coated particles in 5 mM NaCl was analyzed in a quantitative manner, revealing that the potential barrier to aggregation is strongly enhanced when polymer is present.     

Lead hopping using SVM and 3D pharmacophore fingerprints

The combination of 3D pharmacophore fingerprints and the support vector machine classification algorithm has been used to generate robust models that are able to classify compounds as active or inactive in a number of G-protein-coupled receptor assays. The models have been tested against progressively more challenging validation sets where steps are taken to ensure that compounds in the validation set are chemically and structurally distinct from the training set. In the most challenging example, we simulate a lead-hopping experiment by excluding an entire class of compounds (defined by a core substructure) from the training set. The left-out active compounds comprised approximately 40% of the actives. The model trained on the remaining compounds is able to recall 75% of the actives from the "new" lead series while correctly classifying >99% of the 5000 inactives included in the validation set.     

Molecular dynamics simulation study of xyloglucan adsorption on cellulose surfaces: effects of surface hydrophobicity and side-chain variation

The effect of surface hydrophobicity and side-chain variation on xyloglucan adsorption onto cellulose microfibrils (CMF) is investigated via molecular dynamics simulations. A molecular model of CMF with (100), (010), (1–10), (110) and (200) crystal faces was built. We considered xylogluco-oligosaccharides (XGO) with three repeating units, namely (XXXG)_3, (XXLG)₃, and (XXFG)₃ (where each (1,4)-β-d-glucosyl residue in the backbone is given a one-letter code according to its substituents: G = β-d-Glc; X = α-d-Xyl-(1,6)-β-d-Glc; L = β-d-Gal-(1,2)-α-d-Xyl-(1,6)-β-d-Glc; F = α-l-Fuc-(1,2)-β-d-Gal-(1,2)-α-d-Xyl-(1,6)-β-d-Glc). Our work shows that (XXXG)₃ binds more favorably to the CMF (100) and (200) hydrophobic surfaces than to the (110), (010) and (1–10) hydrophilic surfaces. The origin of this behavior is attributed to the topography of hydrophobic CMF surface, which stabilizes (XXXG)₃ in flat conformation. In contrast, on the rough hydrophilic CMF surface (XXXG)₃ adopts a less favorable random-coil conformation to facilitate more hydrogen bonds with the surface. Extending the xyloglucan side chains from (XXXG)₃ to (XXLG)₃ hinders their stacking on the CMF hydrophobic surface. For (XXFG)₃, the interaction with the hydrophobic surface is as strong as (XXXG)₃. All three XGOs have similar binding to the hydrophilic surface. Steered molecular dynamics simulation was performed on an adhesive model where (XXXG)₃ was sandwiched between two CMF hydrophobic surfaces. Our analysis suggests that this sandwich structure might help provide mechanical strength for plant cell walls. Our study relates to a recently revised model of primary cell walls in which extensibility is largely determined by xyloglucan located in limited regions of tight contact between CMFs.     

Painlevé Classification Problems Featuring Essential Singularities

In this article we construct and solve all Painlevé-type differential equations of the second order and second degree that are built upon, in a natural well-defined sense, the "sn-log" equation of Painlevé, the general integral of which admits a movable essential singularity (elliptic function of a logarithm). This equation (which was studied by Painlevé in the years 1893–1902) is frequently cited in the modern literature to elucidate various aspects of Painlevé analysis and integrability of differential equations, especially the difficulty of detecting essential singularities by local singularity analysis of differential equations. Our definition of the Painlevé property permits movable essential singularities, provided there is no branching. While the essential singularity presents no serious technical problems, we do need to introduce new techniques for handling "exotic" Painlevé equations, which are Painlevé equations whose singular integrals admit movable branch points in the leading terms. We find that the corresponding full class of Painlevé-type equations contains three, and only three, equations, which we denote SD-326-I, SD-326-II, and SD-326-III, each solvable in terms of elliptic functions. The first is Painlevé's own generalization of his sn-log equation. The second and third are new, the third being a 15-parameter exotic master equation. The appendices contain results (in general, without uniqueness proofs) of related Painlevé classification problems, including full generalizations of two other second-degree equations discovered by Painlevé, additional examples of exotic Painlevé equations and Painlevé equations admitting movable essential singularities, and third-order equations featuring sn-log and other essential singularities.     

Effects of mechanical stretching on average orientation of cellulose and pectin in onion epidermis cell wall: A polarized FT-IR study

The organization of polysaccharides in plant cell walls is important for the mechanics of plant cells. Spectral analysis of cell walls by polarized IR can reveal polysaccharide organization, but may be complicated by dipoles not aligned with the backbone. For instance, analysis of uniaxially-aligned cellulose Iβ film revealed that the dipole transition vector of the 1160 cm^?1 band involving stretch vibrations of glycosidic C1–O–C4 linkages is approximately at 30° with respect to the backbone of the cellulose chain, because of coupling with C5–O–C1 bonds in the six-membered rings. In the case of homogalacturonan, the dipole transition vector of the ester carbonyl group vibration (ν_C=O, 1745 cm^?1) is expected to be nearly normal to the homogalacturonan backbone. Using this information and the dichroism equation, the change in net orientation of cell wall polymers upon mechanical stretch was determined by polarized IR analysis. Never-dried abaxial outer epidermal cell walls of the second scale of onion bulb were mechanically stretched along longitudinal or transverse directions with respect to the long axis of the cells and then dried while under mechanical stretch. The average orientations of both 1160 and 1745 cm^?1 vibration transition dipoles were rotated by ~5° and ~4°, respectively, along the stretch direction from their initial random distributions upon longitudinal strain by 14%; and by ~4° and ~3°, respectively, upon transverse strain by 12%. These results imply that both cellulose microfibrils and pectins in the cell wall are passively realigned along the stretch direction by external mechanical force. The analytical methodology developed here will be useful to study how cell wall polymers might reorganize during cell wall growth and development.     

Flurbiprofen encapsulation using pluronic triblock copolymers

Pulsed-field gradient stimulated-echo nuclear magnetic resonance (NMR) and surface tension measurements have been used to study the effect of drug addition on the micellization behavior of pluronic triblock copolymers (P103, P123, and L43). The addition of 0.6 wt% flurbiprofen to Pluronic P123 and P103 solutions reduced their cmc and promoted micellization. Also, a substantial increase in the hydrodynamic radius of Pluronic P103 from 5 to 10 nm was observed, along with an increased fraction of polymer micellized, demonstrating that the polymers solubilize this nonsteroidal anti-inflammatory drug.     

All Binomial-Type Painlevé Equations of the Second Order and Degree Three or Higher

In this paper we construct all rational Painlevé-type differential equations which take the binomial form, (d²y/dx²)ⁿ = F(x,y,dy/dx), where n ≥ 3, the case n = 2 having previously been treated in Cosgrove and Scoufis [1]. While F is assumed to be rational in the complex variables y and y′ and locally analytic in x, it is shown that the Painlevé property together with the absence of intermediate powers of y″ forces F to be a polynomial in y and y′. In addition to the six classes of second-degree equations found in the aforementioned paper, we find nine classes of higher-degree binomial Painlevé equations, denoted BP-VII,..., BP-XV, of which the first seven are new. Two of these equations are of the third degree, two of the fourth degree, three of the sixth degree, and two of arbitrary degree n. All equations are solved in terms of the first, second or fourth Painlevé transcendents, elliptic functions, or quadratures. In the appendices, we discuss certain closely related classes of second-order nth equations (not necessarily of Painlevé type) which can also be solved in terms of Painlevé transcendents or elliptic functions.