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.     

Direct In Situ Nitridation of Nanostructured Metal Oxide Deposited Semiconductor Interfaces: Tuning the Response of Reversibly Interacting Sensor Sites

Metal‐oxide nanostructure‐decorated extrinsic semiconductor interfaces modified through in situ nitridation greatly expand the range of sensor interface response. Select metal‐oxide sites, deposited to an n‐type nanopore‐coated microporous interface, direct a dominant electron‐transduction process for reversible chemical sensing, which minimizes chemical‐bond formation. The oxides are modified to decrease their Lewis acidity through a weak interaction to form metal oxynitride sites. Conductometric and X‐ray photoelectron spectroscopy measurements demonstrate that in situ treatment changes the reversible interaction with the analytes NH₃ and NO. The sensor range is extended, which creates a distinct new family of responses determined by the Lewis acidity/basicity of a given analyte relative to that of the nanostructures chosen to decorate the interface. The analyte response, broadened in a substantial and predictable way by nitridation, is explained by the recently developing inverse hard/soft acid/base model (IHSAB) of reversible electron transduction.     

Structural and energetic properties of RMX3-NH3 complexes

We have explored the structural and energetic properties of a series of RMX₃-NH₃ (M=Si, Ge; X=F, Cl; R=CH₃, C₆H₅) complexes using density functional theory and low-temperature infrared spectroscopy. In the minimum-energy structures, the NH₃ binds axially to the metal, opposite a halogen, while the organic group resides in an equatorial site. Remarkably, the primary mode of interaction in several of these systems seems to be hydrogen bonding (C-H--N) rather than a tetrel (N→M) interaction. This is particularly clear for the RMCl₃-NH₃ complexes, and analyses of the charge distributions of the acid fragment corroborate this assessment. We also identified a set of metastable geometries in which the ammonia binds opposite the organic substituent in an axial orientation. Acid fragment charge analyses also provide a clear rationale as to why these configurations are less stable than the minimum-energy structures. Matrix-isolation infrared spectra provide clear evidence for the occurrence of the minimum-energy form of CH₃SiCl₃–NH₃, but analogous results for CH₃GeCl₃–NH₃ are less conclusive. Computational scans of the M-N distance potentials for CH₃SiCl₃–NH₃ and CH₃GeCl₃–NH₃, both in the gas phase and bulk dielectric media, reveal a great deal of anharmonicity and a propensity for condensed-phase structural change.     

Synthesis of Highly Enantioenriched Sulfonimidoyl Fluorides and Sulfonimidamides by Stereospecific Sulfur–Fluorine Exchange (SuFEx) Reaction

Sulfonimidamides present exciting opportunities as chiral isosteres of sulfonamides, with potential for additional directional interactions. Here, we present the first modular enantioselective synthesis of sulfonimidamides, including the first stereoselective synthesis of enantioenriched sulfonimidoyl fluorides, and studies on their reactivity. A new route to sulfonimidoyl fluorides is presented from solid bench-stable, N-Boc-sulfinamide (Boc=tert-butyloxycarbonyl) salt building blocks. Enantioenriched arylsulfonimidoyl fluorides are shown to be readily racemised by fluoride ions. Conditions are developed, which trap fluoride and enable the stereospecific reaction of sulfonimidoyl fluorides with primary and secondary amines (100 % es, es=enantiospecificity) generating sulfonimidamides with up to 99 % ee. Aryl and alkyl sulfonimidoyl fluoride reagents are suitable for mild late stage functionalisation reactions, exemplified by coupling with a selection of complex amines in marketed drugs.     

Application of localized molecular orbitals to the solution of semiempirical self-consistent field equations

When conventional matrix algebra is used to solve the semiempirical self-consistent field equations for large systems, the time required rises as the third power of the size of the system. A consequence of this is that self-consistent calculations of large systems such as enzymes are impractical. By using localized molecular orbitals instead of matrix methods, the time required for these systems can be made almost proportional to the size of the system. In partial geometry optimizations, the time required depends only upon the size of the fragment being optimized and is almost independent of the size of the whole system. © 1996 John Wiley & Sons, Inc.     

Syndioselective propylene polymerizations by ansa-zirconocenium catalyst

The effect of reaction conditions, including catalyst concentration, temperature, and immobilization on support, have been investigated for syndioselective propylene polymerization by the “bare” zirconocenium ion generated from 1,1-diphenyl-methylidene(1-η⁵-cyclopentadienyl)(9-η⁵-fluorenyl)zirconium-dichloride precursor (2). Neither variation of the catalyst concentration nor immobilization of 2 on silica support affect the syndiospecificity of polymerization. The stereoregularity of the syndiotactic polypropylene, as judged from the melting transition temperature and homosteric r-pentad population by ¹³C-NMR, were found to be proportional to polymer molecular weight. These behaviors are compared with a typical isoselective catalyst ethylenebis(4,5,6,7-tetrahydroindenyl) Zr precursor (4). They are in close resemblance in the case of the S-enantiomeric complex of 4, but the racemic mixture of 4 is markedly inferior. The origins of stereo- and regio-errors are discussed. © 1996 John Wiley & Sons, Inc.     

Cyclic antiautomorphisms of directed triple systems

A transitive triple, (a,b,c), is defined to be the set {(a,b), (b,c), (a,c)} of ordered pairs. A directed triple system of order v, DTS(v), is a pair (D,β), where D is a set of v points and β is a collection of transitive triples of pairwise distinct points of D such that any ordered pair of distinct points of D is contained in precisely one transitive triple of β. An antiautomorphism of a Directed triple system, (D,β), is a permutation of D that maps β to β⁻¹, where β ⁻¹ = {(c,b,a)|(a,b,c) E β}. In this article we give necessary and sufficient conditions for the existence of a Directed triple system of order v admitting an antiautomorphism consisting of a single cycle of length d and having v − d fixed points. Further, we give a more general result for partial Directed triple systems in which the missing ordered pairs are precisely those containing two fixed points. © 1996 John Wiley & Sons, Inc.     

Design of aromatic copolyesters for use in microelectronic devices

The next generation of microelectronic devices will require design of new kinds of polymers which are tailored to display a dielectric constant as low as 2.0, a coefficient of thermal expansion (CTE) which matches the metal circuitry as well as the silicon substrate, moisture and chemical resistance, and dimensional stability at processing temperatures of 300-400°C which are required to form an adhesive bond between laminates. In our program, we have made significant progress in all of these areas through the use of liquid crystalline copolyesters (LCP's) laminated with a newly developed crosslinkable copolyester. In this paper, data are presented which illustrate how films and coatings of either system can be made to adhere in the solid state by interchain transesterification reactions (ITR) at the interface. Ability to foam the crosslinkable copolyester films provides direct control over the dielectric constant. The effect of pore size and distribution on the dielectric constant will be discussed. The potential to dramatically increase the melting point of the LCP's through high temperature annealing is also discussed.     

Polymer research directions to meet the needs of the 21st century

The polymer industry in the world has grown from about 15 million pounds in 1923 to over 60 billion pounds in 1990 representing a compound annual growth rate of 15% for nearly 70 years. This dynamic behavior will continue in the 21st century. However, the growth will be faster in the developing nations than inthe developed nations. The basis for this projection of continued growth is provided by looking at the estimated per capita consumption of polymers for different regions of the world shown in Figure 1.