Stabilization of the Alleged 'Bishomoaromatic' Bicyclo[3.2.l]octa-2,6-dienyl Anion by Counterion Interactions and by Hyperconjugation

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution.

Low-temperature stabilization of pure Ni(IT)O

Ordinary nickel (II) oxide prepared by any of the usual methods is always contaminated with some higher valency states of nickel. Such nickel oxide however can be converted to pure nickel (II) oxide by soaking for some hours at 800–850°C in a closed system under a static inert atmosphere and in the presence of pure MnO as oxygen-getter. The resulting Ni(IT)O is highly resistant to oxidation.

Time- and Temperature-Dependent Study in the Three-Component Zinc-Triazolate-Oxybis(benzoate) System: Stabilization of New Topologies

Three new three-dimensional zinc-triazolate-oxybis(benzoate) compounds. [{Zn-3(H2O)(2)}{C12H8O(COO)(2)}(2)-{C2H2N3}(2)]center dot 2H(2)O(I), [Zn-7{C12H8O(COO)(2)}(4){C2H2N3}(6)]center dot H2O, (II), and[{Zn-5(OH)(2)}{C12H8O(COO)(2)}(3){C2H2N3}(2)] (III), synthesized by a hydrothermal reaction of a mixture of Zn(OAc)(2)center dot 2H(2)O, 4,4'-oxybis(benzoic acid), 1,2,4-triazole, NaOH, and water. Compound I has an interpenetrated diamond structure and II and III have pillared-layer related structures. The formation of a hydrated phase (I) at low temperature and a completely dehydrated phase (III) at high temperature suggests the importance of thermodynamic factors in the formation of three compounds. Transformation studies of I in the presence of water shows the formation of a simple Zn-OBA compound, [Zn(OBA)(H2O)] (IV), at 150 and 180 degrees C and compound III at 200 degrees C. The compounds have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction. thermogravimetric analysis, IR, and photoluminescence studies.

Stabilization of linear and nonlinear dynamical systems using an observer-controller configuration

In this paper the problem of stabilization of systems by means of stable compensations is considered, and results are derived for systems using observer�controller structures, for systems using a cascade structure, and for nonlinear systems

Stabilization of the collagen structure by hydroxyproline residues

The molecular structure of collagen is now accepted to be based on a triple-stranded coiled-coil, in which the three strands are held together predominantly by hydrogen bonds. Recent experimental evidence has shown that the presence of hydroxyproline residues in the third position of the repeating tripeptide unit lends additional stability to the collagen structure. In this paper, we report a model structure, which is supported by these observations. In a model structure proposed earlier, there are two hydrogen bonds per tripeptide unit, one of which is a direct interchain hydrogen bond, while the second hydrogen bond can be formedvia a water molecule. It has now been shown that the same water molecule can also form a hydrogen bond with the oxygen of theγ-hydroxyl group of hydroxyproline in the third position in the sequence (Gly-R2-R3). This hydroxyl group can also take part in an inter-triple-helix hydrogen bond. Our studies thus show the role played by hydroxyproline residues in the structure and stability of collagen.

Thermal stabilization of glucose oxidase and glucoamylase by physical entrapment

Physical entrapment was used as an approach to achieve thermal stabilization of enzymes. The ti values for the thermoinactivation of glucose oxidase and glucoamylase were increased several-fold by their entrapment in polyacrylamide gels. In polyacrylate gels the individual enzymes behaved differently, probably owing to microenvironmental effects arising by the polyelectrolyte nature of the carrier.

On the stabilization of uniformly decoupled systems by state variable feedback

Conditions under which the asymptotic stabilization of uniformly decoupled time-varying multivariate systems is possible are explored. This is accomplished by developing a canonical form for integrator uniformly decoupled system in which the coefficient matrices have a simple structure. The procedures developed rely on certain conditions on the given system and yield explicit expressions for the stabilization compensators.

Structural Modes of Stabilization of Permissive Phosphorylation Sites in Protein Kinases: Distinct Strategies in Ser/Thr and Tyr Kinases

Protein kinases phosphorylate several cellular proteins providing control mechanisms for various signalling processes. Their activity is impeded in a number of ways and restored by alteration in their structural properties leading to a catalytically active state. Most protein kinases are subjected to positive and negative regulation by phosphorylation of Ser/Thr/Tyr residues at specific sites within and outside the catalytic core. The current review describes the analysis on 3D structures of protein kinases that revealed features distinct to active states of Ser/Thr and Tyr kinases. The nature and extent of interactions among well-conserved residues surrounding the permissive phosphorylation sites differ among the two classes of enzymes. The network of interactions of highly conserved Arg preceding the catalytic base that mediates stabilization of the activation segment exemplifies such diverse interactions in the two groups of kinases. The N-terminal and the C-terminal lobes of various groups of protein kinases further show variations in their extent of coupling as suggested from the extent of interactions between key functional residues in activation segment and the N-terminal αC-helix. We observe higher similarity in the conformations of ATP bound to active forms of protein kinases compared to ATP conformations in the inactive forms of kinases. The extent of structural variations accompanying phosphorylation of protein kinases is widely varied. The comparison of their crystal structures and the distinct features observed are hoped to aid in the understanding of mechanisms underlying the control of the catalytic activity of distinct subgroups of protein kinases.

Stabilization of an All-Metal Antiaromatic Molecule (Al4Li4) Using BH and C as Caps

It has been reported by Pati et al. (J. Am. Chem. Soc. 2005, 127, 3496) that coordination with a transition metal can stabilize the “antiaromatic”, all-metal compound Al4Li4. Here, we report that it can also be stabilized by capping with a main group element like C and its isoelectronic species BH. Our calculations of binding energy, nuclear independent chemical shift, energy decomposition analysis, and molecular orbital analysis support the capping-induced stability, reduction of bond length alternation, and increase of aromaticity of these BH/C-capped Al4Li4 systems. The interaction between px and py orbitals of BH/C and the HOMO and LUMO of Al4Li4 is responsible for the stabilization. Our calculations suggest that capping can introduce fluxionality at room temperature.

Stabilization of unusual structures in peptides using alpha,beta-dehydrophenylalanine: Crystal and solution structures of Boc-Pro-Delta Phe-Val-Delta Phe-Ala-OMe and Boc-Pro-Delta Phe-Gly-Delta Phe-Ala-OMe

The structures of two dehydropentapeptides, Boc-Pro-Delta Phe-Val-Delta Phe-Ala-OMe (I) and Boc-Pro-Delta Phe-Gly-Delta Phe-Ala-OMe (II) (Boc: t-butoxycarbonyl), have been determined by nuclear magnentic resonance (NMR), circular dichroism (CD), and X-ray, crystallographic studies. The peptide I assumes a S-shaped flat beta-bend structure, characterized by two partially overlapping type II beta-bends and absence of a second 1 <- 4 (N4-H center dot center dot center dot O1') intramolecular hydrogen bond. This is in contrast to the generally observed 3(10)-helical conformation in peptides with Delta Phe at alternate positions. This report describes the novel conformation assumed by peptide I and compares it with that of the conserved tip of the V3 loop of the HIV-1 envelope glycoprotein gp120 (sequence, G:P319 to F:P324, PDB code IACY). The tip of the V3 loop also assumes a S-shaped conformation with Arg:P322, making an intramolecular side-chain-backbone interaction with the carbonyl oxygen of Gly:P319. Interestingly, in peptide I, C(gamma)HVal(3) makes a similar side-chain-backbone C-H center dot center dot center dot O hydrogen bond with the carbonyl oxygen of the Boc group. The observed overall similarity indicates the possible use of the peptide as a viral antagonist or synthetic antigen. Peptide 11 adopts a unique turn followed by a 3(10)-helix. Both peptides I and II are classical examples of stabilization of unusual structures in oligopeptides.

Unconventional mechanism of stabilization of a tetragonal phase in the perovskite ferroelectric (PbTiO3)(1-x)(BiFeO3)(x)

Temperature-dependent x-ray powder-diffraction study of the tetragonal compositions of PbTiO3-BiFeO3 series has revealed that, unlike for all the known ferroelectric perovskites, the compositions exhibiting giant tetragonality is stabilized from the cubic phase via a complex transition pathway which involve (i) formation of minor monoclinic phase with a large pseudotetragonality along with an intermediate tetragonal phase (major) with a small tetragonality, (ii) gradual vanishing of the intermediate tetragonal phase and concomitant increase in the monoclinic regions, and finally (iii) gradual transformation of the monoclinic phase to the tetragonal phase with giant tetragonality.The system seems to adopt such a complex transition pathway to create amicrostructure with very large number of domains and interfaces for stress relief, which would not have been possible in case of a direct cubic-tetragonal transition.

Stabilization of the bio-membrane by small molecules: interaction of trehalose with the phospholipid bilayer

Anhydrobiotic organisms undergo periods of acute dehydration during their life cycle. It is of interest to understand how the biomembrane remains intact through such stress. A disaccharide, trehalose, which is metabolised during anhydrobiosis is found to prevent disruption of model membrane systems. Molecular modelling techniques are used to investigate the possible mode of interaction of trehalose with a model monolayer. The objective is to maximise hydrogen bonding between the two systems. A phospholipid matrix consisting of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) is chosen to represent the monolayer. The crystal structure of DMPC reveals that there are two distinct conformers designated as A and B. An expansion of the monolayer, coplanar with its surface, results in the trehalose molecule being accommodated in a pocket formed by four B conformers. One glucose ring of the sugar rests on the hydrophobic patch provided by the choline methyls of an A conformer. Five hydrogen bonds are formed involving the phosphate oxygens of three of the surrounding B conformers. The model will be discussed with reference to relevant experimental data on the interaction.

Stabilization of α-nickel hydoxide in the presence of organic additives: chemical route to bulk synthesis

Organic molecules such as glucose or lactose mediate the synthesis and stabilize alpha-nickel hydroxide in a simple precipitation reaction, while, in the absence of these additives, beta-nickel hydroxide is formed. The additives are not incorporated in the product phase.