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The first stable stannanetriol and germanetriol bearing sterically congested aryl groups were synthesized by hydrolysis of the corresponding trichloro-stannane and -germane. The stannanetriol is monomeric in solution as well as in the crystalline state, as evidenced by X-ray diffraction analysis and CP-MAS NMR spectroscopy. The stannanetriol reacted with silicone grease to afford a cagelike compound having three Sn-O-Si-O-Sn linkages in the molecule. All the group 14 triols can be converted to the corresponding trihalo compounds in good yields. Key words: stannanetriol, germanetriol, silicone grease, cyclostannasiloxane. Les premieres molecules de stannanetriol et germanetriol stables, portant des groupes aryl steriquement encombres, ont ete synthetisees par hydrolyses respectives des groupes trichloro-stannane et -germane. Le stannanetriol est monomerique aussi bien en solution qu'a l'etat cristallin, comme le montrent l'analyse par diffraction des rayons X et la spectroscopie RMN CP-MAS. Le stannanetriol a reagi avec la graisse de silicone pour laisser apparaitre dans la molecule un compose cage comportant trois liaisons Sn-O-Si-O-Sn. Tous les triols contenant un element du groupe 14 peuvent etre convertis, avec de bons rendements, en leurs composes trihalogenes respectifs. [Traduit par la Redaction] Mots-cles: stannanetriol, germanetriol, graisse de silicone, cyclostannasiloxane., Introduction In contrast to alcohols, heavier group 14 metallanols containing M-OH (M = Si, Ge, or Sn) bonds are relatively unstable species because intermolecular condensation providing the corresponding metalloxanes with [...]

Fusion between the membrane of HIV and the membrane of a host cell is a crucial step in HIV infection and is catalyzed by the binding of the fusion peptide domain (HFP) of the HIV gp41 protein to the host cell membrane. The HFP by itself induces vesicle fusion and is a useful model system to understand the fusion peptide/host cell membrane interaction. This article reports an experimental correlation between the membrane locations of different HFP constructs and their fusogenicities. The constructs were the HFP monomer with Val-2 to Glu-2 mutation (HFPmn_mut), wild type HFP monomer (HFPmn), and wild type HFP trimer (HFPtr). All constructs have predominant [beta] sheet structure in membranes with physiologically relevant cholesterol content. HFPmn_mut does not fuse vesicles, HFPmn has moderate fusion rate, and HFPtr has the putative oligomerization state of HIV gp41 and a very rapid fusion rate. The HFP membrane locations were probed with solid-state NMR measurements of distances between labeled carbonyl ([sup.13]CO) nuclei in the HFP backbone and lipid nuclei in the surface or interior regions of the membrane bilayer. HFPmn_mut is located at the membrane surface, HFPmn is inserted into a single membrane leaflet, and HFPtr is the most deeply inserted construct with contact with the center of the membrane. These results show a clear positive correlation between the insertion depths and the fusion activities of the HFP constructs. Other disease-causing enveloped viruses contain fusion peptides and this correlation may be a general structure-function model for these peptides. NMR | enveloped virus | AIDS | trimer | location

A 'one-pot' process for the preparation of organic-silica hybrid capillary monolithic columns by concurrently using organic monomers and alkoxysilanes was described. In this process, the hydrolyzed alkoxysilanes of tetramethoxysilane (TMOS) and vinyltrimethoxysilane (VTMS) as precursors for the synthesis of a silica-based monolith using the sol--gel method and the organic monomer (allyldimethyldodecylammonium bromide (ADDAB) or acrylamide (AA)) with vinyl groups for free radical polymerization along with the initiator of azobisisobutyronitrile (AIBN) were concurrently introduced into a pretreated capillary; after that, the polycondensation of alkoxysilanes and the copolymerization of organic monomers and asprecondensed siloxanes were subsequently carried out within the confines of a capillary at the proper reaction conditions. Two types of organic-silica hybrid capillary monolithic columns with hydrophobic and hydrophilic properties have been fabricated, respectively, by this 'one-pot' process using two different organic monomers of ADDAB and AA. The morphologies of the synthesized organic-silica hybrid monolithic columns were characterized by scanning electron microscopy (SEM). The performances of these organic-silica monolithic columns were investigated by capillary electrochromatography (CEC). The retention behaviors of the neutral and polar compounds on the resulting hydrophobic and hydrophilic organic-silica hybrid monolithic columns confirmed the successful incorporation of organic monomers in the silica monolithic matrix. In addition, the ADDA-silica hybrid capillary monolithic column was also applied in the analysis of tryptic digests of bovine serum albumin (BSA) and mouse liver extract by microliquid chromatographytandem mass spectrometry ([micro]LC--MS/MS) for demonstrating its potential in proteome analysis.

In the cell, protein complexes form by relying on specific interactions between their monomers. Excluded volume effects due to molecular crowding would lead to correlations between molecules even without specific interactions. What is the interplay of these effects in the crowded cellular environment? We study dimerization of a model homodimer when the mondimers are free and when they are tethered to each other. We consider a structured environment: Two monomers first diffuse into a cavity of size L and then fold and bind within the cavity. The folding and binding are simulated by using molecular dynamics based on a simplified topology based model. The confinement in the cell is described by an effective molecular concentration C ~ [L.sup.-3]. A two-state coupled folding and binding behavior is found. We show the maximal rate of dimerization occurred at an effective molecular concentration [C.sup.op] [equivalent] 1 mM, which is a relevant cellular concentration. In contrast, for tethered chains the rate keeps at a plateau when C < [C.sup.op] but then decreases sharply when C > [C.sup.op]. For both the free and tethered cases, the simulated variation of the rate of dimerization and thermodynamic stability with effective molecular concentration agrees well with experimental observations. In addition, a theoretical argument for the effects of confinement on dimerization is also made. molecular crowding | folding and binding | effective molecular concentration | Arc homodimer monolayer | native topology-based models

The principal histopathological feature of Alzheimer's disease is the presence of [beta]-amyloid (A[beta]) aggregates in the gray matter of the brain, and researchers believe that various environmental factors play significant roles in the conformational change and self-assembly of A[beta] peptides. Therefore, discovering a rapid and convenient analytical method of evaluating the environmental factors on A[beta] aggregation would have a considerable impact. Herein we report our development of a novel microfluidic screening system enabling high-throughput analysis, low-consumption of reagents, and short analytical time. Microchannels with a cross-sectional dimension of 100 [micro]m x 100 [micro]m were immobilized with A[beta] monomers via N-hydroxysuccinimide ester activation of the internal surfaces, and then a fresh A[beta] monomer solution mixed with different small molecules or metal ions was continuously introduced into the microchannels to induce A[beta] aggregation. In this work, we investigated (1) the temporal evolution of A[beta] aggregation within microchannels, (2) the high-throughput screening of the inhibitory effect of 12 small molecules against A[beta] aggregation, and (3) the effect of different metal ions ([Fe.sup.3+], [Cu.sup.2+], [Zn.sup.2+], and [A1.sup.3+]) on A[beta] aggregation by using thioflavin T (ThT)-induced fluorescence microscopy and ex situ atomic force microscopy. The microfluidic system should contribute to a simultaneous analysis of multiple environmental factors affecting amyloid aggregates in a parallel manner and to screen therapeutic small molecules prior to their in vivo evaluation.

The Gram-negative bacterium Vibrio cholerae is the causative agent of a severe diarrheal disease that afflicts three to five million persons annually, causing up to 200,000 deaths. Nearly all V. cholerae strains produce a large multifunctional-autoprocessing RTX toxin (MART[X.sub.Vc]), which contributes significantly to the pathogenesis of cholera in model systems. The actin cross-linking domain (ACD) of MART[X.sub.Vc] directly catalyzes a covalent cross-linking of monomeric G-actin into oligomeric chains and causes cell rounding, but the nature of the cross-linked bond and the mechanism of the actin cytoskeleton disruption remained elusive. To elucidate the mechanism of ACD action and effect on actin, we identified the covalent cross-link bond between actin protomers using limited proteolysis, X-ray crystallography, and mass spectrometry. We report here that ACD catalyzes the formation of an intermolecular iso-peptide bond between residues E270 and K50 located in the hydrophobic and the DNasel-binding loops of actin, respectively. Mutagenesis studies confirm that no other residues on actin can be cross-linked by ACD both in vitro and in vivo. This cross-linking locks actin protomers into an orientation different from that of F-actin, resulting in strong inhibition of actin polymerization. This report describes a microbial toxin mechanism acting via iso-peptide bond cross-linking between host proteins and is, to the best of our knowledge, the only known example of a peptide linkage between nonterminal glutamate and lysine side chains. cross-linking | X-ray crystallography | mass spectrometry

The basic unit in microtubules is [alpha][beta]-tubulin, a heterodimer consisting of an [alpha]- and a [beta]-tubulin monomer. The mechanical characteristics of the dimer as well as of the individual monomers may be used to obtain new insight into the microtubule tensile properties. In the present work, we evaluate the elastic constants of each monomer and the interaction force between them by means of molecular dynamics simulations. Molecular models of [alpha]-, [beta]-, and [alpha][beta]-tubulins were developed starting from the 1TUB.pdb structure from the RCSB database. Simulations were carried out in a solvated environment by using explicit water molecules. In order to measure the monomers' elastic constants, simulations were performed by mimicking experiments carried out with atomic force microscopy. A different approach was used to determine the interaction force between the [alpha]- and [beta]-monomers by using 16 different monomer configurations based on different intermonomer distances. The obtained results show an elastic constant value for [alpha]-tubulin of 3.8-3.9 N/m, while for the [beta]-tubulin, the elastic constant was measured to be 3.3-3.6 N/m. The maximum interaction force between the monomers was estimated to be 11.9 nN. A mechanical model of the tubulin dimer was then constructed and, using the results from MD simulations, Young's modulus was estimated to be 0.6 GPa. A fine agreement with Young's modulus values from literature (0.1-2.5 GPa) is found, thus validating this approach for obtaining molecular scale mechanical characteristics. In perspective, these outcomes will allow exchanging atomic level description with key mechanical features enabling microtubule characterization by continuum mechanics approach. [DOI: 10.1115/1.2913330] Keywords: tubulin, microtubules, molecular dynamics, mechanical properties

This paper analyzes the equilibria involved in the dimerization of monomeric receptors with homo-bifunctional ligands. We provide analytical expressions that can be used to estimate the concentration of each species present in a mixture of homo-bifunctional ligand and monomeric proteins, given initial conditions defining the total concentration of bivalent ligand [[[L.sub.2].sub.0], the total concentration of protein [[P].sub.0], one dissociation constant [K.sub.d], and a parameter to account for cooperativity [alpha]. We demonstrate that the fraction of protein present in a complex of two proteins and one bivalent ligand (P * [L.sub.2] * P) is maximized at [[[L.sub.2].sub.0] = [K.sub.d]/2 + [[P].sub.0]/2.

Gadd45 proteins are recognized as tumor and autoimmune suppressors whose expression can be induced by genotoxic stresses. These proteins are involved in cell cycle control, growth arrest, and apoptosis through interactions with a wide variety of binding partners. We report here the crystal structure of Gadd45[gamma] which reveals a fold comprising an [alpha][beta][alpha] sandwich with a central fivestranded mixed [beta]-sheet with [alpha]-helices packed on either side. Based on crystallographic symmetry we identified the dimer interface of Gadd45[gamma], dimers by generating point mutants that compromised dimerization while leaving the tertiary structure of the monomer intact. The dimer interface comprises a four-helix bundle involving residues that are the most highly conserved among Gadd45 isoforms. Cell-based assays using these point mutants demonstrate that dimerization is essential for growth inhibition. This structural information provides a new context for evaluation of the plethora of protein-protein interactions that govern the many functions of the Gadd45 family of proteins. crystallography | growth inhibition | tumor suppressor

A series of Tn5 transposases (Tnp's) with mutations at the conserved amino acid position W450, which was structurally predicted to be important for synapsis, have been generated and characterized. This study demonstrates that W450 is involved in hydrophobic (and possibly aromatic) contacts within the Tnp monomer that negatively regulate synaptic complex formation.

SecA is an essential component in the Sec-dependent protein translocation pathway and, together with ATP, provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. Previous studies established that SecA undergoes monomer-dimer equilibrium in solution. However, the oligomeric state of functional SecA during the protein translocation process is controversial. In this study, we provide additional evidence that SecA functions as a dimer in the membrane by (i) demonstration of the capability of the presumably monomeric SecA derivative to be cross-linked as dimers in vitro and in vivo, (ii) complementation of the growth of a secA(Ts) mutant with another nonfunctional SecA or (iii) in vivo complementation and in vitro function of a genetically tandem SecA dimer that does not dissociate into monomers, and (iv) formation of similar ring-like structures by the tandem SecA dimer and SecA in the presence of lipid bilayers. We conclude that SecA functions as a dimer in the membrane and dissociation into monomers is not necessary during protein translocation.

Amyloid [beta]-protein (A[beta]) oligomers may be the proximate neurotoxins in Alzheimer's disease (AD). Recently, to elucidate the oligomerization pathway, we studied A[beta] monomer folding and identified a decapeptide segment of A[beta], [sup.21]Ala-[sup.22]Glu-[sup.23]Asp-[sup.24] Val-[sup.25]Gly-[sup.26]Ser-[sup.27]Asn-[sup.28]Lys-[sup.29]Gly-[sup.30]Ala, within which turn formation appears to nucleate monomer folding. The turn is stabilized by hydrophobic interactions between Val-24 and Lys-28 and by long-range electrostatic interactions between Lys-28 and either Glu-22 or Asp-23. We hypothesized that turn destabilization might explain the effects of amino acid substitutions at Glu-22 and Asp-23 that cause familial forms of AD and cerebral amyloid angiopathy. To test this hypothesis, limited proteolysis, mass spectrometry, and solution-state NMR spectroscopy were used here to determine and compare the structure and stability of the A[beta](21-30) turn within wild-type A[beta] and seven clinically relevant homologues. In addition, we determined the relative differences in folding free energies ([DELTA][DELTA][G.sub.f]) among the mutant peptides. We observed that all of the disease-associated amino acid substitutions at Glu-22 or Asp-23 destabilized the turn and that the magnitude of the destabilization correlated with oligomerization propensity. The Ala21Gly (Flemish) substitution, outside the turn proper (Glu-22-Lys-28), displayed a stability similar to that of the wild-type peptide. The implications of these findings for understanding A[beta] monomer folding and disease causation are discussed.

Aggregation properties of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine are examined in buffer and in the presence of electrolytes, beta-cyclodextrin, micelles and DNA. These dyes were found to form aggregates in buffer and methanol-water solutions that have absorption bands blue-shifted to those of the monomeric forms.

The ultrafast pump-probe spectroscopic evidence suggesting the occurrence of excited-state proton transfer in gas-phase clusters of hydrated 7-azaindole monomer was presented. The experiments state that the excited - state proton transfer in the gas-phase hydrated 7-azaindole monomer can occur by means of water proton bridge.

A study is conducted to determine energy gaps for --OMe and --NO2 alpha,alpha'-substituted oligothiophenes up to six monomers using semiempirical, Hartree--Fock and density functional methods. The implications for predictions of HOMO--LUMO gaps of pi-conjugated systems are examined and analyzed in terms of designing new materials with controlled properties.

Full equilibrium free energy interactions between charged surfaces in polyelectrolyte solutions are reported. Perturbations in the isotension ensemble are coupled to a free energy variance minimization scheme to simulate the interactions.

The concept of an ion-sensitive hydrogel ionic reservoir is demonstrated for hydrogel particles of three different size ranges. On the basis of the experimental results, a two-step mechanism affecting the kinetics of proton uptake into the hydrogel particles with ionizable groups is proposed, first being the fast binding of ions to the immediate surface of each particle and second, a slower successive diffusion of bound sites into the nest inner layer of polymer network.

The photophysical properties of the aqueous dispersion of aggregates of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are investigated. Results reveal that the fluorescence of such a system displays a temperature-dependent change in the visible region of the spectrum ratiometrically, with very good performances in terms of reversibility, robustness, and stability, which might be potentially applicable in the design of temperature sensors.

The transition energies of a zinc porphyrin monomer and its meso-meso-linked arrays are calculated by using time-dependent density functional theory. The processes in which transition energies of the dimer array are shifted by an applied electric field are examined.

A study was conducted to examine the structural and optical properties of solution phase monomeric thionine and thionine monomer, and aggregate encapsulated within mesoporous AIMCM-48. The result shows that H-aggregates do not form within the one-dimensional channels of aluminum-containing MCM-41 with pore size of circa 24 Angstrom.

The characterization and shape-selective hydrogenation activity of the Rh-monomer complex catalyst imprinted with alpha-methylbenzylamine at a SiO2 surface is reported. The study revealed that the imprinted Rh is highly active for alpha-methylstyrene hydrogenation, whereas other supported Rh structures before the imprinting have no activity for the reaction.

The quenching of multibubble sonoluminescence, MBSL, by seven organic monomers in aqueous solution using ultrasound at two frequencies is reported. The results at 515 kHz closely parallel those reported for other simple organic compounds in solution. However, at 20 kHz, significant differences from those at the higher frequency are observed with lower degrees of quencing.

The reactions of benzaldehydes with electron-donating substituents on the aromic ring exhibited a more significant nonlinear effect in these reactions than the electron-withdrawing substituents. The modification of Noyori model rationalizes the apparently anomalous results that were observed through a simple extension of the Noyori model to allow for nonthermodynamically controlled monomer/dimer partitioning.

Extremely long molecular chains can self-assemble from two nucleobase-like chemicals, report Nicholas Hud at the Georgia Institute of Technology in Atlanta, and his co-workers. The team showed that three of [...]