Interaction Of Monomers Research Articles

Adsorption of water on graphene: A van der Waals density functional study

The van der Waals density functional (vdW-DF) was used to investigate the interaction of a water monomer with graphene. It was found that a variant of vdW-DF [Hamada and Otani, Phys. Rev. B 82, 153412 (2010)] predicts geometries and energetics of water on graphene which are in good agreement with those obtained using more elaborate random-phase approximation and quantum Monte Carlo approaches. Interfacial electronic structures were also analyzed in detail.

Anomalous Swelling of Polymer Monolayers by Water Vapor

A recent experiment showed that when self-assembled monolayers of single-stranded DNA or PNA are exposed to water vapor, they first shrink and then swell with increasing humidity. In order to understand how a monolayer can shrink by absorbing water, we introduce a three-component lattice model consisting of polymer, water, and vacancies. We find that for moderate grafting densities, attractive monomer–water, and repulsive monomer–monomer interactions, at low water vapor concentrations, the adsorption of water is accompanied by enhanced expulsion of vacancies and compression of the monolayer. As humidity is further increased, continued adsorption of water molecules leads to swelling of the monolayer. The low humidity anomaly is predicted to disappear at high grafting densities.

Precision of Hunchback Expression in the Drosophila Embryo

Activation of the gap gene hunchback (hb) by the maternal Bicoid gradient is one of the most intensively studied gene regulatory interactions in animal development. Most efforts to understand this process have focused on the classical Bicoid target enhancer located immediately upstream of the P2 promoter. However, hb is also regulated by a recently identified distal shadow enhancer as well as a neglected "stripe" enhancer, which mediates expression in both central and posterior regions of cellularizing embryos. Here, we employ BAC transgenesis and quantitative imaging methods to investigate the individual contributions of these different enhancers to the dynamic hb expression pattern. These studies reveal that the stripe enhancer is crucial for establishing the definitive border of the anterior Hb expression pattern, just beyond the initial border delineated by Bicoid. Removal of this enhancer impairs dynamic expansion of hb expression and results in variable cuticular defects in the mesothorax (T2) due to abnormal patterns of segmentation gene expression. The stripe enhancer is subject to extensive regulation by gap repressors, including Kruppel, Knirps, and Hb itself. We propose that this repression helps ensure precision of the anterior Hb border in response to variations in the Bicoid gradient.

Adsorption from Oligomer–Monomer Solutions on the Surfaces Modified with End-Grafted Chains

A density functional theory has been applied to study adsorption from oligomer-monomer solutions on solid surfaces modified with end-grafted polymers. Chains are modeled as freely jointed tangent spheres. All spherical species interact via Lennard-Jones (12-6) potential. The grafted chains are not attracted by the surface. Fluid molecules interact with the substrate via Lennard-Jones (9-3) potential. It is shown how a shape of the relative excess adsorption isotherms depends on such parameters as the oligomer length, the grafting density, oligomer-substrate interactions, oligomer-grafted chains interactions, and oligomer-monomer interactions in the bulk solution. As attractive interactions between free and grafted chains strengthen, the relative excess adsorption isotherm can change from negative, through S-shaped with an adsorption azeotropic point, to positive in the whole concentration region. A change of oligomer-monomer interactions causes the opposite effects. For low grafting densities of attractive bonds, the relative excess of oligomers increases, reaches a maximum, and decreases. The structure of surface layers is also analyzed.

Osteoclasts on bone and dentin in vitro: Mechanism of degradation and comparison of resorption behaviour

Glucorticoids (GCs) belong since their discovery to the standard therapy of rheumatoid arthritis (RA), a severe auto-inflammatory bone disease. One major side effect of GCs affects the bone itself leading to GC induced osteoporosis (GIO), the most secondary osteoporosis. GCs act via a receptor (GR) that can alter gene expression by binding as a dimer to GC responsible elements in the promoter region of target genes or by interacting with and thus interfering with other transcription factors as a monomer. We determined the contribution of molecular mechanisms and cell types critically involved in antiinflammatory effects of GCs in RA and on bone using conditional and function selective GR knockout mice. We showed that dimerization of the GR in IL-17 producing T cells is indispensable for the anti-inflammatory effect in the mouse model antigen-induced arthritis. Now we demonstrate that, surprisingly, in the Kbx/N induced arthritis model the GR dimer in mesenchymal cells is critical for anti-inflammatory effects of GCs. Thus, for immunosuppression of arthritis the GR is required in distinct cell types, which are of hematopoietic and mesenchymal origin. In a model of GIO we showed that unexpectedly interaction of the GR monomer with AP-1, but not NF-kB in osteoblasts is decisive for bone loss. Our findings define new criteria for SEGRM that act as an anti-inflammatory and protect the bone. Indeed we identified one lead-compound that still suppresses NF-kB dependent gene expression but does not affect osteoblast differentiation and activity. Furthermorewe identifiednovel GR target genes by functional genomics anddeveloped a screening platform for novel GR agonist-derivatives not affecting osteoblast function. Taken together, our approach gives new insights into GC action on arthritis and bone that can be translated into new concepts for anti-inflammatory therapies preventing GIO. doi:10.1016/j.bone.2012.08.022 O22 Osteoclasts on bone and dentin in vitro: Mechanism of degradation and comparison of resorption behaviour T. Wurger, P. Roschger, E. Zwettler, P. Fratzl, M.J. Rogers, K. Klaushofer, M. Rumpler Ludwig Boltzmann Institute of Osteology at the Hanusch Hospital of WGKKand AUVATraumaCenter, 1stMed. Dept., HanuschHospital, Vienna, Austria Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany The Garvan Institute of Medical Research, Darlinghurst, New SouthWales,

Novel mechanisms of the glucocorticoid receptor in inflammatory bone disease and osteoporosis — New target genes and selective receptor ligands identified

Glucorticoids (GCs) belong since their discovery to the standard therapy of rheumatoid arthritis (RA), a severe auto-inflammatory bone disease. One major side effect of GCs affects the bone itself leading to GC induced osteoporosis (GIO), the most secondary osteoporosis. GCs act via a receptor (GR) that can alter gene expression by binding as a dimer to GC responsible elements in the promoter region of target genes or by interacting with and thus interfering with other transcription factors as a monomer. We determined the contribution of molecular mechanisms and cell types critically involved in antiinflammatory effects of GCs in RA and on bone using conditional and function selective GR knockout mice. We showed that dimerization of the GR in IL-17 producing T cells is indispensable for the anti-inflammatory effect in the mouse model antigen-induced arthritis. Now we demonstrate that, surprisingly, in the Kbx/N induced arthritis model the GR dimer in mesenchymal cells is critical for anti-inflammatory effects of GCs. Thus, for immunosuppression of arthritis the GR is required in distinct cell types, which are of hematopoietic and mesenchymal origin. In a model of GIO we showed that unexpectedly interaction of the GR monomer with AP-1, but not NF-kB in osteoblasts is decisive for bone loss. Our findings define new criteria for SEGRM that act as an anti-inflammatory and protect the bone. Indeed we identified one lead-compound that still suppresses NF-kB dependent gene expression but does not affect osteoblast differentiation and activity. Furthermorewe identifiednovel GR target genes by functional genomics anddeveloped a screening platform for novel GR agonist-derivatives not affecting osteoblast function. Taken together, our approach gives new insights into GC action on arthritis and bone that can be translated into new concepts for anti-inflammatory therapies preventing GIO. doi:10.1016/j.bone.2012.08.022 O22 Osteoclasts on bone and dentin in vitro: Mechanism of degradation and comparison of resorption behaviour T. Wurger, P. Roschger, E. Zwettler, P. Fratzl, M.J. Rogers, K. Klaushofer, M. Rumpler Ludwig Boltzmann Institute of Osteology at the Hanusch Hospital of WGKKand AUVATraumaCenter, 1stMed. Dept., HanuschHospital, Vienna, Austria Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany The Garvan Institute of Medical Research, Darlinghurst, New SouthWales,

Induction of active epigenetic DNA demethylation in osteoblastic and osteoclastic cells by sulforaphane, a naturally occurring isothiocyanate

Glucorticoids (GCs) belong since their discovery to the standard therapy of rheumatoid arthritis (RA), a severe auto-inflammatory bone disease. One major side effect of GCs affects the bone itself leading to GC induced osteoporosis (GIO), the most secondary osteoporosis. GCs act via a receptor (GR) that can alter gene expression by binding as a dimer to GC responsible elements in the promoter region of target genes or by interacting with and thus interfering with other transcription factors as a monomer. We determined the contribution of molecular mechanisms and cell types critically involved in antiinflammatory effects of GCs in RA and on bone using conditional and function selective GR knockout mice. We showed that dimerization of the GR in IL-17 producing T cells is indispensable for the anti-inflammatory effect in the mouse model antigen-induced arthritis. Now we demonstrate that, surprisingly, in the Kbx/N induced arthritis model the GR dimer in mesenchymal cells is critical for anti-inflammatory effects of GCs. Thus, for immunosuppression of arthritis the GR is required in distinct cell types, which are of hematopoietic and mesenchymal origin. In a model of GIO we showed that unexpectedly interaction of the GR monomer with AP-1, but not NF-kB in osteoblasts is decisive for bone loss. Our findings define new criteria for SEGRM that act as an anti-inflammatory and protect the bone. Indeed we identified one lead-compound that still suppresses NF-kB dependent gene expression but does not affect osteoblast differentiation and activity. Furthermorewe identifiednovel GR target genes by functional genomics anddeveloped a screening platform for novel GR agonist-derivatives not affecting osteoblast function. Taken together, our approach gives new insights into GC action on arthritis and bone that can be translated into new concepts for anti-inflammatory therapies preventing GIO. doi:10.1016/j.bone.2012.08.022 O22 Osteoclasts on bone and dentin in vitro: Mechanism of degradation and comparison of resorption behaviour T. Wurger, P. Roschger, E. Zwettler, P. Fratzl, M.J. Rogers, K. Klaushofer, M. Rumpler Ludwig Boltzmann Institute of Osteology at the Hanusch Hospital of WGKKand AUVATraumaCenter, 1stMed. Dept., HanuschHospital, Vienna, Austria Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany The Garvan Institute of Medical Research, Darlinghurst, New SouthWales,

The melting phenomenon in random-walk model of DNA

The melting phenomenon in a double-stranded homopolypeptide is considered. The relative distance between the corresponding monomers of two polymer chains is modeled by the two-dimensional random walk on the square lattice. Returns of the random walk to the origin describe the formation of hydrogen bonds between complementary units. To take into account the two competing interactions of monomers inside the chains, we obtain a completely denatured state at finite temperature T c .

Degree of Polymerization of Glucan Chains Shapes the Structure Fluctuations and Melting Thermodynamics of a Cellulose Microfibril

A Staggered LATtice (SLAT) model is developed for modeling cellulose microfibrils. The simple representation of molecular packing and interactions employed in SLAT allows simulations of structure fluctuations and phase transition of cellulose microfibrils at sufficiently long and large scales for comparison with experiments. Glucan chains in the microfibril are modeled as connected monomers, each corresponding to a cellobiose subunit, and the surrounding space around the cellulose is composed of solvent cells. Interaction parameters of monomer-monomer interactions were parametrized based on the results of atomistic molecular dynamics simulations. The monomer-solvent interaction was optimized to give a melting temperature of ∼695 K for the 36-glucan chain model cellulose microfibril, which is consistent with the estimation based on experimental data. Monte Carlo simulations of the SLAT model also capture experimentally measured X-ray diffraction patterns of cellulose as a function of temperature, including the region of melting transition, as well as predict the highly flexible regions in the microfibril. Beyond the diameter of ∼3 nm, we found that melting temperature of the cellulose microfibril is not significantly shifted by changing the thickness. On the other hand, a slight decrease in the degree of polymerization of glucan chains is shown to enhance structure fluctuations through the ends of glucan chains, i.e., the defect sites, and thereby significantly reduce the melting temperature. Analysis of the sizes, densities, and lifetimes of defect structures in the microfibril indicates a significant extent of fluctuations on the surfaces even at room temperature and that defect statistics are strong but distinct functions of temperature and solvent quality. The SLAT model is the first of its kind for simulating cellulosic materials, and this work shows that it can be used to incorporate information obtained from atomistic simulations and experimental data to enable the aforementioned findings through computation.

Mouse TSPO in a lipid environment interacting with a functionalized monolayer

Translocator protein TSPO is a membrane protein highly conserved in evolution which does not belong to any structural known family. TSPO is involved in physiological functions among which transport of molecules such as cholesterol to form steroids and bile salts in mammalian cells. Membrane protein structure determination remains a difficult task and needs concomitant approaches (for instance X-ray- or Electron-crystallography and NMR). Electron microscopy and two-dimensional crystallization under functionalized monolayers have been successfully developed for recombinant tagged proteins. The difficulty comes from the detergent carried by membrane proteins that disrupt the lipid monolayer.We identified the best conditions for injecting the histidine tagged recombinant TSPO in detergent in the subphase and to keep the protein stable. Reconstituted recombinant protein into a lipid bilayer favors its adsorption to functionalized monolayers and limits the disruption of the monolayer by reducing the amount of detergent. Finally, we obtained the first transmission electron microscopy images of recombinant mouse TSPO negatively stained bound to the lipid monolayer after injection into the subphase of pre-reconstituted TSPO in lipids. Image analysis reveals that circular objects could correspond to an association of at least four monomers of mouse TSPO.The different amino acid compositions and the location of the polyhistidine tag between bacterial and mouse TSPO could account for the formation of dimer versus tetramer, respectively. The difference in the loop between the first and second putative transmembrane domain may contribute to distinct monomer interaction, this is supported by differences in ligand binding parameters and biological functions of both proteins.

Adhesive Interactions of Polyurethane Monomers with Native Metal Surfaces

Understanding of fundamental adhesion is a key issue for bonding, polymer composites, and coatings. In this study, diluted solutions of polyurethane monomers (methylene diphenyl diisocyanate isomer mixture—MDI, polypropylene ether triol—PPET) are exposed to the native surfaces of Au, Al, and Cu. The adsorption state of the molecules (from multi- to sub-monolayers) is investigated in situ by infrared external reflection absorption spectroscopy (IR-ERAS). A sequence of solvent rinsing steps helps to distinguish physisorbed from chemisorbed molecules. PPET sticks very weakly to the metals. The spectra reveal a bulk-like state. MDI adsorbs weakly on Au but the spectra differ significantly from the bulk state. On Al and Cu, MDI establishes strong adhesion. Rich new spectral features indicate a probably chemical nature of adhesion, and a specific structure in the neighboring adsorbate layer. However, detailed final conclusions on the adhesion mechanisms must not be derived without appropriate quantum mechanical modeling.

Tracking Polypeptide Folds on the Free Energy Surface: Effects of the Chain Length and Sequence

Characterization of the folding transition in polypeptides and assessing the thermodynamic stability of their structured folds are of primary importance for approaching the problem of protein folding. We use molecular dynamics simulations for a coarse grained polypeptide model in order to (1) obtain the equilibrium conformation diagram of homopolypeptides in a broad range of the chain lengths, N = 10, ..., 100, and temperatures, T (in a multicanonical ensemble), and (2) determine free energy profiles (FEPs) projected onto an optimal, so-called "natural", reaction coordinate that preserves the height of barriers and the diffusion coefficients on the underlying free energy hyper-surface. We then address the following fundamental questions. (i) How well does a kinetically determined free energy landscape of a single chain represent the polypeptide equilibrium (ensemble) behavior? In particular, under which conditions might the correspondence be lost, and what are the possible implications for the folding processes? (ii) How does the free energy landscape depend on the chain length (homopolypeptides) and the monomer interaction sequence (heteropolypeptides)? Our data reveal that at low T values equilibrium structures adopted by relatively short homopolypeptides (N < 60) are dominated by α-helical folds which correspond to the primary and secondary minima of the FEP. In contrast, longer homopolypeptides (N > 70), upon quasi-equilibrium cooling, fold preferentially in β-bundles with small helical portions, while the FEPs exhibit no distinct global minima. Moreover, subject to the choice of the initial configuration, at sufficiently low T, essentially metastable structures can be found and prevail far from the true thermodynamic equilibrium. We also show that, by sequence-enabling the polypeptide model, it is possible to restrict the chain to a very specific part of the configuration space, which results in substantial simplification and smoothing of the free energy landscape as compared to the case of the corresponding homopolypeptide.

Studies Using Force, Hydrodynamic Size and Zeta Potential Measurements on Complexation of Polymer and Surfactant on Emulsion Interface

We probe the effect of polymer-surfactant complexation at oil-water emulsion interface on the stability of emulsions using inter-droplet forces, hydrodynamic diameter and zeta potential measurements. The polymer used in our experiment was a statistical copolymer of polyvinyl alcohol and vinyl acetate copolymer (PVA-Vac) and the emulsion was an oil-in-water system with an average droplet diameter of 200 nm. At low surfactant concentrations, the force measurement results show that the interaction of surfactant monomers with the adsorbed polymers at droplet interface drastically changes polymer conformation and the onset of repulsion between the droplets, a condition favourable for enhanced emulsion stability. Interestingly, at low surfactant concentrations, the hydro

Lipid Interaction and Membrane Perturbation of Human Islet Amyloid Polypeptide Monomer and Dimer by Molecular Dynamics Simulations

The aggregation of human islet amyloid polypeptide (hIAPP or amylin) is associated with the pathogenesis of type 2 diabetes mellitus. Increasing evidence suggests that the interaction of hIAPP with β-cell membranes plays a crucial role in cytotoxicity. However, the hIAPP-lipid interaction and subsequent membrane perturbation is not well understood at atomic level. In this study, as a first step to gain insight into the mechanism of hIAPP-induced cytotoxicity, we have investigated the detailed interactions of hIAPP monomer and dimer with anionic palmitoyloleolyophosphatidylglycerol (POPG) bilayer using all-atom molecular dynamics (MD) simulations. Multiple MD simulations have been performed by employing the initial configurations where the N-terminal region of hIAPP is pre-inserted in POPG bilayer. Our simulations show that electrostatic interaction between hIAPP and POPG bilayer plays a major role in peptide-lipid interaction. In particular, the N-terminal positively-charged residues Lys1 and Arg11 make a dominant contribution to the interaction. During peptide-lipid interaction process, peptide dimerization occurs mostly through the C-terminal 20–37 region containing the amyloidogenic 20–29-residue segment. Membrane-bound hIAPP dimers display a pronounced ability of membrane perturbation than monomers. The higher bilayer perturbation propensity of hIAPP dimer likely results from the cooperativity of the peptide-peptide interaction (or peptide aggregation). This study provides insight into the hIAPP-membrane interaction and the molecular mechanism of membrane disruption by hIAPP oligomers.

Depolymerization dynamics of individual filaments of bacterial cytoskeletal protein FtsZ

We report observation and analysis of the depolymerization filaments of the bacterial cytoskeletal protein FtsZ (filament temperature-sensitive Z) formed on a mica surface. At low concentration, proteins adsorbed on the surface polymerize forming curved filaments that close into rings that remain stable for some time before opening irreversibly and fully depolymerizing. The distribution of ring lifetimes (T) as a function of length (N), shows that the rate of ring aperture correlates with filament length. If this ring lifetime is expressed as a bond survival time, (T(b)≡NT), this correlation is abolished, indicating that these rupture events occur randomly and independently at each monomer interface. After rings open irreversibly, depolymerization of the remaining filaments is fast, but can be slowed down and followed using a nonhydrolyzing GTP analogue. The histogram of depolymerization velocities of individual filaments has an asymmetric distribution that can be fit with a computer model that assumes two rupture rates, a slow one similar to the one observed for ring aperture, affecting monomers in the central part of the filaments, and a faster one affecting monomers closer to the open ends. From the quantitative analysis, we conclude that the depolymerization rate is affected both by nucleotide hydrolysis rate and by its exchange along the filament, that all monomer interfaces are equally competent for hydrolysis, although depolymerization is faster at the open ends than in central filament regions, and that all monomer-monomer interactions, regardless of the nucleotide present, can adopt a curved configuration.

Compression behavior of porous dust agglomerates

The early planetesimal growth proceeds through a sequence of sticking collisions of dust agglomerates. Very uncertain is still the relative velocity regime in which growth rather than destruction can take place. The outcome of a collision depends on the bulk properties of the porous dust agglomerates. Continuum models of dust agglomerates require a set of material parameters that are often difficult to obtain from laboratory experiments. Here, we aim at determining those parameters from ab-initio molecular dynamics simulations. Our goal is to improveon the existing model that describe the interaction of individual monomers. We use a molecular dynamics approach featuring a detailed micro-physical model of the interaction of spherical grains. The model includes normal forces, rolling, twisting and sliding between the dust grains. We present a new treatment of wall-particle interaction that allows us to perform customized simulations that directly correspond to laboratory experiments. We find that the existing interaction model by Dominik & Tielens leads to a too soft compressive strength behavior for uni and omni-directional compression. Upon making the rolling and sliding coefficients stiffer we find excellent agreement in both cases. Additionally, we find that the compressive strength curve depends on the velocity with which the sample is compressed. The modified interaction strengths between two individual dust grains will lead to a different behaviour of the whole dust agglomerate. This will influences the sticking probabilities and hence the growth of planetesimals. The new parameter set might possibly lead to an enhanced sticking as more energy can be stored in the system before breakup.

THE HELIX-COIL TRANSITION IN A DOUBLE-STRANDED POLYNUCLEOTIDE AND THE TWO-DIMENSIONAL RANDOM WALK

The helix-coil transition in a double-stranded homopolynucleotide is considered. The new approach to the melted loops account is proposed. The relative distance between the corresponding monomers of two polymer chains is modeled by the two-dimensional random walk on the square lattice. Returns of the random walk to the origin describe the formation of hydrogen bonds between complementary units. To take into account the interaction of monomers inside the chains, we consider various regimes of return to the origin. One of them involves two competing interactions and demonstrates a nontrivial sharp denaturation transition. The rich phase behavior of the double-stranded homopolynucleotide is discussed in terms of the proposed approach.

Effects of solvent interactions on the structure and properties of prepared PAni nanofibers

AbstractThe changes of structure and properties of nanofibers were studied as a function of solubility parameters of the organic solvents that are used in interfacial polymerization of polyaniline (PAni) nanofibers. The presence of UV–visible absorbance at 340, 440, and 800 nm confirmed the formation of emeraldine salt structure of the prepared PAni nanofibers. Fourier transform infrared spectral results indicate an increasing trend of benzenoid to quinoid ratio with the decrease of interaction of the solvents with aniline. This can be correlated to the increase in the degree of conjugation of the polymer chain. Photoluminescence study revealed an increase in the density of defect state with the decrease of interaction. Single‐line approximation technique was used to analyze the broadening of the most intense X‐ray reflection peak corresponding to (110) plane of the nanofibers. The greater the solvent–monomer interaction, the lesser was the domain length and π‐stacking of the PAni chains. The study of this interaction is instrumental to precisely control the internal conformation of the PAni nanofibers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The Impact of Anthocyanin‐Rich Red Raspberry Extract (ARRE) on the Properties of Edible Soy Protein Isolate (SPI) Films

To modify the properties of edible soy protein isolate (SPI) films, 0.5% anthocyanin-rich red raspberry (Rubus strigosus) extract (ARRE) (0.5 g raspberry powder in 95% ethyl alcohol/water/85% lactic acid [80:19:1. v/v/v]) was incorporated into film-forming solutions. ARRE resulted in an SPI film having significantly enhanced tensile strength (P < 0.05) and % elongation at break (P < 0.05), as well as increased water swelling ratio (P < 0.05) and in vitro pepsin digestibility (P < 0.05). The resultant films also showed significantly decreased water solubility and water vapor permeability (P < 0.05). In addition, ARRE increased darkness, redness, and yellowness film appearance as evidenced by a lower L* (P < 0.05), greater positive a* (P < 0.05), and a higher b* (P < 0.05) than the control film. Scanning electron microscopy images revealed that extract-added films had denser and more compact cross-section microstructure. Fourier transform infrared spectra illustrated that ARRE-created hydrogen bonding involved conformational changes of soy protein without destroying its backbone structure. SDS-PAGE electrophoretograms revealed that the extract induced intermolecular interaction of the soy protein monomers. Natural plant extracts would be a promising ingredient to make SPI films with different physicochemical properties and applications. This study characterizes the potential physicochemical changes of SPI film with incorporated raspberry extract. Upon the above modification, the resultant film was found to enhance the applications of pure SPI film in food packaging. For example, SPI-ARRE film could prolong the usage life of SPI film due to increased strength, or could be useful as a desiccant (drying agent) such as a water-absorbing sheet for preserving dried foods due to its increased hydrophilic surface and water-swelling ratio. SPI-ARRE film could also be alternately used as a food wrap with unique color.

Formation of integral membrane protein oligomers

Many transmembrane (TM) proteins form oligomeric structures involving strong homomeric or heteromeric associations with other TM protein subunits. Little is known about the assembly mechanisms of these oligomers. We now report a technique for measuring the association of TM proteins in vitro. Bacteriorhodopsin fragments (2 and 5 TM helices) that are known to associate in detergent/lipid bicelles and form a native structure were separately inserted into nanolipoprotein particles (NLPs) using an E. coli cell‐free expression system. Each fragment contained one engineered cysteine, positioned so that disulfides form when the fragments correctly associate. The fragment/NLP complexes were mixed and disulfide crosslinking was initiated by Cu(II). No crosslinks between 2 and 5 TM helix fragments were detected. However, if the NLP complexes were mixed with bicelles prior to Cu(II) treatment, 2 to 5 TM helix crosslinks were observed. Thus, detergent/lipid bicelles open NLPs and permit protein mixing. The association constant for oligomer formation can be determined from the rate of crosslinking at varying protein concentrations. Future applications of this method include measuring association constants of protomers that form both homo‐ and hetero‐oligomeric channels (e.g. connexins, ligand‐gated ion channels), and studies of interactions of monomers with membrane chaperones.