Potential energy surface of alanine polypeptide chains

The multidimensional potential energy surfaces of the peptide chains consisting of three and six alanine (Ala) residues have been studied with respect to the degrees of freedom related to the twist of these molecules relative to the peptide backbone (these degrees of freedom are responsible for the folding of such peptide molecules and proteins). The potential energy surfaces have been calculated ab initio within the framework of the density functional theory taking into account all electrons in the system. The probabilities of transitions between various stable conformations of polypeptide molecules are evaluated. The results are compared to the data obtained by molecular dynamics simulations and to the available experimental data. The influence of the secondary structure of the polypeptide chain on its conformational properties with respect to rotations has been studied. It is shown that, in a chain of six amino acid (Ala) residues, the secondary structure type (helix or sheet conformation) influences the stable isomer states of the polypeptide.     

Constructing polyamidoamine dendrons from poly(poly(ethylene glycol) monomethacrylate) brushes grafted from planar silicon hydride surfaces for biomedical applications

A facile approach was established to construct polyamidoamine (PAMAM) dendrons from polymer brushes of poly(poly(ethylene glycol) monomethacrylate) (Si-g-P(PEGMA-OH)) grafted from a planar silicon hydride surface. First the Si-g-P(PEGMA-OH) brushes were grown via surface-initiated atom transfer radical polymerization with robust Si–C links on silicon surfaces. The side-chain hydroxyl groups of Si-g-P(PEGMA-OH) were chlorinated with thionyl chloride and further chlorines were substituted with amino groups of ethylenediamine, giving terminal primary amines. Borrowing the solution synthesis approach, we constructed second and third generations of PAMAM dendrons on-chip by surface-initiated alternative growth of two monomers, methyl acrylate and ethylenediamine. Two applications of silicon-based PAMAM dendrons were shown: the dense amino groups were activated via a cross-linker, N-succinimidyl-6-maleimidylhexanoate, to capture a free-thiol-carrying peptide of oxytocin and the third generation of PAMAM dendrons was used as a platform to on-chip synthesize a three amino acid peptide of Arg-Gly-Asp (RGD). The above conclusions were mainly derived from a home-built multiple transmission-reflection infrared spectroscopy, and complemented by X-ray photoelectron spectroscopy, UV–Vis spectroscopy and matrix-assisted laser desorption/ionization-time of flight-mass spectrometry.     

Grafting densely-packed poly(n-butyl methacrylate) chains from an iron substrate by aryl diazonium surface-initiated ATRP: XPS monitoring

Poly(n-butyl methacrylate), PBMA, chains were grafted by atom transfer radical polymerization (ATRP) from the surface of iron plates using electrochemically attached initiators based on diazonium salts providing an iron/polyphenylene/PBMA structure. This surface-initiated ATRP procedure was controlled by the addition of a small proportion of Cu⁺⁺ deactivator, but in the absence of any sacrificial initiator. Combined XPS, IR and AFM experiments provide a powerful means for the characterization of the obtained complex iron/polyphenylene/PBMA layered structure. It is possible to measure the thickness of the brominated aryl structure covalently attached to iron. Concerning the PBMA brushes, their presence on the surface was confirmed by IRRAS. The brominated chain end could be traced by XPS testifying for the ATRP character of the polymerization and the thickness of the polymer brushes was determined. The controlled living ATRP character of the polymerization is confirmed through a linear correlation between the thickness of the layer and the degree of polymerization. Measurement of the grafting density of PBMA chains indicates that they are compactly packed and that, approximately, one brominated aryl chain out of two efficiently initiates ATRP.     

Preparation of well-defined polymer-silicon wafer hybrids via surface-initiated RAFT-mediated process

A simple method was developed for the immobilization of reversible addition-fragmentation chain-transfer (RAFT) initiators on the silicon surface. Well-defined polymer-silicon hybrids, including the tethered brushes of glycidyl methacrylate (GMA) polymer, poly(ethylene glycol) monomethacrylate (PEGMA) polymer and block copolymer on a silicon wafer, were prepared via surface-initiated RAFT living radical polymerization. The “living” chain ends were used as the macroinitiator for the subsequent synthesis of diblock copolymers.     

Controlled structure and density of “living” polystyrene brushes on flat silica surfaces

Thin layers of polystyrene were grown from surface-grafted nitroxide initiators via controlled “living” free radical polymerization. The “reactive” Langmuir-Blodgett deposition method allowed an effective control of the initiator layer density leading to PS brushes with different and high grafting density and stretching. The influence of the grafting density on the layer structure was studied. Comparison with theoretical predictions for monodispersed brushes in bad solvent was discussed. The thickness was found to vary linearly with molecular weight and the density dependence was shown using wetting measurements. Special features of controlled radical nitroxide polymerization from a surface were discussed. A direct comparison of the molecular weight and polydispersity between surface and bulk polymers was made by de-grafting the brushes into a toluene/HF solution. Finally, some evidence of a “surface Fischer” effect was shown from re-initiated layers. Received 20 December 2001     

Precise Design of Surface Nano-texture and Surface Chemistry of Polymeric Solids

Surface properties of polymeric materials were controlled by fabrication of nano-structure and submicronorder structure of poly(fluoroalkyl acrylate) using the chemical and physical surface modification techniques. The relationship between the ordered structure of fluoroalkyl (R _f) groups and water repellency mechanism was precisely investigated by grazing-incidence X-ray diffraction, water contact angle measurements and X-ray photoelectron spectroscopy. It was revealed that poly{2-(perfluorooctyl)ethyl acrylate} (PFA-C₈) was crystallized and formed ordered structure to result in high water repellency. Surface nano-texturing onto PFA-C₈ film surface was carried out using an anodic aluminum oxide (AAO) membrane as a mold. The nano-texture of AAO mold with a pore diameter of 200 nm was transformed onto the PFA-C₈ film surface to fabricate nano-structure, which revealed high water and oil repellency. On the other hand, various types of monomers were grafted on the solid surface through surface-initiated atom transfer radical polymerization to result in high-density polymer brushes. Hydrophobic PFA-C₈brush surface revealed a low friction coefficient and a good wear resistance. Water lubrication and low friction were observed on the hydrophilic polymer brush. These polymer brushes were successfully introduced on the commercial fluoropolymer, which has inherent radical initiator functional groups. These results indicated that nano-layer immobilization through surface-initiated polymerization and surface texturing are promising methods to control the surface chemistry and surface structure at dimensions in the nano-meter range.     

Observation of slow down of polystyrene nanogels diffusivities in contact with swollen polystyrene brushes

The diffusion of dilute colloids in contact with swollen polymer brushes has been studied by evanescent wave dynamic light scattering. Two polystyrene nanogels with 16 nm and 42 nm radius were put into contact with three polystyrene brushes with varying grafting densities. Partial penetration of the nanogels within the brushes was revealed by the evanescent wave penetration depth-dependent scattering intensities. The experimental short-time diffusion coefficients of the penetrating particles were measured and found to strongly slow down as the nanoparticles get deeper into the brushes. The slow down is much more marked for the smaller (16 nm) nanogels, suggesting a size exclusion type of mechanism and the existence of a characteristic length scale present in the outer part of the brush.     

Polymer brushes near the crystallization density

In this paper, polymer brushes are studied via molecular-dynamics simulations at very high grafting densities, where the crossover between the brush regime and the polymer-crystal regime is taking place. This crossover is directly observed with the structure factor and pair-correlation function. With increasing grafting density, this crystallization is progressing from the core layer of the brush towards the surface layer. The same process is analyzed using the lateral fluctuations of the monomers as a signature of their diminishing mobility. Additionally, bond forces and the chain excess free energy indicate a transition from the brush regime to the overstretched regime, which is in agreement with predictions of a modified self-consistent field theory.     

Surface modification of layered silicates. I. Factors affecting thermal stability

The resistance of modification molecules bound to montmorillonite platelet surfaces towards structural damage at high temperature is a major parameter guiding the formation of optimal interface between the filler and polymer phases in a nanocomposite material. As nanocomposites are generated by melt-blending of modified mineral and polymer, it is necessary to quantify the thermal resistance of the filler surface modification at the compounding conditions because different modifications differ in chain length, chemical structure, chain density, and thermal performance. A number of different alkyl ammonium modifications were exchanged on the montmorillonites with cation exchange capacities in the range 680–900?µequiv.?g^?1 and their thermal behaviour was characterised using high resolution thermogravimetric analysis. Quantitative comparisons between different modified minerals were achieved by comparing temperature at 10% weight loss as well peak degradation temperature. Various factors affecting thermal stability, such as length and density (or number) of alkyl chains in the modification, presence of excess modification molecules on the filler surface, the chemical structure of the surface modifications, etc. were studied. The TGA findings were also correlated with X-ray diffraction of the modified platelets.     

Effect of core epitope modification on the antibody recognition of a MUC2 mucin peptide

Identification of protein epitopes via combinatorial chemistry was one of the most important discoveries of the past three decades. Mapping of linear antibody epitopes can be achieved rapidly and cost-effectively by the polymer pin-bound peptide approach. In this article, the determination of the fine epitope structure of MUC2 mucin glycoprotein is described by using specific monoclonal antibody. We have used positional scanning combinatorial approach, and also parallel synthesis. The residues within the MUC2 epitope ¹⁸PTGTQ²² of MAb 996 were replaced by all other proteinogenic amino acids on pin-bound peptide libraries, and their antibody binding was studied in modified ELISA. Thr¹⁹ was the least important of the residues in antibody recognition; most of the other amino acids could be replaced, except Pro. The other residues cannot be replaced without loss of antibody binding, where both the size and character of the amino acids were important. The significance of the non-chiral Gly²⁰ residue was further studied by competitive ELISA of parallelly synthesized soluble peptides containing l- or d-Ala instead of Gly residue. However, the d-Ala-containing oligopeptides showed no antibody binding; therefore, the backbone conformation is characteristic of that of l-amino acid containing peptides in this position as well. With the combinatorial approach we obtained relevant information about the contribution of individual amino acid side chains to the MAb 996 antibody binding within the PTGTQ predominant MUC2 mucin epitope. These results could be utilized for the design of synthetic antigens for detection of MUC2 protein core-specific antibodies related to carcinoma(s).     

Preparing polymer brushes on polytetrafluoroethylene films by free radical polymerization

Films of polytetrafluoroethylene (PTFE) were exposed to sodium naphthalenide (Na/naphtha) etchant so as to defluorinate the surface for obtaining hydroxyl functionality. Surface-initiators were immobilized on the PTFE films by esterification of 4,4′-azobis(4-cyanopentanoic acid) (ACP) and the hydroxyl groups covalently linked to the surface. Grafting of polymer brushes on the PTFE films was carried out by the surface-initiated free radical polymerization. Homopolymers brushes of methyl methacrylate (MMA) were prepared by free radical polymerization from the azo-functionalized PTFE surface. The chemical composition and topography of the graft-functionalized PTFE surfaces were characterized by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance (ATR) FT-IR spectroscopy and atomic force microscopy (AFM). Water contact angles on PTFE films were reduced by surface grafting of MMA.     

Static and dynamic properties of polymer brush with topological ring structures: Molecular dynamic simulation

By defining a topological constraint value(rn),the static and dynamic properties of a polymer brush composed of moderate or short chains with different topological ring structures are studied using molecular dynamics simulation,and a comparison with those of linear polymer brush is also made.For the center-of-mass height of the ring polymer brush scaled by chain length h~N~v,there is no significant difference of exponent from that of a linear brush in the small topological constraint regime.However,as the topological constraint becomes stronger,one obtains a smaller exponent.It is found that there exists a master scaling power law of the total stretching energy scaled by chain length N for moderate chain length regime,F_(ene)~Np~v,for ring polymer brushes,but with a larger exponent v than 5/6,indicating an influence of topological constraint to the dynamic properties of the system.A topological invariant of free energy scaled by(c)~(5/4) is found.     

The fibril structure of type V collagen triple-helical domain

Although the triple-helical structure of fibrillar collagen is regarded in general as being quite similar, each type of collagen molecule has inherent characteristics in the triple-helical domain. Few studies have ever been performed in terms of the aggregate structure of the triple-helical domain of fibrillar collagen. Reconstituted aggregates from the purified triple-helical domain of each type of fibrillar collagen might amplify the subtle differences in the structural characteristics of each type of collagen molecule. In this study, the reconstituted aggregate structure of pepsin-treated type V collagen (type Vp collagen), that is, virtually its triple-helical domain was characterized by transmission electron microscopy. Pepsin-treated type I (type Ip) and type II (type IIp) collagen were compared with type Vp collagen. Unique features of the aggregate structure of the triple-helical domain of the type V collagen can be summarized as follows:These results suggested that the lateral packing of the triple-helical domain of type V collagen is determined by its molecular structure. The characteristics of type Vp collagen fibrils might be explained by their characteristic amino acid composition. A significant feature of the triple-helical domain of type V collagen is the high content of glycosylated hydroxylysine residues. Molecular model building of the collagenous structure suggests that a change in surface roughness is conspicuous by incorporating the glycosylated hydroxylysine residues. More than a ten-fold content of bulky glycosylated hydroxylysine residues in type V collagen compared to that of type I might have a significant influence on both the intermolecular and interfibrillar interactions of the triple-helical domain of type V collagen molecule.     

Solution Behavior of Hydrophobic Cationic Hydroxyethyl Cellulose

The hydrophobic cationic hydroxyethyl cellulose (HEC-g-DA) was prepared by grafting HEC with various alkyl ammonium chlorides (DA) in order to improve the thickening properties of cationic hydroxyethyl cellulose. The solution behavior of HEC-g-DA was studied, and showed that the apparent viscosity of HEC-g-DA increased with polymer concentration, and there existed a critical association concentration (Cp*). The alkyl chain length of DA had a great influence on Cp*, which decreased with increasing alkyl chain length; however, too long an alkyl chain of DA reduced the water solubility of the polymer, resulting in a slight increase of Cp*. The effect of temperature and electrolyte concentration on the thickening properties of HEC-g-DA was investigated; the value of viscous flow activation energy (E_a) was minimum for the sample of HEC-g-DA16 (glycidyl-N-hexadecyl–N,N-dimethyl-ammonium chloride), indicating the weakest sensitivity of the viscosity to temperature. In the whole range of shear rate investigated, the solutions of HEC-g-DA displayed the shear thinning behavior of a pseudoplastic fluid. The values of viscous index (n) from the Ostwald model simulation decreased with polymer concentration, indicating an improvement of the shear thinning property of the solution, whereas the increase of the consistency coefficient (k) indicated the enhancement of the thickening behavior of the polymer. With increasing polymer concentration, the molecular association of HEC-g-DA16 became strong, and high-shear stress was required to remove the association, while the difference between G′ and G″ became small, indicating that the elasticity of the system was enhanced at high polymer concentration. The amphiphilic structure of the HEC-g-DA16 molecules contributed to the low surface tension of the polymer.     

Synthesis and conformational characterization of functional di-block copolymer brushes for microarray technology

Surface initiated polymerization (SIP) coupled with reversible addition-fragmentation chain transfer polymerization (RAFT) was used to functionalize microarray glass slides with block polymer brushes. N,N-dimethylacrylamide (DMA) and N-acryloyloxysuccinimide (NAS) (graft-poly[DMA-b-(DMA-co-NAS)]) brushes, with di-block architecture, were prepared from a novel RAFT chain transfer agent bearing a silanating moiety (RAFT silane) directly anchored onto the glass surfaces. Conformational characterization of the coatings was performed by Self Spectral Interference Fluorescence Microscopy (SSFM), an innovative technique that describes the location of a fluorescent DNA molecule relative to a surface with sub-nanometer accuracy. X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM) were used to characterize the coatings composition and morphology.     

碳酸钙对牛血清白蛋白微结构的影响

用紫外、紫外二阶导数谱和FT－IR光谱及曲线拟合等技术研究了水溶液中碳酸钙对牛血清白蛋白（BSA）微观结构的影响。结果表明,BSA的芳香族氨基酸残基受到干扰;二级结构也发生了变化。这些变化正是生物矿物形成的基础。     

Surface grafting density analysis of high anti-clotting PU-Si-g-P(MPC) films

Well-defined zwitterionic polymer brushes with good blood compatibility were studied, grafted from polyurethane (PU) substrate (PU-Si-g-P(MPC)) by surface-initiated reverse atom transfer radical polymerization (SI-RATRP). We found that the structure of polymer brushes and hence their properties greatly depend on the grafting density. To solve the problems of the normal method for grafting density measurement, i.e., more requirements for qualified and proficient instrument operator, we established an effective and feasible way instead of the conventional method of spectroscopic ellipsometer combined with gel permeation chromatograph (ELM/GPC) to calculate the grafting density of PU-Si-g-P(MPC) films by using a software named ImageJ 1.44e in combination with scanning electronic microscope (SEM) or atomic microscope (AFM). X-ray photoelectron spectroscopy (XPS), SEM and AFM were employed to analyze the surface topography and changes of elements before and after graft modification of the synthetic PU-Si-g-P(MPC) biofilms.     

A versatile method for the preparation of end-functional polymers onto SiO2 nanoparticles by a combination of surface-initiated ATRP and Huisgen [3 + 2] cycloaddition

A versatile method was developed for the chain-end functionalization of the grafted polymer chains for surface modification of nanoparticles with functionalized groups through a combination of surface-initiated atom-transfer radical polymerization (ATRP) and Huisgen [3 + 2] cycloaddition. First, the surface of SiO₂ nanoparticles was modified with poly(methyl methacrylate) (PMMA) brushes via the “grafting from” approach. The terminal bromides of PMMA-grafted SiO₂ nanoparticles were then transformed into an azide function by nucleophilic substitution. These azido-terminated PMMA brushes on the nanoparticle surface were reacted with alkyne-terminated functional end group via Huisgen [3 + 2] cycloaddition. FTIR and ¹H NMR spectra indicated quantitative transformation of the chain ends of PMMA brushes onto SiO₂ nanoparticles into the desired functional group. And, the dispersibility of the end-functional polymer-grafted SiO₂ nanoparticles was investigated with a transmission electron microscope (TEM).     

Surface modification of PVDF membrane via AGET ATRP directly from the membrane surface

This contribution demonstrates a method for PVDF microporous membrane modification via surface-initiated activators generated by electron transfer atom transfer radical polymerization (AGET ATRP) directly from the membrane surface. Three hydrophilic polymers, poly(2-(N,N-dimethylamino) ethyl methacrylate) (PDMAEMA), poly(2-oligo (ethylene glycol) monomethyl ether methacrylate) (POEGMA), and poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted from the PVDF membrane surface in aqueous solution at room temperature. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the successful covalent tethering of the polymer chains onto the PVDF membrane surface. The gravimetry results indicated an approximately linear increase of the graft yields, up to about 330 μg/cm² for DMAEMA and 470 μg/cm² for both HEMA and OEGMA, with the polymerization time. Block copolymer brushes were prepared by chain extension. Water contact angle decreased over 50% for high yields, indicating improved surface hydrophilicity. The effects of the graft polymerization on membrane surface morphology, pore structure and permeability were investigated. It was found that the surface roughness was decreased and the pore size distribution was narrowed. The membrane permeability increased at low graft yields due to the enhanced hydrophilicity and decreased at high graft yields due to the overall reduction of the pore diameters.     

Contraction and reswelling of a polymer chain near the critical point of a binary liquid mixture

We have studied the conformation change of a flexible linear polymer chain near the critical point of a binary liquid mixture using fluorescence correlation spectroscopy, which measured the hydrodynamic radius of the chains. Our results indicate that as the critical temperature (Tc) is approached, the chain size decreases. The polymer attains its most compact conformation when the correlation length of the critical fluctuations becomes comparable to the coil size. At very close to Tc, the polymer reexpands dramatically. To our knowledge, this is the first experimental evidence supporting the prediction of Brochard and de Gennes that a polymer chain will collapse and subsequently reswell on approaching Tc.