Biomedical subjects

Sergei S Sheiko

Publications and source records attributed to Sergei S Sheiko.

10 recordsLinked to original sources

PubMed2006

Adsorption-induced scission of carbon-carbon bonds.

Covalent carbon-carbon bonds are hard to break. Their strength is evident in the hardness of diamonds and tensile strength of polymeric fibres; on the single-molecule level, it manifests itself in the need for forces of several nanonewtons to extend and mechanically rupture one bond. Such forces have been generated using extensional flow, ultrasonic irradiation, receding meniscus and by directly stretching a single molecule with nanoprobes. Here we show that simple adsorption of brush-like macromolecules with long side chains on a substrate can induce not only conformational deformations, but also spontaneous rupture of covalent bonds in the macromolecular backbone. We attribute this behaviour to the fact that the attractive interaction between the side chains and the substrate is maximized by the spreading of the side chains, which in turn induces tension along the polymer backbone. Provided the side-chain densities and substrate interaction are sufficiently high, the tension generated will be strong enough to rupture covalent carbon-carbon bonds. We expect similar adsorption-induced backbone scission to occur for all macromolecules with highly branched architectures, such as brushes and dendrimers. This behaviour needs to be considered when designing surface-targeted macromolecules of this type-either to avoid undesired degradation, or to ensure rupture at predetermined macromolecular sites.

Journal Article↗

PubMed2006

Flow-enhanced epitaxial ordering of brush-like macromolecules on graphite.

Long-range orientational order in monolayers of brush-like macromolecules was achieved during spreading of a polymer melt on the surface of highly oriented pyrolytic graphite. The combination of wetting-induced flow and epitaxial adsorption of poly(n-butylacrylate) side chains on graphite led to the formation of large domains of uniaxially oriented rodlike molecules. The domain size varied from ca. 1 to 10 microm which is noticeably larger than the submicrometer-sized mosaic domains typically observed upon adsorption from solution. The increase in the degree of order is attributed to the flow-enhanced diffusion of the macromolecules within spreading monolayers which facilitates the epitaxial alignment of the large macromolecules. The diffusion coefficient was shown to increase linearly with the spreading rate. Even though the ordering occurred during flow, no correlation was observed between the molecular orientation and the flow direction. Thus, the role of the flow was not to induce the molecular orientation but to facilitate the intrinsic ordering process. This finding can inspire and lead to new strategies for constructing large scale ordered structures on surfaces.

Journal Article↗

PubMed2005

Molecular visualization of conformation-triggered flow instability.

A new type of flow fingering instability was observed in monolayer-thick polymer films as they spread on a solid substrate. Tracing the movement of individual molecules by atomic force microscopy enabled us to follow the development of the flow instability on the molecular level and to understand the underlying physical mechanism. The fingering instability was observed to be triggered by conformational changes of brushlike macromolecules in response to the pressure gradient driving the flow.

Journal Article↗

PubMed2005

AFM study of micelle chaining in surface films of polystyrene-block-poly(ethylene oxide) stars at the air/water interface.

A series of three-arm star block copolymers were examined using atomic force microscopy (AFM). These stars consisted of a polystyrene core composed of ca. 111 styrene units/branch with poly(ethylene oxide) (PEO) chains at the star periphery. Each star contained different amounts of PEO, varying from 107 to 415 ethylene oxide units/branch. The stars were spread as thin films at the air/water interface on a Langmuir trough and transferred onto mica at various surface pressures. Circular domains representing 2D micelle-like aggregated molecules were observed at low pressures. Upon further compression, these domains underwent additional aggregation in a systematic manner, including micellar chaining. At this point, domain area and the number of molecules/domain increased with increasing pressure. In addition, it was found that longer PEO chains led to greater intermolecular separation and less aggregation. These AFM results correspond to attributes seen in the surface pressure-area isotherms of the stars. In addition, they demonstrate the viability of AFM as a quantitative characterization technique.

Journal Article↗

PubMed2005

Peptide mediated formation of hierarchically organized solution and solid state polymer nanostructures.

Biologically-inspired peptide sequences have been explored as auxiliaries to mediate self-assembly of synthetic macromolecules into hierarchically organized solution and solid state nanostructures. Peptide sequences inspired by the coiled coil motif and "switch" peptides, which can adopt both amphiphilic alpha-helical and beta-strand conformations, were conjugated to poly(ethylene glycol) (PEG). The solution and solid state self-assembly of these materials was investigated using a variety of spectroscopic, scattering and microscopic techniques. These experiments revealed that the folding and organization properties of the peptide sequences are retained upon conjugation of PEG and that they provide the driving force for the formation of the different nanoscale structures which were observed. The possibility of using defined peptide sequences to direct structure formation of synthetic polymers together with the potential of peptide sequences to induce a specific biological response offers interesting prospects for the development of novel self-assembled and biologically active materials.

Amino Acid Sequence↗

PubMed2004

Molecular motion in a spreading precursor film.

Spreading of a polymer drop on a solid substrate was monitored with molecular resolution. Three characteristic rates, i.e., the spreading rate of the precursor film D(spread)=(3.9+/-0.2)x10(3) nm(2)/s, the flow-induced diffusion rate of molecules within the film D(induced)=1.3+/-0.1 nm(2)/s, and the thermal diffusion coefficient of single molecules D(therm)</=0.10+/-0.03 nm(2)/s, were independently measured. Since D(spread)>>D(induced), the plug flow of polymer chains was identified as the main mass-transport mechanism of spreading with an insignificant contribution from the molecular diffusion.

Journal Article↗

PubMed2004

Reversible collapse of brushlike macromolecules in ethanol and water vapours as revealed by real-time scanning force microscopy.

Environment-controlled scanning force microscopy allowed us to study adsorption and desorption of single poly(methacrylate)-graft-poly(n-butyl acrylate) brush molecules on mica in real time. The molecules transform reversibly from a two-dimensional, extended wormlike state to a compact globular state. The dynamics of the conformational transition was sufficiently slow in order to allow its observation by scanning force microscope in real time. The reversible transformation is effected by coadsorption of water or ethanol, the latter introduces the collapse. Adsorbing ethanol and water from the vapour atmosphere results in a change of the surface properties of mica, either favouring adsorption or desorption of the graft polymer. When the extended, tightly adsorbed poly(n-butyl acrylate) brush molecules are exposed to ethanol vapour, the macromolecules swell and contract to form compact globules. Exchanging the ethanol vapour to a humid atmosphere caused the molecules to extend again to a wormlike two-dimensional conformation. Coexistence of collapsed and extended strands within the same molecule indicates a single-molecule first-order transition in agreement with observations on Langmuir films previously reported.

Ethanol↗

PubMed2004

Multiarm molecular brushes: effect of the number of arms on the molecular weight polydispersity and surface ordering.

Individual molecules of multiarm starlike molecular brushes were visualized by atomic force microscopy. In the studied series of brushes, the number of arms varied from one for a linear chain to four, while the length of the side chains was kept approximately constant. Molecular visualization provided a unique opportunity for independent size characterization of the brush arms separately from that of the entire molecule. In agreement with the Schulz-Flory theory for chain coupling, the polydispersity of the total length was significantly lower than that of the arm length. The variation in polydispersity had an effect on molecular ordering. Lateral compression of the starlike brushes caused a transition from an extended dendritic-like conformation to a compact disklike conformation. In contrast to one-, two-, and three-arm brushes, the four-arm molecules with a lower polydispersity index of 1.04 demonstrated local hexagonal order.

Journal Article↗

PubMed2004

Structure and dynamics of self-assembled poly(ethylene glycol) based coiled-coil nano-objects.

Herein we describe the structure and dynamics of self-assembled nano-objects generated from poly(ethylene glycol) based (PEG-ylated) coiled-coil hybrid block copolymers. Electron paramagnetic resonance (EPR) experiments on spin-labeled samples provided a strong indication for a parallel alignment of the peptide helices in at least the dimeric coiled-coil nano-object and indicated that the PEG chains are folded rather closely around the peptide core of the nano-objects. The EPR results were supported by AFM studies, which revealed the presence of discrete nanosized objects in thin, spin cast films of the block copolymers on mica substrates. Since their size and structure may be engineered via directed mutations in the amino acid sequence, these nano-objects may be interesting building blocks for the development of supramolecular materials with various potential applications.

Amino Acid Sequence↗

PubMed2003

Measuring molecular weight by atomic force microscopy.

Absolute-molecular-weight distribution of cylindrical brush molecules were determined using a combination of the Langmuir Blodget (LB) technique and Atomic Force Microscopy (AFM). The LB technique gives mass density of a monolayer, i.e., mass per unit area, whereas visualization of individual molecules by AFM enables accurate measurements of the molecular density, i.e., number of molecules per unit area. From the ratio of the mass density to the molecular density, one can determine the absolute value for the number average molecular weight. Assuming that the structure of brush molecules is uniform along the backbone, the length distribution should be virtually identical to the molecular weight distribution. Although we used only brush molecules for demonstration purpose, this approach can be applied for a large variety of molecular and colloidal species that can be visualized by a microscopic technique.

Journal Article↗