Spreading and Dewetting of Single Bottle‐Brush Macromolecules on Nanofacetted SrTiO3 Substrate as Induced by Different Vapours

Responsive bottle‐brush macromolecules undergo vapour‐induced spreading and dewetting cycles on facetted SrTiO₃ surfaces with a saw‐toothed topography. The macromolecular transformations are followed by environmental scanning force microscopy in real time. A set of the wafers with different groove periods are tested as the substrates for the transforming macromolecules. In order to estimate the orienting influence of the anisotropic patterned substrate, an asymmetry factor is measured for the dewetting macromolecules at different groove periods. The most evident template influence of the substrate occurs when the period is in range of 20–40 nm. This range is commensurable with the typical lateral dimension of the adsorbed bottle‐brush macromolecules with a dense flat corona of extended side chains.

     

Facile Surface Modification and Application of Temperature Responsive Poly(N‐isopropylacrylamide‐co‐dopamine methacrylamide)

The temperature‐responsive poly(N‐isopropylacrylamide) [PNIPAAm] has been exploited for various biomedical applications. In this work, poly(N‐isopropylacrylamide‐co‐dopamine methacrylamide) [P(NIPAAm‐co‐DMAAm)] was synthesized for facile surface modification and application to cell sheets. ¹H NMR, FT‐IR, and GPC confirmed the successful synthesis of P(NIPAAm‐co‐DMAAm). The lower critical solution temperature was measured to be ca. 29.2 °C by UV–Vis spectroscopy. AFM imaging clearly visualized the transient phase transition of the temperature‐responsive polymer bound on silicon substrate by coordination bond formation. Furthermore, the adhesive and temperature responsive P(NIPAAm‐co‐DMAAm) could be successfully applied to the facile preparation of NIH‐3T3 fibroblast cell sheets.     

Direct Observation of Poly(propylene)‐block‐Poly(ethylene‐co‐propylene) Molecules by Atomic Force Microscopy

Summary: Atomic force microscopy (AFM) has been applied to get molecular images of diblock poly(propylene)‐block‐poly(ethylene‐co‐propylene) copolymers deposited on mica from diluted solutions at elevated temperatures. Both isolated molecules and their small aggregates have been visualized as compact particles of various sizes with outspreading poly(ethylene‐co‐propylene) chains. On the base of the height, volume and morphological analysis AFM images were divided into three groups. In the first group the compact particles are suggested to be small regular‐shaped crystallites formed by a few poly(propylene) blocks. Some isolated particles of this group were connected with single copolymer chains. In the second group the compact particles have larger dimensions and irregular or round shapes implying unordered packing of constituents. The third group were represented by isolated poly(ethylene‐co‐propylene) coils. The two‐dimensional expansion of coils on mica both isolated and included in aggregates exceeds several times their dimensions in a solution. The probable mechanism of such an expansion is proposed relying on the existence of van‐der‐Waals surface force field of a sufficient strength in the vicinity of the crystal surface.

Enlarged AFM height images of block‐copolymer aggregates of group A with small compact particles of regular shapes (frames a, b, c) and group B (frame d) with a large globular compact particle.     

Self‐Assembly of Arborescent Polystyrene‐graft‐Poly(ethylene oxide) Copolymers at the Air/Water Interface

AFM imaging was used to investigate the influence of molecular structure and composition on the formation of Langmuir monolayers by arborescent PS‐graft‐PEO copolymers at the air/water interface. Even without compression, these molecules associate spontaneously at the air/water interface and form stable monolayers that are easily transferred onto solid substrates as Langmuir/Blodgett films. The copolymers either form aggregates with a low association level, large island‐like clusters, or ribbon‐like superstructures. The different behaviors of the copolymers are rationalized in terms of the influence of their composition and of the structure of the arborescent PS cores on the assembly process.

     

Asymmetric Behavior of Temperature‐Responsive Poly(N‐isopropylacrylamide) Ultrathin Layers Observed by Atomic Force Microscopy

During characterization of a temperature‐responsive poly(N‐isopropylacrylamide) (PIPAAm) layer grafted onto a Si(100) substrate, atomic force microscopy (AFM) is able to probe the interactions between the microscope tip and the polymer. The modification of the AFM tip surface with octadecyltrichlorosilane (OTS) changes the interaction between the PIPAAm surface and the tip. Although a repulsive interaction is observed between a commercially available Si tip and the PIPAAm surface, a strong attractive interaction between the OTS‐modified Si tip and the surface is observed. Adhesion‐force analysis shows changes in the hydrophilic/hydrophobic character of ultrathin PIPAAm surfaces immediately after a change in temperature. The PIPAAm surface becomes hydrophobic less than 30 min after temperature increase, but requires 120 min to become hydrophilic after temperature reduction.

     

Real‐Time Observations of Oriented Crystallization of Poly(ε‐caprolactone) Thin Film,Induced by an AFM Tip

Atomic force microscopy (AFM) was used for modifying the surface structures of poly(ε‐caprolactone) (PCL) thin film. Oriented growth of PCL crystals at a desired area of the film surface was induced by scanning with a strong, normal load. PCL crystals were first grown edge‐on from the induction line and then their orientation changed to flat‐on at a lamellar length. The effects of molecular weight, crystallization temperature, scanning rate, and normal load on the AFM‐tip‐induced crystallization were examined. The growth kinetics of lamellar crystals in the AFM‐tip‐induced crystallization was the same as that in spherulitic crystallization. It was found that the formation of precursors strongly depends on the applied tip load and is facilitated when the applied load is higher than a threshold.

     

Investigation of Thermoresponsive Nano‐Confined Polymer Brushes by AFM‐Based Force Spectroscopy

PNIPAM polymer brushes are synthesized in nanopores of track‐etched membranes. The internal structure of these nano‐confined brushes is studied by AFM force spectroscopy experiments as a function of the synthesis conditions. The approach force vs. distance profiles are fitted using the Alexander‐de Gennes model to determine the grafting density, and the worm‐like chain model is used to estimate the chain length from the retract profiles. The grafting density for brushes synthesized within 80 nm pores is ten times lower than that obtained in 330 nm pores. For the same polymerization time, the chain lengths are much lower if the synthesis is carried out in small pores. The elastic properties of the brushes are also directly related to the confinement conditions.     

Ultra‐Small‐Angle X‐Ray Scattering Study of PET/PC Nanolayers and Comparison to AFM Results

The forced assembly of two immiscible polymers, produced by layer‐multiplying co‐extrusion, is analyzed by means of USAXS. Comparison of scattering and AFM results sheds light on many details of the nanolayered structure in PET/PC films. The role played by the volume concentration and cold crystallization of PET on the experimental scattering is discussed. The appearance of at least two scattering maxima in all cases, corresponding to higher orders of the same repeating distance, accounts for the high regularity of the developed nanostructure. It is finally shown that long spacing values, derived from a localized area in AFM, are in a good agreement with the USAXS values averaged over much larger areas.

     

Crystalline Monolayer Ordering at Substrate/Polymer Interfaces in Poly(3‐hexylthiophene) Ultrathin Films

The nanocrystalline structure of thermally annealed P3HT ultrathin films was studied as a function of the substrate properties using AFM, GIXRD, and static contact angle measurements which showed that an edge‐on structure is formed on both types of substrate. Substrate‐induced ordering is observed at the interface with air only when the film matches the average lamellae thickness, ≈30 nm in the present case, while, when the film thickness is higher than a single lamella size, i.e., ≈60 nm the substrate nature does not affect the lamellar ordering at the film/air interface. In other words, the substrate properties seem able to induce the ordering of the only first crystalline layer. These results are discussed in terms of simple relationships between interface free energy and chain mobility.

     

Direct Preparation of Nanoscale Thin Films of Poly(vinylidene fluoride) Containing β‐Crystalline Phase by Heat‐Controlled Spin Coating

Nanoscale thin films of PVDF containing the β‐crystalline phase were directly prepared by heat‐controlled spin coating without the influence of external stimuli either in the form of additives or post‐treatments. Sample preparation was carried out at different temperatures, ranging from 10 to 70 °C. At elevated temperatures (40, 50, 60 and 70 °C), PVDF was crystallized into the β‐phase, while at near‐ambient conditions (20 and 30 °C) it was crystallized into the α‐phase. Some samples exhibited a phase‐segregated morphology, with varying particle sizes depending on the preparation temperature. The ferroelectric nature of a typical sample, prepared at 40 °C, was visualized by piezoresponse imaging studies.

     

Light‐Directed Anisotropic Reorientation of Mesopatterns in Block Copolymer Monolayers

Photoreorientation of a stripe pattern consisting of an MPS structure in a Langmuir‐Blodgett monolayer by irradiation with linearly polarized light is achieved for the first time using a PDMS/poly(methacrylate) diblock copolymer with a liquid crystalline azobenzene‐containing side chain. The stripe MPS pattern in the trans azobenzene state is diminished by photoisomerization to the cis form upon UV light irradiation. By the erasure of the MPS structure, the anisotropic photo‐oriented stripe pattern is generated upon irradiation with linearly polarized visible light, the orientation of the stripe pattern being orthogonal to the electric vector of the light.

     

Super‐resolution imaging reveals changes in Escherichia coli SSB localization in response to DNA damage

The E. coli single‐stranded DNA‐binding protein (SSB) is essential to viability. It plays key roles in DNA metabolism where it binds to nascent single strands of DNA and to target proteins known as the SSB interactome. There are >2,000 tetramers of SSB per cell with 100–150 associated with the genome at any one time, either at DNA replication forks or at sites of repair. The remaining 1,900 tetramers could constantly diffuse throughout the cytosol or be associated with the inner membrane as observed for other DNA metabolic enzymes. To visualize SSB localization and to ascertain potential spatiotemporal changes in response to DNA damage, SSB‐GFP chimeras were visualized using a novel, super‐resolution microscope optimized for the study of prokaryotic cells. In the absence of DNA damage, SSB localizes to a small number of foci and the excess protein is associated with the inner membrane where it binds to the major phospholipids. Within five minutes following DNA damage, the vast majority of SSB disengages from the membrane and is found almost exclusively in the cell interior. Here, it is observed in a large number of foci, in discreet structures or, in diffuse form spread over the genome, thereby enabling repair events.     

Gene conversion causing human inherited disease: Evidence for involvement of non‐B‐DNA‐forming sequences and recombination‐promoting motifs in DNA breakage and repair

A variety of DNA sequence motifs including inverted repeats, minisatellites, and the χ recombination hotspot, have been reported in association with gene conversion in human genes causing inherited disease. However, no methodical statistically based analysis has been performed to formalize these observations. We have performed an in silico analysis of the DNA sequence tracts involved in 27 nonoverlapping gene conversion events in 19 different genes reported in the context of inherited disease. We found that gene conversion events tend to occur within (C+G)‐ and CpG‐rich regions and that sequences with the potential to form non‐B‐DNA structures, and which may be involved in the generation of double‐strand breaks that could, in turn, serve to promote gene conversion, occur disproportionately within maximal converted tracts and/or short flanking regions. Maximal converted tracts were also found to be enriched (P<0.01) in a truncated version of the χ‐element (a TGGTGG motif), immunoglobulin heavy chain class switch repeats, translin target sites and several novel motifs including (or overlapping) the classical meiotic recombination hotspot, CCTCCCCT. Finally, gene conversions tend to occur in genomic regions that have the potential to fold into stable hairpin conformations. These findings support the concept that recombination‐inducing motifs, in association with alternative DNA conformations, can promote recombination in the human genome. Hum Mutat 30:1–10, 2009. © 2009 Wiley‐Liss, Inc.     

Conformational Behaviour of Comb‐Like Poly(4‐vinylpyridinium) Salts and their Complexes with Surfactants in Solution and on a Flat Surface

We synthesised P4VP bromides with octyl and dodecyl pendant groups; the bromide anions in these polymer salts were further substituted with dodecylsulfate or bis(2‐ethylhexylsuccinate) anions. Direct SFM visualisation of the original P4VP chains, of the macromolecular salts and of their complexes, showed that the attachment of the pendant groups and especially the complexation with the surfactants promotes stretching of the macromolecules while adsorbing on mica substrate. On the other hand, viscosimetry showed that in solution (chloroform) the comb‐like chains and their complexes with dodecylsulfate had more compact conformations than those of the original P4VP. The scaling exponents describing the correlation between the contour length and the end‐to‐end distance of the adsorbed macromolecules were found to be ca. 3/4, which is indicative of 2D self‐avoiding walk statistics.

     

Specific basic patch‐dependent multimerization of Saccharomyces cerevisiae ORC on single‐stranded DNA promotes ATP hydrolysis

Replication initiation at specific genomic loci dictates precise duplication and inheritance of genetic information. In eukaryotic cells, ATP‐bound origin recognition complexes (ORCs) stably bind to double‐stranded (ds) DNA origins to recruit the replicative helicase onto the origin DNA. To achieve these processes, an essential region of the origin DNA must be recognized by the eukaryotic origin sensor (EOS) basic patch within the disordered domain of the largest ORC subunit, Orc1. Although ORC also binds single‐stranded (ss) DNA in an EOS‐independent manner, it is unknown whether EOS regulates ORC on ssDNA. We found that, in budding yeast, ORC multimerizes on ssDNA in vitro independently of adenine nucleotides. We also found that the ORC multimers form in an EOS‐dependent manner and stimulate the ORC ATPase activity. An analysis of genomics data supported the idea that ORC‐ssDNA binding occurs in vivo at specific genomic loci outside of replication origins. These results suggest that EOS function is differentiated by ORC‐bound ssDNA, which promotes ORC self‐assembly and ATP hydrolysis. These mechanisms could modulate ORC activity at specific genomic loci and could be conserved among eukaryotes.     

Method of Preparation of Soluble PEDOT: Self‐Polymerization of EDOT without Oxidant at Room Temperature

The preparation of soluble conducting polymers proceeds by the chemical oxidation method in the presence of water‐soluble polyelectrolytes. Among conducting polymers, polyethylene‐(3,4‐dioxythiophene) (PEDOT) is the most investigated due to its intrinsic properties. In this work, for the first time a simple method of ethylene‐(3,4‐dioxythiophne) self‐polymerization without applying any oxidant and with the formation of PEDOT solution at room temperature with a yield of 100% is presented. This PEDOT solution could be deposited on many desirable surfaces (by simple evaporation of the solvent) for various applications from photovoltaic cell to pseudocapacitors. Moreover, it is discovered that the self‐polymerization method does not produce byproducts, which makes the method environmentally friendly. The effect of light and different acids is explored. Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy confirm the formation of PEDOT by the self‐polymerization method. Moreover, this method provides a way to obtain and study individual PEDOT chains. The self‐polymerization method may be applied for the preparation of other conducting polymers.     

Reversible DNA damage checkpoint activation at the presenescent stage in telomerase‐deficient cells of Saccharomyces cerevisiae

The telomere protects the ends of eukaryotic linear chromosomes, and its shortening or erosion is recognized as DNA damage, leading to loss of proliferation activity and, thus, cellular senescence at the population level. Here, using a GFP‐based DNA damage checkpoint marker suited for single‐cell observation of Saccharomyces cerevisiae cells, we correlated the checkpoint status of telomere‐shortened cells with their behavior. We show that some cells possessing short telomeres retain proliferation capacity even after the DNA damage checkpoint is activated. At the presenescent stage, the activation of the checkpoint causes cell cycle delay, but does not induce permanent cell cycle arrest, eventually leading to the expansion of cell size that is characteristic of cellular senescence. Moreover, the proliferation capacity of checkpoint‐activated cells is not dependent on homologous recombination or the checkpoint adaptation pathway. The retention of proliferation capacity is specific to the telomere‐derived DNA damage response, suggesting that damaged telomeres differ functionally from other types of DNA damage. Our data establish the role of the presenescent stage in telomere shortening‐induced senescence, which proceeds gradually and is associated with a variety of changes, including altered cell morphology and metabolism.     

Formation of Multiple Structural Formats of DNA in a Bio‐Deep Eutectic Solvent

A solution of DNA (salmon testes, 2.5% w/w) in a bio‐based deep eutectic solvent consisting of the mixture of choline chloride and ethylene glycol (Chol.Cl–EG 1:2) is turned to a soft ion gel upon addition of 6 n HCl (pH 2.9) at room temperature (25 °C). The gel structure is lost when 6 n NaOH (pH 7.3) is added, and the structure could be regained at pH 2.9, and vice versa. The DNA regenerated from the solution at pH 7.3 shows the presence of spheroid‐shaped micro‐structured assemblies under scanning electron (SEM), atomic force (AFM), and transition electron (TEM) microscopes, and forms pH responsive micro‐hydrogel in water.

     

Surface Patterning of Poly(ε‐caprolactone): Epitaxial Crystallization and Enzymatic Degradation

Surface patterning was carried out by the epitaxial crystallization of biodegradable PCL on a HOPG, and the surface morphologies were observed by atomic force microscopy. Edge‐on view lamellae were aligned along the HOPG lattice to display stripe patterns in the threefold symmetry. The intervals of stripe patterns composed of ridges and valleys increased with an increase in the crystallization temperature. Enzymatic degradation of the PCL nanopattern allowed the different depth profiles of the fringed structure. The persistence length of the nanopattern could be tuned by the molecular weight of PCL.

     

Stabilizing the Microphase Separation of Block Copolymers by Controlled Photo‐crosslinking

Photo‐crosslinkable material is produced via self‐assembly of poly(methyl methacrylate)‐block‐poly(n‐butyl acrylate) (PMMA‐b‐PBA) block copolymers, in which the MMA block is decorated with coumarin moieties. The block copolymer is synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization. Atomic force microscopy (AFM) reveals microphase separation into nanodomains of controlled and regular morphology. The local elastic modulus of different nanodomains is determined using the PeakForce Quantitative Nano‐Mechanics (PFQNM) mode of AFM. The global modulus, as determined by mechanical spectroscopy, agrees well with the local values. The crosslinking of the block copolymer, via photodimerization of the incorporated coumarin moieties using UV light, is traced by UV–vis absorption measurements and resulted in an increase of the global elastic modulus. The crosslinking reaction proceeding in the solid state is surprisingly effective and stabilizes the morphology to such an extent that equilibrium phase separation, as targeted by annealing, is effectively suppressed.