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Hans G Börner

Publications and source records attributed to Hans G Börner.

9 recordsLinked to original sources
PubMed2006

Peptide-directed microstructure formation of polymers in organic media.

Synthesis and peptide-guided self-assembly of an organo-soluble peptide-polymer conjugate, comprising a sequence-defined polypeptide and a poly(n-butyl acrylate), are described. The amino acid sequence of the peptide encodes a high tendency to adopt an antiparallel beta-sheet motif, and thus programs the formation of tapelike microstructures. Easy synthesis and controllable self-assembly is ensured by the incorporation of structure breaking switch defects into the peptide segment. This suppresses temporarily the aggregation tendency of the conjugate as shown by circular dichroism, infrared spectroscopy (FT-IR), and atomic force microscopy (AFM). A pH-controlled rearrangement in the switch segments restores the native peptide backbone, triggering the self-assembly process and leading to the formation of densely twisted tapelike microstructures as could be observed by AFM and transmission electron microscopy. The resulting helical superstructures, when deposited on a substrate, are 2.9 nm high, 10 nm wide, and up to 2.3 mum long. The helical pitch is about 37 nm, and the pitch angle is 48 degrees . The helical superstructures undergo defined entanglement to form superhelices, leading to the formation of soft, continuous organo-gels. A twisted two-dimensional core-shell tape is proposed as a structure model, in which the peptide segments form an antiparallel beta-sheet with a polymer shell.

Circular Dichroism↗
PubMed2006

Synthesis of DL-alanine hollow tubes and core-shell mesostructures.

Three double hydrophilic block copolymers were used as crystal-growth modifiers of DL-alanine to generate amorphous precursor nanoparticles that undergo subsequent mesoscopic transformation to core-shell mesostructures and hollow tubes with quadratic cross-sections. The growth sequence can be stopped at various stages so that a series of intermediates between amorphous core- and crystalline-shell particles and tubes can be obtained. Time-dependent conductivity, TEM, SEM, and environmental scanning electron microscopy (ESEM) measurements were used to obtain a better understanding of the crystallization process, and a formation mechanism for the generation of the tubes is proposed. Na2SO4, NaCl, and NaNO3 as salts differ in their influence on the crystallization behavior of alanine by changing the solubility of alanine and by decreasing the stability of the intermediate particles. Core-shell mesostructures that formed in the dissolution-recrystallization process were captured as the transformation rate was decreased by the addition of copolymers or salts. Hollow tubes with quadratic cross-sections are the final product of the transformation process.

Alanine↗
PubMed2006

Switch-peptides to trigger the peptide guided assembly of poly(ethylene oxide)-peptide conjugates into tape structures.

A strong beta-sheet forming peptide was conjugated to PEO and utilized to guide the structure formation process toward well-defined, tape-like structures with millimeters in length, about 2 mum width, and approximately 50 nm height. The aggregation tendency of the peptide was temporarily suppressed for ease of synthesis by the integration of multiple switch-peptide defect segments into the peptide backbone. A subsequent rearrangement in the defects re-establishes the native peptide backbone and triggers the assembly by switching the aggregation properties on.

Microscopy, Electron, Scanning↗
PubMed2006

Solid-phase supported polymer synthesis of sequence-defined, multifunctional poly(amidoamines).

A novel synthesis route toward multifunctional, sequence-defined polyamides is described. A fully automated, solid-phase supported polymer synthesis was developed and utilized to obtain linear poly(amidoamine) segments (PAAs) that exhibit the absence of molecular weight and chemical distribution. This was achieved by an alternating assembly of diacids and diamines, using a forced step-growth mechanism, and driving each coupling step to completion. Within the monodisperse PAA segment, functionalities can be precisely positioned along the polymer chain allowing local control of the chain properties. The versatility of the approach was demonstrated by the conjugation of the monodisperse PAA segment toward an oligopeptide, leading to a single component block copolymer as verified by mass spectrometry. Moreover, two different poly(ethylene oxide)-PAA conjugates were synthesized utilizing the direct, solid-phase supported route. By varying the PAA repeat unit, the cationic nature of the PAA segment was adjusted, demonstrating the potential of the approach. The products were characterized by means of 1H NMR and matrix-assisted laser desorption mass spectrometry (MALDI-TOF-MS) methods, which confirmed the chemical structures conclusively.

Cations↗
PubMed2005

World Year of Physics: a direct test of E=mc2.

One of the most striking predictions of Einstein's special theory of relativity is also perhaps the best known formula in all of science: E=mc(2). If this equation were found to be even slightly incorrect, the impact would be enormous--given the degree to which special relativity is woven into the theoretical fabric of modern physics and into everyday applications such as global positioning systems. Here we test this mass-energy relationship directly by combining very accurate measurements of atomic-mass difference, Delta(m), and of gamma-ray wavelengths to determine E, the nuclear binding energy, for isotopes of silicon and sulphur. Einstein's relationship is separately confirmed in two tests, which yield a combined result of 1-Delta(mc2)/E=(-1.4+/-4.4)x10(-7), indicating that it holds to a level of at least 0.00004%. To our knowledge, this is the most precise direct test of the famous equation yet described.

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↗
PubMed2002

Quantum states of neutrons in the Earth's gravitational field.

The discrete quantum properties of matter are manifest in a variety of phenomena. Any particle that is trapped in a sufficiently deep and wide potential well is settled in quantum bound states. For example, the existence of quantum states of electrons in an electromagnetic field is responsible for the structure of atoms, and quantum states of nucleons in a strong nuclear field give rise to the structure of atomic nuclei. In an analogous way, the gravitational field should lead to the formation of quantum states. But the gravitational force is extremely weak compared to the electromagnetic and nuclear force, so the observation of quantum states of matter in a gravitational field is extremely challenging. Because of their charge neutrality and long lifetime, neutrons are promising candidates with which to observe such an effect. Here we report experimental evidence for gravitational quantum bound states of neutrons. The particles are allowed to fall towards a horizontal mirror which, together with the Earth's gravitational field, provides the necessary confining potential well. Under such conditions, the falling neutrons do not move continuously along the vertical direction, but rather jump from one height to another, as predicted by quantum theory.

Journal Article↗