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Scientific highlights in polymer science

Here you will find selected scientific highlights of the collaborative research center in the fields of polymer physics, polymer chemistry and biophysics. Our transregional research center is a joint initiative of Martin Luther University Halle-Wittenberg and Leipzig University.

Aggregation of biological macromolecules

Highlights 2016

Coupling and Decoupling of Rotational and Translational Diffusion of Proteins under Crowding Conditions

Cataract, that is, the loss of vision due to growing opacity of the eye lens, is one of the most frequent deseases with a correspondingly huge social (and economic) impact on humankind. It is associated with either phase separation or denaturation and ensuing aggregation in the concentrated solution of various lens proteins, the so-called crystallins. Our understanding of the molecular underpinnings of these processes is as yet incomplete, and it is one of the goals of project A08 to test in how far amyloid formation may be relevant in this context. As a step towards this goal, the constrained diffusive motion of proteins in such a highly crowded environment should be characterized first. It is well established that the large-scale translational diffusion is governed by the viscosity of the system, in agreement with the Stokes-Einstein relation. However, the applicability of the coresponding Stokes-Einstein-Debye relation for rotational diffusion, which is more localized short-time process, is still poorly understood. Using a combination of NMR and fluorescence spectroscopy techniques, we investigate the (de)coupling of the rotational from the translational diffusion in a series of crowded model proteins, revealing that proteins can span the whole range of phenomena in dependence of their complex mutual interactions.

Highlights 2015

Single Molecules Trapped by Dynamic Inhomogeneous Temperature Fields

Holding single molecules in liquids is still a challenge as Brownian motion fueled by thermal energy is randomizing molecular position quickly and common forces to counteract this erratic motion commonly scale with the volume. The benefits of hold and manipulating a single molecule are however huge as it would allow for long time observations of molecular conformation detecting rare events, biomolecular reactions and true bi-molecular interaction studies. Here we report on a method which employs the fuel of Brownian motion - thermal energy - to confine Brownian motion of single molecules in liquid. By generating time-dependent feedback controlled inhomogeneous temperature fields with a laser heated gold structure, we create thermophoretic drive fields which allow the confinement of single molecules in solution. The feedback control and the inhomogeneous character of the temperature field even allow for a trapping a well controlled number of multiple molecules.  We expect that this simple method and an extension to large arrays of traps will pave the way for controlled molecular interactions studies.

See also

Highlights 2012

Solid-state NMR Reveals a Close Structural Relationship between Amyloid-β Protofibrils and Oligomers

Proteins, essential components of every living being, have been posing riddles to researchers worldwide for decades. The function of these long-chain biomolecules is determined by the spatial configuration. Strictly speaking a protein folds into a specific three-dimensional structure. Properly folded, the protein can fulfill the specific biological function in our body, e.g., the control of cell growth.
However, if anything is wrong with the folding - known as the misfolding of proteins - toxic substances can arise that might lead to diseases such as Alzheimer's disease and diabetes. Through intermediate stages, a gradual aggregation of misfolded proteins results in the so-called mature fibrils. This final stage possesses the form of elongated rods which are composed of helically wound, zipper-like interconnected strips.

Holger Scheidt, from the group of Daniel Huster at University of Leipzig, and his colleagues have now succeeded in finding a greater similarity of the internal three-dimensional structure of the two intermediates for the first time. Mature fibrils, however, show an altered spatial arrangement. In detail, the protein amyloid-beta (1-40) was examined by using high-resolution nuclear magnetic resonance (NMR) spectroscopy within the CRC project A06.

Crystallization of synthetic polymers

Highlights 2017

Induction of Chirality in β-Turn Mimetic Polymer Conjugates via Postpolymerization “Click” Coupling

Chirality is one of the most determining principles in chemistry and biology, dominating the assembly of large synthetic and biological macromolecules. Polymers with artificial (geometrical) constraints can - similar to alpha-helices or proteins - fold into well defined helical structures, especially those containing a rigid backbone with limited conformational flexibility. We here report on the transfer of chirality, induced by an artificial beta-turn-mimetic element, which is directly linked to a synthetic (helical) polymer. The distance over which chirality is transferred from the geometrical constraint onto the helicity of the polymers is studied, revealing that transfer of chirality is possible up to three chemical bonds between the helix and the chiral center on the beta-turn element.

Intracrystalline Jump Motion in Poly(ethylene oxide) Lamellae of Variable Thickness: A Comparison of NMR Methods

It has long been known that the degree of crystallinity of linear polymers depends on the presence or absence of chain motion within and through the crystalline lamellae. While this fact has not yet found widespread appreciation or even explicit consideration in theoretical accounts of polymer crystallization, recent results from project A01 demonstrate that even the morphology is qualitatively different in the two cases. That is, polymers with intracrystalline mobility display lamellar stacks with well-defined amorphous-phase thickness but a wider distribution of crystallite thickness, in some contradiction to conventional wisdom. It is thus of high interest to develop an understanding of the relation between the intra-crystalline chain motion and morphological parameters, such as the crystallite thickness. Here, we report on a comprehensive NMR study of intracrystalline chain dynamics in poly(ethylene oxide), and find that the weak but notable thickness dependence of the monomer jump correlation time within the crystalline stems is in agreement with a model based on the diffusive motion of defects that are likely generated within the fold surface.

Highlights 2016

Dynamic Ordering and Phase Segregation in Hydrogen-Bonding Polymers

Hydrogen bonds constitute highly relevant structural units of molecular self-assembly. They bridge biological and synthetic sciences, implementing dynamic properties into materials and molecules. Phase segregation and crystallization on the other hand represent important assembly-principle, responsible for eg. cell compartimentation, membrane-formation and microphase segregation in polymers. We here discuss the phase segregation of H-bonding polymers in both, the solution and solid state, focusing on the specific aggregation of different H-bonding polymers in competition to phase segregation and crystallization. We illustrate that a rational architectural design within H-bonding polymer systems in interplay with phase segregation in both, the amorphous and crystalline state, opens perspectives to develop artificial supramolecular systems approaching the level of complexities and properties present in nature’s biomaterials.

Highlights 2014

Direct Observation of Prefreezing at the Interface Melt-Solid in Polymer Crystallization

The microscopic ordering process that a liquid undergoes during crystallization is often initiated at an interface to a solid. Different processes have been suggested by theory to occur at this interface. Of special interest is prefreezing—the formation of a thin crystalline layer at the interface already at temperatures above the melting temperature. Because of the difficult accessibility of the buried interface, experimental proof of crystallization by prefreezing has been elusive in molecular systems. We here present direct in situ observations of such a process in a polymeric model system. The results not only contribute to our fundamental understanding of crystallization but might also be useful for the preparation of well-ordered oriented thin films of crystalline organic materials.

Kinetic mechanism of chain folding in polymer crystallization

Polymer crystallization, which was discovered more than 50 years ago, is a particular case of solidification of matter which occurs by supercooling a polymer melt. The specific feature of polymer crystallization is the back-folding of chains in a polymer crystal leading to lamellae with a thickness of around 10-15 nm consisting of straight chain segments. The conventional description, which is based on phenomenological nucleation theory, ignores the polymeric structure and conformational interactions, and has been questioned in some recent experiments and numerical work. We develop a description of the crystallization kinetics that explains chain folding in polymer crystallization in terms of conformational interactions and the coil shape of polymer chains in the melt. The fundamental relation between lamellar thickness and supercooling is derived from the interplay between the formation time of rod shaped stems, which are induced by supercooling, and the relaxation time of coiled polymer parts of the same length. Our work suggests the existence of two separate time scales in polymer crystallization: the chain dynamics time scale at which the fold length is selected, and the much larger time scale associated with the crystal growth.

Highlights 2013

Determination of the Crystallinity of Semicrystalline Poly(3-Hexylthiophene) by Means of Wide-Angle X-Ray Scattering

Poly(3-hexylthiophene) (P3HT) is often studied as a model system for the more general class of semiconducting polymers. It has been shown in the past that the electronic transport properties of this material strongly correlate with crystallinity, which can depend on the details of chemical structure, molecular weight and processing. In most cases, crystallinity is measured by Differential Scanning Calorimetry (DSC), which requires calibration in order to deliver absolute values. Unfortunately, the available calibration value is uncertain and under debate. We therefore here present small-angle and wide-angle X-ray scattering measurements on a series of chemically well-defined poly(3-hexylthiophenes), which were analyzed to determine absolute values of the crystallinities. The resulting values for the crystallinity are substantially higher than assumed in the past. An extrapolated reference melting enthalpy for a 100% crystalline material was determined by comparison with DSC measurements. The observed decrease of the crystallinity for higher molecular weights can be explained by the onset of chain folding. Additionally we show that the crystalline regions of P3HT exhibit a large amount of internal disorder.

Crystallization of supramolecular pseudo block copolymers

Due to the presence of supramolecular bonds the crystallization process of supramolecular pseudo block copolymers (SPBCP) is more complex in comparison to conventional covalently bonded block copolymers (BCP). Thus supramolecular binding motives included on the polymer chain-ends display additional dynamic effects as well as possible nuclei for the crystallization. In this article we systematically study non-isothermal crystallization processes in SPBCP’s consisting of a crystallizable poly(ε-caprolactone) (PCL) connected via triple hydrogen bonds to either a short alkyl-modified 2,4-diaminotriazine, or bound to a large block of amorphous poly(isobutylene) (PIB). The crystallization of the PCL is studied with both groups acting as supramolecular barriers for the crystallization process, either during nucleation or during crystal growth. A strong influence of the short alkyl-modified 2,4-diaminotriazine barrier on the crystallization temperature of the PCL compared to the control sample devoid of this compound is observed. In contrast, the large polymer block (PIB) acting as a barrier causes a strong decrease of the crystallization temperature and fractionated crystallization of SPBCP consisting of smaller PCL-chains is observed.