NMR investigations of amyloid formation

M. Baumann1, M. Gopalswamy1, J. Adler2, B. Voigt1, D. Huster2,
and J. Balbach1

1Institute of Physics, Biophysics, University Halle-Wittenberg, Germany
2Institute for Medical Physics and Biophysics, University Leipzig, Germany

Amyloids are well ordered protein aggregates involved in many functional and pathogenic processes of life. Various structural models of the molecular architecture of amyloids have been derived in the last decade mainly driven by advances in solid state NMR. This talk will summarize our NMR efforts to study not only structural features of mature fibrils but the amyloid formation mechanism. Two systems will be covered: the Alzheimer peptide Aβ(1-40) and variants as well as the human parathyroid hormone PTH(1-84). For Aβ(1-40) we show that backbone hydrogen bonds are the main driving force for fibril structure formation overwriting side chain effect and buffer conditions [1,2]. Additionally, we show that morphological properties of fibril seeds do not necessarily propagate towards the growing fibril [3]. Several molecular observations during the formation of PTH(1-84) fibrils will be presented [4] to classify them as functional amyloids and possible storage form of the hormone.Amyloids are well ordered protein aggregates involved in many functional and pathogenic processes of life. Various structural models of the molecular architecture of amyloids have been derived in the last decade mainly driven by advances in solid state NMR. This talk will summarize our NMR efforts to study not only structural features of mature fibrils but the amyloid formation mechanism. Two systems will be covered: the Alzheimer peptide Aβ(1-40) and variants as well as the human parathyroid hormone PTH(1-84). For Aβ(1-40) we show that backbone hydrogen bonds are the main driving force for fibril structure formation overwriting side chain effect and buffer conditions [1,2]. Additionally, we show that morphological properties of fibril seeds do not necessarily propagate towards the growing fibril [3]. Several molecular observations during the formation of PTH(1-84) fibrils will be presented [4] to classify them as functional amyloids and possible storage form of the hormone.

References
[1] M. Garvey, M. Baumann, M. Wulff, M. Fändrich, J. Balbach et al., Amyloid 23, 76 (2016). (link)
[2] J. Adler, M. Baumann, B. Voigt, H. A. Scheidt, D. Bhowmik, T. Haupl, B. Abel, P. K. Madhu, J. Balbach, S. Maiti, and D. Huster, ChemPhysChem 17, 2744 (2016). (link)
[3] M. Wulff, M. Baumann, J. Balbach, M. Fändrich et al., Angew. Chemie Int. Ed. 55, 5081 (2016). (link)
[4] M. Gopalswamy, A. Kumar, J. Adler, M. Baumann, M. Henze, S. T. Kumar, M. Fändrich, H. A. Scheidt, D. Huster, and J. Balbach, Biochim. Biophys. Acta 1854, 249 (2015). (link)

Interface-induced crystallization via prefreezing: A first order prewetting transition

Ann-Kristin Flieger, M. Schulz, and T. Thurn-Albrecht

Experimental Polymer Physics, Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany

Interface-induced crystallization of a liquid on a solid substrate can either occur via heterogeneous nucleation or via prefreezing. Whereas heterogeneous nucleation takes place at finite supercooling below the melting temperature Tm, in prefreezing a crystalline layer is formed at the surface of a solid substrate already above Tm. Wetting theory predicts a jump in thickness at the formation and a divergence upon approaching coexistence. However, the thickness of the prefreezing layer has not been experimentally measured so far.

We studied ultrathin films of polycaprolactone (PCL) during the crystallization on graphite. With in-situ AFM-measurements we observe prefreezing instead of heterogeneous nucleation. The corresponding crystalline layer is formed at a temperature above the bulk melting temperature. Similar observations were already made for polyethylene on graphite [1]. In that case however, a direct measurement of the thickness of the prefreezing layer was not possible. Here, we show directly the finite thickness of the prefreezing layer for PCL. It forms with a thickness of a few nanometers which further increases during cooling. This observation demonstrates the transition is of first order, as expected for a prewetting transition.

The results prove that prefreezing can be described by common wetting theory. The studied system PCL-graphite is of importance for applications since graphitic materials are widely used as fillers for PCL.

References
[1] A.-K. Löhmann, T. Henze, and T. Thurn-Albrecht, PNAS 49, 17368-17372 (2014). (link)

Crystallization in melts of semi-flexible hard polymer chains: An interplay of entropies and dimensions

T. Shakirov, W. Paul

Institut für Physik, Martin Luther Universität Halle-Wittenberg, 06099 Halle 

Stochastic Approximation Monte Carlo simulations [1] are employed to obtain the complete thermodynamic equilibrium information for a melt of short, semi-flexible polymer chains with purely repulsive intermolecular interactions. Thermodynamics is obtained based on the density of states of our simple coarse-grained model, which varies by up to 5000 orders of magnitude. We show that our polymer melt undergoes a first-order crystallization transition upon increasing the chain stiffness at fixed density [2]. The lyotropic three-dimensional orientational ordering transition drives the crystallization and is accompanied by a two-dimensional hexagonal ordering transition in the plane perpendicular to the chains. While the three-dimensional ordering can be understood in terms of Onsager theory, the two-dimensional transition is similar to the liquid-hexatic transition of hard disks. Due to the domination of lateral two-dimensional translational entropy over the one-dimensional translational entropy connected with columnar displacements, the chains form a lamellar phase. The tilt of the chain axis with respect to the lamella surface makes this a rotator II phase.

References
[1] B. Werlich, T. Shakirov, M. P. Taylor, W. Paul, Comput. Phys. Commun. 186, 65, (2015) (link)
[2] T. Shakirov, W. Paul, preprint

The Functional Role of Nanocrystals in Native and Artificial Spider Silk

A. M. Anton and F. Kremer

Peter Debye Institute for Soft Matter Physics, University of Leipzig, Linnéstr. 5, 04103 Leipzig

Spider dragline silk exhibits remarkable characteristics such as exceptional toughness arising from high tensile strength combined with great elasticity. Its mechanical properties are based on a refined architecture on the molecular scale: Proteins with highly repetitive core motifs aggregate into nanometer-sized crystals, rich non alanine in β-sheet secondary structure, surrounded by an amorphous, glycine rich matrix. During spinning the less ordered parts are elongated, which orients both substructures and gives rise to an inherent non-equilibrium state. Thus, external stress is directly transferred to the crystallites, as demonstrated by FTIR experiments in combination with uniaxial stress [1] or hydrostatic pressure [2].Spider dragline silk exhibits remarkable characteristics such as exceptional toughness arising from high tensile strength combined with great elasticity. Its mechanical properties are based on a refined architecture on the molecular scale: Proteins with highly repetitive core motifs aggregate into nanometer-sized crystals, rich non alanine in β sheet secondary structure, surrounded by an amorphous, glycine rich matrix. During spinning the less ordered parts are elongated, which orients both substructures and gives rise to an inherent non-equilibrium state. Thus, external stress is directly transferred to the crystallites, as demonstrated by FTIR experiments in combination with uniaxial stress [1] or hydrostatic pressure [2].Even though the protein structure has been thoroughly studied, until recently it was not possible to artificially re-create this exceptional (morphological and functional) architecture. We show that wet spinning of a novel biomimetic protein results in fibers with a similar nanostructure as the natural template. However, only post spinning strain induces a microscopic non-equilibrium that gives rise to a similar mechanism of energy dissipation as in natural spider silk and comparable macroscopic toughness [3,4].

References
[1] P. Papadopoulos, J. Sölter, and F. Kremer, Eur. Phys. J. E 24, 193 (2007). (link)
[2] A. M. Anton, C. Kuntscher, F. Kremer et al., Macromolecules 46, 4919 (2013). (link)
[3] A. Heidebrecht, T. Scheibel et al., Adv. Mater. 27, 2189 (2015). (link)
[4] A. M. Anton, M. Beiner, T. Scheibel, F. Kremer et al, manuscript submitted

Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds

Julia A. Kornfield

California Institute of Technology, Chemistry & Chemical Engineering,  Pasadena CA 91125

Poly(L-lactide), PLLA, is the structural material of the first clinically approved bioresorbable vascular scaffold (BVS), a promising alternative to permanent metal stents for treatment of coronary heart disease. BVSs are transient implants that support the occluded artery for 6 months, and are completely resorbed in 2 years. Clinical trials of BVSs report restoration of arterial vasomotion and elimination of serious complications such as Late Stent Thrombosis. It is remarkable that a scaffold made from PLLA, known as a brittle polymer, does not fracture when crimped onto a balloon catheter or during deployment in the artery. X-ray microdiffraction revealed how PLLA acquired ductile character and that the crimping process creates localized regions of extreme anisotropy; PLLA chains in the scaffold change orientation from the hoop direction to the radial direction over micron-scale distances. The multiplicity of morphologies in the crimped scaffold enable a low-stress response during deployment, which avoids fracture of the PLLA hoops and leaves them with the strength needed to support the artery. Thus, the transformations of the semicrystalline PLLA microstructure during crimping explain the unexpected strength and ductility of the current BVS and point the way to thinner resorbable scaffolds in the future.

References:
[1] Ailianou, A.; Ramachandran, K.; Kossuth, M.; Oberhauser, J.P.; Kornfield, J.A.*; “Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds,” PNAS, 113, 11670-11675 (2016). (link)

Crystallization of Supramolecular Polymers Linked by Multiple Hydrogen Bonds

Pengju Pan, Jianna Bao, Xiaohua Chang, Ruoxing Chang, Guorong Shan, Yongzhong Bao

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China.

Supramolecular polymers (SMPs) have different crystallization behavior from conventional polymers. Crystallization of SMPs occurs in a “confined” and “dynamic” manner. Because of the reversible and stimuli-responsive natures of non-covalent bonds in SMPs, crystalline structure and crystallization kinetics of SMPs depend strongly on crystallization conditions (e.g., crystallization temperature, Tc). This offers a feasible way to tune the physical properties and functions of SMPs in processing. We first selected the 2-ureido-4[1H]-pyrimidione (UPy)-bonded poly(L-lactic acid) (PLLA) as a model SMP and investigated the crystallization kinetics, polymorphic crystalline structure, and phase transition of supramolecular PLLAs (SM-PLLAs). Crystallization rate and crystallinity of SM-PLLAs were strongly depressed as compared to the non-functionalized PLLA precursors. Crystalline structure of SM-PLLAs was sensitive to Tc. A low Tc (80~100 °C) facilitated the formation of metastable β crystals of PLLA in SM-PLLAs. The β crystals formed in SM-PLLAs transformed into the more stable α crystals in the following heating process. We further studied the stereocomplex crystallization between UPy-functionalized PLLA and poly(D-lactic acid) (PDLA). It was found that the stereocomplexation ability of PLLA and PDLA was highly improved after UPy end functionalization; this was ascribed to the enhanced interchain interaction.

References
[1] Chang, R. X., Pan, P. J., et al. Macromolecules 2015, 48, 7872. (link)
[2] Bao, J. N., Pan, P. J., et al. Polym. Chem. 2016, 7, 4891. (link)
[3] Bao, J. N.; Pan, P. J., et al. Cryst. Growth Des. 2016, 16, 1502. (link)

The Isothermal Crystallization of Polyamide 11 Investigated from Low to High Supercooling by Simultaneous Fast Scanning (Chip) Calorimetry (FSC) and Synchrotron SAXS/WAXD

D. Baeten1, V.B.F. Mathot1, M.F.J. Pijpers1, O. Verkinderen1, P. Van Puyvelde2, B. Goderis1

1Polymer Chemistry and Materials, KU Leuven, Heverlee, Belgium
2Soft Matter, Rheology and Technology, KU Leuven, Heverlee, Belgium

An in-situ FSC-SAXS/WAXD approach was used to study the isothermal crystallization of polyamide 11 (PA11) at different degrees of supercooling in order to elucidate its bimodal crystallization rate behavior with temperature [1]. Time resolved WAXD analyses over the complete range of supercoolings revealed that mesomorphic material was produced in less than a second at high supercooling, whereas at very low supercooling crystals were obtained [2]. The crystalline to mesomorphic ratio was found to increase gradually with increasing crystallization temperature. Analysis of the SAXS data supported the existence of a crystallization temperature dependent semicrystalline morphology composed of alternating solid and liquid-like layers with the solid layers made from crystalline, mesomorphic and rigid amorphous patches. Moreover, the crystalline or mesomorphic patches alternate with rigid amorphous patches in neighboring solid layers. The relation between details of this peculiar morphology and the crystallization rate as a function of the crystallization temperature will be discussed.

References
[1] A. Mollova, R. Androsch, D. Mileva, C. Schick, A. Benhamida, Macromolecules 46, 828–835 (2013) (link)
[2] D. Baeten, V.B.F. Mathot, M.F.J. Pijpers, O. Verkinderen, G. Portale, P. Van Puyvelde, B. Goderis, Macromol. Rapid Commun. 36, 1184–1191 (2015) (link)

The non-equilibrium phase diagrams of flow-induced crystallization and melting of polymer

Zhen Wang, Youxin Ji, Jianzhu Ju, Xiaoliang Tang, Liangbin Li

National Synchrotron Radiation Lab and CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China

With a combination of extensional rheology and in-situ synchrotron ultrafast x-ray scattering measurements, we have studied the flow-induced phase behaviors of polyethylene (PE) [1], isotactic polypropylene (iPP) [2] and Poly(1-butene) (PB-1) [3] over a wide temperature and flow strength range. Non-equilibrium phase diagrams are constructed in temperature-stress space for PE, and in temperature-strain rate space for iPP and PB-1, which reflect the non-equilibrium natures of flow-induced crystallization (FIC). Applying flow is recognized to favor the formation of structure with high entropy and low conformational order. The interplay of kinetic competitions and thermodynamic stabilities between different phases leads to rich kinetic pathways for FIC and diverse final structures. The non-equilibrium flow diagrams provide a detailed roadmap for precisely processing of polymers with designed structures and properties. It demonstrates that the non-equilibrium process stimulated by flow is fundamentally different from the equilibrium phase behaviors, where a rich source of physics is still waiting for us to dig out.

References
[1] Z. Wang, J. Ju, J. Yang, Z. Ma, D. Liu, K. Cui, H. Yang, J. Chang, N. Huang, and L. Li, Scientific Reports 6, 32968 (2016). (link)
[2] J. Ju, Z. Wang, F. Su, Y. Ji, H. Yang, J. Chang, S. Ali, X. Li, and L. Li, Macromolecular Rapid Communications 37, 1441 (2016). (link)
[3] Z. Wang, J. Ju, L. Meng, N. Tian, J. Chang, H. Yang, Y. Ji, F. Su, L. Li, and L. Li, Soft Matter 13, 3639 (2017). (link)

Multi-Shape Memory Effect of Columnar Side-Chain Liquid Crystalline Polymer

R. Y. Zhao, S. Yang, and E. Q. Chen

Department of Polymer Science and Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China

Recently, we are interested in side-chain liquid crystalline (LC) polymers bearing the hemiphasmid side-chain that contains a rod-like mesogen linked with a half-disk end group [1-3]. We found that they could self-organize into the hexagonal and/or rectangular columnar LC phase when the size of flexible tails on the half-disk was properly chosen and the dimension of columnar lattice could approach to 10 nm easily. It is identified that the supramolecular column in the columnar phase shall contain several chains (e.g., ~5 chains) laterally associated together rather than a single chain. This “multi-chain column” provides a new type of physical crosslinking. Namely, within the confined space of the column the backbones and pendant groups of the polymer can get entangled. Using hemiphasmid side-chain LC polynorbornene as the example [4], we demonstrate that such physical crosslinks can be rather robust, giving the polymer with the typical properties of thermal plastic elastomer. Furthermore, taking the physical crosslinks to define the permanent shape and the LC formation to fix the temporary shape, we realized the side-chain polynorbornene with excellent shape memory effect. For the dual shape memory, both the shape fixity (Rf) and shape recovery (Rr) are admirably high (approaching 100%), even when a large strain of 600% was applied. Benefited from a broad LC transition, the polymer can present the high-strain multi-shape memory effect, exampled by its triple- and quadruple-shape memory with high Rf and Rr at each step.

References
[1] J. F. Zheng, X. Liu, X. F. Chen, X. K. Ren, S. Yang, E. Q. Chen, ACS Macro. Lett. 1, 641 (2012). (link)
[2] X. Q. Liu, J. Wang, S. Yang, E. Q. Chen, ACS Macro Lett. 3, 834 (2014). (link)
[3] Y. S. Xu, D. Shi, J. Gu, Z. Lei, H. L. Xie, T. P. Zhao, S. Yang, E. Q. Chen, Polym. Chem. 7, 462 (2016). (link)
[4] R. Y. Zhao, T. P. Zhao, X. Q. Jiang, X. Liu, D. Shi, C. Y. Liu, S. Yang, E. Q. Chen, Adv. Mater. DOI 10.1002/adma.201605908 (2017) (link)

The role of entanglements for polymer crystallization

J.-U. Sommer

Leibniz-Insitut für Polymerforschung Dresden, Institut für Theorie der Polymere, Hohe Straße 6, 01069 Dresden
TU Dresden, Institut für Theoretische Physik, Zellescher Weg 17, 01069 Dresden

We use a coarse-grained polymer model to study the crystallization and melting behavior of long polymer chains in the dense state under various conditions. Our primary goal was to investigate the impact of the entanglement length on the properties of the lamellar crystal, in particular on the thickness selection, using algorithms to calculate the primitive path of the chains. In situ analysis of the local entanglement length prior to crystallization and the stem length and crystallinity at the same location reveals a direct correlation between the entanglement length and crystallization properties in the nucleation-controlled regime [1]. We have investigated various scenarios to change the apparent entanglement length in the system such as rapid cooling and cold-crystallization protocols, dilution of the melt by short chains, and the influence of solid substrates [2-4]. All studies confirm the correlation between entanglement length and thickness selection in the dense state. First results on non-concatenated ring polymer melts display a strong increase of the lamellar thickness and the degree of crystallization as compared to otherwise identical linear counterparts subject to the same thermal history [5].

References
[1] C.-F. Luo and J.-U. Sommer; Physical Review Letters 112, 195701 (2014). (link)
[2] C.-F. Luo, M. Kröger and J.-U. Sommer; Polymer 109, 71 (2017). (link)
[3] C.-F. Luo, M. Kröger and J.-U. Sommer; Macromolecules 49, 9017 (2016). (link)
[4] C.-F. Luo and J.-U. Sommer; ACS Macro Letters 5, 30 (2016). (link)
[5] H.-Y. Xiao, C.-F. Luo, D. Yan and J.-U. Sommer, manuscript (2017).