Effects of Hofmeister Ion Series on Stability of a Salt Bridge

S. Pylaeva, H. Elgabarty, and D. Sebastiani

Theoretical Chemistry, Institute of Chemistry, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 4, 06120, Halle, Germany

Salt bridges are important components of protein structure stability. They can be defined as an interaction of two aminoacid side chains of opposite charge [1]. Such Coulomb attraction interaction is sensitive to presence of other charged species in the vicinity. Concentration of charged species – free ions can be significant in a crowded environment of a living cell. Additionally Hofmeister ion series have been shown to have a significant impact on structure and dynamics of water and solvated proteins [2, 3].

We have investigated effects of Hofmeister ion series on an arginine – aspartic acid salt bridge by means of computer simulations [4]. Changes in thermodynamic properties of a salt bridge and dynamic properties of their solvation shells will be discussed in a poster.

[1] J.E. Donald, D.W. Kulp, W.F. DeGrado, Proteins, 79(3), 898 (2011). (link)
[2] C. Allolio, N. Salas-Illanes, Y.S. Desmukh, M.R. Hansen, D. Sebastiani, JPCB 117(34), 9939 (2013) (link)
[3] M.D. Smith, L. Cruz, JPCB 117, 6614 (2013) (link)
[4] M. Fyta, R. Netz, JCP 136, 124103 (2012). (link)

Amyloid peptide aggregation near interfaces

T. John1,2,3, L.L. Martin3, H.J. Risselada1,4, and B. Abel1,2

1Leibniz Institute of Surface Modification, Leipzig (Germany)
2Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Leipzig University, Leipzig (Germany)
3School of Chemistry, Monash University, Clayton (Australia)
4Department of Theoretical Physics, Georg-August-University Göttingen, Göttingen (Germany)

Amyloid peptides aggregate into characteristic fibrils with cross-β-sheet structure, also known as amyloid plaque. They are associated with several diseases such as Alzheimer’s disease or type II diabetes. However, there is evidence that indicates the soluble transient oligomers, instead of mature fibrils, as the toxic species. Amyloid-forming peptides are natively soluble and only aggregate under certain circumstances. Comprehensive knowledge on the aggregation mechanism and a detailed characterisation of the transient species is essential to understand the physiological role of these peptides [1].

Interfaces, such as nanoparticles, can act to accelerate or inhibit peptide aggregation. Experimental studies and molecular dynamics simulations (MD) presented an accelerated fibril formation in the presence of citrate-stabilised gold nanoparticles [2,3]. The role of gold surfaces in oligomer formation and peptide aggregation is discussed in this study. Moreover, possible mechanisms for the observed acceleration of the peptide aggregation by a reduction of the conformational space that is sampled are presented.

[1] F. Chiti, and C.M. Dobson, Annu. Rev. Biochem., 75, 333 (2006). (link)
[2] A. Gladytz, B. Abel, and H.J. Risselada, Angew. Chem., Int. Ed., 55, 11242 
(2016). (link)
[3] A. Gladytz, M. Wagner, T. Häupl, C. Elsner, and B. Abel, Part. Part. Syst. 
Charact., 32, 573 (2015). (link)

Crystallization-Driven Reversible Actuation in Cross-Linked Poly(ε-caprolactone)

O. Dolynchuk1, I. Kolesov2, D. Jehnichen3, U. Reuter3, H.-J. Radusch4, and J.-U. Sommer3

1Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany
2Interdisciplinary Center for Transfer-oriented Research, Martin Luther University Halle-Wittenberg, 06099 Halle, Germany
3Leibniz-Institut für Polymerforschung Dresden e.V., 01069 Dresden, Germany
4Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle, Germany

Crystallization of the pre-deformed polymer network under constant load can result in a non-trivial macroscopic elongation accompanied by network stiffening, which is reversible upon melting. Such actuation, known as the reversible shape-memory effect (rSME), is in focus due to fundamental interest of underlying molecular mechanisms and numerous potential applications. The rSME was studied in cross-linked linear poly(ε-caprolactone) (PCL) under various constant loads [1]. A striking rSME under stress-free conditions was found in PCL with the highest obtained cross-link density. The viscoelastic and thermal properties of the material as well as size and orientation of the crystals formed in PCL networks under and without load were investigated. As concluded, the directed growth of crystals is the origin of the reversible actuation in both loaded and free-standing PCL. The mean field approach was employed to calculate the free energy change during non-isothermal crystallization of PCL networks under load, whereby the possible morphology and orientation of crystals were analyzed. The analytical results on the nanocrystalline structure along with fitting curves of the temperature dependent strain, which were obtained by modeling the SME in PCL under load, are in good accordance with experimental findings.

[1] O. Dolynchuk, I. Kolesov, D. Jehnichen, U. Reuter, H.-J. Radusch, J.-U. Sommer, Macromolecules 50, 3841 (2017). (link)

Transition from Ring- to Chain-Dominated Phases in Supramolecular Polymer Networks

E. Lee, T. Shakirov, and W. Paul

Institut für Physik, Martin-Luther Universität Halle-Wittenberg, 06099 Halle (Saale), Germany

Rheological properties of supramolecular polymers depend on their structures including the size, the number, and the topology of aggregates. A linear polymer with hydrogen bonding units at both ends is one of widely used precursors to build the supramolecular polymer networks. Due to complex interplay between chain stiffness, hydrogen bonding interaction, and polymer conformational entropy it is difficult to theoretically predict the structure of the supramolecular polymer. In this work, we investigate structures of supramolecular polyethylene glycol and polybutylene glycols whose ends are capable of the hydrogen bond using a coarse-grained (CG) model via stochastic approximation Monte Carlo simulation (SAMC) method. Our CG force field is constructed by Boltzmann inversion of the probability distributions of all-atom polymer conformations. SAMC provides all the thermodynamic information of the system, which allows one to investigate supramolecular structures in a wide temperature range. This work especially focuses on the transition from ring- to chain-dominated phases since the contaminant of rings in a melt is known to significantly influence its rheology. In a limit of dilute concentration, the transition temperature (T*) shows non-monotonous behavior as molecular weight of the precursor increases due to competition between chain stiffness and hydrogen bonding. We also investigate the polymer concentration (c) dependence on T* to construct a c-T phase diagram.

Polythiophene Adsorption and Restructuring on Gold Surfaces

E. Schreck1, T. Simon1, S. Förster1, R. Hammer1, and W. Widdra1,2

1Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany
2Max Planck Institute of Microstructure Physics, Halle, Germany

Scanning tunneling microscopy is used to study the complex adsorption behavior of poly-3-hexyl-thiophene (P3HT) on Au(001) [1] and Au(011). Upon electrospray deposition under ultrahigh vacuum conditions, weakly adsorbed polymer chains are found on Au(001), which exhibit a truly 2D random-coil-like chain backbone structure. Their end-to-end distance and their radius of gyration are reported as function of the polymer length. Additionally, a fraction of the P3HT molecules is constraint into a fully stretched configuration along the high-symmetry [110] crystal direction, indicating a stronger molecule-substrate interaction. This adsorption is accompanied by local lifting of the Au(001) surface reconstruction [2] underneath the polymer chains [3].
For the more open Au(011) surface that exhibits a missing-row (2×1) reconstruction, we find a stronger interaction between P3HT and the gold surface. Polymer chains align predominantly along the missing rows by changing their conformation into an all-trans state. The presence of the polymer is inducing a local reorganization process changing the surface reconstruction from 2×1 to a 3×1 reconstruction. These results will be compared with similar findings for smaller thiophene oligomers (sexithiophene, 6T) on both substrates [4,5].

[1] S. Förster, E. Kohl, M. Ivanov, J. Gross, W. Widdra, and W. Janke, J. Chem. Phys. 141, 164701 (2014). (link)
[2] R. Hammer, A. Sander, S. Förster, M. Kiel, K. Meinel, and W. Widdra, Physical Review B, 2014, 90, 035446. (link)
[3] S. Förster and W. Widdra, J. Chem. Phys. 141, 054713 (2014). (link)
[4] K. Duncker, M. Kiel, A. Höfer, and W. Widdra, Phys. Rev. B 77, 155423 (2008). (link)
[5] M. Kiel, K. Duncker, C. Hagendorf, and W. Widdra, Phys. Rev. B 75, 195439 (2007). (link)

Dynamic and Structural Properties of Polyglutamine

P. Enke, M. Schleeger, and T. Kiefhaber

Institut für Biochemie und Biotechnologie, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Straße 3, 06120 Halle

More than ten diseases are known that are based on the expansion of polyglutamine sequences within the disease-related proteins, which leads to the formation of amyloids. We tested, whether the propensity to form amyloids is due to distinct structural and/or dynamic properties of the monomeric state of polyglutamine (polyQ) chains. Using triplet-triplet energy transfer (TTET) and time-resolved fluorescence resonance energy transfer (trFRET) we were able to characterize the structure (end-to-end distribution) and the intrachain dynamics of monomeric polyQ chains of different length. The results were compared to the properties of the previously characterized model chains poly-(glycine-serine) and poly-serine [1-4] and of fragments from IDPs, which do not form amyloids. The results show that intrachain dynamics in polyQ chains are slower than the dynamics of all other investigated chains and that longer polyQ chains contain a fraction of pre-formed loop structures in the conformational ensemble, which may explain their tendency to form oligomers of β-hairpins.

[1] O. Bieri, T. Kiefhaber, Biol. Chem. 380, 923 (1999). (link)
[2] F. Krieger, B. Fierz, O. Bieri, M. Drewello, T. Kiefhaber, J. Mol. Biol. 332, 265 (2003). (link)
[3] A. Möglich, K. Joder, T. Kiefhaber, PNAS 103, 12394 (2006). (link)
[4] B. Fierz, H. Satzger, C. Root, P. Glich, W. Zinth, T. Kiefhaber, PNAS 104, 2163 (2007). (link)

The confined crystallization of polymer in anodized aluminum oxide template

Xiaoli Sun, Xiying Dai, Shouke Yan

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

The one dimensional (1D) polymer nanostructures with controlled morphologies and properties can be easily achieved from Anodized aluminum oxide (AAO) template. In AAO template, confinement effect and interface effect determine the crystallization and dynamical behavior.[1-4] Some aspects of questions are worthwhile for deeper understanding: (1) discrimination between the interfacial and confinement effect on the dynamics of multiscale chain-motions; (2) discrimination between segmental dynamics and chain dynamics on the crystallization of polymer. Moreover, the studies on the comparison of confinement effect between 1D polymer nanostructures and two dimensional (2D) polymer thin film by employing the same polymer is also less explored. In the present study, poly(3-hydroxybutyrate) (PHB) was selected as a model to explore the above aspects. Interfacial effect slows down the dynamics of PHB segmental mobility and shows strong dependence on pore size. Spatial confinement accelerates the dynamics of segmental mobility. Both effects slow down the chain mobility which consequently slow down the crystallization kinetics of PHB. The inhibited crystallization of PHB in AAO nanopores can be attributed to both segmental and chain mobility. By contrast, only chain mobility plays the role on the crystallization of thin film. Segmental mobility does not change with film thickness.

[1] L. Li, D. Zhou, D. Huang, G. Xue, Macromolecules 47, 297 (2014). (link)
[2] M. Steinhart, P. Göring, H. Dernaika, M. Prabhukaran, U. Gösele,  E. Hempel, and T. Thurn-Albrecht, Phys. Rev. Lett. 97, 027801 (2006). (link)
[3] X. Sun, Q. Fang, H. Li, Z. Ren, S. Yan, Langmuir 32, 3269 (2016). (link)
[4] X. Dai, J. Niu, Z. Ren, X. Sun, S. Yan, J. Phys. Chem B 120, 843 (2016). (link)

Effects Of Phase Separation and Crystallization on Morphology of Poly(propylene carbonate)/Poly(3-Hydroxybutyrate) Blend Thin Films

Huihui Li, Shujing Zhang and Shouke Yan

Beijing University of Chemical Technology, Beijing, China

For partially miscible crystalline/amorphous polymer blends, the combined effects of phase separation and crystallization can result in multiple morphology. In this study, the effects of composition and melting time on the morphology and structures of poly(propylene carbonate)/poly(3-hydroxybutyrate) blend thin films was investigated. A low PPC content in the film resulted in compact PHB spherulites, filling the whole space, whereas the amorphous PPC spherical microdomains scatter in the PHB region. With increasing PPC component and melting time, a large amount of PPC aggregates to the surface to form a network uplayer, whereas the PHB thick domains connected by its thin layer form a continuous PHB region, leading to a superimposed bilayer structure. If PPC content reached 70 wt%, a PPC-top and microporous PHB-bottom bilayer structure can be developed. We suggested that phase separation can take place mainly along the normal direction of the film surface at 190 ºC, attributed to the different surface energies of the two components. After cooling, the crystallization occurred, leading to further segregation and solidification along film planes. This superimposed bilayer by interplay between phase separation and crystallization may provide availability to tailor the final structure and properties of crystalline/amorphous polymer films.

[1] S. Zhang, X. Sun, Z. Ren, H. Li and S. Yan Phys. Chem. Chem. Phys. 17, 32225 (2015). (link)
[2] S. Zhang, Z. Ren, X. Sun, H. Li and S. Yan Langmuir 33, 1202 (2017). (link)

Development of improved materials from poly(lactic acid) with the aid of plasticizers and crystallization-promoting gelators

M. Colaers1, W. Thielemans2, and B. Goderis1

1Polymer Chemistry and Materials, KU Leuven, 3001 Heverlee, Belgium
2Chemical Engineering, Campus Kulak Kortrijk, 8500 Kortrijk, Belgium

Polylactic acid (PLA) is a bio-based polymer which might become an alternative for petroleum-based plastics such as polypropylene and polystyrene. However, at present, the properties of PLA do not meet the requirements for a number of applications. The challenge to be addressed is to develop transparent PLA with an increased heat distortion temperature, balanced stiffness and toughness and increased barrier properties. The method explored to reach this multi-dimensional goal is to blend PLA with combinations of plasticizers (for reducing the brittleness) and crystallization-promoting gelators (to increase the crystallinity). Upon cooling, the gelator forms a fine fibrillary network, which nucleates the PLA crystallization. The resulting, small-sized crystal aggregates limit the scattering of visible light and enhance transparency, which is desired for packaging purposes. An increased crystallinity is required to enhance the barrier properties and increase the heat distortion temperature.

The semicrystalline morphology and efficiency of the gelator fibrillary network depends on the cooling conditions and the addition of plasticizers. Both aspects are addressed using Differential Scanning Calorimetry, optical microscopy and time-resolved synchrotron SAXS/WAXD.

Effects of thermal denaturation and UV-B irradiation on eye lens crystallin proteins

M. Camilles, S. Link, A. Krushelnitsky, J. Balbach, K. Saalwächter

Institute of Physics, Martin Luther University Halle-Wittenberg, Germany

Crystallins are the major vision-related (i.e. refractive) proteins found in the eye lens. The mammalian lens consist of three classes of proteins, α-, β- and γ-crystallins, the former also acts as chaperone [1]. Commonly, proteins are subject to a continuous degradation and replacement process, but the eye lens proteins have no turnover and hence have to remain stable and soluble for a lifetime. So far, most studies have focused on single eye lens proteins and their interactions at low concentrations [2]. Here we combine NMR and other biophysical techniques to monitor stress induced aggregation and changes of the interactions of crystallins at various concentrations [3]. This allows us to investigate molecular effects which might lead to cataract in a highly concentrated eye lens surrounding.

[1] H. Bloemendal et. al., Progress in Biophysics & Molecular Biology 86 407-485 (2004). (link)
[2] CN Kingsley, R.W. Martin et.al., Structure 21, 2221 (2013). (link)
[3] M. Roos, S. Link et. al., Biophysical Journal 108, 98 (2015). (link)