publish.UP Institut für Chemie

New synthetic routes towards well-defined polypeptides, morphologies and hydrogels (2016)

Charlotte Vacogne

Proteins are natural polypeptides produced by cells; they can be found in both animals and plants, and possess a variety of functions. One of these functions is to provide structural support to the surrounding cells and tissues. For example, collagen (which is found in skin, cartilage, tendons and bones) and keratin (which is found in hair and nails) are structural proteins. When a tissue is damaged, however, the supporting matrix formed by structural proteins cannot always spontaneously regenerate. Tailor-made synthetic polypeptides can be used to help heal and restore tissue formation. Synthetic polypeptides are typically synthesized by the so-called ring opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCA). Such synthetic polypeptides are generally non-sequence-controlled and thus less complex than proteins. As such, synthetic polypeptides are rarely as efficient as proteins in their ability to self-assemble and form hierarchical or structural supramolecular assemblies in water, and thus, often require rational designing. In this doctoral work, two types of amino acids, γ-benzyl-L/D-glutamate (BLG / BDG) and allylglycine (AG), were selected to synthesize a series of (co)polypeptides of different compositions and molar masses. A new and versatile synthetic route to prepare polypeptides was developed, and its mechanism and kinetics were investigated. The polypeptide properties were thoroughly studied and new materials were developed from them. In particular, these polypeptides were able to aggregate (or self-assemble) in solution into microscopic fibres, very similar to those formed by collagen. By doing so, they formed robust physical networks and organogels which could be processed into high water-content, pH-responsive hydrogels. Particles with highly regular and chiral spiral morphologies were also obtained by emulsifying these polypeptides. Such polypeptides and the materials derived from them are, therefore, promising candidates for biomedical applications.

Untersuchung des Verhaltens von thermoresponsiven Polymeren auf Elektroden in Interaktion mit biomolekularen Systemen (2016)

Artur Fandrich

Diese Arbeit befasst sich mit der Herstellung und Charakterisierung von thermoresponsiven Filmen auf Goldelektroden durch Fixierung eines bereits synthetisierten thermoresponsiven Polymers. Als Basis für die Entwicklung der responsiven Grenzfläche dienten drei unterschiedliche Copolymere (Polymere I, II und III) aus der Gruppe der thermisch schaltbaren Poly(oligo(ethylenglykol)methacrylate). Die turbidimetrischen Messungen der Copolymere in Lösungen haben gezeigt, dass der Trübungspunkt vom pH-Wert, der Gegenwart von Salzen sowie von der Ionenstärke der Lösung abhängig ist. Nach der Charakterisierung der Polymere in Lösung wurden Experimente der kovalenten Kopplung der Polymere I bis III an die Oberfläche der Gold-Elektroden durchgeführt. Während bei Polymeren I und II die Ankopplung auf einer Amidverbrückung basierte, wurde bei Polymer III als alternative Methode zur Immobilisierung eine photoinduzierte Anbindung unter gleichzeitiger Vernetzung gewählt. Der Nachweis der erfolgreichen Ankopplung erfolgte bei allen Polymeren elektrochemisch mittels Cyclovoltammetrie und Impedanzspektroskopie in K3/4[Fe(CN)6]-Lösungen. Wie die Ellipsometrie-Messungen zeigten, waren die erhaltenen Polymer-Filme unterschiedlich dick. Die Ankopplung über Amidverbrückung lieferte dünne Filme (10 – 15 nm), während der photovernetzte Film deutlich dicker war (70-80 nm) und die darunter liegende Oberfläche relativ gut isolierte. Elektrochemische Temperaturexperimente an Polymer-modifizierten Oberflächen in Lösungen in Gegenwart von K3/4[Fe(CN)6] zeigten, dass auch die immobilisierten Polymere I bis III responsives Temperaturverhalten zeigen. Bei Elektroden mit den immobilisierten Polymeren I und II ist der Temperaturverlauf der Parameterwerte diskontinuierlich – ab einem kritischen Punkt (37 °C für Polymer I und 45 °C für Polymer II) wird zunächst langsame Zunahme der Peakströme wird deutlich schneller. Das Temperaturverhalten von Polymer III ist dagegen bis 50 °C kontinuierlich, der Peakstrom sinkt hier durchgehend. Weiterhin wurde mit den auf Polymeren II und III basierten Elektroden deren Anwendung als responsive Matrix für Bioerkennungsreaktionen untersucht. Es wurde die Ankopplung von kleinen Biorezeptoren, TAG-Peptiden, an Polymer II- und Polymer III-modifizierten Elektroden durchgeführt. Das hydrophile FLAG-TAG-Peptid verändert das Temperaturverhalten des Polymer II-Films unwesentlich, da es die Hydrophilie des Netzwerkes nicht beeinflusst. Weiterhin wurde der Effekt der Ankopplung der ANTI-FLAG-TAG-Antikörper an FLAG-TAG-modifizierte Polymer II-Filme untersucht. Es konnte gezeigt werden, dass die Antikörper spezifisch an FLAG-TAG-modifiziertes Polymer II binden. Es wurde keine unspezifische Anbindung von ANTI-FLAG-TAG an Polymer II beobachtet. Die Temperaturexperimente haben gezeigt, dass die thermische Restrukturierung des Polymer II-FLAG-TAG-Filmes auch nach der Antikörper-Ankopplung noch stattfindet. Der Einfluss der ANTI-FLAG-TAG-Ankopplung ist gering, da der Unterschied in der Hydrophilie zwischen Polymer II und FLAG-TAG bzw. ANTI-FLAG-TAG zu gering ist. Für die Untersuchungen mit Polymer III-Elektroden wurde neben dem hydrophilen FLAG-TAG-Peptid das deutlich hydrophobere HA-TAG-Peptid ausgewählt. Wie im Falle der Polymer II Elektrode beeinflusst das gekoppelte FLAG-TAG-Peptid das Temperaturverhalten des Polymer III-Netzwerkes nur geringfügig. Die gemessenen Stromwerte sind geringer als bei der Polymer III-Elektrode. Das Temperaturverhalten der FLAG-TAG-Elektrode ähnelt dem der reinen Polymer III-Elektrode – die Stromwerte sinken kontinuierlich bis die Temperatur von ca. 40 °C erreicht ist, bei der ein Plateau beobachtet wird. Offensichtlich verändert FLAG-TAG auch in diesem Fall nicht wesentlich die Hydrophilie des Polymer III-Netzwerkes. Das an Polymer III-Elektroden gekoppelte hydrophobe HA-TAG-Peptid beeinflusst dagegen im starken Maße den Quellzustand des Netzwerkes. Die Ströme für die HA-TAG-Elektroden sind deutlich geringer als die für die FLAG-TAG-Polymer III-Elektroden, was auf geringeren Wassergehalt und dickeren Film zurückzuführen ist. Bereits ab 30 °C erfolgt der Anstieg von Stromwerten, der bei Polymer III- bzw. bei Polymer III-FLAG-TAG-Elektroden nicht beobachtet werden kann. Das gekoppelte hydrophobe HA-TAG-Peptid verdrängt Wasser aus dem Polymer III-Netzwerk, was in der Stauchung des Films bereits bei Raumtemperatur resultiert. Dies führt dazu, dass der Film im Laufe des Temperaturanstieges kaum noch komprimiert. Die Stromwerte steigen in diesem Fall entsprechend des Anstiegs der temperaturabhängigen Diffusion des Redoxpaares. Diese Untersuchungen zeigen, dass das HA-TAG-Peptid als Ankermolekül deutlich besser für eine potentielle Verwendung der Polymer III-Filme für sensorische Zwecke geeignet ist, da es sich deutlich in der Hydrophilie von Polymer III unterscheidet.

pH and concentration dependence of the optical properties of thiol-capped CdTe nanocrystals in water and D2O (2016)

Ralf Schneider Florian Weigert Vladimir Lesnyak Susanne Leubner Tommy Lorenz Thomas Behnke Aliaksei Dubavik Jan-Ole Joswig Ute Resch-Genger Nikolai Gaponik Alexander Eychmuller

The optical properties of semiconductor nanocrystals (SC NCs) are largely controlled by their size and surface chemistry, i.e., the chemical composition and thickness of inorganic passivation shells and the chemical nature and number of surface ligands as well as the strength of their bonds to surface atoms. The latter is particularly important for CdTe NCs, which – together with alloyed CdxHg1−xTe – are the only SC NCs that can be prepared in water in high quality without the need for an additional inorganic passivation shell. Aiming at a better understanding of the role of stabilizing ligands for the control of the application-relevant fluorescence features of SC NCs, we assessed the influence of two of the most commonly used monodentate thiol ligands, thioglycolic acid (TGA) and mercaptopropionic acid (MPA), on the colloidal stability, photoluminescence (PL) quantum yield (QY), and PL decay behavior of a set of CdTe NC colloids. As an indirect measure for the strength of the coordinative bond of the ligands to SC NC surface atoms, the influence of the pH (pD) and the concentration on the PL properties of these colloids was examined in water and D2O and compared to the results from previous dilution studies with a set of thiol-capped Cd1−xHgxTe SC NCs in D2O. As a prerequisite for these studies, the number of surface ligands was determined photometrically at different steps of purification after SC NC synthesis with Ellman's test. Our results demonstrate ligand control of the pH-dependent PL of these SC NCs, with MPA-stabilized CdTe NCs being less prone to luminescence quenching than TGA-capped ones. For both types of CdTe colloids, ligand desorption is more pronounced in H2O compared to D2O, underlining also the role of hydrogen bonding and solvent molecules.

Real-time monitoring of plasmon induced dissociative electron transfer to the potential DNA radiosensitizer 8-bromoadenine (2016)

Robin Schürmann Ilko Bald

The excitation of localized surface plasmons in noble metal nanoparticles (NPs) results in different nanoscale effects such as electric field enhancement, the generation of hot electrons and a temperature increase close to the NP surface. These effects are typically exploited in diverse fields such as surface-enhanced Raman scattering (SERS), NP catalysis and photothermal therapy (PTT). Halogenated nucleobases are applied as radiosensitizers in conventional radiation cancer therapy due to their high reactivity towards secondary electrons. Here, we use SERS to study the transformation of 8-bromoadenine (8BrA) into adenine on the surface of Au and AgNPs upon irradiation with a low-power continuous wave laser at 532, 633 and 785 nm, respectively. The dissociation of 8BrA is ascribed to a hot-electron transfer reaction and the underlying kinetics are carefully explored. The reaction proceeds within seconds or even milliseconds. Similar dissociation reactions might also occur with other electrophilic molecules, which must be considered in the interpretation of respective SERS spectra. Furthermore, we suggest that hot-electron transfer induced dissociation of radiosensitizers such as 8BrA can be applied in the future in PTT to enhance the damage of tumor tissue upon irradiation.

Metathesis polymerization of cystine-based macrocycles (2016)

Felix N. Behrendt Helmut Schlaad

Macrocycles based on L-cystine were synthesized by ring-closing metathesis (RCM) and subsequently polymerized by entropy-driven ring-opening metathesis polymerization (ED-ROMP). Monomer conversion reached ∼80% in equilibrium and the produced poly(ester-amine-disulfide-alkene)s exhibited apparent molar masses (Mappw) of up to 80 kDa and dispersities (Đ) of ∼2. The polymers can be further functionalized with acid anhydrides and degraded by reductive cleavage of the main-chain disulfide.

Quantum chemistry and surface hopping dynamics of azobenzenes (2017)

Evgenii Titov

This cumulative doctoral dissertation, based on three publications, is devoted to the investigation of several aspects of azobenzene molecular switches, with the aid of computational chemistry. In the first paper, the isomerization rates of a thermal cis → trans isomerization of azobenzenes for species formed upon an integer electron transfer, i.e., with added or removed electron, are calculated from Eyring’s transition state theory and activation energy barriers, computed by means of density functional theory. The obtained results are discussed in connection with an experimental study of the thermal cis → trans isomerization of azobenzene derivatives in the presence of gold nanoparticles, which is demonstrated to be greatly accelerated in comparison to the same isomerization reaction in the absence of nanoparticles. The second paper is concerned with electronically excited states of (i) dimers, composed of two photoswitchable units placed closely side-by-side, as well as (ii) monomers and dimers adsorbed on a silicon cluster. A variety of quantum chemistry methods, capable of calculating molecular electronic absorption spectra, based on density functional and wave function theories, is employed to quantify changes in optical absorption upon dimerization and covalent grafting to a surface. Specifically, the exciton (Davydov) splitting between states of interest is determined from first-principles calculations with the help of natural transition orbital analysis, allowing for insight into the nature of excited states. In the third paper, nonadiabatic molecular dynamics with trajectory surface hopping is applied to model the photoisomerization of azobenzene dimers, (i) for the isolated case (exhibiting the exciton coupling between two molecules) as well as (ii) for the constrained case (providing the van der Waals interaction with environment in addition to the exciton coupling between two monomers). For the latter, the additional azobenzene molecules, surrounding the dimer, are introduced, mimicking a densely packed self-assembled monolayer. From obtained results it is concluded that the isolated dimer is capable of isomerization likewise the monomer, whereas the steric hindrance considerably suppresses trans → cis photoisomerization. Furthermore, the present dissertation comprises the general introduction describing the main features of the azobenzene photoswitch and objectives of this work, theoretical basis of the employed methods, and discussion of gained findings in the light of existing literature. Also, additional results on (i) activation parameters of the thermal cis → trans isomerization of azobenzenes, (ii) an approximate scheme to account for anharmonicity of molecular vibrations in calculation of the activation entropy, as well as (iii) absorption spectra of photoswitch–silicon composites obtained from time-demanding wave function-based methods are presented.

Thermoresponsive poly(2-oxazoline)s, polypeptoids, and polypeptides (2016)

Richard Hoogenboom Helmut Schlaad

This review covers the recent advances in the emerging field of thermoresponsive polyamides or polymeric amides, i.e., poly(2-oxazoline)s, polypeptoids, and polypeptides, with a specific focus on structure–thermoresponsive property relationships, self-assembly, and applications.

How surface reparation prevents catalytic oxidation of carbon monoxide on atomic gold at defective magnesium oxide surfaces (2016)

Kai Töpfer Jean Christophe Tremblay

In this contribution, we study using first principles the co-adsorption and catalytic behaviors of CO and O2 on a single gold atom deposited at defective magnesium oxide surfaces. Using cluster models and point charge embedding within a density functional theory framework, we simulate the CO oxidation reaction for Au1 on differently charged oxygen vacancies of MgO(001) to rationalize its experimentally observed lack of catalytic activity. Our results show that: (1) co-adsorption is weakly supported at F0 and F2+ defects but not at F1+ sites, (2) electron redistribution from the F0 vacancy via the Au1 cluster to the adsorbed molecular oxygen weakens the O2 bond, as required for a sustainable catalytic cycle, (3) a metastable carbonate intermediate can form on defects of the F0 type, (4) only a small activation barrier exists for the highly favorable dissociation of CO2 from F0, and (5) the moderate adsorption energy of the gold atom on the F0 defect cannot prevent insertion of molecular oxygen inside the defect. Due to the lack of protection of the color centers, the surface becomes invariably repaired by the surrounding oxygen and the catalytic cycle is irreversibly broken in the first oxidation step.

Quantification of the push–pull effect in 2-alkylidene-4-oxothiazolidines by using NMR spectral data and barriers to rotation around the C=C bond (2016)

Aleksandar Rasovic Vladimir Blagojevic Marija Baranac-Stojanovic Erich Kleinpeter Rade Markovic Dragica M. Minic

Information about the strength of donor–acceptor interactions in push–pull alkenes is valuable, as this so-called “push–pull effect” influences their chemical reactivity and dynamic behaviour. In this paper, we discuss the applicability of NMR spectral data and barriers to rotation around the C[double bond, length as m-dash]C double bond to quantify the push–pull effect in biologically important 2-alkylidene-4-oxothiazolidines. While olefinic proton chemical shifts and differences in 13C NMR chemical shifts of the two carbons constituting the C[double bond, length as m-dash]C double bond fail to give the correct trend in the electron withdrawing ability of the substituents attached to the exocyclic carbon of the double bond, barriers to rotation prove to be a reliable quantity in providing information about the extent of donor–acceptor interactions in the push–pull systems studied. In particular all relevant kinetic data, that is the Arrhenius parameters (apparent activation energy Ea and frequency factor A) and activation parameters (ΔS‡, ΔH‡ and ΔG‡), were determined from the data of the experimentally studied configurational isomerization of (E)-9a. These results were compared to previously published related data for other two compounds, (Z)-1b and (2E,5Z)-7, showing that experimentally determined ΔG‡ values are a good indicator of the strength of push–pull character. Theoretical calculations of the rotational barriers of eight selected derivatives excellently correlate with the calculated C[double bond, length as m-dash]C bond lengths and corroborate the applicability of ΔG‡ for estimation of the strength of the push–pull effect in these and related systems.

On the role of monomer drops and swelling in aqueous heterophase polymerization (2016)

Chunxiang Wei

Institut für Chemie

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