Oligomer Stacks Research Articles

Formation of TiO2 nanostructures modified Eumelanin films with enhanced properties for biopolymer implementations

Thin films of hybrid melanin-TiO2 nanoparticles (eumelanin: TiO2) deposited from solution by electro-spray were accurately inspected to unveil modified structural and electronic properties for device implementations. Based on the reorganization of the melanin electronic valence orbital, among other interesting behaviors, we observed up to a two orders increase in the absorption coefficient in the visible range. Furthermore the red-shifted absorption features, disclose that the extended π- stacking of oligomers is mainly responsible for the tunability of the optical gap as confirmed by photoluminescence. TiO2 nanostructures via oxidative polymerization, further improve the oligomeric character observed by means of Raman scattering. Finally, the photocatalytic activity of TiO2 nanoparticles helps the saturation of shallow trap states in melanin structure and consequently enhances the charge carrier transport.

Unraveling the molecular nature of melanin changes in metastatic cancer.

.More people die from melanoma after a stage I diagnosis than after a stage IV diagnosis, because the tools available to clinicians do not readily identify which early-stage cancers will be aggressive. Near-infrared pump-probe microscopy detects fundamental differences in melanin structure between benign human moles and melanoma and also correlates with metastatic potential. However, the biological mechanisms of these changes have been difficult to quantify, as many different mechanisms can contribute to the pump-probe signal. We use model systems (sepia, squid, and synthetic eumelanin), cellular uptake studies, and a range of pump and probe wavelengths to demonstrate that the clinically observed effects come from alterations of the aggregated mode from “thick oligomer stacks” to “thin oligomer stacks” (due to changes in monomer composition) and (predominantly) deaggregation of the assembled melanin structure. This provides the opportunity to use pump-probe microscopy for the detection and study of melanin-associated diseases.

Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin.

In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules), and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation is key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum and produce drastic changes in the transient absorption spectrum.

Backbone‐Directed Perylene Dye Self‐Assembly into Oligomer Stacks

Armdrücken: Die Rückgrat-dirigierte „Arm-in-Arm“-Aggregation einer neuartigen Perylenbisimid(PBI)-Dyade mit einem festgelegten intramolekularen Abstand führt zur Bildung von kinetisch stabilen erweiterten PBI-π-Stapeln. Es wird gezeigt, dass sich die PBI-Dyade zu Oligomeren aus bis zu 21 Einheiten anordnet. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Backbone‐Directed Perylene Dye Self‐Assembly into Oligomer Stacks

Arm wrestling: Backbone-directed "arm-to-arm" aggregation of a newly designed perylene bisimide (PBI) dyad with a defined intramolecular space leads to the growth of kinetically stable extended PBI π-stacks. This PBI dyad was shown to assemble into oligomers up to 21 units in length.

Nine compounds containing high-nuclearity [CunX2n+2]2−(n= 4, 5 or 7;X= Cl or Br) quasi-planar oligomers

The structures of seven A(2)Cu(4)X(10) compounds containing quasi-planar oligomers are reported: bis(1,2,4-trimethylpyridinium) hexa-μ-chlorido-tetrachloridotetracuprate(II), (C(8)H(12)N)(2)[Cu(4)Cl(10)], (I), and the hexa-μ-bromido-tetrabromidotetracuprate(II) salts of 1,2,4-trimethylpyridinium, (C(8)H(12)N)(2)[Cu(4)Br(10)], (II), 3,4-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (III), 2,3-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (IV), 1-methylpyridinium, (C(6)H(8)N)(2)[Cu(4)Br(10)], (V), trimethylphenylammonium, (C(9)H(14)N)(2)[Cu(4)Br(10)], (VI), and 2,4-dimethylpyridinium, (C(7)H(10)N)(2)[Cu(4)Br(10)], (VII). The first four are isomorphous and contain stacks of tetracopper oligomers aggregated through semicoordinate Cu···X bond formation in a 4(5/2,1/2) stacking pattern. The 1-methylpyridinium salt also contains oligomers stacked in a 4(5/2,1/2) pattern, but is isomorphous with the known chloride analog instead. The trimethylphenylammonium salt contains stacks of oligomers arranged in a 4(3/2,1/2) stacking pattern similar to the tetramethylphosphonium analog. These six structures feature inversion-related organic cation pairs and hybrid oligomer/organic cation layers derived from the parent CuX(2) structure. The 2,4-dimethylpyridinium salt is isomorphous with the known (2-amino-4-methylpyridinium)(2)Cu(4)Cl(10) structure, in which isolated stacks of organic cations and of oligomers in a 4(1/2,1/2) pattern are found. In bis(3-chloro-1-methylpyridinium) octa-μ-bromido-tetrabromidopentacuprate(II), (C(6)H(7)ClN)[Cu(5)Br(12)], (VIII), containing the first reported fully halogenated quasi-planar pentacopper oligomer, the oligomers stack in a 5(3/2,1/2) stacking pattern as the highest nuclearity [Cu(n)X(2n+2)](2-) oligomer compound known with isolated stacking. Bis(2-chloro-1-methylpyridinium) dodeca-μ-bromido-tetrabromidoheptacuprate(II), (C(6)H(7)ClN)(2)[Cu(7)Br(16)], (IX), contains the second heptacopper oligomer reported and consists of layers of interleaved oligomer stacks with a 7[(7/2,1/2)][(-9/2,-1/2)] pattern isomorphous with that of the known 1,2-dimethylpyridinium analog. All the oligomers reported here are inversion symmetric.

Self-assembly of vesicles from the stacking of a dipodal F⋯H–N hydrogen bonded arylamide foldamer

In this paper, we report the synthesis and self-assembling behavior of an F⋯H–N hydrogen bonding-induced arylamide-based dipodal foldamer. SEM, AFM, TEM, and XRD studies reveal that this preorganized oligomer stacks to form vesicles in methanol–chloroform (10–70%) binary solvents due to the strong stacking interaction of the folded segments and the supramolecular polymeric feature of the dipodal molecule in the stacked state. In contrast, a simple folded molecule can give rise to vesicles only when the chloroform content is 45–55%.

Spectroscopic studies of 9-hydroxyellipticine binding to DNA.

The binding of 9-hydroxyellipticine to calf thymus DNA, poly[d(A-T)]2, and poly[d(G-C)]2 has been studied in detail by means of CD, linear dichroism, resonance light scattering, and molecular dynamics. The transition moment polarizations of 9-hydroxyellipticine were determined in polyvinyl alcohol stretched film. Spectroscopic solution studies of the DNA/drug complex are combined with theoretical CD calculations using the final 50 ps of a series of molecular dynamics simulations as input. The spectroscopic data shows 9-hydroxyellipticine to adopt two main binding modes, one intercalative and the other a stacked binding mode involving the formation of drug oligomers in the DNA major groove. Analysis of the intercalated binding mode in poly[d(A-T)]2 suggests the 9-hydroxyellipticine hydroxyl group lies in the minor groove and hydrogen bonds to water with the pyridine ring protruding into the major groove. The stacked binding mode was examined using resonance light scattering and it was concluded that the drug was forming small oligomer stacks rather than extended aggregates. Reduced linear dichroism measurements suggested a binding geometry that precluded a minor groove binding mode where the plane of the drug makes a 45 degrees angle with the plane of the bases. Thus it was concluded that the drug stacks in the major groove. No obvious differences in the mode of binding of 9-hydroxyellipticine were observed between different DNA sequences; however, the stacked binding mode appeared to be more favorable for calf thymus DNA and poly[d(G-C)]2 than for poly[d(A-T)]2, an observation that could be explained by the slightly greater steric hindrance of the poly[d(A-T)]2 major groove. A strong concentration dependence was observed for the two binding modes where intercalation is favored at very low drug load, with stacking interactions becoming more prominent as the drug concentration is increased. Even at DNA: drug mixing ratios of 70:1 the stacked binding mode was still important for GC-rich DNAs.

STM investigations on bridged macrocyclic metal complexes

The non-crystallizable bridged phthalocyaninatoiron complexes, [(tBu) 4PcFe(dib)] n ( 1) and [(tBu) 4PcFe(me 2pyNC)] n ( 2), were spread onto highly oriented pyrolytic graphite and imaged using scanning tunnelling microscopy in air. Dominating parallel arrangements of the oligomeric molecules in bundles of two to five stacks have been found. The individual building blocks, namely, the macrocycles (tBu) 4PcFe and the bridging ligands 1,4-diisocyanobenzene (dib) and 4-isocyano-3,5-dimethylpyridine (me 2pyNC), however, could not be resolved. Possible reasons include the non-pronounced molecular structure, a non-perfect solvent removal and the strong electric force of the tip to the weakly bonded molecule. The width (about 15 Å) and the length (80 to 450 Å) of the single oligomer stacks agree well with the structure model and, with respect to the chain lengths, the results of non-spatially resolving measuring methods.