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Homochirality is an obvious feature of life on Earth. On the other hand, extraterrestrial samples contain largely racemic compounds. The same is true for any common organic synthesis. Therefore, it has been a perplexing puzzle for decades how these racemates could have formed enantiomerically enriched fractions as a basis for the origin of homochiral life forms. Numerous hypotheses have been put forward as to how preferentially homochiral molecules could have formed and accumulated on Earth. In this article, it is shown that homochirality of the abiotic organic pool at the time of formation of the first self-replicating molecules is not necessary and not even probable. It is proposed to abandon the notion of a molecular ensemble and to focus on the level of individual molecules. Although the formation of the first self-replicating, most likely homochiral molecule, is a seemingly improbable event, on a closer look, it is almost inevitable that some homochiral molecules have formed simply on a statistical basis. In this case, the non-selective leap to homochirality would be one of the first steps in chemical evolution directly out of a racemic “ocean”. Moreover, most studies focus on the chirality of the primordial monomers with respect to an asymmetric carbon atom. However, any polymer with a minimal size that allows folding to a secondary structure would spontaneously lead to asymmetric higher structures (conformations). Most of the functions of these polymers would be influenced by this inherently asymmetric folding. Furthermore, a concept of physical compartmentalization based on rock nanopores in analogy to nanocavities of digital immunoassays is introduced to suggest that complex cell walls or membranes were also not required for the first steps of chemical evolution. To summarize, simple and universal mechanisms may have led to homochiral self-replicating systems in the context of chemical evolution. A homochiral monomer pool is deemed unnecessary and probably never existed on primordial Earth.
The controlled production of supramolecular aggregates formed by the self-assembly of dendritic amphiphiles is of great interest owing to their potential application in the fields of nanotechnology and nanomedicine. Dendritic amphiphiles as building blocks offer the advantage that their structure and size can be precisely tuned through organic synthesis. This synthetic flexibility enables the fine-tuning of the hydrophobic to hydrophilic ratio of the dendritic segments, which mainly controls the morphology of the self-assembled structures.
A promising method for the controlled preparation of supramolecular assemblies is based on the use of micromixers.[5,6] Due to their mixing times in the range of milliseconds at the microscale level, the application of such microfluidic systems benefits from a high mixing efficiency, a low mixing time and a reproducible synthesis compared to conventional batch-based techniques such as the solvent injection method or the film hydration method.
Herein, we report on the microfluidic-assisted self-assembly of several dendritic amphiphiles and the impact of the mixing parameters on the self-assembly process.
Cylindrical poly(styrene sulfonate) brushes were combined with a tetravalent double-DABCO based counterion. With increasing addition of counterion, hydrodynamic radius and radius of gyration decrease and AFM reveals a conformation change from wormlike to curled structures. Rg/Rh changes from 1.3 to 0.9. Above charge stoichiometry, brushes become interconnected into finite size assemblies, which are stable in aqueous solution for charge ratios (ratio of the molar concentration of charged counterion groups to the molar concentration of charged monomer units of the polymer brush) 1 < Ι < 1.2. They are compact and consist of the brushes in curled (bent) conformation, which is in difference to network-like aggregates from cylindrical poly(styrene sulfonate) brushes with tetravalent porphyrin counterions (Gröhn et al. Macromolecules 2009) and to a wormlike — bent conformation — helix transition found for another brush counterion system (Müller et al. Soft Matter 2009).
Polystyrene sulfonate - porphyrin assemblies: influence of polyelectrolyte and porphyrin structure
(2011)
In this study, electrostatic self-assembly of different polystyrene sulfonates and a set of tetravalent cationic porphyrins is investigated. It is shown that association of linear polystyrene sulfonates of different molar masses yields finite size nanoscale assemblies that are stable in aqueous solution. Aggregates are compared to the ones of cylindrical brushes, revealing that both form assemblies in the 100 nm range with the charge ratio (molar ratio of porphyrin charges to polyelectrolyte charges) being determining, while the morphology of the resulting network-like assemblies is different for both polyelectrolyte architectures. For the smallest 8k polystyrene sulfonate, in addition, stoichiometric conditions differ. The influence of the molecular porphyrin structure was investigated by comparing meso-tetrakis(4-(trimethyl-ammonium)phenyl)porphyrin (TAPP) with its Cu(II) and Zn(II) loaded analogues and meso-tetrakis(4-N-methylpyridinium)porphyrin (TMPyP), revealing differences in stacking tendency and geometry. Additionally, the TMPyP accumulates more in the inside of the brush than the other porphyrins, likely due to the different position of its charged groups. The supramolecular nanostructures formed were characterized by UV–vis spectroscopy, light scattering, atomic force microscopy, cryo transmission electron microscopy, and small-angle neutron scattering. Results may build a valuable basis for the use of polyelectrolyte–porphyrin assemblies in medicine, catalysis, or energy conversion.
DNA origami nanostructures are a versatile tool that can be used to arrange functionalities with high local control to study molecular processes at a single-molecule level. Here, we demonstrate that DNA origami substrates can be used to suppress the formation of specific guanine (G) quadruplex structures from telomeric DNA. The folding of telomeres into G-quadruplex structures in the presence of monovalent cations (e.g. Na+ and K+) is currently used for the detection of K+ ions, however, with insufficient selectivity towards Na+. By means of FRET between two suitable dyes attached to the 3'- and 5'-ends of telomeric DNA we demonstrate that the formation of G-quadruplexes on DNA origami templates in the presence of sodium ions is suppressed due to steric hindrance. Hence, telomeric DNA attached to DNA origami structures represents a highly sensitive and selective detection tool for potassium ions even in the presence of high concentrations of sodium ions.
The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-L-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200±42 nm composed of densely packed PBLG helices.
High photoluminescence of shortwave infrared-emitting anisotropic surface charged gold nanoclusters
(2019)
Incorporating anisotropic surface charges on atomically precise gold nanoclusters (Au NCs) led to a strong absorption in the nearinfrared region and could enable the formation of self-assembled Au NCs xhibiting an intense absorption band at ∼1000 nm. This surface modification showed a striking enhancement of the photoluminescence in the Shortwave Infrared (SWIR) region with a quantum yield as high as 6.1% in water.
Formation Mechanism of a Nano-Ring of Bismuth Cations and Mono-Lacunary Keggin-Type Phosphomolybdate
(2022)
A new hetero-bimetallic polyoxometalate (POM) nano-ring was synthesized in a one-pot procedure. The structure consists of tetrameric units containing four bismuth-substituted monolacunary Keggin anions including distorted [BiO8] cubes. The nano-ring is formed via self-assembly from metal precursors in aqueous acidic medium. The compound (NH4)16[(BiPMo11O39)4] ⋅ 22 H2O; (P4Bi4Mo44) was characterized by single-crystal X-ray diffraction, extended X-ray absorption fine structure spectroscopy (EXAFS), Raman spectroscopy, matrix-assisted laser desorption/ionisation-time of flight mass spectrometry (MALDI-TOF), and thermogravimetry/differential scanning calorimetry mass spectrometry (TG-DSC-MS). The formation of the nano-ring in solution was studied by time-resolved in situ small- and wide-angle X-ray scattering (SAXS/WAXS) and in situ EXAFS measurements at the Mo−K and the Bi−L3 edge indicating a two-step process consisting of condensation of Mo-anions and formation of Bi−Mo-units followed by a rapid self-assembly to yield the final tetrameric ring structure.
The radical polymerization of three monomers bearing nucleobases 1-(4-vinylbenzyl)thymine (VBT), 1-(4-vinylbenzyl)uracil (VBU) and 9-(4-vinylbenzyl)adenine (VBA) was investigated. The corresponding homopolymers could be prepared in high yields via conventional radical polymerization. However, the resulting polymers were found to be only soluble in a few polar solvents. On the other hand, copolymers of dodecyl methacrylate (DMA) with either VBT or VBA could be prepared via both free radical polymerization and atom transfer radical polymerization and could be dissolved in a large variety of organic solvents. Moreover, the formed complementary copolymers P(VBT-co-DMA) and P(VBA-co-DMA) were found to self-assemble in dilute solutions in dioxane or chloroform via base recognition, as evidenced by a significant hypochromicity effect in UV spectroscopy. Nevertheless, at higher concentrations in chloroform, both dynamic light scattering and optical microscopy indicate that P(VBT-co-DMA), P(VBA-co-DMA), or P(VBT-co-DMA)/P(VBA-co-DMA) mixtures spontaneously self-assemble into micron size spherical aggregates. 1H NMR and FTIR studies confirmed that the self-assembly process is driven in all cases via H-bond formation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4805-4818, 2005