Heating red phosphorus in sealed ampoules in the presence of a Sn/SnI4 catalyst mixture has provided bulk black phosphorus at much lower pressures than those required for allotropic conversion by anvil cells. Herein we report the growth of ultra-long 1D red phosphorus nanowires (>1 mm) selectively onto a wafer substrate from red phosphorus powder and a thin film of red phosphorus in the present of a Sn/SnI4 catalyst. Raman spectra and X-ray diffraction characterization suggested the formation of crystalline red phosphorus nanowires. FET devices constructed with the red phosphorus nanowires displayed a typical I-V curve similar to that of black phosphorus and a similar mobility reaching 300 cm(2) V(-1) s with an Ion /Ioff ratio approaching 10(2) . A significant response to infrared light was observed from the FET device.
Hydrogels formed by polypeptides could be much-favored tools for drug delivery because their main ingredients are generally biodegradable. However, the gelation of peptides in aqueous solution generally requires a minimal length of the peptide as well as distinct sequences of hydrophilic and hydrophobic residues. The aggregation of short peptides like tripeptides, which are relatively cheap and offer a high degree of biodegradability, are generally thought to require a high hydrophobicity of their residues. We found that contrary to this expectation cationic glycylalanylglycine in 55 mol% ethanol/45 mol% water forms a gel below a melting temperature of ca. 36 °C. A pure hydrogel state can be obtained after allowing the ethanol component to evaporate. The gel phase consists of crystalline fibrils of several 100 μm, which form a sample-spanning network. Rheological data reveal a soft elastic solid gel. We investigated the kinetics of the various processes that lead to the final gel state of the ternary mixture by a unique combination of UV circular dichroism, infrared, vibrational circular dichroism (VCD) and rheological measurements. A mathematical analysis of our data show that gelation is preceded by the formation of peptide β-sheet like tapes or ribbons, which give rise to a significant enhancement of the amide I' VCD signal, and the subsequent formation of rather thick and long fibrils. The VCD signals indicate that the tapes exhibit a right-handed helicity at temperatures above 16 °C and a left-handed helicity below. The tapes'/ribbons' helicity change occurs at a temperature where the UVCD data reflect a relatively long nucleation process. The kinetics of gel formation probed by the storage and loss moduli are composed of a fast process that follows tape/ribbon/fibril formation and is clearly identifiable in a movie that shows the gelation process and a slow process that causes an additional gel stabilization. The rheological data indicate that left-handed fibrils observed at low temperatures form a more solid-like structure than their right-handed counterparts formed at higher temperatures. Taken together our data reveal GAG as an unexpected gelator, the formation of which is underlied by a set of distinguishable kinetic processes.
The conformational propensity of amino acid residues is determined by an intricate balance of peptide-solvent and solvent-solvent interactions. To explore how the systematic replacement of water by a cosolvent affects the solvation of both the amino acid backbone and side chains, we performed a combined vibrational spectroscopy and NMR study of cationic glycylalanylglycine (GAG) in different ethanol/water mixtures of between 0 and 42 mol percent ethanol. Classical model peptide N'-methylacetamide was used as a reference system to probe solvent-induced spectroscopic changes. The alanine residue of GAG in water is known to exhibit a very high propensity for polyproline II (pPII). Adding up to 30 mol % ethanol at room temperature leads only to minor changes in the Ramachandran distribution of alanine, which mostly changes within the individual conformational subspaces. A further increase in the ethanol fractions leads to a destabilization of pPII and a stabilization of β-strand conformations. At higher temperatures, different degrees of enthalpy-entropy compensations lead to a much stronger influence of ethanol on the peptide's conformational distribution. Ethanol-induced changes in chemical shifts and amide I wavenumbers strongly suggest that ethanol replaces water preferentially in the solvation shell of the polar C-terminal peptide group and of the alanine side chain, whereas the N-terminal group remains mostly hydrated. Furthermore, we found that ethanol interacts more strongly with the peptide if the latter adopts β-strand conformations. This leads to an unusual positive temperature coefficient for the chemical shift of the C-terminal amide proton. Our data suggests a picture in which GAG eventually accumulates at water-ethanol interfaces if the ethanol fractions exceed 0.3, which explains why the further addition of ethanol eventually causes self-aggregation and the subsequent formation of a hydrogel.
Short peptides have emerged as versatile building blocks for supramolecular structures and hydrogels. In particular, the presence of aromatic amino acid residues and/or aromatic end groups is generally considered to be a prerequisite for initiating aggregation of short peptides into nanotubes or cross β-sheet type fibrils. However, the cationic GAG tripeptide surprisingly violates these rules. Specifically, in water/ethanol mixtures, GAG peptides aggregate into very long crystalline fibrils at temperatures below 35 °C where they eventually form a spanning network structure and, thus, a hydrogel. Two gel phases are formed in this network, and they differ substantially in chirality and thickness of the underlying fibrils, their rheological parameters, and the kinetics of oligomerization, fibrilization, and gel formation. The spectroscopic data strongly suggests that the observed fibrils do not exhibit canonical cross β-sheet structures and are indicative of a yet unknown secondary conformation. To complement our unusual experimental observations in this perspective article, we performed large-scale DFT calculations to probe the geometry and spectroscopic properties of these GAG oligomers. Most importantly, our experimental and computational results yield rather unconventional structures that are not reminiscent of classical cross-β-sheet structures, and we give two extremely likely candidates for oligomer structures that are consistent with experimental amide I' profiles in IR and vibrational circular dichroism (VCD) spectra of the two gel phases.
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