The hydrated electron represents a "super-reductant" in water, providing 2.9 eV of reductive power, which suffices to decompose nonactivated aliphatic halides. We show that 3-amino-perylene in SDS micelles, when combined with the bioavailable ascorbate as an extramicellar sacrificial donor, sustainably produces hydrated electrons through photoredox catalysis with green light, from a metal-free system, and at near-physiological pH. Photoionization of the amine with a 532 nm laser yields an extremely long-lived radical cation as the by-product, and a subsequent reaction of the latter with the sacrificial donor across the micelle/water interface regenerates the catalyst. The regeneration step involves parallel reactions between differently protonated forms, causing a bell-shaped pH dependence in basic medium. We have separated these processes kinetically. Employing this catalytic cycle for the laboratory-scale decomposition of chloroacetate, an accepted model compound for toxic and persistent halo-organic waste, gave turnover numbers of about 170. Even though both the substrate and the sacrificial donor compete for the hydrated electron, their consumption ratio is practically independent of the initial concentration ratio because the formal radical anion of the ascorbate undergoes secondary scavenging by the chloroacetate. In the course of the reaction, the initial hydrophobic catalyst is converted into a secondary species that is hydrophilic and still exhibits catalytic activity.
Using an improved methodology, we have carefully reinvestigated the title reaction by laser flash photolysis and disproved an earlier study (J. K. Thomas and P. Piciulo, J. Am. Chem. Soc., 1978, 100, 3239), which claimed this green-light ionization to be monophotonic, the only instance of such a scenario ever reported for a stable compound. We show it to be biphotonic instead, in accordance with thermodynamic considerations, and present a photokinetic model that accurately represents the intensity dependences throughout the whole excitation range in the green (532 nm) and the near UV (355 nm), up to near-quantitative electron release in the latter case. A major artifact deceptively similar to a chemical decay arises from an SDS-related laser-induced turbidity but can be eliminated by difference experiments or careful selection of excitation intensities and temporal windows. The ionization step is not accompanied by side processes, and affords an extremely long-lived (0.35 s) radical cation remaining solubilized. The micelles completely block attacks of hydrated electrons or hydroxyl radicals on the starting material and its radical cation but allow a post-ionization regeneration by high concentrations of the hydrophilic ascorbate monoanion.
Upon irradiation with ns laser pulsesa t3 55 nm, 2aminoanthracene in SDS micelles readily produces hydrated electrons. These "super-reductants" rapidly attack substrates such as chloro-organics and convert them into carbon-centred radicals through dissociative electron transfer.F or ac atalytic cycle, the aminoanthracene needs to be restored from its photoionizationb y-product,t he radical cation, by as acrificial donor.T he ascorbate monoanion can only achieve this across the micelle-water interface, but the monoanion of ascorbyl palmitate resultsi nafully micelle-contained regenerative electron source.T he shielding by the micelle in the latter case not only increases the life of the catalystb ut also strongly suppresses the interception of the carbon-centred radicals by the hydrogen-donating ascorbate moiety;a nd in conjunction with the high local concentrations effected by the pulsed laser,t ermination by radicald imerizationt hus dominates. We have obtained ac omplete and consistent picture through monitoring the individual steps and the assembled systemb yf lash photolysis on fast ands low timescales, from microseconds to minutes;a nd in preparative studies on av ariety of substrates, we have achieved up to quantitative dimerizationw ith at urnover on the order of 1mmol per hour.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.
It ain’t necessarily so—existing theories of combined quenching in micelles are flawed. We derive a consistent model, analyze its properties, and apply it to obtain information on ground-state complexes between fluorophore F and quencher Q.
We here report a novel strategy to control the bioavailability of the fibrillizing parathyroid hormone (PTH)-derived peptides, where the concentration of the bioactive form is controlled by an reversible, photoswitchable peptide. PTH1–84, a human hormone secreted by the parathyroid glands, is important for the maintenance of extracellular fluid calcium and phosphorus homeostasis. Controlling fibrillization of PTH1–84 represents an important approach for in vivo applications, in view of the pharmaceutical applications for this protein. We embed the azobenzene derivate 3-{[(4-aminomethyl)phenyl]diazenyl}benzoic acid (3,4′-AMPB) into the PTH-derived peptide PTH25–37 to generate the artificial peptide AzoPTH25–37 via solid-phase synthesis. AzoPTH25–37 shows excellent photostability (more than 20 h in the dark) and can be reversibly photoswitched between its cis/trans forms. As investigated by ThT-monitored fibrillization assays, the trans-form of AzoPTH25–37 fibrillizes similar to PTH25–37, while the cis-form of AzoPTH25–37 generates only amorphous aggregates. Additionally, cis-AzoPTH25–37 catalytically inhibits the fibrillization of PTH25–37 in ratios of up to one-fifth. The approach reported here is designed to control the concentration of PTH-peptides, where the bioactive form can be catalytically controlled by an added photoswitchable peptide.
During CASP10 in summer 2012, we tested BCL::Fold for prediction of free modeling (FM) and template-based modeling (TBM) targets. BCL::Fold assembles the tertiary structure of a protein from predicted secondary structure elements (SSEs) omitting more flexible loop regions early on. This approach enables the sampling of conformational space for larger proteins with more complex topologies. In preparation of CASP11, we analyzed the quality of CASP10 models throughout the prediction pipeline to understand BCL::Fold’s ability to sample the native topology, identify native-like models by scoring and/or clustering approaches, and our ability to add loop regions and side chains to initial SSE-only models. The standout observation is that BCL::Fold sampled topologies with a GDT_TS score > 33% for 12 of 18 and with a topology score > 0.8 for 11 of 18 test cases de novo. Despite the sampling success of BCL::Fold, significant challenges still exist in clustering and loop generation stages of the pipeline. The clustering approach employed for model selection often failed to identify the most native-like assembly of SSEs for further refinement and submission. It was also observed that for some β-strand proteins model refinement failed as β-strands were not properly aligned to form hydrogen bonds removing otherwise accurate models from the pool. Further, BCL::Fold samples frequently non-natural topologies that require loop regions to pass through the center of the protein.
We demonstrate that the 3-aminoperylene radical cation is a near-ideal probe for investigating kinetic and transport processes in SDS micellar systems. Its isolated generation by two-photon ionization at a wavelength where most quenchers are transparent (532 nm) is free from side reactions; no exit from the micelles is detectable on a millisecond timescale; and its unquenched lifetime is as long as 350 ms, thus allowing the study of quenching processes over a time frame spanning at least 7 orders of magnitude. The lipophilic antioxidant ascorbyl palmitate reconverts it to its parent compound through the interplay of static and fast dynamic intramicellar quenching as well as through subsequent slow intermicellar migration. Using this radical-cation probe, we have successfully validated closed-form expressions which we derived for the probe decay in all these situations. From these functions, we also obtained an exact and closed-form analytical result for Stern-Volmer experiments with combined static and dynamic intramicellar quenching.
We have studied the combined static and dynamic quenching of pyrene by methyl viologen in sodium alkyl sulfate micelles varying in volume by a factor of more than 4. Size...
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