Simplification in the Acquisition and Analysis of Fluorescence Decays Acquired with Polarized Emission for Time-Resolved Fluorescence Anisotropy Measurements.

This study introduces a global fluorescence decay analysis that substantially simplifies the acquisition and analysis of time-resolved fluorescence decays acquired with a vertically polarized excitation and vertically (IVV(t)) and horizontally (IVH(t)) polarized emission for time-resolved fluorescence anisotropy (TRFA) measurements. TRFA measurements were conducted whereby the IVV(t) and IVH(t) fluorescence decays of a series of oligoquinolines labeled at one end with an oligo(phenylenevinylene) dye (OPV-Qn with n = 4, 7, 17, 24, 33) were acquired according to the standard protocol that is currently accepted in the scientific literature which involves toggling the emission polarizer before fitting linear combinations of the IVV(t) and IVH(t) decays or acquiring the IVV(t) and IVH(t) decays with static polarizers before fitting them globally. The rotational time (ϕ) and initial anisotropy (r0) retrieved from these analyses were identical within experimental error regardless of whether the decays were acquired with toggling or static polarizers and fitted according to the standard protocol or globally. These experimental results were further supported by retrieving the parameters used to generate mono-, bi-, and tri-exponential TRFAs from the global analysis of simulated IVV(t) and IVH(t) fluorescence decays which were found to match perfectly the values that were inputted. Together, these experiments and simulations demonstrated that the parameters describing any type of TRFA can be extracted directly from the analysis of the IVV(t) and IVH(t) fluorescence decays acquired with a standard time-resolved fluorometer, a substantial simplification compared to the protocols currently in place to determine the TRFA.

Multi-dimensional charge transport in supramolecular helical foldamer assemblies.

Aromatic foldamers are bioinspired architectures whose potential use in materials remains largely unexplored. Here we report our investigation of vertical and horizontal charge transport over long distances in helical oligo-quinolinecarboxamide foldamers organized as single monolayers on Au or SiO2. Conductive atomic force microscopy showed that vertical conductivity is efficient and that it displays a low attenuation with foldamer length (0.06 Å-1). In contrast, horizontal charge transport is found to be negligible, demonstrating the strong anisotropy of foldamer monolayers. Kinetic Monte Carlo calculations were used to probe the mechanism of charge transport in these helical molecules and revealed the presence of intramolecular through-space charge transfer integrals approaching those found in pentacene and rubrene crystals, in line with experimental results. Kinetic Monte Carlo simulations of charge hopping along the foldamer chain evidence the strong contribution of multiple 1D and 3D pathways in these architectures and their dependence on conformational order. These findings show that helical foldamer architectures may provide a route for achieving charge transport over long distance by combining multiple charge transport pathways.

Selenocysteine as a Latent Bioorthogonal Electrophilic Probe for Deubiquitylating Enzymes.

Deubiquitylating enzymes (DUBs) remove ubiquitin (Ub) from various cellular proteins and render eukaryotic ubiquitylation a dynamic process. The misregulation of protein ubiquitylation is associated with many human diseases, and there is an urgent need to identify specific DUBs associated with therapeutically relevant targets of Ub. We report the development of two facile selenocysteine-based strategies to generate the DUB probe dehydroalanine (Dha). Optimized oxidative or alkylative elimination of Se yielded Dha at the C-terminus of Ub. The high utility of alkylative elimination, which is compatible with multiple thiols in Ub targets, was demonstrated by generating a probe derived from the Ub ligase tripartite motif protein 25 (TRIM-25). Successful capture of the TRIM-25-associated DUB, ubiquitin-specific protease 15, demonstrated the versatility of our chemical strategy for identifying target-specific DUBs.

Photoinduced Electron Transfer and Hole Migration in Nanosized Helical Aromatic Oligoamide Foldamers.

A series of photoactive triads have been synthesized and investigated in order to elucidate photoinduced electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of two chromophores, an N-terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylene bis-imide electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following photoexcitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the acceptor chromophore on the picosecond time scale. The oligo(para-phenylenevinylene) electron donor is oxidized after a time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state that is finally generated was found to be remarkably long-lived (>80 µs). While the initial charge injection rate is largely invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1 × 109 s-1 for the longest sequence to 17 × 109 s-1 for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole transport through the foldamer helix, with individual hops occurring on the subpicosecond time scale (kET = 2.5 × 1012 s-1 in CH2Cl2). This work demonstrates the possibility of fast long-range hole transfer over 300 Å (through bonds) across a synthetic modular bridge, an achievement that had been previously observed principally with DNA structures.

A one-pot azido reductive tandem mono-N-alkylation employing dialkylboron triflates: online ESI-MS mechanistic investigation.

An efficient one-pot reductive tandem mono-N-alkylation of both aromatic and aliphatic azides using dialkylboron triflates as alkylating agents has been examined under standardized reaction conditions. This methodology after optimization has been employed toward the syntheses of various secondary alkyl as well as aryl amines, including the synthesis of N10-butylated pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones via in situ azido reductive-cyclization process. This protocol is particularly attractive to provide an environmentally benign and practical method for mono-N-alkylation from organic azides without the use of toxic catalysts or corrosive alkylating agents. In addition, the mechanistic aspects have been investigated and the intermediates associated with this selective transformation have been intercepted and characterized by online monitoring of the reaction by ESI-MS/MS.

Chemoselective aromatic azido reduction with concomitant aliphatic azide employing Al/Gd triflates/NaI and ESI-MS mechanistic studies.

Aluminium and gadolinium triflates catalyze the chemoselective reduction of aromatic azides to the corresponding amines in combination with sodium iodide. This mild chemoselective method has been applied to the synthesis of various aryl amines, C2-azido-substituted pyrrolo[2,1-c][1,4]benzodiazepines, and fused[2,1-b]quinazolinones by an intramolecular azido reduction tandem cyclization reaction. Interestingly, this methodology selectively reduces aryl azides with enhanced yields and proceeds in shorter reaction times than previous strategies. The mechanistic aspects have been investigated and the intermediates associated with this selective transformation have been intercepted and characterized by online monitoring of the reaction by ESI-MS.