Effect of Controlled Deposition of ZnS Shell on the Photostability of CdTe Quantum Dots as Studied by Conventional Fluorescence and FCS Techniques

The effect of one and two monolayers of ZnS shells on the photostability of CdTe quantum dots (QDs) in aqueous and nonaqueous media has been studied by monitoring the fluorescence behavior of the QDs under ensemble and single‐molecule conditions. ZnS capping of the CdTe QDs leads to significant enhancement of the fluorescence brightness of these QDs. Considerable enhancement of the photostability of the shell‐protected QDs, including the suppression of photoactivation, is also observed. Fluorescence correlation spectroscopy measurements reveal an increase in the number of particles undergoing reversible fluorescent on–off transitions in the volume under observation with increasing excitation power; this effect is found to be more pronounced in the case of core‐only QDs than for core–shell QDs.     

Synthesis of 8‐Aminoquinolines by Using Carbamate Reagents: Facile Installation and Deprotection of Practical Amidating Groups

Described herein is the development of practical routes to 8‐aminoquinolines by using readily installable and easily deprotectable amidating reagents. Two scalable procedures were optimized under Rh^III‐catalyzed conditions: i) the use of pre‐generated chlorocarbamates and ii) a two‐step one‐pot process that directly employs carbamates. Both approaches are highly convenient for the gram‐scale synthesis of 8‐aminoquinolines under mild conditions. Facile deprotection of the synthetically versatile amidating groups was achieved under the Pd‐catalyzed transfer hydrogenation conditions with simultaneous deoxygenation of quinoline N‐oxides, thus yielding 8‐aminoquinolines in excellent overall efficiency.     

Contrasting Response of Two Dipolar Fluorescence Probes in a Leucine‐Based Organogel and Its Implications

The microenvironments of a leucine‐based organogel are probed by monitoring the fluorescence behavior of coumarin 153 (C153) and 4‐aminophthalimide (AP). The steady‐state data reveals distinctly different locations of the two molecules in the gel. Whereas AP resides close to the hydroxyl moieties of the gelator and engages in hydrogen‐bonding interactions, C153 is found in bulk‐toluene‐like regions. In contrast to C153, AP exhibits excitation‐wavelength‐dependent emission, indicating that the environments of the hydrogen‐bonded AP molecules are not all identical. A two‐component fluorescence decay of AP in gel, unlike C153, supports this model. A time‐resolved fluorescence anisotropy study of the rotational motion of the molecules also reveals the strong association of only AP with the gelator. That AP influences the critical gelation concentration implies its direct involvement in the gel‐formation process. The results highlight the importance of guest–gelator interactions in gels containing guest molecules.     

New beginnings for matrix metalloproteinase inhibitors: identification of high-affinity zinc-binding groups

In an effort to identify promising non-hydroxamate inhibitors of matrix metalloproteinases (MMPs), several new zinc-binding groups (ZBGs) based on pyrone, pyrothione, hydroxypyridinone, and hydroxypyridinethione chelators have been examined. Structural studies with tris(pyrazolyl)borate model complexes show that these ligands bind to the MMP active site zinc(II) ion in a bidentate fashion, similar to that found with hydroxamate-based inhibitors. Fluorescence- and colorimetric-based enzyme assays have been used to determine the IC50 values for these ZBGs against MMP-3; mixed O,S-donor ligands were found to be remarkably potent, with IC50 values as much as 700-fold lower than that found for acetohydroxamic acid. Inhibitory activity was found to parallel metal binding affinity as determined in titrations with model complexes. These results demonstrate that MPIs based on new ZBGs are feasible and may indeed improve the overall performance of inhibitors designed against these important medicinal targets.     

Synthesis, ionisation potentials and electron affinities of hexaazatrinaphthylene derivatives

Several hexaazatrinaphthylene derivatives and a tris(thieno)hexaazatriphenylene derivative have been synthesised by reaction of the appropriate diamines with hexaketocyclohexane. The crystal structure of 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene has been determined by X-ray diffraction; this reveals a molecular structure in good agreement with that predicted by density functional theory (DFT) calculations and pi-stacking with an average spacing between adjacent molecular planes of 3.18 A. Solid-state ionisation potentials have been measured by using UV photoelectron spectroscopy and fall in the range of 5.99 to 7.76 eV, whereas solid-state electron affinities, measured using inverse photoelectron spectroscopy, vary in the range -2.65 to -4.59 eV. The most easily reduced example is a tris(thieno)hexaazatriphenylene substituted with bis(trifluoromethyl)phenyl groups; DFT calculations suggest that the highly exothermic electron affinity is due both to the replacement of the outermost phenylene rings of hexaazatrinaphthylene with thieno groups and to the presence of electron-withdrawing bis(trifluoromethyl)phenyl groups. The rather exothermic electron affinities, the potential for adopting pi-stacked structures and the low intramolecular reorganisation energies obtained by DFT calculations suggest that some of these molecules may be useful electron-transport materials.     

Structure of 4-[4-(Dimethylamino)-Phenylazo] Benzeneboronic Acid and Its Cyclic Esters with D-Glucose: A Computational Study

We present results from a computational study of 4-[4-(dimethylamino)-phenylazo] benzene boronic acid (DABBA) (the 4'-boronic acid isomer of the aminoazobenzene dye N,N-dimethylaminoazobenzene) and its associated anion, as well as, several cyclic esters formed from these azoborates and various conformers of D-glucose. Azo dyes that also contain one or more boronic acid functional groups are of practical importance in the development of chemical sensors for saccharide recognition because of their ability to induce a visible color change upon binding. The lowest-energy DABBA:D-glucose esters found in this investigation consistently involved at least one of the exocyclic hydroxymethyl groups on the D-glucose moiety rather than vicinal cis or trans diol arrangements of hydroxyl groups on the ring.     

Conformationally Homogeneous Heterocyclic Pseudotetrapeptides as Three‐Dimensional Scaffolds for Rational Drug Design: Receptor‐Selective Somatostatin Analogues

A would‐be amide : A 1,4‐disubstituted 1,2,3‐triazole was used as a surrogate for a trans amide bond to create a library of 16 diastereomeric pseudotetrapeptides as β‐turn mimetics. High‐resolution structural analysis indicated that these scaffolds adopt distinct, rigid, conformationally homogeneous β‐turn‐like structures (see example), some of which bind somatostatin receptor subtypes selectively, and some of which show broad‐spectrum activity.

     

Activity-based chemical proteomics accelerates inhibitor development for deubiquitylating enzymes

Converting lead compounds into drug candidates is a crucial step in drug development, requiring early assessment of potency, selectivity, and off-target effects. We have utilized activity-based chemical proteomics to determine the potency and selectivity of deubiquitylating enzyme (DUB) inhibitors in cell culture models. Importantly, we characterized the small molecule PR-619 as a broad-range DUB inhibitor, and P22077 as a USP7 inhibitor with potential for further development as a chemotherapeutic agent in cancer therapy. A striking accumulation of polyubiquitylated proteins was observed after both selective and general inhibition of cellular DUB activity without direct impairment of proteasomal proteolysis. The repertoire of ubiquitylated substrates was analyzed by tandem mass spectrometry, identifying distinct subsets for general or specific inhibition of DUBs. This enabled identification of previously unknown functional links between USP7 and enzymes involved in DNA repair.     

Total synthesis and biological evaluation of pederin, psymberin, and highly potent analogs

The potent cytotoxins pederin and psymberin have been prepared through concise synthetic routes (10 and 14 steps in the longest linear sequences, respectively) that proceed via a late-stage multicomponent approach to construct the N-acyl aminal linkages. This route allowed for the facile preparation of a number of analogs that were designed to explore the importance of the alkoxy group in the N-acyl aminal and functional groups in the two major subunits on biological activity. These analogs, including a pederin/psymberin chimera, were analyzed for their growth inhibitory effects, revealing several new potent cytotoxins and leading to postulates regarding the molecular conformational and hydrogen bonding patterns that are required for biological activity. Second generation analogs have been prepared based on the results of the initial assays and a structure-based model for the binding of these compounds to the ribosome. The growth inhibitory properties of these compounds are reported. These studies show the profound role that organic chemistry in general and specifically late-stage multicomponent reactions can play in the development of unique and potent effectors for biological responses.     

Oxime Side‐Chain Cross‐Links in an α‐Helical Coiled‐Coil Protein: Structure,Thermodynamics, and Folding‐Templated Synthesis of Bicyclic Species

Covalent side‐chain cross‐links are a versatile method to control peptide folding, particularly when α‐helical secondary structure is the target. Here, we examine the application of oxime bridges, formed by the chemoselective reaction between aminooxy and aldehyde side chains, for the stabilization of a helical peptide involved in a protein–protein complex. A series of sequence variants of the dimeric coiled coil GCN4‐p1 bearing oxime bridges at solvent‐exposed positions were prepared and biophysically characterized. Triggered unmasking of a side‐chain aldehyde in situ and subsequent cyclization proceed rapidly and cleanly at pH 7 in the folded protein complex. Comparison of folding thermodynamics among a series of different oxime bridges show that the cross links are consistently stabilizing to the coiled coil, with the extent of stabilization sensitive to the exact size and structure of the macrocycle. X‐ray crystallographic analysis of a coiled coil with the best cross link in place and a second structure of its linear precursor show how the bridge is accommodated into an α‐helix. Preparation of a bicyclic oligomer by simultaneous formation of two linkages in situ demonstrates the potential use of triggered oxime formation to both trap and stabilize a particular peptide folded conformation in the bound state.