FRET induced by an ‘allosteric’ cycloaddition reaction regulated with exogenous inhibitor and effectors

Self-Propagating Amplification Reactions for Molecular Detection and Signal Amplification: Advantages, Pitfalls, and Challenges

Self-propagating cascade reactions are a recent development for chemo-sensing protocols. These cascade reactions, in principle, offer low limits of detection by virtue of exponential signal amplification, and are initiated by a specific, pre-planned molecular detection event. This combination of selectivity for a detection event followed by in situ signal amplification is achieved by exploitation of mechanistic organic chemistry, and thus has resulted in various chemo-sensing protocols that employ one or more reagents to achieve the desired selectivity and sensitivity for an assay. Species such as hydrogen peroxide, thiols, and fluoride, have been used as active reagents to initiate the first examples of self-propagating signal amplification reactions, although many other active reagents should be compatible with the approaches. A common feature of the reagents that support the self-propagating signal amplification reactions is the involvement of quinonemethide intermediates resulting from elimination of optical reporters and/or active reagents, where the latter propagates the signal amplification reaction. The early examples of these amplification sequences, however, are slow to reach full signal, thus leaving time for background reactions to generate non-specific signals. This issue of background has limited practical applications of these self-propagating signal amplification reactions, as has challenging synthetic routes to the reagents, as well as the potential for other chemical species to interfere with the detection and signal amplification processes. Thus, the goal of this review is to summarize the progress of self-propagating signal amplification technology, identify the pitfalls of current designs, and by doing so, to stimulate future studies in this growing and promising research area.

New Autoinductive Cascade for the Optical Sensing of Fluoride: Application in the Detection of Phosphoryl Fluoride Nerve Agents

A new autoinductive cascade employing benzoyl fluoride as a latent source of fluoride is reported for signal amplification and optical detection of fluoride. The autoinduction leads to a maximum 4-fold signal enhancement for each fluoride generated, as well as a self-propagating cycle that generates three fluorophores for each single fluoride released. A two-step integrated protocol creates a more rapid autoinductive cascade than previously reported, as well as a highly sensitive diagnostic assay for the ultratrace quantitation of a phosphoryl fluoride nerve agent surrogate.

Wheel-shaped copper containing polyoxotungstate as an efficient catalyst in the three-component synthesis of 1,2,3-triazoles

The catalytic effect of wheel-shaped polyoxotungstate [Cu₂₀Cl(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]²⁵⁻ was investigated in the preparation of triazoles from the corresponding alkyl halides, sodium azide, and alkynes.

Design of catalytically amplified sensors for small molecules

Catalytically amplified sensors link an allosteric analyte binding site with a reactive site to catalytically convert substrate into colored or fluorescent product that can be easily measured. Such an arrangement greatly improves a sensor’s detection limit as illustrated by successful application of ELISA-based approaches. The ability to engineer synthetic catalytic sites into non-enzymatic proteins expands the repertoire of analytes as well as readout reactions. Here we review recent examples of small molecule sensors based on allosterically controlled enzymes and organometallic catalysts. The focus of this paper is on biocompatible, switchable enzymes regulated by small molecules to track analytes both in vivo and in the environment.

Zn(II)-coordination modulated ligand photophysical processes - the development of fluorescent indicators for imaging biological Zn(II) ions

Molecular photophysics and metal coordination chemistry are the two fundamental pillars that support the development of fluorescent cation indicators. In this article, we describe how Zn(II)-coordination alters various ligand-centered photophysical processes that are pertinent to developing Zn(II) indicators. The main aim is to show how small organic Zn(II) indicators work under the constraints of specific requirements, including Zn(II) detection range, photophysical requirements such as excitation energy and emission color, temporal and spatial resolutions in a heterogeneous intracellular environment, and fluorescence response selectivity between similar cations such as Zn(II) and Cd(II). In the last section, the biological questions that fluorescent Zn(II) indicators help to answer are described, which have been motivating and challenging this field of research.

A benzothiazole alkyne fluorescent sensor for Cu detection in living cell

A new type of alkyne dye, 6-dimethylaminobenzothiazole alkyne (1), was developed for Cu sensing in biological system. Dye (1) offered excellent selective over a panel of ions, only Cu(I) could change the fluorescence of dye (I) by forming copper acetylide between the terminal alkyne and Cu(I). Its potential of detecting Cu in biological system was demonstrated in cell culture.

Experimental investigation on the mechanism of chelation-assisted, copper(II) acetate-accelerated azide-alkyne cycloaddition

A mechanistic model is formulated to account for the high reactivity of chelating azides (organic azides capable of chelation-assisted metal coordination at the alkylated azido nitrogen position) and copper(II) acetate (Cu(OAc)(2)) in copper(II)-mediated azide-alkyne cycloaddition (AAC) reactions. Fluorescence and (1)H NMR assays are developed for monitoring the reaction progress in two different solvents, methanol and acetonitrile. Solvent kinetic isotopic effect and premixing experiments give credence to the proposed different induction reactions for converting copper(II) to catalytic copper(I) species in methanol (methanol oxidation) and acetonitrile (alkyne oxidative homocoupling), respectively. The kinetic orders of individual components in a chelation-assisted, copper(II)-accelerated AAC reaction are determined in both methanol and acetonitrile. Key conclusions resulting from the kinetic studies include (1) the interaction between copper ion (either in +1 or +2 oxidation state) and a chelating azide occurs in a fast, pre-equilibrium step prior to the formation of the in-cycle copper(I)-acetylide, (2) alkyne deprotonation is involved in several kinetically significant steps, and (3) consistent with prior experimental and computational results by other groups, two copper centers are involved in the catalysis. The X-ray crystal structures of chelating azides with Cu(OAc)(2) suggest a mechanistic synergy between alkyne oxidative homocoupling and copper(II)-accelerated AAC reactions, in which both a bimetallic catalytic pathway and a base are involved. The different roles of the two copper centers (a Lewis acid to enhance the electrophilicity of the azido group and a two-electron reducing agent in oxidative metallacycle formation, respectively) in the proposed catalytic cycle suggest that a mixed valency (+2 and +1) dinuclear copper species be a highly efficient catalyst. This proposition is supported by the higher activity of the partially reduced Cu(OAc)(2) in mediating a 2-picolylazide-involved AAC reaction than the fully reduced Cu(OAc)(2). Finally, the discontinuous kinetic behavior that has been observed by us and others in copper(I/II)-mediated AAC reactions is explained by the likely catalyst disintegration during the course of a relatively slow reaction. Complementing the prior mechanistic conclusions drawn by other investigators, which primarily focus on the copper(I)/alkyne interactions, we emphasize the kinetic significance of copper(I/II)/azide interaction. This work not only provides a mechanism accounting for the fast Cu(OAc)(2)-mediated AAC reactions involving chelating azides, which has apparent practical implications, but suggests the significance of mixed-valency dinuclear copper species in catalytic reactions where two copper centers carry different functions.

cat-ELCCA: a robust method to monitor the fatty acid acyltransferase activity of ghrelin O-acyltransferase (GOAT)

Assays armed with catalytic signal amplification have arisen as superior systems for ultrasensitive detection of analytes. Here we describe a conceptually new enzyme assay based on cat-ELISA, catalytic assay using enzyme-linked click chemistry assay (cat-ELCCA), where an enzyme-linked azide is utilized to arm the assay with catalytic fluorescence signal amplification. Using this assay technology, we have developed the first potentially high-throughput screen for the recently disclosed acyltransferase, ghrelin O -acyltransferase (GOAT).

Organic azides: an exploding diversity of a unique class of compounds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aubé rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Signal Amplification by Allosteric Catalysis

In this article we unify a series of recent studies on bio- and chemosensors under a single signaling strategy: signal amplification by allosteric catalysis (SAAC). The SAAC strategy mimics biological signal transduction processes, where molecular recognition between an external signal and a protein receptor is allosterically transduced into catalytically amplified chemical information (usually second messengers). Several recent biosensing and chemosensing studies apply this nature-inspired strategy by using engineered allosteric enzymes, ribozymes, or regulatable organic catalysts. The factors pertinent to achieving high sensitivity and specificity in SAAC strategies are analyzed. The authors believe that these early studies from a variety of research groups have opened up a new venue for the development of sensing technologies where molecular recognition and catalysis can be coupled for practical purposes.

Signal amplification and transduction by photo-activated catalysis

A simple flavin-based catalytic system is able to transform light into chemical output with amplified response utilizing a Cu(I)-catalyzed cycloaddition reaction.

Multivalent, bifunctional dendrimers prepared by click chemistry

Unsymmetrical dendrimers, containing both mannose binding units and coumarin fluorescent units, have been prepared using click chemistry and shown to be highly efficient, dual-purpose recognition/detection agents for the inhibition of hemagglutination.

Catalytic Signal Amplification Using a Heck Reaction. An Example in the Fluorescence Sensing of Cu(II)

Catalytic signal enhancement using an organometallic reaction is demonstrated. The reactivity of a Heck cross-coupling reaction that creates a fluorophore is modulated by the addition of a polyazacyclam inhibitor. The inhibitor will complex with Cu(II), which restores the activity of the Pd(II). The addition of Cu(II) therefore leads to the generation of fluorescence, thereby creating a very sensitive assay for Cu(II). The rate of the Heck reaction is followed by monitoring emission as a function of time. The rate is proportional to the Cu(II) concentration and correlates to the affinity of the inhibitor to various metals. This strategy represents a general technique that can be exploited with other catalytic organometallic reactions.