A photocaged, cyclopropene-containing analog of the amino acid neurotransmitter glutamate

Cell-selective bioorthogonal labeling

In classic bioorthogonal labeling experiments, the cell's biosynthetic machinery incorporates bioorthogonal tags, creating tagged biomolecules that are subsequently reacted with a corresponding bioorthogonal partner. This two-step approach labels biomolecules throughout the organism indiscriminate of cell type, which can produce background in applications focused on specific cell populations. In this review, we cover advances in bioorthogonal chemistry that enable targeting of bioorthogonal labeling to a desired cell type. Such cell-selective bioorthogonal labeling is achieved in one of three ways. The first approach restricts labeling to specific cells by cell-selective expression of engineered enzymes that enable the bioorthogonal tag's incorporation. The second approach preferentially localizes the bioorthogonal reagents to the desired cell types to restrict their uptake to the desired cells. Finally, the third approach cages the reactivity of the bioorthogonal reagents, allowing activation of the reaction in specific cells by uncaging the reagents selectively in those cell populations.

Pyranthiones/Pyrones: "Click and Release" Donors for Subcellular Hydrogen Sulfide Delivery and Labeling

Hydrogen sulfide (H2 S), one of the most important gasotransmitters, plays a critical role in endogenous signaling pathways of many diseases. However, developing H2 S donors with both tunable release kinetics and high release efficiency for subcellular delivery has been challenging. Here, we describe a click and release reaction between pyrone/pyranthiones and bicyclononyne (BCN). This reaction features a release of CO2 /COS with second-order rate constants comparable to Strain-Promoted Azide-Alkyne Cycloaddition reactions (SPAACs). Interestingly, pyranthiones showed enhanced reaction rates compared to their pyrone counterparts. We investigated pyrone biorthogonality and demonstrated their utility in protein labeling applications. Moreover, we synthesized substituted pyranthiones with H2 S release kinetics that can address the range of physiologically relevant H2 S dynamics in cells and achieved quantitative H2 S release efficiency in vitro. Finally, we explored the potential of pyranthiones as H2 S/COS donors for mitochondrial-targeted H2 S delivery in living cells.

Precise spatiotemporal control of voltage-gated sodium channels by photocaged saxitoxin

Here we report the pharmacologic blockade of voltage-gated sodium ion channels (Na_Vs) by a synthetic saxitoxin derivative affixed to a photocleavable protecting group. We demonstrate that a functionalized saxitoxin (STX-eac) enables exquisite spatiotemporal control of Na_Vs to interrupt action potentials in dissociated neurons and nerve fiber bundles. The photo-uncaged inhibitor (STX-ea) is a nanomolar potent, reversible binder of Na_Vs. We use STX-eac to reveal differential susceptibility of myelinated and unmyelinated axons in the corpus callosum to Na_V-dependent alterations in action potential propagation, with unmyelinated axons preferentially showing reduced action potential fidelity under conditions of partial Na_V block. These results validate STX-eac as a high precision tool for robust photocontrol of neuronal excitability and action potential generation.

Design, Synthesis, and Biological Evaluation of Light-Activated Antibiotics

The spatial and temporal control of bioactivity of small molecules by light (photopharmacology) constitutes a promising approach for study of biological processes and ultimately for the treatment of diseases. In this study, we investigated two different "caged" antibiotic classes that can undergo remote activation with UV-light at λ = 365 nm, via the conjugation of deactivating and photocleavable units through a short synthetic sequence. The two widely used antibiotics vancomycin and cephalosporin were thus enhanced in their performance by rendering them photoresponsive and thereby suppressing undesired off-site activity. The antimicrobial activity against Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC 29213, S. aureus ATCC 43300 (MRSA), Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 could be spatiotemporally controlled with light. Both molecular series displayed a good activity window. The vancomycin derivative displayed excellent values against Gram-positive strains after uncaging, and the next-generation caged cephalosporin derivative achieved good and broad activity against both Gram-positive and Gram-negative strains after photorelease.

Modular activatable bioorthogonal reagents

The bioorthogonal reaction toolbox contains approximately two-dozen unique chemistries that permit selective tagging and probing of biomolecules. Over the past two decades, significant effort has been devoted to optimizing and discovering bioorthogonal reagents that are faster, fluorogenic, and orthogonal to the already existing bioorthogonal repertoire. Conversely, efforts to explore bioorthogonal reagents whose reactivity can be controlled in space and/or time are limited. The "activatable" bioorthogonal reagents that do exist are often unimodal, meaning that their reagent's activation method cannot be easily modified to enable activation with red-shifted wavelengths, enzymes, or metabolic-byproducts and ions like H₂O₂ or Fe³⁺. Here, we summarize the available activatable bioorthogonal reagents with a focus on our recent addition: modular caged cyclopropenes. We designed caged cyclopropenes to be unreactive to their bioorthogonal partner until they are activated through the removal of the cage by light, an enzyme, or another reaction partner. To accomplish this, their structure includes a nitrogen atom at the cyclopropene C3 position that is decorated with the desired caging group through a carbamate linkage. This 3-N cyclopropene system can allow control of cyclopropene reactivity using a multitude of already available photo- and enzyme-caging groups. Additionally, this cyclopropene scaffold can enable metabolic-byproduct or ion activation of bioorthogonal reactions.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Lipidated cyclopropenes via a stable 3-N spirocyclopropene scaffold

Lipidated cyclopropenes serve as useful bioorthogonal reagents for imaging cell membranes due to the cyclopropene's small size and ability to ligate with pro-fluorescent tetrazines. Previously, the lipidation of cyclopropenes required modification at the C3 position because methods to append lipids at C1/C2 were not available. Herein, we describe C1/C2 lipidation with the biologically active lipid ceramide and a common phospholipid using a cyclopropene scaffold whose reactivity with 1,2,4,5-tetrazines has been caged.