Live-cell imaging of cyclopropene tags with fluorogenic tetrazine cycloadditions

trans-Cyclooctene--a stable, voracious dienophile for bioorthogonal labeling

Discussed herein is the development and advancement of trans-cyclooctene as a tool for facilitating bioorthogonal labeling through reactions with s-tetrazines. While a number of strained alkenes have been shown to combine with tetrazines for applications in bioorthogonal labeling, trans-cyclooctene enables fastest reactivity at low concentration with rate constants in excess of k2=10(6) M(-1) s(-1). In the present article, we describe advances in computation and synthesis that have enabled applications in chemical biology and nuclear medicine.

Expanding room for tetrazine ligations in the in vivo chemistry toolbox

There is tremendous interest in developing and refining methods to predictably perform chemical reactions within the framework of living systems. Here we review recent advances in applying tetrazine cycloadditions to live cell and in vivo chemistry. We highlight new syntheses of the tetrazine and dienophile precursors useful for in vivo studies. We briefly overview the use of this reaction in combination with unnatural amino acid technology and discuss applications involving the imaging of glycans on live cells. An emerging area is the use of tetrazine ligations for the development of in vivo imaging probes such as those used for positron emission tomography. We summarize recent applications involving tetrazine cycloadditions performed in live mice for pretargeted imaging of cancer cell biomarkers.

BODIPY-tetrazine derivatives as superbright bioorthogonal turn-on probes

The fastest and the brightest: A new design that intimately connects tetrazine to a BODIPY fluorophore enables exceptionally efficient energy transfer and quenching. Upon reaction of the tetrazine, the brightness of the fluorophore increases more than a thousand-fold, a fluorogenic activation up to two orders of magnitude greater than previously described.

Rapid oligonucleotide-templated fluorogenic tetrazine ligations

Template driven chemical ligation of fluorogenic probes represents a powerful method for DNA and RNA detection and imaging. Unfortunately, previous techniques have been hampered by requiring chemistry with sluggish kinetics and background side reactions. We have developed fluorescent DNA probes containing quenched fluorophore-tetrazine and methyl-cyclopropene groups that rapidly react by bioorthogonal cycloaddition in the presence of complementary DNA or RNA templates. Ligation increases fluorescence with negligible background signal in the absence of hybridization template. Reaction kinetics depend heavily on template length and linker structure. Using this technique, we demonstrate rapid discrimination between single template mismatches both in buffer and cell media. Fluorogenic bioorthogonal ligations offer a promising route towards the fast and robust fluorescent detection of specific DNA or RNA sequences.

A biocompatible in vivo ligation reaction and its application for noninvasive bioluminescent imaging of protease activity in living mice

The discovery of biocompatible reactions had a tremendous impact on chemical biology, allowing the study of numerous biological processes directly in complex systems. However, despite the fact that multiple biocompatible reactions have been developed in the past decade, very few work well in living mice. Here we report that D-cysteine and 2-cyanobenzothiazoles can selectively react with each other in vivo to generate a luciferin substrate for firefly luciferase. The success of this "split luciferin" ligation reaction has important implications for both in vivo imaging and biocompatible labeling strategies. First, the production of a luciferin substrate can be visualized in a live mouse by bioluminescence imaging (BLI) and furthermore allows interrogation of targeted tissues using a "caged" luciferin approach. We therefore applied this reaction to the real-time noninvasive imaging of apoptosis associated with caspase 3/7. Caspase-dependent release of free D-cysteine from the caspase 3/7 peptide substrate Asp-Glu-Val-Asp-D-Cys (DEVD-(D-Cys)) allowed selective reaction with 6-amino-2-cyanobenzothiazole (NH(2)-CBT) in vivo to form 6-amino-D-luciferin with subsequent light emission from luciferase. Importantly, this strategy was found to be superior to the commercially available DEVD-aminoluciferin substrate for imaging of caspase 3/7 activity. Moreover, the split luciferin approach enables the modular construction of bioluminogenic sensors, where either or both reaction partners could be caged to report on multiple biological events. Lastly, the luciferin ligation reaction is 3 orders of magnitude faster than Staudinger ligation, suggesting further applications for both bioluminescence and specific molecular targeting in vivo.

Fluorescent live-cell imaging of metabolically incorporated unnatural cyclopropene-mannosamine derivatives

Sugar coated: We recently developed methylcyclopropenes as low-molecular-weight tetrazine coupling partners. Here, we demonstrate that methylcyclopropenes can meet the stringent steric demands required for metabolic imaging of unnatural mannosamines on live cells. Using sequential azide-alkyne chemistry, we also demonstrate multicolor imaging of two different metabolically incorporated unnatural sugars.

Genetically encoded cyclopropene directs rapid, photoclick-chemistry-mediated protein labeling in mammalian cells

We just click: Genetic incorporation of a cyclopropene amino acid CpK (see scheme) site-specifically into proteins in E. coli and mammalian cells was achieved using an orthogonal aminoacyl-tRNA synthetase/tRNA(CUA) pair (CpKRS/MbtRNA(CUA)). Cyclopropene exhibited fast reaction kinetics in the photoclick reaction and allowed rapid (ca. 2 min) labeling of proteins.