Coumarin-Based Fluorogenic Probes for No-Wash Protein Labeling

Site-Specific Anchoring a Luminescent Tag in a Protein with Non-Emissive Iridium(III) Complex

Site-specific modification of proteins with synthetic fluorescent tag effectively improves the resolution of imaging, and such a labeling method with negligible three-dimensional structural perturbations and minimal impact on the biological functions of proteins is of high interest to dissect the high-resolution activities of biomolecules in complex systems. To this end, several non-emissive iridium(III) complexes [Ir(C-N)2 (H2 O)2 ]+ OTF- (C-N denotes various cyclometalated ligands) were designed and synthesized. These complexes were tested for attaching a protein by coordinating to H/X (HisMet, HisHis, and HisCys) that are separated by i and i+4 in α-helix. Replacement of the two labile water ligands in the iridium(III) complex by a protein HisHis pair increases the luminescent intensity up to over 100 folds. This labeling approach has been demonstrated in a highly specific and efficient manner in a number of proteins, and it is also feasible for labeling target proteins in cell lysates.

Phosphorogenic Iridium(III) bis-Tetrazine Complexes for Bioorthogonal Peptide Stapling, Bioimaging, Photocytotoxic Applications, and the Construction of Nanosized Hydrogels

The dual functionality of 1,2,4,5-tetrazine as a bioorthogonal reactive unit and a luminescence quencher has shaped tetrazine-based probes as attractive candidates for luminogenic labeling of biomolecules in living systems. In this work, three cyclometalated iridium(III) complexes featuring two tetrazine units were synthesized and characterized. Upon photoexcitation, the complexes were non-emissive but displayed up to 3900-fold emission enhancement upon the inverse electron-demand Diels-Alder (IEDDA) [4+2] cycloaddition with (1R,8S,9s)-bicyclo[6.1.0]non-4-yne (BCN) substrates. The rapid reaction kinetics (k₂ up to 1.47×10⁴  M^-1  s^-1 ) of the complexes toward BCN substrates allowed effective peptide labeling. The complexes were also applied as live cell bioimaging reagents and photocytotoxic agents. One of the complexes was utilized in the preparation of luminescent nanosized hydrogels that exhibited interesting cargo delivery properties.

A Protein-Binding Molecular Photothermal Agent for Tumor Ablation

Photothermal therapy usually requires a high power density to activate photothermal agent for effective treatment, which inevitably leads to damage to normal tissues and inflammation in tumor tissues. Herein, we rationally design a protein-binding strategy to build a molecular photothermal agent for photothermal ablation of tumor. The synthesized photothermal agent can covalently bind to the thiol groups on the intracellular proteins. The heat generated by the photothermal agent directly destroyed the bioactive proteins in the cells, effectively reducing the heat loss and the molecular leakage. Under a low power density of 0.2 W cm^-2 , the temperature produced by the photothermal agent was sufficient to induce apoptosis. In vitro and in vivo experiments showed that the therapeutic effect of photothermal therapy can be efficiently improved with the protein-binding strategy.

Engineering of a Red Fluorogenic Protein/Merocyanine Complex for Live-Cell Imaging

A reengineered human cellular retinol binding protein II (hCRBPII), a 15-kDa protein belonging to the intracellular lipid binding protein (iLBP) family, generates a highly fluorescent red pigment through the covalent linkage of a merocyanine aldehyde to an active site lysine residue. The complex exhibits "turn-on" fluorescence, due to a weakly fluorescent aldehyde that "lights up" with subsequent formation of a strongly fluorescent merocyanine dye within the binding pocket of the protein. Cellular penetration of merocyanine is rapid, and fluorophore maturation is nearly instantaneous. The hCRBPII/merocyanine complex displays high quantum yield, low cytotoxicity, specificity in labeling organelles, and compatibility in both cancer cell lines and yeast cells. The hCRBPII/merocyanine tag is brighter than most common red fluorescent proteins.

A Near-Infrared Photoswitchable Protein-Fluorophore Tag for No-Wash Live Cell Imaging

FR-1V, a fluorene-based aldehydic chromophore, binds its target protein as an imine to yield a highly bathochromic pigment, CF-2, a prototypic protein-dye tagging system whose NIR emission can be spatiotemporally switched ON by rapid UV-light activation. This is achieved through photoisomerization of the imine and its subsequent protonation. We demonstrate a no-wash protocol for live cell imaging of subcellular compartments in a variety of mammalian cell lines with minimal fluorescence background.

Emerging Applications of Fluorogenic and Non-fluorogenic Bifunctional Linkers

Homo- and hetero-bifunctional linkers play vital roles in constructing a variety of functional systems, ranging from protein bioconjugates with drugs and functional agents, to surface modification of nanoparticles and living cells, and to the cyclization/dimerization of synthetic polymers and biomolecules. Conventional approaches for assaying conjugation extents typically rely on ex situ techniques, such as mass spectrometry, gel electrophoresis, and size-exclusion chromatography. If the conjugation process involving bifunctional linkers was rendered fluorogenic, then in situ monitoring, quantification, and optical tracking/visualization of relevant processes would be achieved. In this review, conventional non-fluorogenic linkers are first discussed. Then the focus is on the evolution and emerging applications of fluorogenic bifunctional linkers, which are categorized into hetero-bifunctional single-caging fluorogenic linkers, homo-bifunctional double-caging fluorogenic linkers, and hetero-bifunctional double-caging fluorogenic linkers. In addition, stimuli-cleavable bifunctional linkers designed for both conjugation and subsequent site-specific triggered release are also summarized.

Bistetrazine-Cyanines as Double-Clicking Fluorogenic Two-Point Binder or Crosslinker Probes

Fluorogenic probes can be used to minimize the background fluorescence of unreacted and nonspecifically adsorbed reagents. The preceding years have brought substantial developments in the design and synthesis of bioorthogonally applicable fluorogenic systems mainly based on the quenching effects of azide and tetrazine moieties. The modulation power exerted by these bioorthogonal motifs typically becomes less efficient on more conjugated systems; that is, on probes with redshifted emission wavelength. To reach efficient quenching, that is, fluorogenicity, even in the red range of the spectrum, we present the synthesis, fluorogenic, and conjugation characterization of bistetrazine-cyanine probes with emission maxima between 600 and 620 nm. The probes can bind to genetically altered proteins harboring an 11-amino acid peptide tag with two appending cyclooctyne motifs. Moreover, we also demonstrate the use of these bistetrazines as fluorogenic, covalent cross-linkers between monocyclooctynylated proteins.