In Situ Synthesis of Alkenyl Tetrazines for Highly Fluorogenic Bioorthogonal Live‐Cell Imaging Probes

In spite of the wide application potential of 1,2,4,5‐tetrazines, particularly in live‐cell and in vivo imaging, a major limitation has been the lack of practical synthetic methods. Here we report the in situ synthesis of (E)‐3‐substituted 6‐alkenyl‐1,2,4,5‐tetrazine derivatives through an elimination–Heck cascade reaction. By using this strategy, we provide 24 examples of π‐conjugated tetrazine derivatives that can be conveniently prepared from tetrazine building blocks and related halides. These include tetrazine analogs of biological small molecules, highly conjugated buta‐1,3‐diene‐substituted tetrazines, and a diverse array of fluorescent probes suitable for live‐cell imaging. These highly conjugated probes show very strong fluorescence turn‐on (up to 400‐fold) when reacted with dienophiles such as cyclopropenes and trans‐cyclooctenes, and we demonstrate their application for live‐cell imaging. This work provides an efficient and practical synthetic methodology for tetrazine derivatives and will facilitate the application of conjugated tetrazines, particularly as fluorogenic probes for live‐cell imaging.     

Alkene–Tetrazine Ligation for Imaging Cellular DNA

5‐Vinyl‐2′‐deoxyuridine (VdU) is the first reported metabolic probe for cellular DNA synthesis that can be visualized by using an inverse electron demand Diels–Alder reaction with a fluorescent tetrazine. VdU is incorporated by endogenous enzymes into the genomes of replicating cells, where it exhibits reduced genotoxicity compared to 5‐ethynyl‐2′‐deoxyuridine (EdU). The VdU–tetrazine ligation reaction is rapid (k≈0.02 M ^?1 s^?1) and chemically orthogonal to the alkyne–azide “click” reaction of EdU‐modified DNA. Alkene–tetrazine ligation reactions provide the first alternative to azide–alkyne click reactions for the bioorthogonal chemical labeling of nucleic acids in cells and facilitate time‐resolved, multicolor labeling of DNA synthesis.     

An Organotrifluoroborate for Broadly Applicable One‐Step 18F‐Labeling

A new zwitterionic organotrifluoroborate is appended to three radiosynthons that afford undergo facile bioconjugation to several clinically relevant peptides and one enzyme inhibitor. Molecularly complex bioconjugates are ¹⁸F‐labeled in a single aqueous step in rapid time (<15 min) without HPLC purification to afford tracers in good yields (>200 mCi, 20–40 %) at high specific activity (≥3 Ci/μmol) and at >98 % purity. PET imaging shows in vivo stability and tumor uptake.     

Probing the Anticancer Mechanism of (−)‐Ainsliatrimer A through Diverted Total Synthesis and Bioorthogonal Ligation

Herein, we report an efficient approach for exploring the novel anticancer mechanism of (?)‐ainsliatrimer A, a structurally complex and unique trimeric sesquiterpenoid, through a combined strategy of diverted total synthesis (DTS) and bioorthogonal ligation (TQ ligation), which allowed us to visualize the subcellular localization of this natural product in live cells. Further biochemical studies facilitated by pretarget imaging revealed that PPARγ, a nucleus receptor, was a functional cellular target of ainsliatrimer A. We also confirmed that the anticancer activity of ainsliatrimer A was caused by the activation of PPARγ.     

An Isotope‐Coded Fluorogenic Cross‐Linker for High‐Performance Target Identification Based on Photoaffinity Labeling

A photoaffinity labeling (PAL)‐based method for the rapid identification of target proteins is presented in which a high‐performance chemical tag, an isotope‐coded fluorescent tag (IsoFT), can be attached to the interacting site by irradiation. Labeled peptides can be easily distinguished among numerous proteolytic digests by sequential detection with highly sensitive fluorescence spectroscopy and mass spectrometry. Subsequent MS/MS analysis provides amino acid sequence information with a higher depth of coverage. The combination of PAL and heterogeneous target‐selecting techniques significantly reduces the amount of time and protein required for identification. An additional photocleavable moiety successfully accelerated proteomic analysis using cell lysate. This method is a widely applicable approach for the rapid and accurate identification of interacting proteins.     

金属标记结合高效液相色谱-选择离子监测质谱的蛋白质绝对定量新方法

建立了金属标记结合高效液相色谱-选择性离子监测质谱(SIM)的蛋白质绝对定量新方法。实验考察了金属标记效率、金属标记的稳定性、标记后肽段的色谱保留和质谱行为、新定量方法的线性范围和准确度。实验结果表明金属标记具有标记效率高,稳定性好,色谱保留行为一致等优点。另外,金属标记-选择离子监测质谱绝对定量方法灵敏度高,其定量限低至1 fmol,线性范围为1~500 fmol,线性范围内R2值大于0.99,具有良好的线性关系;经过测量,标准肽段的回收率为117.01%,说明该方法具有较高的准确度。将该方法应用于腾冲嗜热菌中烯醇酶蛋白的定量分析,相对标准偏差为5.47%,表明该方法的精密度高。以上结果表明该方法可以用于生物样本中的蛋白质的绝对定量分析,为比较简单的生物样本中蛋白质的绝对定量方法提供了一种新的选择。     

Chromophore-Assisted Proximity Labeling of DNA Reveals Chromosomal Organization in Living Cells

The spatial arrangement of chromosome within the nucleus is linked to genome function and gene expression regulation. Existing genome-wide mapping methods often rely on chemically crosslinking DNA with protein baits, which raises concerns of artifacts being introduced during cell fixation. By genetically targeting a photosensitizer protein to specific subnuclear locations, we achieved blue-light-activated labeling of local DNA with a bioorthogonal functional handle for affinity purification and sequence identification through next-generation sequencing. When applied to the nuclear lamina in human embryonic kidney 293T cells, it revealed lamina-associated domains (LADs) that cover 37.6 % of the genome. These LADs overlap with heterochromatin hallmarks and are depleted with CpG islands. This simple labeling method avoids the harsh treatment of chemical crosslinking and is generally applicable to the genome-wide high-resolution mapping of the spatial chromosome organization in living cells.     

Live Monitoring of Strain-Promoted Azide Alkyne Cycloadditions in Complex Reaction Environments by Inline ATR-IR Spectroscopy

The strain-promoted azide alkyne cycloaddition (SPAAC) is a powerful tool for forming covalent bonds between molecules even under physiological conditions, and therefore found broad application in fields ranging from biological chemistry and biomedical research to materials sciences. For many applications, knowledge about reaction kinetics of these ligations is of utmost importance. Kinetics are commonly assessed and studied by NMR measurements. However, these experiments are limited in terms of temperature and restricted to deuterated solvents. By using an inline ATR-IR probe we show that the cycloaddition of azides and alkynes can be monitored in aqueous and even complex biological fluids enabling the investigation of reaction kinetics in various solvents and even human blood plasma under controlled conditions in low reaction volumes.     

Bioorthogonal Turn-On BODIPY-Peptide Photosensitizers for Tailored Photodynamic Therapy

Photodynamic therapy (PDT) leads to cancer remission via the production of cytotoxic species under photosensitizer (PS) irradiation. However, concomitant damage and dark toxicity can both hinder its use. With this in mind, we have implemented a versatile peptide-based platform of bioorthogonally activatable BODIPY-tetrazine PSs. Confocal microscopy and phototoxicity studies demonstrated that the incorporation of the PS, as a bifunctional module, into a peptide enabled spatial and conditional control of singlet oxygen (¹O₂) generation. Comparing subcellular distribution, PS confined in the cytoplasmic membrane achieved the highest toxicities (IC₅₀=0.096±0.003 μm ) after activation and without apparent dark toxicity. Our tunable approach will inspire novel probes towards smart PDT.     

LC-MS Quantification of Site-Specific Phosphorylation Degree by Stable-Isotope Dimethyl Labeling Coupled with Phosphatase Dephosphorylation

Protein phosphorylation is a crucial post-translational modification that plays an important role in the regulation of cellular signaling processes. Site-specific quantitation of phosphorylation levels can help decipher the physiological functions of phosphorylation modifications under diverse physiological statuses. However, quantitative analysis of protein phosphorylation degrees is still a challenging task due to its dynamic nature and the lack of an internal standard simultaneously available for the samples differently prepared for various phosphorylation extents. In this study, stable-isotope dimethyl labeling coupled with phosphatase dephosphorylation (DM + deP) was tried to determine the site-specific degrees of phosphorylation in proteins. Firstly, quantitation accuracy of the (DM + deP) approach was confirmed using synthetic peptides of various simulated phosphorylation degrees. Afterwards, it was applied to evaluate the phosphorylation stoichiometry of milk caseins. The phosphorylation degree of Ser130 on α-S1-casein was also validated by absolute quantification with the corresponding synthetic phosphorylated and nonphosphorylated peptides under a selected reaction monitoring (SRM) mode. Moreover, this (DM + deP) method was used to detect the phosphorylation degree change of Ser82 on the Hsp27 protein of HepG2 cells caused by tert-butyl hydroperoxide (t-BHP) treatment. The results showed that the absolute phosphorylation degree obtained from the (DM + deP) approach was comparable with the relative quantitation resulting from stable-isotope dimethyl labeling coupled with TiO₂ enrichment. This study suggested that the (DM + deP) approach is promising for absolute quantification of site-specific degrees of phosphorylation in proteins, and it may provide more convincing information than the relative quantification method.