Inspired by nature’s use of simple and powerful connecting reactions, the concept of click chemistry was first introduced by Kolb, Finn and Sharpless as a new molecular assembly strategy in 20011. Click chemistry has had a profound impact on many fields, such as synthetic chemistry, modern drug discovery, biological research, nanotechnology and materials science, providing remarkable alternatives to conventional chemistry (Figure 1a-c)2. Nevertheless, several well-established click reactions and other click-like transformations have unique advantages, yet with inherent limitations3. Clearly, developing new versatile systems to fulfill the click chemistry criteria remains highly desirable yet challenging.
Light-activation is an ideal way to drive chemical transformations in chemical and biological processes. It has unique advantages including simple operations without use of ligands and metal catalysts, temporal and spatial control. Despite many exciting applications have been found, currently, only limited photoclick transformations have been developed4, and photoclick chemistry is still in its infancy. Indeed, several challenges are associated with the development of light-induced click reactions, such as, merging photon utilization with chemical transformations to initiate the proposed reactions remains largely elusive in complex biological environments, designing of photo-responsible substrates with structure diversity as modular units to fulfill the criteria of click chemistry, and the requirement of functionalized photoclick handles.
Figure 1. Design of light-induced PANAC conjugation enables modular functionalization of small molecules, and native biomolecules in temporal control. a, Archetypal click reaction, CuAAC. b, Diazotransfer/CuAAC double-click reactions for modular synthesis of triazole libraries from primary amines. c, Toolkit of representative click reactions. d, o-Nitrobenzyl alcohol (o-NBA) was designed as molecular plugin and amide formations as general linkage to rapidly access diverse reactants as modular units, and primary amines as straightforward click handle for PANAC photoclick reaction in vitro and in living systems. e, Diverse applications of this PANAC photoclick chemistry.
Although the light-induced indazolone formations from o-nitrobenzyl alcohol (o-NBA) derivatives and primary amines were previous reported by others and our group, the generality (i.g. efficiency, functionality and accessibility) of these formations do not meet the criteria of click chemistry5-8. Based on our previous research6, 8, we recognized that the light-activated indazolone formations (photo-crosslinking of protein complexes) is biocompatible, and the o-NBA backbones are principally inert before light activation within protein complexes and living systems, thus the o-NBA moieties could serve as general masked reactants and photo-responsible handles in complex environments. Inspired by the residue-selective photo-crosslinking technology in living cells, we questioned whether the light-induced primary amines and o-nitrobenzyl alcohols cyclization (PANAC) might be successfully evolved as a photoclick reaction (Fig. 1d), where primary amines--one of the most abundant functional groups--would be ideal and direct click handles, thus for rapid conjugations of diverse primary-amine-containing molecules and functional o-NBA motifs, in temporal and spatial control.
In this research, we have designed o-NBA derivatives with amide linkage as modular reactants to achieve the accessibility, established the optimal procedure for high efficiency, and systematic investigated the reactivity and diverse chemical function of the reaction. We showed that the PANAC conjugation is sufficiently fast and highly efficient for straightforward late-stage diversification of pharmaceuticals and bio-relevant molecules, lysine-specific labeling and cyclization of unprotected peptides as potential medicinal agents, functionalization of diagnostic nanobody as imaging probe and modification of other native proteins in vitro. The generality of PANAC conjugation was further validated by broad-spectrum profiling endogenous kinases and organelle-targeted temporal labeling in living systems. Furthermore, as spatial control for live cell labeling, we have demonstrated the cell surface labeling with the HER2 specific nanobody and site-specifically labeling of histone protein via the developed PANAC photo-click reaction.
As the PANAC photoclick reaction is very simple and practical under mild conditions, we believe this approach will be easy to implement for researchers in multi-discipline fields. Given the intrinsic advantages of temporal control, high efficiency, good biocompatibility, exquisite chemoselectivity, wide scope in reactants and operational simplicity in primary-amine modular conjugation processes, the PANAC photoclick reaction would provide a powerful and reliable chemical tool for modular synthesis, bioconjugations, and beyond.
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