Chemical modification of proteins and peptides represents a challenge of reaction design as well as an important biological tool. In contrast to side-chain modification, synthetic methods to alter backbone structure are extremely limited. In this communication, copper-mediated backbone N-alkenylation or N-arylation of peptides and proteins by direct modification of natural sequences is described. Histidine residues direct oxidative coupling of boronic acids at the backbone NH of a neighboring amino acid. The mild reaction conditions in common physiological buffers, at ambient temperature, are compatible with proteins and biological systems. This simple reaction demonstrates the potential for directed reactions in complex systems to allow modification of N-H bonds that directly affect polypeptide structure, stability, and function.

Preparation of antibody-drug conjugates (ADCs), an emerging novel class of highly targeted biological hybrid agents, necessitates precise control of conjugation reactivity. Antibodies have complex multistranded architectures, and specific modification of natural antibodies has proven quite challenging. Here, we demonstrate that cooperative activity of a multimetallic metallopeptide enables efficient site-specific antibody functionalization, based on molecular recognition of the constant Fc region. This interplay of multiple metal centers enables introduction of an orthogonal alkyne handle into monoclonal or polyclonal antibodies from different species in an Fc-specific fashion. Elaboration of this simple functionalization allows preparation of conjugates with fluorophore, affinity handle, and pharmacological agents. This method opens a new opportunity for quick and easy production of well-defined antibody conjugates from a variety of antibody sequence and species of origin.

Nickel salts catalyze fast cysteine arylation with 2-nitroarylboronic acids. The process uses cheap, readily-available reagents and allows introduction of diverse chemical handles.

Carbon is central to the chemistry of life, and in addition to its fundamental roles as a static component of all major biomolecules spanning proteins, nucleic acids, sugars, and lipids, emerging evidence shows that small and transient carbon-based metabolites, termed reactive carbon species (RCS), are dynamic signaling/stress agents that can influence a variety of biological pathways. Recent examples include the identification of carbon monoxide (CO) as an ion channel blocker and endogenous formaldehyde (FA) as a one-carbon metabolic unit formed from the spontaneous degradation of dietary folate metabolites. These findings motivate the development of analytical tools for transient carbon species that can achieve high specificity and sensitivity to further investigate RCS signaling and stress pathways at the cell, tissue, and wholeorganism levels. This Account summarizes work from our laboratory on the development of new chemical tools to monitor two important one-carbon RCS, CO and FA, through activity-based sensing (ABS), where we leverage the unique chemical reactivities of these small and transient analytes, rather than lock-and-key binding considerations, for selective detection. Classic inorganic/organometallic and organic transformations form the basis for this approach. For example, to distinguish CO from other biological diatomics of similar shape and size (e.g., nitric oxide and oxygen), we exploit palladium-mediated carbonylation as a synthetic method for CO sensing. The high selectivity of this carbonylation approach successfully enables imaging of dynamic changes in intracellular CO levels in live cells. Likewise, we apply the aza-Cope reaction for FA detection to provide high selectivity for this one-carbon unit over other larger biological aldehydes that are reactive electrophiles, such as acetaldehyde and methylglyoxal. By relying on an activity-based trigger as a design principle for small-molecule detection, this approach can be generalized to create a toolbox of selective FA imaging reagents, as illustrated by a broad range of FA probes spanning turn-on and ratiometric fluorescence imaging, positron emission tomography imaging, and chemiluminescence imaging modalities. Moreover, these chemical tools have *