Bioorthogonal reactions, chemical transformations engineered to proceed inside living organisms without disturbing native biochemistry, have reshaped how scientists study and manipulate biological systems. Since the rise of click chemistry and related methods, researchers have used them to image sugars on cell surfaces, track proteins through their life cycles, and map molecular interactions in environments that conventional probes cannot reach. Yet the field has struggled with a persistent limitation: most bioorthogonal reactions occur wherever their two partners happen to meet. Once the reagents enter a cell, the chemist largely forfeits the ability to dictate when and where the reaction takes place, and background labeling can blur the biological picture. Light-activated chemistry offers one route to control, but light scatters in tissue, contends with cellular autofluorescence, and can harm living material. Electrical stimulation, by contrast, is precise, tunable, and already central to technologies that interface with biology, from pacemakers to brain implants, making it an alluring candidate for a chemical switch.

Cyclopropanol sits at the heart of the new strategy. The molecule consists of a three-membered carbon ring bearing a hydroxyl group, and that ring is a repository of strain energy. Its carbon–carbon bonds are bent far from their natural resting geometry, storing energy the way a compressed spring stores mechanical work. Synthetic chemists have long exploited this stored strain as a driving force for constructing complex ring systems, but the same property usually makes strained molecules fragile in water. What makes cyclopropanol unusual is that, despite its latent reactivity, it is remarkably stable under physiological conditions. The authors describe it as an energy-loaded handle: compact enough to be attached to probes and biomolecules, quiet enough to survive in a living cell, yet loaded with sufficient strain that a modest push will set it off. In this study, the push comes from electrochemistry, delivered by an electrode rather than by heat, light, or an added reagent.

When a suitable electrical potential is applied, the cyclopropanol is oxidized at the electrode surface, and the strained ring springs open. The resulting open-chain intermediate is then intercepted by halide to yield a β-haloketone, a functional group that pairs a ketone with an adjacent carbon–halogen bond. This is the dual-functional warhead of the paper’s title: an electrophile generated on the spot, carrying two complementary reactive handles that can engage the nucleophilic side chains of proteins. Crucially, the reactive species does not exist until the electrode says so. Instead of flooding a cell with a chemically aggressive compound and hoping it finds the right targets, the researchers create the electrophile only at the chosen moment, sharply reducing unwanted background reactions. The electrode thus behaves as a chemical switch, converting a stable, benign molecule into a protein-labeling agent on demand, and converting a long-standing aspiration of bioorthogonal chemistry, spatial and temporal command over a reaction, into an experimental reality.

The most unexpected finding concerns what the activated warhead actually labels. Electrophilic protein-labeling reagents typically pursue the most nucleophilic residues in a proteome, above all the thiol of cysteine and the amine of lysine. The cyclopropanol-derived β-haloketones did something counterintuitive: they preferentially modified acidic amino acids, chiefly glutamate and aspartate, and did so within hydrophobic regions of proteins. Acidic residues are ordinarily unattractive targets for covalent probes. At physiological pH their carboxylate side chains are deprotonated and negatively charged, and they tend to sit in polar environments rather than at reactive interfaces. The observation suggests that local context can overturn classical reactivity expectations: a hydrophobic pocket appears to change the behavior of the carboxylate, positioning and priming it to attack the electrophile. In effect, the warhead reads the three-dimensional environment of a protein rather than merely its chemical composition. That quality grants chemists access to labeling sites that conventional reagents overlook, and it hints at new ways to probe the hidden, water-excluding interiors of proteins.

Protein labeling inside cells is an old ambition, and the field has accumulated an impressive arsenal: enzyme-mediated proximity labeling recruits engineered enzymes that tag nearby proteins; photoaffinity probes use light to create short-lived reactive species that capture fleeting molecular neighbors; and classical electrophilic reagents exploit the inherent nucleophilicity of cysteine or lysine. Each approach involves trade-offs between selectivity, speed, and perturbation of the very system under study. The cyclopropanol platform enters this landscape with a distinctive profile. Its reactive species is generated only when an electrode is switched on; its reaction partner is a small, unobtrusive molecule rather than a bulky enzyme or aggressive photochemistry; and its target profile, centered on acidic residues in hydrophobic protein regions, complements rather than duplicates existing methods. That combination of compactness, dormancy, and external control addresses several long-standing limitations at once, which is why the authors regard the strategy as a general platform rather than a single-purpose reagent.

The team then asked whether the chemistry could survive the leap from purified proteins to living cells, and it could. Applying mild electrochemical stimulation to cells incubated with cyclopropanol-based probes triggered efficient protein labeling while the cells remained healthy, evidence that the voltages involved are gentle enough to avoid significant damage and that the probes themselves are not intrinsically disruptive. To watch the reaction unfold in real time, the researchers turned to fluorogenic probes, dyes engineered to remain nearly dark until they react, at which point they become brightly fluorescent. With such probes, protein labeling could be visualized as it happened, turning the electrochemical trigger into something like a live broadcast of covalent bond formation inside cells. The combination of a caged reagent, an external electrical switch, and a turn-on fluorescent readout offers a level of oversight that few bioconjugation methods can match, and it allows experiments to be repeated, delayed, or localized with relative ease.

To demonstrate the platform’s practical reach, the group designed a probe built around choline, a small molecule that living cells treat as a precious commodity. Choline is a structural cornerstone of phosphatidylcholine, the most abundant phospholipid in animal cell membranes, and cells actively import it and channel it into their membrane lipid metabolism. By furnishing choline with the cyclopropanol handle, the researchers created a kind of molecular Trojan horse. The cell willingly metabolizes the probe, incorporating it into its lipid-building pathways and thereby positioning the latent, electrically activatable warhead within membrane territories. When the electrical trigger was applied, the activated warhead labeled membrane-associated proteins as well as cytoplasmic proteins, yielding a snapshot of the protein populations that dwell in and around the lipid compartments where the probe had traveled. The experiment shows how a caged handle can be smuggled into a specific metabolic route and then unleashed on command, a design logic that could in principle be adapted to other metabolites, opening routes to label different organelles or protein communities.

The consequences for chemical biology could be substantial. Protein labeling under electrochemical control offers a way to interrogate proteomes with a degree of temporal precision that is difficult to achieve with genetically encoded tags or permanently reactive probes. Because the labeling event can be initiated at a chosen moment, researchers could in principle capture fast-changing protein states, follow proteins as they relocate within a cell, or restrict analysis to the proteins present in a particular compartment at a particular time. Mass spectrometry-based proteomics stands to benefit especially, since the unusual preference of the warhead for acidic residues in hydrophobic contexts would map a portion of the proteome that existing enrichment methods, which overwhelmingly target cysteines and lysines, largely miss. Adding glutamate and aspartate sites to the chemist’s address book broadens the observable chemical landscape of proteins and may reveal functional information encoded in acidic, membrane-embedded regions that current tools simply cannot see.

Beyond fundamental research, the approach hints at applications in biotechnology and medicine. Controllable bioconjugation underpins many therapeutic technologies, from antibody–drug conjugates to targeted imaging agents, and a method that activates only under an external electrical signal could add a layer of precision to how such conjugates are prepared or deployed. The strategy also resonates with the growing interface between electrochemistry and biology: bioelectronics, electrophysiology, and implantable devices all operate through electrodes, raising the possibility of chemistries that communicate directly with such hardware, releasing or activating a labeling reagent exactly where a device sits in the body. Much work remains before the method becomes routine. The rules that determine which acidic residues are modified are only beginning to be understood, and delivering electrode-based stimulation to tissues deep within an organism poses engineering challenges that a culture dish does not. Even so, the conceptual advance is striking. A molecule that stores strain like a compressed spring, conceals a reactive warhead like a sprung trap, and waits for a whisper of electrons to release it is a genuinely new kind of tool for biology. In a discipline where timing and placement often determine what can be known, cyclopropanol offers a deceptively simple answer: keep the chemistry switched off until the moment you are ready to learn something, and then let a small jolt of electricity do the rest.

Researchers led by Professor Yousung Jung at Seoul National University have proposed a catalyst-design principle that selectively suppresses hydrogen evolution while preserving the electrochemical reaction that converts nitrogen into ammonia. Their approach relies not on removing protons from the reaction environment, but on controlling how easily those protons can physically reach the electrode surface. The concept could offer a new route toward cleaner ammonia production powered by renewable electricity.

Ammonia is increasingly viewed as a practical carrier for hydrogen energy. It stores a large amount of hydrogen, remains liquid at room temperature under moderate conditions, and is easier to transport than gaseous hydrogen. Today, most ammonia is manufactured through the Haber–Bosch process, which requires high temperatures and pressures and consumes substantial amounts of energy. Because the process is commonly powered by fossil fuels, it also produces significant carbon dioxide emissions.

Electrochemical nitrogen reduction has emerged as a possible alternative. In principle, the process uses nitrogen, water and electricity to produce ammonia, potentially allowing production facilities to operate near wind or solar farms rather than relying on massive centralized chemical plants. The central obstacle, however, is that the electrode preferentially produces hydrogen instead of ammonia. This competing hydrogen evolution reaction consumes electrons and protons that would otherwise contribute to nitrogen reduction, sharply reducing ammonia yields.

Previous strategies have often attempted to control hydrogen evolution by changing the proton concentration or acidity of the electrolyte. That solution creates a fundamental trade-off: protons are needed not only to generate hydrogen but also to convert nitrogen into ammonia. Reducing their availability can therefore suppress both reactions. Jung’s team instead focused on the reaction interface—the narrow region where the electrolyte, catalyst and electrode meet—and asked whether the two reactions could be separated geometrically.

The researchers designed a reaction environment in which proton donors encounter greater steric hindrance as they approach the electrode surface. Steric hindrance occurs when the size and arrangement of molecules make it physically difficult for another molecule to reach a reactive site. In this case, the researchers effectively create a molecular gate around the electrode. Proton donors can still exist in the surrounding solution, but their paths toward the surface become restricted, making the initial step of hydrogen production less favorable.

That initial step is known as the Volmer reaction. During the Volmer reaction, a proton receives an electron and forms an adsorbed hydrogen atom on the electrode. Two such hydrogen atoms can subsequently combine to form molecular hydrogen, or an adsorbed hydrogen atom can react with another proton and electron. By making it harder for proton donors to reach the electrode, the new strategy raises the energy barrier for the Volmer reaction and slows the entire hydrogen evolution pathway.

The geometry of nitrogen reduction is different. According to the researchers, nitrogen molecules protrude outward from the catalyst environment, allowing protons to interact with nitrogen rather than needing to reach the electrode surface directly. This difference means that the same steric barrier that obstructs hydrogen evolution has a much smaller effect on nitrogen reduction. The result is a form of molecular selectivity based on access and positioning rather than solely on chemical composition.

The team used microkinetic modeling to examine how these competing pathways respond to changes in steric hindrance and applied voltage. Microkinetic models describe the rates of individual elementary reactions by tracking intermediates and solving differential equations for the overall reaction network. The simulations indicated that increasing steric hindrance can maintain high Faradaic efficiency across a broad voltage range. Faradaic efficiency measures the fraction of supplied electrical charge that produces the desired product—in this case, ammonia rather than hydrogen.

The findings could address a major limitation in electrochemical ammonia research, where Faradaic efficiencies have often remained near 70 percent or lower. The researchers report that their design principle could raise the value toward nearly 100 percent under modeled conditions, although future catalyst development and experimental validation will be essential. The strategy may also be transferable to other electrochemical systems in which a desired reaction competes with an unwanted one, including carbon dioxide reduction.

The study, published in the Journal of the American Chemical Society, presents the reaction interface itself as a programmable component of catalyst design. Rather than treating the electrolyte as a passive medium, the approach uses molecular shape to control which reactants can reach specific locations. Jung’s team plans to identify highly active catalyst materials that can incorporate this principle and to test the concept across additional reactions. If successful, the work could help bring renewable-powered ammonia production closer to practical use while offering a broader blueprint for controlling chemical reactions at the atomic scale.

Subject of Research: Electrochemical nitrogen reduction and selective suppression of the hydrogen evolution reaction

Article Title: Selective Suppression of Hydrogen Evolution in Electrochemical Nitrogen Reduction through Steric Control of Proton Donors

News Publication Date: July 22

Web References: https://doi.org/10.1021/jacs.6c07080

References: Journal of the American Chemical Society, DOI: 10.1021/jacs.6c07080

Image Credits: Seoul National University College of Engineering

Keywords

Electrochemical ammonia production, nitrogen reduction reaction, hydrogen evolution reaction, steric hindrance, catalyst design, green hydrogen, renewable energy, Faradaic efficiency, microkinetic modeling, Seoul National University