Chemists Deploy Palladium to Switch Off and On a Key Amino Acid in Living Cells

Phenylalanine rarely gets top billing in discussions of molecular biology, yet this unassuming aromatic amino acid quietly underpins some of the most fundamental processes in the cell. Its benzene ring governs the hydrophobic character of protein surfaces, drives the pi-stacking and cation-pi interactions that hold molecular complexes together, and anchors the recognition events that allow peptides and proteins to find their partners. Now, a team of chemists at Peking University has developed a way to switch phenylalanine’s function off and then back on again inside living systems, using nothing more exotic than an iodine atom and a spark of palladium chemistry. The achievement, published in Nature Chemistry, opens the door to chemically manipulating one of the most widespread and stubbornly inert structural motifs in biology.

The problem the researchers set out to solve has long frustrated chemical biologists. Most strategies for controlling amino acid function in living cells rely on caging groups built around heteroatoms — oxygen, nitrogen, boron, or iodine-bearing linkages that can be cleaved by light, enzymes, or reactive chemicals. These approaches work beautifully for residues like serine, lysine, cysteine, tyrosine, and histidine, all of which carry reactive heteroatoms in their side chains. Phenylalanine, by contrast, is a purely hydrocarbon residue: a nonpolar benzene ring dangling from the protein backbone with no convenient chemical handle. Traditional caging chemistry simply has nothing to grab onto. As a result, although phenylalanine is one of the twenty canonical amino acids and is functionally critical in contexts ranging from amyloid formation to immune recognition, its activity could not previously be masked and restored at will in a living system.

The Peking University group, led by Peng R. Chen and Xinyuan Fan, with Yuchao Zhu, Shibo Liu, and Shan Qin as co-first authors, approached the challenge from an unconventional angle. Rather than trying to attach a bulky protecting group to an impossible target, they systematically evaluated caging strategies based on exogenous heteroatoms — oxygen, nitrogen, boron, and iodine — under physiological conditions. The winning design turned out to be remarkably simple: iodine atoms installed directly onto phenylalanine’s aromatic ring. This haloatom-assisted caging strategy exploits the fact that aryl iodides can undergo clean, traceless reduction, stripping the iodine away and regenerating the native phenylalanine residue without leaving any molecular scar behind.

The trigger for this restoration is palladium, a transition metal that has become a workhorse of bioorthogonal chemistry over the past decade. Palladium catalysts can mediate deprotection and bond-cleavage reactions inside living cells because they operate through mechanisms that native biochemistry simply does not use — no enzyme, no metabolite, and no cellular component competes for the reaction. When the researchers delivered palladium alongside a mild reducing system to iodinated phenylalanine residues, the aromatic cage was lifted and the amino acid sprang back to life in its native form. Crucially, the team demonstrated that the reaction works not only in solution but also in cell lysates and inside living cells, a benchmark that few bioorthogonal decaging reactions have reached.

To show that the chemistry could control real biology, the researchers first turned their attention to small molecules and peptides. They demonstrated that iodination could silence the fluorescence of a fluorophore and that palladium-triggered decaging could restore it, providing a convenient optical readout for the reaction. They then applied the strategy to peptide self-assembly, showing that iodinated phenylalanine residues could direct the disassembly of peptide structures — a finding with implications for the growing field of peptide-based nanomedicine, where the phenylalanine-phenylalanine motif is a celebrated driver of supramolecular assembly. By removing the iodine on demand, the researchers could toggle assembly states at will, effectively writing and erasing structural information in a peptide system.

The most striking demonstrations, however, came at the level of proteins and cells. Using genetic code expansion — the technique of engineering cells to site-specifically incorporate unnatural amino acids into proteins at chosen positions — the team installed iodinated phenylalanine into engineered HER2-targeting affibodies, small binding proteins directed against the HER2 receptor that is overexpressed in many breast cancers. lodinating a phenylalanine at the binding interface crippled the affibody’s ability to engage its receptor. When palladium was added, the cage was lifted, the native phenylalanine was restored, and ligand-receptor binding on the cell surface surged back to full strength. This dynamic control of a protein-protein interaction on a living cell membrane represents exactly the kind of precise, externally triggered molecular switch that the bioorthogonal chemistry community has pursued for years.

The team then extended the strategy into immunology, an area where the stakes are particularly high. Antigenic peptides presented on the surface of tumor cells by major histocompatibility complex class I molecules are the signals that tell cytotoxic T cells to attack. The researchers showed that iodinating phenylalanine residues within antigenic peptides could reshape their immunogenicity, dampening the presentation landscape until palladium-triggered decaging flipped it back on. In practical terms, this means tumor cells could be chemically tuned in their engagement with T cells — a concept that suggests future therapeutic strategies in which the immune visibility of a tumor is masked or unmasked on demand. The experiments demonstrated temporally controlled reshaping of the immunopeptidome, rewiring the tumor-T cell interface from the outside in, with chemistry rather than genetics as the controlling hand.

What makes this work especially significant is its scope. Phenylalanine is not a niche residue; it is ubiquitous, appearing in roughly four percent of protein sequences on average and clustering disproportionately at binding interfaces, active sites, and recognition motifs. Previous decaging efforts from the same laboratory and others had conquered tryptophan, tyrosine, lysine, and other functionalized residues, but the hydrophobic aromatic core of phenylalanine had remained out of reach. By establishing that a halogen atom can serve as both a functional disruptor and a removable cage — and that palladium chemistry can reverse the modification under fully physiological conditions — the study unlocks an entire class of nonpolar groups for chemical manipulation in living systems. The authors note that the strategy complements photocaged amino acids and other genetically encoded approaches, adding a small-molecule trigger that can penetrate cells and act without light.

The technical achievements underlying the paper are considerable. The researchers systematically compared oxygen-, nitrogen-, boron-, and iodine-based caging groups, finding that monoidinated phenylalanine offered the best balance of stability under physiological conditions and clean, traceless decaging. Computational modeling helped them understand how iodination perturbs binding energetics at protein interfaces, and LC-MS analysis confirmed complete consumption of caged substrates with well-defined products. In living cells, the palladium-mediated decaging restored roughly half of the fluorescence of a caged pyrene reporter, a substantial efficiency for intracellular bioorthogonal catalysis. The work also builds on a decade of progress in palladium-mediated intracellular chemistry, from early demonstrations of palladium-mediated deprotection on cell surfaces to nanopalladium catalysts operating inside living animals, and it extends that legacy into a domain — nonpolar aromatic residues — that was previously considered chemically inaccessible.

Looking forward, the implications ripple outward across chemical biology, drug development, and immunotherapy. Prodrug strategies could exploit palladium-triggered phenylalanine decaging to activate therapeutics at disease sites where catalysts are delivered. Synthetic biologists could build protein circuits whose interactions are gated by a small-molecule cue rather than by transcription or light. Cancer immunologists now have a chemical tool to modulate how tumor cells present themselves to the immune system, potentially improving the precision of adoptive cell therapies and vaccine design. And because the reaction is traceless, the decaged product is indistinguishable from the native biomolecule, sidestepping concerns about residual chemical artifacts. What the Peking University team has delivered is not merely a new reaction but a new degree of freedom: the ability to silence and restore, on command, one of biology’s most essential and least manipulable building blocks. In a field where the grand ambition is to exert the precision of synthetic chemistry inside the messiness of living systems, that is a milestone worth pausing over.

Subject of Research: Palladium-triggered bioorthogonal decaging of iodine-caged phenylalanine for controlling protein and cell functions in living systems

Article Title: Palladium-triggered bioorthogonal phenylalanine decaging

Article References: Zhu, Y., Liu, S., Qin, S., Wang, X., Liu, Y., Zhang, X., Shan, Y., Fan, X., & Chen, P. R. (2026). Palladium-triggered bioorthogonal phenylalanine decaging. Nature Chemistry. https://doi.org/10.1038/s41557-026-02226-2

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02226-2

Keywords: bioorthogonal chemistry, palladium catalysis, phenylalanine decaging, genetic code expansion, protein-protein interactions, iodination, chemical biology, tumor immunology, peptide self-assembly, HER2 affibody, amino acid caging, Nature Chemistry