Macrocycles are ring-shaped molecules built from at least twelve atoms, large enough to create an internal cavity that can grab onto other molecules or metal ions with striking precision. They show up everywhere from your own bloodstream to the latest cancer drugs, and more than sixty macrocyclic drugs have received FDA approval since the twentieth century, spanning infections, cancer, immunosuppression, and neurological disorders.1PubMed Central. FDA-approved drugs featuring macrocycles or medium-sized rings What makes these rings so versatile is their combination of size, flexibility, and the ability to organize themselves around a target in ways that smaller molecules and large proteins cannot.
Macrocycles Already Running Your Body
Before chemists ever learned to build macrocycles in a lab, nature had been using them for billions of years. The most familiar example is heme, the iron-containing porphyrin ring at the heart of hemoglobin. Heme is what makes blood red and what ferries oxygen from your lungs to your tissues. Plants rely on a closely related macrocycle, chlorophyll, to capture sunlight during photosynthesis.2PubMed Central. Chemistry of porphyrins in fossil plants and animals Vitamin B12, another porphyrin-family macrocycle, is essential for nerve function and DNA synthesis. In each case, the ring’s cavity holds a metal ion (iron in heme, magnesium in chlorophyll, cobalt in B12) in exactly the right geometry to carry out its biochemical job. The ring doesn’t just passively house the metal; its electronic structure fine-tunes the metal’s reactivity so it can do things a bare metal ion in solution never could.
Bacteria also produce macrocyclic natural products, including lasso peptides like microcin J25, whose unusual threaded-ring topology gives them stability against heat and digestive enzymes. Researchers have studied these structures for potential biotechnological applications, including antimicrobial development.3PubMed. Microcin J25, from the macrocyclic to the lasso structure: implications for biosynthetic, evolutionary and biotechnological perspectives The recurring theme across biology is that forming a large ring imposes just enough structural order to make molecular recognition reliable without sacrificing the flexibility that lets the molecule adapt to changing conditions.
An Accidental Discovery That Launched a Field
The modern science of synthetic macrocycles traces back to a laboratory accident in 1961. Charles J. Pedersen, a chemist at DuPont, was trying to make a simple phenolic compound when a trace contaminant led to the formation of a strange byproduct in just 0.4% yield. That byproduct turned out to be dibenzo-18-crown-6, an 18-atom ring containing six oxygen atoms, and it had an unexpected talent: it could wrap around a sodium ion and hold it tightly.4PubMed. The discovery of crown ethers Pedersen recognized the significance immediately and spent years systematically making dozens of related ring compounds, which he named “crown ethers” because of their regal-looking shapes when complexed with metal ions.
This discovery did more than add a new entry to the catalog of known molecules. It sparked the entire field of supramolecular chemistry, the study of how molecules recognize and bind each other through non-covalent forces. Pedersen shared the 1987 Nobel Prize in Chemistry with Donald J. Cram and Jean-Marie Lehn for this work.5Angewandte Chemie International Edition in English. The Discovery of Crown Ethers (Noble Lecture) Pedersen’s serendipitous finding fueled decades of advances in macrocyclic and supramolecular chemistry that continue today.6PubMed. Charles J. Pedersen’s legacy to chemistry
How a Ring Grabs an Ion
Crown ethers illustrate the defining trick of macrocyclic chemistry: size-selective binding. The cavity of a crown ether has a fixed diameter, determined by how many atoms make up the ring. If a metal ion is the right size, it drops neatly into the center and is stabilized by interactions with the oxygen atoms lining the cavity. Spectroscopic studies in cold ion traps confirm this picture in detail. When sodium sits inside a 15-crown-5 ring, or potassium inside an 18-crown-6 ring, the ether opens up fully and cradles the ion at its center. Smaller ions cause the ring to pucker inward, squeezing down to maintain contact. Ions too large to fit inside simply perch on top of the ring, leaving one face exposed.7PubMed. Ion selectivity of crown ethers investigated by UV and IR spectroscopy in a cold ion trap Computational studies agree, finding that the binding strength of dibenzo-18-crown-6 peaks for potassium and drops off for ions that are either larger or smaller.8PubMed Central. Binding selectivity of dibenzo-18-crown-6 for alkali metal cations in aqueous solution: A density functional theory study using a continuum solvation model
This selectivity isn’t just a curiosity. It underpins practical applications from lithium extraction to medical diagnostics. And the thermodynamic reasons it works are worth noting briefly. When a simple, open-chain molecule with the same donor atoms binds a metal ion, the resulting complex is reasonably stable. When you close that chain into a ring, the complex becomes significantly more stable, a phenomenon called the macrocyclic effect. The extra stability comes from several reinforcing factors: the ring is already partly pre-organized for binding, its donor atoms become slightly more electron-rich upon ring closure, and there are entropy advantages because the ring doesn’t lose as many degrees of freedom when it wraps around the ion.9Coordination Chemistry Reviews. Factors affecting stabilities of chelate, macrocyclic and macrobicyclic complexes in solution
Cyclodextrins and Drug Delivery
Crown ethers are not the only macrocycles with a useful cavity. Cyclodextrins are ring-shaped sugars, typically made of six, seven, or eight glucose units, that form a bucket-like structure with a water-friendly exterior and a greasy interior. That hydrophobic pocket can encapsulate drug molecules that are otherwise too insoluble to work well in the body. Beta-cyclodextrin, the seven-glucose version, is the most widely used in pharmaceuticals. Molecular dynamics simulations of beta-cyclodextrin complexed with the cancer drug 5-fluorouracil show that the encapsulated drug can be released in two stages: a fast release from molecules sitting near the rim of the bucket and a slower release from molecules deeply buried inside.10PubMed Central. Different Drug Mobilities in Hydrophobic Cavities of Host-Guest Complexes between β-Cyclodextrin and 5-Fluorouracil at Different Stoichiometries: A Molecular Dynamics Study in Water This kind of staged release can be useful for controlling how quickly a drug enters the bloodstream.
Pairing beta-cyclodextrin with polysaccharide-based carriers takes this a step further, creating composite vehicles that achieve better drug release rates through host-guest interactions.11PubMed. Host-guest drug delivery by β-cyclodextrin assisted polysaccharide vehicles: A review Cyclodextrins are already found in dozens of commercial drug formulations, from eye drops to injectable solutions, where their main role is simply making a poorly soluble drug dissolve well enough to be administered.
Why Drug Designers Love Macrocycles
Many of the most important biological targets for new medicines are proteins with large, flat, or groove-shaped binding surfaces. Conventional small-molecule drugs, the pills most people are familiar with, follow a set of guidelines often called the “rule of five” that limit molecular weight and other properties. Those guidelines work well for typical enzyme targets with deep, well-defined pockets, but they leave out targets whose binding surfaces are shallow and extended. Macrocycles can cover more surface area and make more contacts with these difficult targets, reaching into chemical space beyond what traditional small molecules can access.12Journal of Medicinal Chemistry. How Beyond Rule of 5 Drugs and Clinical Candidates Bind to Their Targets
The obvious problem with making molecules bigger is that they tend to become worse at getting into cells. A molecule that is too polar dissolves fine in blood but bounces off cell membranes. One that is too greasy slips through membranes but won’t dissolve. Some macrocyclic drugs solve this paradox with what researchers call chameleonic behavior: they change their three-dimensional shape depending on their environment. In water, they expose their polar groups for good solubility. When approaching a lipid membrane, they fold those polar groups inward, presenting a greasier surface that lets them pass through. Analysis of twenty approved oral macrocyclic drugs suggests that some degree of this shape-shifting is essentially required for oral drugs above a certain molecular weight.13PubMed Central. Quantifying the chameleonic properties of macrocycles and other high-molecular-weight drugs
Cyclosporin A, the immunosuppressant that transformed organ transplantation in the 1980s, is one of the best-studied chameleonic macrocycles. Work on cyclosporin O derivatives has shown that specific modifications to the ring’s backbone and side chains can dial chameleonicity up or down. For instance, adding a hydroxyl group at one position enhanced both the shape-shifting ability and membrane permeability, while adding an extra internal hydrogen bond locked the ring into a rigid shape that crossed membranes much more slowly.14PubMed. Controlling the Chameleonic Behavior and Membrane Permeability of Cyclosporine Derivatives via Backbone and Side Chain Modifications In a separate study, cyclosporin O adopted a conformation similar to cyclosporin A in nonpolar environments but showed less chameleonic behavior in polar environments, which reduced its membrane permeability while not necessarily hurting its overall pharmacokinetic profile.15PubMed. Interplay among Conformation, Intramolecular Hydrogen Bonds, and Chameleonicity in the Membrane Permeability and Cyclophilin A Binding of Macrocyclic Peptide Cyclosporin O Derivatives These findings suggest that membrane permeability and target binding can sometimes be tuned independently, giving chemists more room to optimize drug candidates.
Tackling Protein-Protein Interactions
One class of targets where macrocycles have particularly strong potential is protein-protein interactions. Proteins constantly form partnerships inside cells, and when those partnerships go wrong they drive diseases from cancer to neurodegeneration. But the surfaces where two proteins meet are typically broad and flat, with no deep pocket for a conventional small molecule to grab onto. Macrocyclic peptides have emerged as a promising class of drug candidates for disrupting these interactions, filling a gap between small molecules (which are too small to cover enough surface) and antibodies (which are too large to get inside cells).16PubMed Central. Macrocycles as protein-protein interaction inhibitors
The main hurdle has been getting these peptide macrocycles through cell membranes. Macrocyclic peptides are often highly effective at blocking protein-protein interactions in test-tube experiments, and combinatorial library screening can find potent binders quickly, but most of these hits cannot enter living cells. Recent advances in peptide engineering are changing that picture, enabling the development of macrocyclic peptides that can reach intracellular targets in cell culture and animal models.17PubMed Central. Targeting intracellular protein-protein interactions with macrocyclic peptides Much of this progress draws on the chameleonic design principles described above, along with strategies like N-methylation and backbone modifications borrowed from natural cyclic peptides.
The Challenge of Building Large Rings
Making a macrocycle in the lab is harder than it sounds. The basic problem is entropic: you need the two ends of a long, floppy chain to find each other and react. Given that the chain can adopt countless conformations with its ends far apart, the odds of the right conformation occurring at any given moment are low. Meanwhile, one chain’s reactive end can easily find and react with a different chain’s end, producing oligomers and polymers instead of a ring. Chemists have developed several strategies to tilt the odds in favor of cyclization.
Ring-closing metathesis, a reaction that stitches two carbon-carbon double bonds together using a metal catalyst, is one of the most widely used. It works because the catalyst tolerates a wide range of other functional groups on the molecule, and the product double bond can be further modified.18PubMed. Macrocyclization by ring-closing metathesis in the total synthesis of natural products: reaction conditions and limitations A persistent problem, though, is that the unwanted oligomers form right alongside the desired ring. Some newer catalyst designs address this by accelerating the “backbiting” step, where an oligomer chain folds back on itself to form the ring. One approach incorporates a hemilabile chelate into the catalyst that speeds up macrocyclization of conformationally flexible chains, even at room temperature, which is a significant improvement over earlier catalysts that needed high dilution and elevated temperatures.19PubMed. Chelate-Assisted Ring-Closing Metathesis: A Strategy for Accelerating Macrocyclization at Ambient Temperatures
Another powerful approach is template-directed synthesis, where a rigid molecular scaffold holds the building blocks in the right geometry for ring closure. In one striking demonstration, three building blocks bearing terminal alkyne groups were attached to a trimesic acid template, then coupled to form a 54-membered macrocycle in 89% yield. Without the template, the same reaction gave only about 20% of the desired ring, with the rest being oligomers and polymers.20PubMed Central. Covalent Template-Directed Synthesis: A Powerful Tool for the Construction of Complex Molecules – Section: Ester Covalent Attachment After ring closure, the template is removed by hydrolysis, leaving the free macrocycle. The yield difference is dramatic and illustrates how much pre-organization matters.
Screening Trillions of Rings at Once
Designing macrocyclic drug candidates one at a time is slow. A faster route is to generate enormous libraries of macrocyclic peptides and then fish out the ones that bind a given target. A technique called mRNA display does this by physically linking each peptide to the strand of mRNA that encodes it, creating a molecular barcode for each ring. After several rounds of binding selection against a target protein, researchers sequence the surviving mRNA to identify which peptide sequences won.
This approach was used to find cyclic peptide ligands to the SARS-CoV-2 spike protein’s receptor-binding domain. Using three parallel libraries, including both thioether-closed and disulfide-closed macrocyclic peptides, each containing more than a trillion unique sequences, researchers identified high-affinity binders after seven rounds of selection.21PubMed Central. Discovery of Cyclic Peptide Ligands to the SARS-CoV-2 Spike Protein Using mRNA Display More recently, the same general platform has been extended to target not just proteins but structured RNA. Researchers discovered macrocyclic peptides that bind to an iron-responsive element in the mRNA that regulates alpha-synuclein, a protein implicated in Parkinson’s disease. Some of these peptides could even covalently modify or degrade the target RNA, opening a frontier where macrocycles act on nucleic acids rather than proteins.22Journal of the American Chemical Society. Discovery of Macrocyclic Peptide Binders, Covalent Modifiers, and Degraders of a Structured RNA by mRNA Display
Cleaning Up Water and Purifying Gas
The same host-guest chemistry that makes macrocycles useful in drug delivery also makes them effective at capturing pollutants. Crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes each have different cavity sizes and chemical preferences, and they can selectively grab heavy metals, organic pollutants, and even perfluoroalkyl substances (the “forever chemicals” known as PFAS) from water. When these discrete macrocyclic hosts are incorporated into extended porous frameworks, the result is materials that combine molecular recognition with high surface area for bulk adsorption.23Coordination Chemistry Reviews. Porous materials for adsorption of aqueous pollutants: from discrete macrocyclic hosts to extended porous frameworks The selectivity afforded by host-guest interactions becomes especially important when a pollutant is present at trace concentrations, where a non-selective adsorbent might be overwhelmed by harmless dissolved species.
Macrocyclic porous crystals are also showing promise in gas separation. A recently reported macrocycle called CaC9 forms crystals with intrinsic pores just 4.2 angstroms wide, lined with saturated alkyl chains that make them extremely water-repellent. These crystals can purify methane to greater than 99.99% purity from mixtures with ethane and propane, even under conditions of 97% relative humidity, where most porous materials would lose performance because water clogs their pores. The crystals also self-heal, meaning minor damage to the material can be repaired without replacing it.24PubMed. Superhydrophobic and Self-Healing Porous Organic Macrocycle Crystals for Methane Purification under Humid Conditions
Molecular Machines Built from Interlocked Rings
Macrocycles also serve as the moving parts in molecular machines, devices where mechanical motion at the molecular scale is designed to perform work. Two macrocycles threaded through each other form a catenane; a macrocycle threaded onto a dumbbell-shaped axle forms a rotaxane. By applying chemical, electrochemical, or photochemical stimuli, researchers can make the macrocyclic component shuttle back and forth along the axle or rotate within a catenane. The 2016 Nobel Prize in Chemistry went to Jean-Pierre Sauvage, Fraser Stoddart, and Bernard Feringa for pioneering this field. Despite remarkable prototypes demonstrated in academic labs, translating these molecular machines into useful products remains a major open challenge.25PubMed Central. Simplicity in the Design, Operation, and Applications of Mechanically Interlocked Molecular Machines Current research is focused on finding applications where the unique properties of molecular-level mechanical motion offer advantages that no conventional material can match, such as stimuli-responsive surfaces or molecular-scale transport systems.
Recyclable Polymers from Macrocyclic Monomers
An emerging application that would have seemed unlikely a decade ago is using macrocycles as the starting materials for recyclable plastics. The idea exploits a thermodynamic feature of large rings: because they contain a certain amount of strain and entropy stored in their cyclic structure, they can be opened up and strung together into long polymer chains. Crucially, the reaction is reversible. Researchers have recently demonstrated polar-olefin macrocycles that undergo ring-opening polymerization to form useful polymers, and then depolymerize back to the original monomers when the conditions are shifted, for instance by diluting the solution. This closed-loop design enables efficient monomer recovery, offering a potential route to plastics that can be chemically recycled with high fidelity rather than downcycled or landfilled.26National Science Review. Lifecycle of dynamic covalent polar-olefin macrocycles via entropy-driven ring-opening polymerization and closed-loop chemical recycling The field is still in its early stages, but the principle is attractive: the same thermodynamic balance between ring and chain that makes macrocycle synthesis tricky is precisely what enables clean recycling.