Peptides are chains of amino acids, and most people picture them as relatively simple, linear molecules. Macrocyclic peptides are different. They are looped back on themselves, forming a ring, and that closed shape gives them properties that straight-chain peptides lack. A recent paper published in the Journal of the American Chemical Society examined exactly how the atoms inside these rings move and rearrange in solution, using a combination of advanced magnetic-resonance measurements and computer simulations.
The motivation behind the research is practical. Macrocyclic peptides are attracting serious scientific attention because they may be able to do two things that most peptide drugs cannot: survive being swallowed as a pill and latch onto protein targets that smaller molecules cannot reach. Understanding the precise three-dimensional shape these rings adopt, and how that shape shifts over time, is a prerequisite for designing molecules that work reliably in a biological setting.
The study focused specifically on what happens when aromatic heterocycles, which are flat ring-shaped chemical groups containing atoms like nitrogen or oxygen alongside carbon, are built into the macrocyclic backbone. The researchers found that these aromatic motifs act almost like molecular switches, biasing the larger ring toward certain shapes and away from others. Capturing that behavior required tools that go well beyond what standard laboratory analysis can offer.
The challenge of flexible ring structures
A molecule that holds a single fixed shape is relatively straightforward to study. Macrocyclic peptides are rarely that simple. In solution, they can interconvert between multiple distinct backbone conformations, meaning the same molecule can look structurally different from one moment to the next. Chemists call this a conformational ensemble, and it matters because the shape a molecule presents to a target protein determines whether the two will interact.
Traditional nuclear magnetic resonance, or NMR, spectroscopy has long been the tool of choice for studying molecular structure in solution. It works by detecting magnetic signals from atomic nuclei and using those signals to infer distances and angles within a molecule. However, when a molecule is rapidly flipping between shapes, conventional NMR measurements tend to report an average that can obscure what is actually happening. The researchers note that this averaging problem has made accurate structural characterization of macrocycles genuinely difficult.
Residual dipolar couplings as a sharper lens
To get around the averaging problem, the research team added a second type of NMR measurement called residual dipolar coupling, or RDC. Where conventional NMR relies on a phenomenon called the nuclear Overhauser effect to estimate distances between nearby atoms, RDCs capture directional information about how pairs of atomic nuclei are oriented relative to an external magnetic field.
The distinction matters because RDCs are sensitive to the full population of shapes a molecule visits, not just the most common one. By combining both types of NMR data, the researchers assembled a much more complete picture of each macrocycle's conformational behavior. They describe the approach as integrating complementary isotropic and anisotropic NMR observables, meaning measurements that either do or do not depend on molecular orientation.
The team also ran the experiments in different solvent systems. Because a solvent can stabilize or destabilize particular molecular shapes through its own chemical interactions, testing multiple solvents allowed the researchers to see which conformational preferences were intrinsic to the macrocycle itself and which were environmentally driven.
Computational modeling alongside the experiments
NMR measurements alone could not fully decode the conformational ensembles. The researchers paired the experimental data with two types of computational work. First, they used enhanced sampling simulations, which are specialized computer algorithms designed to explore the full range of shapes a molecule can adopt without getting stuck in any single configuration. Second, they applied density functional theory, or DFT, calculations to predict the energy of each possible conformation with quantum-chemical accuracy.
The interplay between experiment and computation was essential. The simulations generated candidate structures, DFT refined their energies, and the NMR data served as a filter, confirming which structures were actually present in solution and in what proportions. The researchers found that, for each ring system studied, up to three distinct backbone conformers had to be included to fully account for the observed NMR signals. No single structure was sufficient.
Aryl and heterobiaryl motifs as conformational governors
One of the study's central findings is that the aromatic heterocycle units embedded in the macrocyclic rings are not passive structural passengers. They actively govern which conformations the ring can access and how populated each one is. The paper describes these aryl and heterobiaryl motifs as structural elements that modulate the populations of multiple thermodynamically accessible states.
The mechanism behind this influence involves two phenomena. First, the flat, rigid nature of aromatic rings imposes geometric constraints on the adjacent parts of the backbone, limiting the angles those segments can adopt. Second, each distinct conformation is stabilized or destabilized by a different network of intramolecular hydrogen bonds, meaning the molecule's own internal chemistry changes depending on its shape. The researchers characterize some of the resulting backbone geometries as unusual, suggesting these molecules can adopt arrangements that standard peptide chemistry does not predict.
Fast and slow exchange regimes
An additional technical contribution of the work is its ability to handle two fundamentally different situations. In some cases, a macrocycle switches between conformations faster than the NMR instrument can time-resolve the individual states. This is called fast exchange, and it tends to produce averaged signals. In other cases, the interconversion is slow enough that each conformer produces its own distinct NMR signature. This is slow exchange.
Many analytical methods are designed to work well in one regime but not the other. The integrative framework developed in this study, combining RDC measurements with enhanced sampling and DFT, proved capable of extracting meaningful structural information in both situations. The researchers present this as a general strategy that should be applicable to a broad range of macrocyclic scaffolds beyond those specifically examined in the paper.
Implications for rational peptide design
The researchers position their methodology as a tool for informing synthetic decisions. If a drug-design team wants a macrocyclic peptide to adopt a particular shape when it encounters its target, they first need to know what shapes the molecule naturally prefers and what structural features push it toward or away from any given conformation. The framework described in the paper provides a route to that knowledge.
The study also connects to a broader conversation in the field about so-called undruggable targets. Many proteins that play important roles in disease have surfaces that are too large, too flat, or too featureless for conventional small molecules to grip. Macrocyclic peptides are large enough to cover more of that surface while remaining small enough to potentially enter cells. The literature suggests that better conformational control, guided by tools like those described in this research, could accelerate the development of macrocyclic candidates aimed at these challenging targets.
Early-stage structural research of this kind does not produce treatments directly. What it produces is a more precise map of chemical space, one that researchers can use to decide which structural modifications are worth making and which are likely to be dead ends. The authors describe the goal as paving a way for structure-based rational design of next-generation peptide therapeutics, a phrase that underscores the foundational rather than clinical nature of the work.
