In our first article celebrating the 30th anniversary of smFRET (30 Years of smFRET: A Chronological Journey), we traced how the ‘spectroscopic ruler’ from Stryer & Haugland in 19671 and the landmark paper by Ha et al. in 19962 evolved from a specialised physics laboratory curiosity into robust benchtop technology. We explored how overcoming technical bottlenecks, from bespoke optical setups to standardised open-data formatting, set the stage for a dynamic revolution across the life sciences.
For researchers spanning academia and biopharma, the critical question now turns from feasibility to functionality: How do we deploy this tool to solve complex biological questions?
The answer lies in a shift away from static endpoints. 3D coordinates from Cryo-EM or generative AI models like AlphaFold-3 provide invaluable blueprints, but they remain fundamentally static. If we want to truly understand mechanism and function, we need to bring these structures to life.
To understand allosteric signalling, enzymatic catalysis, and disease progression, the industry must start exploring dynamic molecular characterisation – the ability to analyse individual molecules in solution and map their precise structural conformations, binding kinetics and dynamic heterogeneity in real time.
In this article, we explore how this single-molecule biophysics toolkit is actively transforming our understanding of core biological pathways and how these mechanistic insights can be translated into next-generation therapeutics.
Transcription, splicing and translation intermediates
The processing of genetic information requires macromolecular machines to coordinate complex mechanical steps. Because these assemblies transition through heterogeneous, multi-state pathways, ensemble biochemistry often presents a blurred average of competing mechanisms. Single-molecule FRET has served as a definitive biophysical tool to deconstruct these processes step by step.
Transcription initiation and ‘DNA scrunching’
smFRET famously settled the textbook debate surrounding bacterial transcription initiation. By monitoring real-time distance changes between the RNA polymerase (RNAP) and DNA promoter elements, smFRET provided evidence for the ‘DNA scrunching’ model3. Here, RNAP remains stationary and pulls downstream DNA into itself. Further smFRET characterisation mapped this entire lifecycle, defining the precise opening and closing kinetics of the mobile bacterial RNAP ‘clamp’ across each step of initiation, promoter escape, and elongation4.
The ‘DNA scrunching’ mechanism for RNAP transcription, as demonstrated by Kapanidis et al.
Splicing
Early single-molecule investigations using yeast Ubc4 pre-mRNA demonstrated that splicing does not follow a rigid, deterministic path; instead, individual pre-mRNAs undergo a multitude of highly reversible, time- and ATP-dependent conformational transitions5.
More insights came when researchers transitioned from highly purified systems into complex physiological matrices. By tracking single-molecule FRET trajectories directly inside unpurified, whole-cell yeast extracts, researchers proved that spliceosome activation and structural rearrangements actually precede the stable physical approach of the 5′ splice site and the branch site6. This demonstrated that smFRET could successfully deliver high-fidelity dynamic molecular characterisation within chaotic, native environments.
If you’d like to know more, see our article on performing single-molecule biophysics in cell lysates and serum here.
Translation intermediates
Once processed, mRNA must be accurately translated by the ribosome, a process dictated by the coordinated assembly and disassembly of initiation factors.
Using solution-phase smFRET, biophysicists captured the millisecond-scale kinetics of universally conserved eukaryotic initiation factors eIF1A and eIF5B7.The real-time trajectories revealed how the synchronised association and departure of these factors actively reorient initiator tRNA within the ribosomal complex, creating a structural gateway that allows proper ribosomal subunit joining. By capturing these fleeting transitions, smFRET characterised a highly transient mechanistic checkpoint that would have been challenging to see using other methods.
Mechanisms for maintaining genome integrity
Maintaining genome fidelity requires cells to execute rapid, highly coordinated structural transactions on DNA. When a replication fork stalls, a double-strand break occurs, or chromatin needs to be restructured, multi-protein complexes must assemble and manipulate the double helix with sub-nanometre precision. Because these interactions are highly dynamic and often transient, smFRET has become indispensable for capturing these vital structural intermediates.
The rate-limiting step of DNA synthesis
DNA polymerases rely on massive domain reorientations to verify base-pairing accuracy before adding a nucleotide. Historically, the field struggled to pinpoint the exact rate-limiting step of polymerisation because the fastest chemical transitions are obscured in bulk assays.
By placing FRET pairs on the moving ‘fingers’ domain of a replication polymerase, researchers tracked these conformational changes across the entire DNA synthesis reaction in real time8. The single-molecule trajectories revealed a surprise: the true rate-limiting bottleneck of DNA synthesis is not the chemical step of nucleotide addition, nor is it the initial binding event. Instead, the enzyme spends the majority of its active cycle in a post-chemistry, fingers-closed conformation. Capturing this elusive structural state in solution provided a brand-new mechanical framework for understanding polymerase fidelity and drug inhibition.
smFRET captured the ‘fingers open’ and ‘fingers closed’ conformations of RNAP, giving clarity to the DNA synthesis mechanism. Figure taken from Evans et al., 2022.8
Double-strand break repair
When a chromosome suffers a double-strand break, the non-homologous end-joining (NHEJ) machinery must hold two broken, freely diffusing DNA ends in close proximity and align them for ligation.
To understand how this alignment is coordinated, researchers monitored the real-time assembly of NHEJ filament machinery via smFRET9. The single-molecule traces showed that the protein filaments forming at either end of the broken DNA do not form a rigid, static bridge. Instead, they interact via highly flexible, constantly shifting configurations. This structural fluidity allows the two broken ends to dynamically sample multiple orientations, ensuring they locate the optimal spatial configuration for precise end-to-end positioning and successful ligation.
Histone exchange
The spatial layout of the genome is tightly regulated by chromatin remodelling complexes, which manually swap histone variants to activate or repress specific genes. The SWR1 remodelling enzyme, for example, replaces a canonical H2A-H2B dimer with a histone H2A.Z-H2B dimer. How an enzyme coordinates the mechanical unwrapping of DNA from the nucleosome core while simultaneously managing histone swapping was a long-standing biophysical mystery.
Using three-colour smFRET, biophysicists successfully tracked DNA unwrapping and histone displacement simultaneously10. The real-time three-coordinate data demonstrated that SWR1 directly coordinates macro-scale DNA unwrapping. By applying tension to the DNA strand, SWR1 creates a transient structural window that allows for the placement of H2A.Z on chromatin. This study highlighted how multi-colour dynamic molecular characterisation can demystify complex, multi-component epigenetic remodelling events.
Macromolecular energetics: Folding landscapes and transmembrane signalling
Molecules operate on a highly rugged free-energy landscape, continually buffeted by thermal noise. By monitoring individual molecules as they navigate these landscapes, smFRET allows researchers to map thermodynamic boundaries, quantify internal friction, and track allosteric signalling networks in native-like environments.
Mapping the rugged free-energy surface of folding
Understanding how a linear chain of amino acids or nucleotides folds into a functional, three-dimensional structure is one of the foundational questions across the life sciences. Often, only a few conformations are captured by common techniques, given that the transitions between states happen too fast and the intermediate populations are too sparse to measure.
Single-molecule FRET is hugely valuable for addressing this problem. For example, by studying individual hairpin ribozymes, researchers directly correlated real-time structural fluctuations with enzymatic function, mapping the precise energetic steps required for catalytic activation11.
Concurrently, pioneering protein-folding work on small cold-shock proteins demonstrated that smFRET could expose the equilibrium structural collapse of a polypeptide chain as it begins to fold12. By tracking these rapid solution-phase events, biophysicists calculated the exact physical limits of polypeptide reconfiguration times, providing the raw thermodynamic data needed to validate modern computational folding models.
Quantifying internal friction in intrinsically disordered proteins
Not all functional biomolecules possess a fixed, folded structure. Intrinsically disordered proteins (IDPs) and unfolded states remain fluid, playing crucial roles in cellular signalling and neurodegenerative disease. However, describing the physics of a completely unstructured protein requires measuring how fast its polymer chain can move through space – a property governed by internal friction.
By combining smFRET with photoinduced electron transfer (PET), polymer theory, and molecular simulations, researchers mapped the internal friction loops of unfolded protein L. The single-molecule data provided an integrated view of how the local amino acid sequence resists rapid conformational changes13. Quantifying this internal friction is crucial, as it dictates how fast an unstructured protein can segmentally reconfigure, collapse, or interact with therapeutic targets.
Deciphering transmembrane allostery
Some of the most challenging targets for dynamic molecular characterisation are membrane proteins like G-protein coupled receptors (GPCRs) and ion channels. Vafabakhsh et al. used smFRET to probe the activation mechanism of full-length mammalian group II metabotropic glutamate receptors14. The real-time trajectories revealed a clear allosteric mechanism: agonist binding drives the rapid closure of the extracellular ligand-binding domains, which induces a sub-nanometre reorientation of the dimer interface. This study linked crystal structures, ligand binding and closure of the ligand binding domain.
Why dynamic molecular characterisation is the blueprint for modern medicine
Ultimately, the boundary between fundamental biophysics and clinical translation is entirely artificial; every modern therapeutic is an attempt to alter a biological mechanism. For decades, the pharmaceutical pipeline was constrained by static models of drug design, viewing targets as rigid locks to be fit with molecular keys.
However, the rise of modern medicine’s most sophisticated interventions, such as allosteric modulators that trap hidden conformations, proximity-induced degraders like PROTACs and molecular glues that must bring three components together to form a complex, and therapeutics targeting the volatile free-energy landscapes of neurodegenerative aggregates, proves that affinity alone is no longer enough.
To engineer molecules that can predictably intervene in complex disease pathways, the life sciences require dynamic molecular characterisation. By mapping the time-resolved, dynamic structural conformations of individual molecules in solution, single-molecule FRET strips away the ensemble averages that obscure rare, short-lived intermediates that are often vital for function.
Understanding exactly how a molecular machine breathes, moves, and functions under physiological conditions provides the definitive mechanical blueprint needed to translate basic cellular mechanisms into targeted, sophisticated therapeutics.
The evolution of single-molecule FRET over the last thirty years mirrors the broader arc of the life sciences: moving from isolated observations of fundamental physics to high-throughput, actionable data that is valuable across basic research and drug discovery pipelines.
By bringing smFRET out of specialised darkrooms and into an automated, benchtop platform like the EI-FLEX Pro, the barrier to entry has been remarkably reduced, opening up the field to thirty more years of discoveries and beyond.
Welcome to the era of dynamic molecular characterisation.
One thing’s for certain – it’s going to be exciting.
References
- Stryer, L. & Haugland, R. P. Energy transfer: a spectroscopic ruler. Proceedings of the National Academy of Sciences 58, 719–726 (1967).
- Ha, T. et al. Probing the interaction between two single molecules: fluorescence resonance energy transfer between a single donor and a single acceptor. Proceedings of the National Academy of Sciences 93, 6264–6268 (1996).
- Kapanidis, A. N. et al. Initial Transcription by RNA Polymerase Proceeds Through a DNA-Scrunching Mechanism. Science (1979). 314, 1144–1147 (2006).
- Chakraborty, A. et al. Opening and Closing of the Bacterial RNA Polymerase Clamp. Science (1979). 337, 591–595 (2012).
- Abelson, J. et al. Conformational dynamics of single pre-mRNA molecules during in vitro splicing. Nat. Struct. Mol. Biol. 17, 504–512 (2010).
- Crawford, D. J., Hoskins, A. A., Friedman, L. J., Gelles, J. & Moore, M. J. Single-molecule colocalization FRET evidence that spliceosome activation precedes stable approach of 5′ splice site and branch site. Proceedings of the National Academy of Sciences 110, 6783–6788 (2013).
- Lapointe, C. P. et al. eIF5B and eIF1A reorient initiator tRNA to allow ribosomal subunit joining. Nature 607, 185–190 (2022).
- Evans, G. W., Craggs, T. & Kapanidis, A. N. The Rate-limiting Step of DNA Synthesis by DNA Polymerase Occurs in the Fingers-closed Conformation. J. Mol. Biol. 434, 167410 (2022).
- Reid, D. A. et al. Organization and dynamics of the nonhomologous end-joining machinery during DNA double-strand break repair. Proceedings of the National Academy of Sciences 112, E2575–E2584 (2015).
- Poyton, M. F. et al. Coordinated DNA and histone dynamics drive accurate histone H2A.Z exchange. Sci. Adv. 8, eabj5509 (2026).
- Zhuang, X. et al. Correlating Structural Dynamics and Function in Single Ribozyme Molecules. Science (1979). 296, 1473–1476 (2002).
- Schuler, B., Lipman, E. A. & Eaton, W. A. Probing the free-energy surface for protein folding with single-molecule fluorescence spectroscopy. Nature 419, 743–747 (2002).
- Soranno, A. et al. Integrated view of internal friction in unfolded proteins from single-molecule FRET, contact quenching, theory, and simulations. Proceedings of the National Academy of Sciences 114, E1833–E1839 (2017).
- Vafabakhsh, R., Levitz, J. & Isacoff, E. Y. Conformational dynamics of a class C G-protein-coupled receptor. Nature 524, 497–501 (2015).
