smFRET

What is single-molecule FRET?

What is smFRET?

Single-molecule Förster Resonance Energy Transfer (smFRET) is a powerful biophysical technique that enables researchers to measure nanoscale distances (3-10 nm), study conformational dynamics, and biomolecular interactions with single-molecule sensitivity. Unlike traditional ensemble methods that average signals over many molecules, smFRET probes individual molecules, revealing rare events, transient intermediates, and distinct subpopulations that are typically obscured in bulk measurements. As illustrated in the figure below, the average FRET efficiency determined from a sample comprising a mixture of two subpopulations (a low- and a high-FRET state) yields a mid-FRET value that does not actually exist in the sample.

How does smFRET work?

Förster resonance energy transfer (FRET) is a non-radiative energy transfer process in which an excited donor fluorophore transfers energy to a nearby acceptor fluorophore. The efficiency of this transfer depends on the distance between the donor and the acceptor, and their properties, and typically occurs over a distance of ~3–10 nm. In FRET experiments, excitation of the donor results in acceptor emission in a highly distance-dependent manner. A requirement of the technique is therefore the labelling of the biomolecule(s) under study with both a donor and an acceptor. The efficiency of the energy transfer between the donor and acceptor is called the FRET efficiency (E). FRET efficiency serves as an indicator of the distance between fluorophores: a smaller distance between the donor and acceptor leads to greater energy transfer and higher FRET efficiencies, while lower FRET efficiencies reflect a greater distance.

Discover everything you need to understand smFRET in detail and use it to advance your research, including the experimental workflows and analytical strategies that underpin it, in our practical handbook. Download it here.

What are the benefits of smFRET?

Single-molecule resolution:
Provides insights into the behaviours of individual molecules, avoiding ensemble averaging and revealing transient events
Real-time monitoring:
Captures dynamic processes as they occur
High sensitivity:
Detects subtle conformational changes and interactions
Versatility:
Applicable to a wide range of fluorescently labelled biomolecules, including DNA, RNA, proteins and peptides, lipids and membrane components, viruses and virus-like particles

What applications can smFRET be used for?

smFRET has become a powerful tool in molecular biology, biophysics, and structural biology due to its ability to resolve dynamic molecular processes at the nanometer scale. The ability to resolve molecular heterogeneity, dynamic transitions, and transient intermediates one molecule at a time is no longer a niche advantage – it is rapidly becoming a standard for mechanistic insight. Some of its key applications include:

Protein Folding and Conformational Dynamics

smFRET tracks the folding and unfolding of individual protein molecules. It distinguishes intermediate states, folding pathways, and misfolded species that are invisible in ensemble studies. Some recent examples include:

Doxsey and Shen used smFRET to resolve the conformational changes of a highly dynamic heterodimeric protein, Rag GTPase that binds both nucleotides and the serine/threonine protein kinase complex mTORC1, observing how the global structure of Rag GTPase changes and drives downstream signalling in response to cellular amino acid sensing¹.
Howard et al. used smFRET to reveal two hidden, salt-influenced conformational states on the bacterial helicase Rep in addition to the two open and closed structures that X-ray crystallography had previously identified².

Enzyme Mechanisms and Catalysis

By attaching a FRET dye pair near active sites, smFRET can reveal conformational cycles and dynamics that enzymes undergo during catalysis.

Sanabria et al. characterised three transient conformational states of the T4 Lysozyme by combining smFRET with other biochemical and biophysical techniques; they captured 33 FRET-derived distances, uncovering a minor state that had not been observed in crystal structures and is thought to be involved in the product release step during catalysis³.
Stella et al. used smFRET to capture the kinetic and thermodynamic activity of Cas12a, characterising the conformational activation that induces hydrolysis of phosphodiester bonds. This complemented cryo-EM structures of Cas12a intermediates during the cleavage reaction⁴.

Nucleic Acid Structure and Folding 

smFRET can resolve structural transitions in DNA and RNA such as hairpin opening, pseudoknot formation, or hybridisation events.

Betts et al. resolved a previously unknown RNA virus riboswitch mechanism using smFRET. Combined with X-ray crystallography and small-angle X-ray scattering (SAXS), they uncovered the formation of a protein-dependent RNA pseudoknot that acts as a stimulatory element, regulating programmed –1 ribosomal frameshifting (PRF) in Theiler’s murine encephalitis virus⁵.

Biomolecular Complex Assembly and Binding

Observing multi-subunit assembly, binding kinetics, and induced-fit dynamics at the single-molecule level.

Chadda et al. performed smFRET on ssDNA molecules of varying lengths, investigating the impact of binding to replication protein A (RPA). Existing models using crystal structures or bulk methods have so far failed to reach consensus on whether binding causes DNA wrapping or stretching; smFRET data indicate that ssDNA wraps around RPA⁶ .

Membrane Protein Dynamics and Signalling

smFRET enables functional studies of membrane-embedded proteins by tracking their conformational states directly within lipid bilayers or nanodiscs, for example, upon ligand binding.

Ren et al. used a range of techniques to investigate the molecular mechanisms of the membrane-embedded Enzyme IIC that transports sugars into bacterial cells and phosphorylates them. In this work, they use smFRET to validate conformational changes that were proposed by crystal structures and all-atom molecule dynamics⁷.

Studying Heterogeneity in Ensembles of Molecules

smFRET reveals molecule-to-molecule variation that ensemble techniques average out. This is particularly useful for identifying intermediate structures or heterogenous populations where clear definition of stoichiometries matter, such as when designing and characterising DNA nanoswitches.

D’Rozario et al. combined smFRET with circular dichroism spectroscopy to determine the pH responsiveness of a DNA triplex nanoswitch prior to immobilisation onto a 2-dimensional DNA monolayer and characterisation of its behaviour on a surface⁸.

Method Development and Tool Validation

Researchers also use smFRET to benchmark new fluorescent labels, validate biosensors, or refine analysis algorithms.

smFRET is applicable wherever molecular distance, motion, or heterogeneity matters – especially when real-time, single-molecule sensitivity is needed. Its versatility makes it invaluable across fields ranging from basic biochemistry to therapeutic drug development.

What does a typical smFRET experiment involve?

A typical smFRET experiment involves:

Fluorophore labelling:
Attaching donor and acceptor fluorophores to specific sites on the molecule of interest
Functional checks: Confirming that the fluorophore labelling hasn’t disrupted biomolecular structure or function
Excitation and detection:
Using a laser to excite the donor fluorophore, detecting emissions from both donor and acceptor
Data analysis: Calculating FRET efficiency to infer distance changes and molecular dynamics

Do I need a dark room to run smFRET experiments myself?

Most single-molecule fluorescence spectrometers require a dedicated dark room and optical table. However, the EI-FLEX is designed to be used on a benchtop in ambient light conditions.

Which dye pairs are suitable for smFRET experiments?

Fluorophore Pairs

Donors
TAMRA 
Cy3 
Cy3B 
Atto 532 
Atto 550 
Alexa Fluor 532 
Alexa Fluor 546 
Alexa Fluor 555

Acceptors
Cy5
Atto 647
Atto 647N
Alexa Fluor 647

What kinds of experiments complement smFRET?

smFRET is a versatile technique, but it becomes even more powerful when used in combination with complementary experimental methods. These companion approaches can validate, extend, or contextualise smFRET observations, providing deeper insight into molecular structures and dynamics.

1. Structural biology methods like Cryo-Electron Microscopy (Cryo-EM) and X-ray Crystallography

Cryo-EM and X-ray crystallography capture high-resolution 3D structural models of biomolecules in frozen-hydrated or crystalline states, respectively, making them ideal for resolving large and stable complexes with atomic detail.

How smFRET complements it:

When used together, Cryo-EM or X-ray crystallography delivers the static structural framework, while smFRET reveals the dynamic processes that structures undergo, offering a more holistic understanding of molecular mechanisms.

2. Fluorescence Correlation Spectroscopy (FCS)

Provides diffusion, size and concentration measurements with sub-millisecond resolution, characterising biomolecular complex formation.