Single-Molecule FRET: Watching a Molecule Switch Shape

Single-molecule Forster resonance energy transfer (smFRET) lets biophysicists watch one protein or nucleic-acid molecule change shape in real time by tracking the color of the photons it emits. This simulator renders a donor/acceptor dye pair tethered to a molecule that hops stochastically between an open (low-FRET) and a closed (high-FRET) conformation, computes the true distance-dependent FRET efficiency E = 1/(1+(r/R0)^6) every frame, and simulates individual photon detections split between the donor and acceptor channels according to that efficiency — reproducing both the physics and the photon shot noise of a real single-molecule fluorescence experiment. Adjust the Forster radius, conformational switching rate, state population bias and detector photon rate to see how each shapes the live time trace and the underlying molecular motion.