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Conformational dynamics of biomolecules are of fundamental importance for their function. Single-molecule studies of Förster Resonance Energy Transfer (smFRET) between a tethered donor and acceptor dye pair are a powerful tool to investigate the structure and dynamics of labeled molecules. However, capturing and quantifying conformational dynamics in intensity-based smFRET experiments remains challenging when the dynamics occur on the sub-millisecond timescale. The method of multiparameter fluorescence detection addresses this challenge by simultaneously registering fluorescence intensities and lifetimes of the donor and acceptor. Together, two FRET observables, the donor fluorescence lifetime τ D and the intensity-based FRET efficiency E, inform on the width of the FRET efficiency distribution as a characteristic fingerprint for conformational dynamics. We present a general framework for analyzing dynamics that relates average fluorescence lifetimes and intensities in two-dimensional burst frequency histograms. We present parametric relations of these observables for interpreting the location of FRET populations in E–τ D diagrams, called FRET-lines. To facilitate the analysis of complex exchange equilibria, FRET-lines serve as reference curves for a graphical interpretation of experimental data to (i) identify conformational states, (ii) resolve their dynamic connectivity, (iii) compare different kinetic models, and (iv) infer polymer properties of unfolded or intrinsically disordered proteins. For a simplified graphical analysis of complex kinetic networks, we derive a moment-based representation of the experimental data that decouples the motion of the fluorescence labels from the conformational dynamics of the biomolecule. Importantly, FRET-lines facilitate exploring complex dynamic models via easily computed experimental observables. We provide extensive computational tools to facilitate applying FRET-lines.
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Linear Behavior of the Phase Lifetime in Frequency-Domain Fluorescence Lifetime Imaging of FRET Constructs
We utilize a cost-effective frequency-domain fluorescence lifetime imaging microscope to measure the phase lifetime of mTFP1 in mTFP1-mVenus fluorescence resonance energy transfer (FRET) constructs relevant to the VinTS molecular tension probe. Our data were collected at 15 modulation frequencies ω/2π selected between 14 and 70 MHz. The lifetime of mTFP1 was τ D = 3.11 ± 0.02 ns in the absence of acceptor. For modulation frequencies, ω, such that (ω · τ D ) < 1.1, the phase lifetime of mTFP1in the presence of acceptor (mVenus), τ ϕ D A , was directly related to the amplitude-weighted lifetime τ a v e D A inferred from the known FRET efficiency ( E FRET true ) of the constructs. A linear fit to a plot of ( ω · τ ϕ D A ) v s . ( ω · τ a v e D A ) yielded a slope of 0.79 ± 0.05 and intercept of 0.095 ± 0.029 (R 2 = 0.952). Thus, our results suggest that a linear relationship exists between the apparent E FRET app based on the measured phase lifetime and E FRET true for frequencies such that (ω · τ D ) < 1.1. We had previously reported a similar relationship between E FRET app and E FRET true at 42 MHz. Our current results provide additional evidence in support of this observation, but further investigation is still required to fully characterize these results. A direct relationship between τ ϕ D A and τ a v e D A has the potential to simplify significantly data acquisition and interpretation in fluorescence lifetime measurements of FRET constructs.
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- Award ID(s):
- 1825433
- PAR ID:
- 10285501
- Date Published:
- Journal Name:
- Frontiers in Physics
- Volume:
- 9
- ISSN:
- 2296-424X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Fӧrster (or fluorescence) resonance energy transfer (FRET) is a quantifiable energy transfer in which a donor fluorophore nonradiatively transfers its excitation energy to an acceptor fluorophore. A change in FRET efficiency indicates a change of proximity and environment of these fluorophores, which enables the study of intermolecular interactions. Measurement of FRET efficiency using the sensitized emission method requires a donor–acceptor calibrated system. One of these calibration factors named theGfactor, which depends on instrument parameters related to the donor and acceptor measurement channels and on the fluorophores quantum efficiencies, can be determined in several different ways and allows for conversion of the raw donor and acceptor emission signals to FRET efficiency. However, the calculated value of the G factor from experimental data can fluctuate significantly depending on the chosen experimental method and the size of the sample. In this technical note, we extend the results of Gates et al. (Cytometry Part A 95A (2018) 201–213) by refining the calibration method used for calibration of FRET from image pixel data. Instead of using the pixel histograms of two constructs with high and low FRET efficiency to determine theGfactor, we use pixel histogram data from one construct of known efficiency. We validate this method by determining theGfactor with the same constructs developed and used by Gates et al. and comparing the results from the two approaches. While the two approaches are equivalent theoretically, we demonstrate that the use of a single construct with known efficiency provides a more precise experimental measurement of theGfactor that can be attained by collecting a smaller number of images. © 2020 International Society for Advancement of Cytometrymore » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fphy.2021.648016
