Under our experimental conditions, the confocal volume is most often devoid of fluorescent molecules, with an average occupancy of less than 0.01 molecules. Nevertheless, individual fluorescently labeled molecules will inevitably diffuse through the confocal volume, where they will be excited by the two alternating lasers, resulting in a transient burst of fluorescent photons (
Fig. 2B). The detection records of the photons emitted from these freely diffusing fluorescently labeled RNA molecules were analyzed using
Mathematica 12.0 (Wolfram Alpha) in conjunction with Fretica, a C++ based MATHLINK module for the analysis of time-correlated single-photon-counting single-molecule FRET data (40 ). The data analysis workflow employed during this research is as follows: first, time-gating was used to determine which laser (either 515 or 642 nm) was active for every detected photon in each of the four streams. Then, photons were assigned to 500
μs time bins based on their absolute arrival time (
Fig. 2C). Time bins with a total photon count rate (
Totn =
1n515 +
1n642 +
2n515 +
2n642 +
3n515 +
3n642 +
4n515 +
4n642) of less than 20 photons per bin were used to calculate the average background photon count rate for each of the four streams during either 515 or 642 nm excitation. Then, corrected photon count rates (
N) were determined for all time bins by accounting for background, spectral cross talk, direct excitation of the acceptor, and nonidentical excitation and detection efficiencies of the donor and acceptor fluorophores. Those time bins with a corrected total photon count rate (
TotN =
1N515 +
1N642 +
2N515 +
2N642 +
3N515 +
3N642 +
4N515 +
4N642) of more than 20 photons per bin were considered bursts of fluorescence arising from single molecules diffusing through the confocal volume (41 (
link)). Corrected photon count rates associated with the acceptor and donor fluorophores (
AccN =
1N +
3N and
DonN =
2N +
4N, where
TotN =AccN +
DonN) during 515 and 642 nm excitation were used to calculate values for both the FRET efficiency (
E) and fluorescence stoichiometry (
S) using
Eq. 1 and
Eq. 2, respectively (
Fig. 2D).
The fluorescence stoichiometry (
S) of a burst arising from a molecule containing only donor or acceptor fluorophores will yield values near
S = 1 and
S = 0, respectively. Therefore, values of fluorescence stoichiometry are used to restrict our analysis and interpretation of FRET efficiencies to only those bursts arising from molecules containing active donor and acceptor fluorophores (i.e., 0.25 <
S < 0.75). This allows us to effectively filter out unwanted contributions from any potential donor-only or acceptor-only molecules that, for example, may not have been removed during the HPLC purification of the RNA constructs.
The FRET efficiency (
E) values from bursts with
TotN > 50 were then compiled into histograms (
Fig. 2D), in which the widths of the resulting distributions were largely limited by shot noise (
Fig. S1). Histograms were then fitted using Gaussian distributions to determine the mean FRET efficiency, ⟨
E⟩, and fractional abundance,
Θ, of the folded and unfolded subpopulations. Based on several repeated measurements under identical conditions, typical experimental uncertainties associated with ⟨
E⟩ and
Θ are ±0.02 and ±0.03, respectively. The fractional abundance of each subpopulation was used to calculate the equilibrium constant for folding (
Kfold =
Θf/
Θu) and thus the standard state Gibbs free energy difference (Δ
G°
fold = −
RT ln
Kfold, where
R is the gas constant) between the two subpopulations at
T = 294.2 K. For the ease of data interpretation, all Δ
G°
fold values in apolar solvent conditions were referenced to aqueous conditions, i.e., ΔΔ
G°
fold = Δ
G°
fold (mixture) − Δ
G°
fold (H
2O).
Gunawardhana S.M, & Holmstrom E.D. (2021). Apolar chemical environments compact unfolded RNAs and can promote folding. Biophysical Reports, 1(1), 100004.