Optically Modulated and Optically Activated Delayed Fluorescent Proteins through Dark State Engineering

Impact of triplet state population on GFP-type fluorescence and photobleaching

BACKGROUND INFORMATION

Based on recently published parameters (Rane et al. 2023, JPCB 127, 5046-5054) for (rs)EGFP triplet state formation and decay rates and yields, we consider the power density dependence of triplet state population dynamics and its consequences for the application of green fluorescent proteins in biological single molecule fluorescence microscopy.

RESULTS

We find that under certain conditions, the photon budget of GFP type fluorescent proteins can be linearly dependent on power density and we propose a possible explanation for such a non-Hirschfeld photobleaching behavior. Moreover, illumination with millisecond pulses at sub-kHz rates is shown to improve photostability.

CONCLUSIONS

We stipulate that a judicious choice of excitation wavelength should take into account the triplet state absorption spectrum along with the singlet state absorption spectrum. Formulas are given for the estimation of the effects of such choice as function of the experimental parameters.

SIGNIFICANCE

The linear photobleaching model as proposed by Hirschfeld 50 years ago with power-independent photon budget is not generally applicable to fluorescent proteins with millisecond-lived triplet states.

Near-infrared co-illumination of fluorescent proteins reduces photobleaching and phototoxicity

Here we present a method to reduce the photobleaching of fluorescent proteins and the associated phototoxicity. It exploits a photophysical process known as reverse intersystem crossing, which we induce by near-infrared co-illumination during fluorophore excitation. This dual illumination method reduces photobleaching effects 1.5-9.2-fold, can be easily implemented on commercial microscopes and is effective in eukaryotic and prokaryotic cells with a wide range of fluorescent proteins.

Light-Induced Forward and Reverse Intersystem Crossing in Green Fluorescent Proteins at Cryogenic Temperatures

Combining fluorescence and phosphorescence kinetics, we characterize forward and reverse intersystem crossing (FISC and RISC, respectively) between the singlet and triplet manifolds S ↔ T in photoswitchable (rsEGFP2) and non-photoswitchable (EGFP) green fluorescent proteins upon continuous 488 nm laser excitation at cryogenic temperatures (CTs). Both proteins behave very similarly, with T₁ absorption spectra showing a visible peak at 490 nm (10 mM^-1 cm^-1) and a vibrational progression in the near-infrared (720 to 905 nm). The dark lifetime of T₁ is 21-24 ms at 100 K and very weakly temperature-dependent up to 180 K. Above 180 K, T₁ lifetimes reduce rapidly to few milliseconds as found at room temperature (RT). FISC and RISC quantum yields are 0.3 and 0.1%, respectively, for both proteins. The light-induced RISC channel becomes faster than the dark reversal at power densities as low as 20 W cm^-2. We discuss implications for fluorescence (super resolution-) microscopy at CT and RT.

Sequential Two-Photon Delayed Fluorescence Anisotropy for Macromolecular Size Determination

Time-resolved fluorescence anisotropy (FA) uses the fluorophore depolarization rate to report on rotational diffusion, conformation changes, and intermolecular interactions in solution. Although FA is a rapid, sensitive, and nondestructive tool for biomolecular interaction studies, the short (∼ns) fluorescence lifetime of typical dyes largely prevents the application of FA on larger macromolecular species and complexes. By using triplet shelving and recovery of optical excitation, we introduce optically activated delayed fluorescence anisotropy (OADFA) measurements using sequential two-photon excitation, effectively stretching fluorescence anisotropy measurement times from the nanosecond scale to hundreds of microseconds. We demonstrate this scheme for measuring slow depolarization processes of large macromolecular complexes, derive a quantitative rate model, and perform Monte Carlo simulations to describe the depolarization process of OADFA at the molecular level. This setup has great potential to enable future biomacromolecular and colloidal studies.

Characterizing Dark State Kinetics and Single Molecule Fluorescence of FusionRed and FusionRed-MQ at Low Irradiances

The presence of dark states causes fluorescence intermittency of single molecules due to transitions between “on” and “off” states. Genetically encodable markers such as fluorescent proteins (FPs) exhibit dark states...