Copyright@ László Bene | Biomed J Sci & Tech Res | BJSTR. MS.ID.004370. 20078 Review Article ISSN: 2574 -1241 Rotationally Gated Fluorescence: Common Theme Behind Molecular Rotors, Photoactivation and Förster Resonance Energy Transfer (FRET) László Bene* and László Damjanovich Department of Surgery, Faculty of Medicine, University of Debrecen, Hungary *Corresponding author: László Bene, Department of Biophysics and Cell Biology, University of Debrecen, Debrecen, H-4032 Egyetem tér 1; Mail: H-4002 Debrecen P.O.Box 400, Hungary DOI: 10.26717/BJSTR.2020.26.004370 Introduction Viscosity represents the strength of interaction of an object or a molecule with other objects or molecules within immediate vi- cinity in a solution. Its importance follows from the fact that via its involvement in the lateral and rotational diffusion coefficients it de- termines the rates of biochemical-biological processes in almost ev- ery area in life sciences. From another point of view, it determines also the rates of energy dissipation of moving bodies, molecules. An interesting feature is that while lateral diffusion depends on the inverse 1st power of the object radius, the rotational one depends on the inverse 3rd power of the radius, i.e. on the volume [1], sug- gesting a rule – which is a rather general one, having significance also in fluorescence – that by increasing dimensionality of motion, the degrees of freedom for energy dissipation is also increased, ex- emplified by translation and rotation as 1- and 3-D motions (Figure 1, Panel A). Tradional approaches of viscosity measurements in cell biology rest on Fluorescence Recovery After Photobleaching (FRAP) mea surements of translational diffusion and fluorescence or phospho- rescence anisotropy measurements of rotational diffusion [2-6]. In FRAP, lateral diffusion is quantified by the speed of reappearance of fluorescence in a target volume after selectively destroying by intense light a portion of fluorophores initially present there [2,3]. In fluorescence anisotropy measurements, rotational diffusion is quantified by the speed of randomization of polarization direc- tions of the emitted photons after excitation by linearly polarized light [4-6]. These two approaches, and even their combinations in non-imaging and imaging modes, could be realized equally well in the cytoplasm, on the cell membrane, and in the different intracel- lular organelles and/or in their membranes [2,6]. Substantial inter- est has been arisen by a class of special dyes called molecular rotors [1] (Figure 1, Panel B). Received: March 03, 2020 Published: March 12, 2020 Citation: L. Bene, L. Damjanovich. Ro- tationally Gated Fluorescence: Common Theme Behind Molecular Rotors, Photo- activation and Förster Resonance Energy Transfer (FRET). Biomed J Sci & Tech Res 26(4)-2020. BJSTR. MS.ID.004370. ARTICLE INFO Abstract About the common features behind the mechanism of molecular rotors, pho- toswitching of engineered fluorescent proteins (GFPs), and Förster resonance energy transfer (FRET). Besides explining the common mechanism, we also offer a new method for increasing sensitivity of viscosity determination even in living cells, by the combina- tion of polarization energy transfer (polFRET) method and a molecular rotor dye either as an energy donor or acceptor. Keywords: Internal Charge Transfer (TICT) State; Viscosity; Fluorescence Lifetime; Quantum Yield; Photoactivation; Polarization Energy Transfer (polFRET) Abbreviations: FRET: Förster Resonance Energy Transfer; FRAP: Fluorescence Recov- ery After Photobleaching; LE: Locally Excited State