Effect of Ca2+ on the Steady-State and Time-Resolved Emission Properties of the Genetically Encoded Fluorescent Sensor CatchER

We previously designed a calcium sensor CatchER (a GFP-based Calcium sensor for detecting high concentrations in the high calcium concentration environment such as ER) with a capability for monitoring calcium ion responses in various types of cells. Calcium binding to CatchER induces the ratiometric changes in the absorption spectra, as well as an increase in fluorescence emission at 510 nm upon excitation at both 395 and 488 nm. Here, we have applied the combination of the steady-state and time-resolved optical methods and Hydrogen/Deuterium isotope exchange to understand the origin of such calcium-induced optical property changes of CatchER. We first demonstrated that calcium binding results in a 44% mean fluorescence lifetime increase of the indirectly excited anionic chromophore. Thus, CatchER is the first protein-based calcium indicator with the single fluorescent moiety to show the direct correlation between the lifetime and calcium binding. Calcium exhibits a strong inhibition on the excited-state proton transfer nonadiabatic geminate recombination in protic (vs deuteric) medium. Analysis of CatchER crystal structures and the MD simulations reveal the proton transfer mechanism in which the disrupted proton migration path in CatchER is rescued by calcium binding. Our finding provides important insights for a strategy to design calcium sensors and suggests that CatchER could be a useful probe for FLIM imaging of calcium in situ

Designing redder and brighter fluorophores by synergistic tuning of ground and excited states

We strategically modified the GFP core via chemical synthesis to make redder and brighter biomimetic fluorophores. Based on quantum calculations, solvatochromism analysis, and femtosecond Raman, we unveiled the additive effect of tuning the electronic ground and excited states, respectively, to achieve a dramatic emission redshift with a "double-donor-one-acceptor" structure

Dual Asymptotic Behavior in Geminate Diffusion-Influenced Reaction

The kinetics of excited-state proton transfer to solvent from the strong photoacid 5-cyano-2-naphthol exhibits two different power-law asymptotic tails for acid and base, arising from competition between reversible and irreversible geminate reprotonation. We show that the data are in quantitative agreement with a recently developed theory for this diffusion-influenced reaction. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction The study of diffusion-influenced reactions in solution was initiated by Smoluchowski longer than 80 wx years ago 1 . The simplest case is geminate recom- Z. bination GR of isolated pairs. In spite of intensive work in the field of GR, there is scarcity of convincing quantitative experimental evidence for diffusional effects even in the most comprehensive chemical kinetics accounts of diffusion-influenced reac- wx tions 2 . Indeed, diffusional effects are often masked by complicating factors: iodine GR -- by vibrational wx relaxation 3 , GR of l..

Themed issue on shape-responsive fluorophores

Fluorogens are molecules whose fluorescence can be ''turned on'' as a response to external action. In some cases, off/on ratios as large as ~10-4 have been observed, suggesting that such chromophores may have utility beyond static ones as reporter molecules. The recovery and/or intensification of fluorescence, through a variety of structural modifications of the chromophores and their complexation with proteins, DNA/RNA and other important biological hosts via host–guest interactions which freeze conformation or inhibit intramolecular fluorescence quenching, present a versatile and low-cost approach to high specificity in host identification if binding is highly specific. Indeed, the presence of recognition elements in the fluorogens