Excited-state proton transfer of firefly dehydroluciferin.

Steady-state and time-resolved emission techniques were used to study the protolytic processes in the excited state of dehydroluciferin, a nonbioluminescent product of the firefly enzyme luciferase. We found that the ESPT rate coefficient is only 1.1 × 10(10) s(-1), whereas those of d-luciferin and oxyluciferin are 3.

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin.

Optical steady-state and time-resolved spectroscopic methods were used to study the photoprotolytic reaction of oxyluciferin, the active bioluminescence chromophore of the firefly's luciferase-catalyzed reaction. We found that like D-luciferin, the substrate of the firefly bioluminescence reaction, oxyluciferin is a photoacid with pK(a)* value of ∼0.5, whereas the excited-state proton transfer (ESPT) rate coefficient is 2.

Ultrafast proton transfer of three novel quinone cyanine photoacids.

Steady-state and time-resolved emission techniques were used to study the photoprotolytic properties of three recently synthesized strong quinone cyanine photoacids (QCy7 and its sulfonated derivatives). The rate coefficient of the excited-state proton transfer (ESPT), k(PT), of the three dyes is roughly 1.5 × 10(12) s(-1), a high value that is comparable to the solvation dynamics rate of large polar organic molecules in H(2)O and D(2)O.

The effect of a mild base on curcumin in methanol and ethanol.

Steady-state and time-resolved emission techniques were employed to study the effect of acetate, a mild base, on the luminescence of curcumin in methanol and ethanol. We found that the steady-state emission intensity as well as the average fluorescence decay time are reduced by a factor of 5 when the acetate concentration is raised to about 1.8 M.

Excited-state proton transfer in N-methyl-6-hydroxyquinolinium salts: solvent and temperature effects.

The excited-state proton transfer (ESPT) reaction of the "super"photoacid N-methyl-6-hydroxyquinolinium (MHQ) was studied using both fluorescence upconversion and time-correlated single photon counting (TCSPC) techniques. The ultrafast ESPT kinetics were investigated in various alcohols and water and determined to be solvent-controlled. The ESPT temperature dependence of MHQ was also studied in various alcohols and compared to that observed for another "super"photoacid, 5,8-dicyano-2-naphthol (DCN2).

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes.

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1).

Structure and excited-state proton transfer in the GFP S205A mutant.

To further explore excited state proton transfer (ESPT) pathways within green fluorescent protein (GFP), mutagenesis, X-ray crystallography, and time-resolved and steady-state optical spectroscopy were employed to create and study the GFP mutant S205A. In wild type GFP (wt-GFP), the proton transfer pathway includes the hydroxyl group of the chromophore, a water molecule, Ser205, and Glu222. We found that the ESPT rate constant of S205A is smaller by a factor of 20 than that of wt-GFP and larger by a factor of 2 in comparison to the ESPT rate of S205V mutant which we previously characterized.

Temperature dependence of the fluorescence properties of curcumin.

Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175-250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer.

Excited-state intermolecular proton transfer of firefly luciferin V. Direct proton transfer to fluoride and other mild bases.

We studied the direct proton transfer (PT) from electronically excited D-luciferin to several mild bases. The fluorescence up-conversion technique is used to measure the rise and decay of the fluorescence signals of the protonated and deprotonated species of D-luciferin. From a base concentration of 0.

Time-resolved emission of flavin adenine dinucleotide in water and water-methanol mixtures.

Time-resolved fluorescence decay of flavin adenine dinucleotide (FAD) was studied at room temperature in water and water-methanol mixtures by a fluorescence upconversion technique. The observations were focused on the most initial decay phase (200 ps), before the residual fluorescence assumes a single exponential decay, typical for an extended conformation of the fluorophore. Within the first few picoseconds, where most of the electron transfer coupled quenching takes place, the emission decay curves could be fitted by a stretched exponent, compatible with the inhomogeneous distance dependent electron transfer model.

Excited-state intermolecular proton transfer of firefly luciferin IV. Temperature and pH dependence.

Time-resolved emission as well as steady-state UV-vis techniques were employed to study the photoprotolytic processes that d-luciferin, the natural substrate of the firefly luciferase, undergoes in both acidic aqueous solutions and ice. The emission spectrum of D-luciferin in a 20 mM HCl aqueous solution or higher has an additional emission band at 590 nm red-shifted with respect to the strongest emission band positioned at 530 nm of the deprotonated NRO(-*) form in a pH-neutral aqueous solution. We attribute this emission band to the zwitterion form designated as (+)HNRO(-).

Excited-state intermolecular proton transfer of firefly luciferin III. Proton transfer to a mild base.

Steady-state and time-resolved techniques were employed to study the excited-state proton transfer (ESPT) from d-luciferin, the natural substrate of the firefly luciferase, to the mild acetate base in aqueous solutions. We found that in 1 M aqueous solutions of acetate or higher, a proton transfer (PT) process to the acetate takes place within 30 ps in both H(2)O and D(2)O solutions. The time-resolved emission signal is composed of three components.

Excited-state intermolecular proton transfer of the firefly's chromophore D-luciferin. 2. Water-methanol mixtures.

Steady-state emission and time-resolved techniques were employed to study the photoprotolytic processes d-luciferin undergoes in water-methanol mixtures over a wide range of molar fractions (chi(MeOH)) of methanol. We found that in the concentration range of 0 < chi(MeOH) < 0.8 the rate constant of the excited-state proton transfer (ESPT) to the solvent decreases nearly exponentially with increasing chi(MeOH).

Indication of a very large proton diffusion in ice I(h). III. Fluorescence quenching of 1-naphthol derivatives.

The effects of excess protons on the fluorescence quenching process of 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-3-sulfonate (1N3S) in methanol-doped ice samples were studied by employing a time-resolved emission technique. We found that the fluorescence quenching of the deprotonated form RO(-)* of both photoacids by protonation is very efficient in ice, whereas in liquid water the proton fluorescence quenching is rather small. Using the Smoluchowski diffusion-assisted binary collision model under certain assumptions and approximations, we found that the calculated proton diffusion constant in ice in the temperature range of 240-260 K was 10 times greater than that of water at 295 K.