Photoprotolytic Processes of Lumazine

Steady-state and time-resolved UV–vis spectroscopies were used to study the photoprotolytic properties of lumazine, which belongs to a class of biologically important compoundsthe petridines. We found that in water an excited-state proton transfer occurs with a time constant of ∼70 ps and competes with a nonradiative rate of about the same value. The nonradiative rate of the protonated form of lumazine in polar and nonpolar solvents is large <i>k</i>_nr ≥ 1.5 × 10¹⁰s^–1. The fluorescence properties indicate that in water, the ground-state neutral form of lumazine is already stable in two tautomeric forms. The fluorescence of the deprotonated form is quenched by protons in acidic solutions with a diffusion-controlled reaction rate. We conclude that the neutral form of lumazine is an irreversible mild photoacid

Photoprotolytic Processes of Umbelliferone and Proposed Function in Resistance to Fungal Infection

The photoprotolytic processes of 7-hydroxy-coumarin (Umb) were investigated by steady-state and time-resolved-fluorescence techniques. We found that the Umb compound is a photoacid with p<i>K</i>_a* ≈ 0.4 and a rate constant of the excited-state proton transfer (ESPT) to water of 2 × 10¹⁰ s^–1. Umb is also a photobase and accepts an excess proton in solution and also directly from weak acids like acetic acid. When Umb is adsorbed on cellulose it also functions as a photoacid and a photobase. Hydroxycoumarins are known to accumulate next to fungal-, bacterial-, and viral-infected regions in the leaves and stems of plants in general and also in trees. We propose that these compounds when irradiated by sunlight UV, combat the fungi or bacteria by excited-state proton-transfer reactions. These photoprotolytic reactions provide a universal resistance mechanism to infections in plants

Excited-State Proton Transfer in Resveratrol and Proposed Mechanism for Plant Resistance to Fungal Infection

Steady-state and time-resolved fluorescence techniques were employed to study the photophysics and photochemistry of <i>trans</i>-resveratrol. <i>trans</i>-Resveratrol is found in large quantities in fungi-infected grapevine-leaf tissue and plays a direct role in the resistance to plant disease. We found that <i>trans</i>-resveratrol in liquid solution undergoes a trans–cis isomerization process in the excited state at a rate that depends partially on the solvent viscosity, as was found in previous studies on <i>trans</i>-stilbene. The hydroxyl groups of the phenol moieties in resveratrol are weak photoacids. In water and methanol solutions containing weak bases such as acetate, a proton is transferred to the base within the lifetime of the excited state. When resveratrol is adsorbed on cellulose (also a component of the plant’s cell wall), the cis–trans process is slow and the lifetime of the excited state increases from several tens of picoseconds in ethanol to about 1.5 ns. Excited-state proton transfer occurs when resveratrol is adsorbed on cellulose and acetate ions are in close proximity to the phenol moieties. We propose that proton transfer from excited resveratrol to the fungus acid-sensing chemoreceptor is one of the plant’s resistance mechanisms to fungal infection

Auramine‑O as a Fluorescence Marker for the Detection of Amyloid Fibrils

There is an indispensable need for a fluorescence marker for the detection of amyloid fibrils, where, at present, the most used marker is thioflavin-T (ThT). Here, we present the use of auramine-O (AuO) as a possible alternative to ThT. As with ThT, the increase in the emission of AuO upon binding to amyloid fibrils is the result of inhibition of the free rotation of the two dimethylamino arms of the molecule. This inhibition prevents the excited-state electronic wave function from moving from the emissive locally excited state to the dark charge-transfer state. We further show that not only AuO is comparable to ThT as a fluorescent marker for amyloid fibrils but also it has a unique spectroscopic signature. AuO has distinct two modes that are characterized by a large shift in the absorption and emission peak positions between its unbound and bound states (before and after the fibrils formation, respectively). In this context, we show that, whereas the emission band position is red-shifting, the absorption peak shifts to the blue and the spectrum exhibits an isosbestic point. The large shifts in emission and absorption peak positions can be explained by the photoacid activity of AuO exhibiting an excited-state proton-transfer process

Excited-State Intramolecular Proton Transfer of the Natural Product Quercetin

Intramolecular proton-transfer dynamics in the lowest excited state (ESIHT) were studied in the natural product quercetin. We found that in all seven solvents used in this study, the ESIHT rate is ultrafast. We estimate that the ESIHT rate is about 70 fs or less. We found that in deuterated protic solvents, such as methanol-<i>d</i> or ethanol-<i>d</i>, the ESIHT rate is slower and the proton-transfer time constant is about 110 fs. The tautomeric form fluorescence quantum yield of quercetin is very low, of the order of the normal form

Excited-State Proton Transfer of Weak Photoacids Adsorbed on Biomaterials: Proton Transfer on Starch

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a photoacid adsorbed on starch to a nearby water molecule. Starch is composed of ∼30% amylose and ∼70% amylopectin. We found that the ESPT rate of adsorbed 8-hydroxy-1,3,6-pyrene­trisulfonate (HPTS) on starch arises from two time constants of 300 ps and ∼3 ns. We explain these results by assigning the two different ESPT rates to HPTS adsorbed on amylose and on amylopectin. When adsorbed on amylose, the ESPT rate is ∼3 × 10⁹s^–1, whereas on amylopectin, it is only ∼3 × 10⁸ s^–1

Excited-State Proton Transfer of Weak Photoacids Adsorbed on Biomaterials: 8‑Hydroxy-1,3,6-pyrenetrisulfonate on Chitin and Cellulose

Time-resolved and steady-state florescence measurements were used to study the photoprotolytic process of an adsorbed photoacid on cellulose and chitin. For that purpose we used the 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) photoacid which transfers a proton to water with a time constant of 100 ps, but is incapable of doing so in methanol or ethanol. We found that both biopolymers accept a proton from the electronically excited acidic ROH form of HPTS. The excited-state proton-transfer (ESPT) rate of HPTS adsorbed on chitin is greater than that on cellulose by a factor of 5. The ESPT on chitin also occurs in the presence of methanol or ethanol, but at a slower rate. The transferred protons can recombine efficiently with the conjugate excited base, the RO^– form of HPTS

Excited-State Proton Transfer of Weak Photoacids Adsorbed on Biomaterials: Proton Transfer to Glucosamine of Chitosan

UV–vis steady-state and time-resolved techniques were employed to study the excited-state proton-transfer process from two weak photoacids positioned next to the surface of chitosan and cellulose. Both chitosan and cellulose are linear polysaccharides; chitosan is composed mainly of d-glucosamine units. In order to overcome the problem of the high basicity of the glucosamine, we chose 2-naphthol (p<i>K</i>_a* ≈ 2.7) and 2-naphthol-6-sulfonate (p<i>K</i>_a* ≈ 1.7) as the proton emitters because of their ground state p<i>K</i>_a (≈9). Next to the 1:1 cellulose:water weight ratio, the ESPT rate of these photoacids is comparable to that of bulk water. We found that the ESPT rate of 2-naphthol (2NP) and 2-naphthol-6-sulfonate (2N6S) next to chitosan in water (1:1) weight ratio samples is higher than in bulk water by a factor of about 5 and 2, respectively. We also found an efficient ESPT process that takes place from these photoacids in the methanol environment next to the chitosan scaffold, whereas ESPT is not observed in methanolic bulk solutions of these photoacids. We therefore conclude that ESPT occurs from these photoacids to the d-glucosamine units that make up chitosan

Anomalous H⁺ and D⁺ Excited-State Proton-Transfer Rate in H₂O/D₂O Mixtures

We used the time-resolved fluorescence technique to measure the excited-state proton-transfer (ESPT) rates from 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) to solvent mixtures of H₂O and D₂O. We found an anomalous deviation from linear mole-fraction behavior of the ESPT rate in H₂O/D₂O mixtures. We provide a chemical model based on equilibrium constant of the reaction H₂O + D₂O ↔ 2HOD and rate constants of the ESPT process of H and D transfers from HPTS to the mixed solvent. Anomalous deviation from linear mole-fraction behavior was previously found for H⁺/D⁺ conductance in these mixtures

Ultrafast Excited-State Intramolecular Proton Transfer of Aloesaponarin I

Time-resolved emission of aloesaponarin I was studied with the fluorescence up-conversion and time-correlated single-photon-counting techniques. The rates of the excited-state intramolecular proton transfer, of the solvent and molecular rearrangements, and of the decay from the excited proton-transferred species were determined and interpreted in the light of time-dependent density functional calculations. These results were discussed in conjunction with UV protection and singlet-oxygen quenching activity of aloe