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

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

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 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

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

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

How Fast Can a Proton-Transfer Reaction Be beyond the Solvent-Control Limit?

In this article, we review the field of photoacids. The rate of excited-state proton transfer (ESPT) to solvent spans a wide range of time scales, from tens of nanoseconds for the weakest photoacids to short time scales of about 100 fs for the strongest photoacids synthesized so far. We divide the photoacid strength into four regimes. Regime I includes the weak photoacids 0 < p<i>K</i>_a* < 3. These photoacids can transfer a proton only to water or directly to a mild-base molecule in solution. The ESPT rate to other protic solvents, like methanol or ethanol, is too small in comparison with the radiative rate. The second regime includes stronger photoacids whose p<i>K</i>_a*’s range from −4 to 0. They are capable of transferring a proton to other protic solvents and not only to water. The third regime includes even stronger photoacids. Their p<i>K</i>_a* is ∼ –6, and the ESPT rate constant, <i>k</i>_PT, is limited by the orientational time of the solvent which is characterized by the average solvation correlation function ⟨<i>S</i>(<i>t</i>)⟩. The fourth regime sets a new time limit for the ESPT rate of the strongest photoacids synthesized so far. The <i>k</i>_PT value of such photoacids is 10¹³ s^–1, and τ_PT = 100 fs. We attribute this new time limit (beyond the solvent control) to intermolecular vibration between the two heavy atoms of the proton donor and the proton acceptor, which assist the ESPT by lowering the height and width of the potential barrier, thus enhancing the ESPT rate

Temperature Dependence of the Excited-State Proton-Transfer Reaction of Quinone-cyanine‑7

Steady-state and time-resolved fluorescence techniques were used to study the temperature dependence of the photoprotolytic process of quinone-cyanine-7 (QCy7), a very strong photoacid, in H₂O and D₂O ice, over a wide temperature range, 85–270 K. We found that the excited-state proton-transfer (ESPT) rate to the solvent decreases as the temperature is lowered with a very low activation energy of 10.5 ± 1 kJ/mol. The low activation energy is in accord with free-energy-correlation theories that predict correlation between Δ<i>G</i> of reaction and the activation energy. At very low temperatures (<i>T</i> < 150 K), we find that the emission band of the RO^–*, the deprotonated form of QCy7, is blue-shifted by ∼1000 cm^–1. We attributed this band to the RO^–*···H₃O⁺ ion pair that was suggested to be an intermediate in the photoprotolytic process but has not yet been identified spectroscopically