Excited-State Proton Transfer of Phenol Cyanine Picolinium Photoacid

In situ generation of a Zwitterionic fluorescent probe for detection of human serum albumin protein

In this article, a new approach for human serum albumin selective fluorophore design has been reported. The fluorophore reported here comprises a substituted phenol donor and a cationic benzo[e]indolium acceptor connected with a π bond. Originally, the cationic fluorophore did not bind with human serum albumin. Upon deprotonation of the phenolic-OH by a water molecule the cationic form was transformed into an active zwitterionic form. Spectroscopic studies and theoretical calculations revealed that the new active form remained in a zwitterionic state in neutral aqueous solution, and it formed a strong supramolecular complex with human serum albumin. The spontaneous complexation resulted multi-fold increase of fluorescence intensity which increased linearly with the concentrations of the protein, thus giving an analytical tool to monitor human serum albumin in aqueous samples. We believe, this simple strategy applied on appropriate fluorogenic scaffolds would prove useful to develop new and improved turn-on fluorescent probes for pH regulated biological applications.

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine "strap". In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.

Photochemistry of Cannabidiol (CBD) Revised. A Combined Preparative and Spectrometric Investigation

Cannabis is a plant with an astonishing ability to biosynthesize cannabinoids, and more than 100 molecules belonging to this class have been isolated. Among them in recent years cannabidiol (CBD) has received the interest of pharmacology as the major nonpsychotropic cannabinoid with many potential clinical applications. Although the reactivity of CBD has been widely investigated, only little attention has been given to the possible photodegradation of this cannabinoid, and the data available in the literature are outdated and, in some cases, conflicting. The aim of the present work is providing a characterization of the photochemical behavior of CBD in organic solvents, through a detailed GC-MS analyses, isolation, and NMR characterization of the photoproducts obtained.

H2O-Bridged Proton-Transfer Channel in Emitter Species Formation in Obelin Bioluminescence

Bioluminescence of a number of marine organisms is conditioned by Ca²⁺-regulated photoprotein (CaRP) with coelenterazine as the reaction substrate. The reaction product, coelenteramide, at the first singlet excited state (S₁) is the emitter of CaRP. The S₁-state coelenteramide is produced via the decomposition of coelenterazine dioxetanone. Experiments suggested that the neutral S₁-coelenteramide is the primary emitter species. This supposition contradicts with theoretical calculations showing that the anionic S₁-coelenteramide is a primary product of the decomposition of coelenterazine dioxetanone. In this study, applying molecular dynamic (MD) simulations and the hybrid quantum mechanics/molecular mechanics (QM/MM) method, we investigated a proton-transfer (PT) process taking place in CaRP obelin from Obelia longissima for emitter formation. Our calculations demonstrate a concerted PT process with a water molecule as a bridge between anionic S₁-coelenteramide and the nearest histidine residue. The low activation barrier as well as the strong hydrogen-bond network between the proton donor and the proton acceptor suggests a fast PT process comparable with that of the lifetime of excited anionic S₁-coelenteramide. The existence of the PT process eliminates the discrepancy between experimental and theoretical studies. The fast PT process at emitter formation can also take place in other CaRPs.

Predicting Absorption and Emission Maxima of Polycyclic Aromatic Azaborines: Reliable Transition Energies and Character

Polycyclic aromatic azaborines have potential applications as luminophores, novel fluorescent materials, organic light-emitting diodes, and fluorescent sensors. Additionally, their relative structural simplicity should allow the use of computational techniques to design and screen novel compounds in a rapid manner. Herein, the absorption and emission maxima of twelve polycyclic aromatic BN-1,2-azaborine analogues containing the N-BOH moiety were examined to determine a methodology for reliably predicting both the energy and character (local excitation [LE] vs charge transfer [CT]) of the absorption and emission maxima for these compounds. The necessity of implicit solvation models was also investigated. The cam-QTP(01) functional with a small, double-ζ quality basis set provides reliable data compared to EOM-CCSD/cc-pVDZ single-point computations. Of note, commonly used functionals for these applications (B3LYP and ωB97xD) struggle to provide reliable results for both the energy and LE character of the transitions relative to EOM-CCSD computations.

Excited-State Proton Transfer to H2O in Mixtures of CH3CN-H2O of a Superphotoacid, Chlorobenzoate Phenol Cyanine Picolinium (CBCyP)

Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a p K_a* of ∼-7, the chlorobenzoate phenol cyanine picolinium salt (CBCyP) in acetonitrile-water mixtures. We found that the time-resolved fluorescence is bimodal. The amplitude of the short-time component depends on χ_water; the larger χ_water, the greater the amplitude. We found that the excited-state proton-transfer (ESPT) rate constant, k_PT, is ≥5 × 10¹² s^-1 in mixtures of χ_water ≥ 0.08, whereas in neat water, k_PT = 6 × 10¹² s^-1. The long-time component has a lifetime of 50 ps at χ_water = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to H₂O clusters. The results suggest that the ESPT rate constant to water in acetonitrile-water mixtures depends only slightly on the water cluster size and structure surrounding the CBCyP molecule. We attribute the independence of the ESPT rate on the average water-cluster size to the large photoacidity of CBCyP. QM TD-DFT calculations found that in the excited-state the RO^-(S₁) species that is formed by the ESPT process is more stable than the ROH(S₁) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO^-(S₁) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CH₃CN-H₂O solvent mixtures of dielectric constant of ε ≥ 45.