Solvent structural effects on proton dissociation

Radical Hydroarylation of Functionalized Olefins and Mechanistic Investigation of Photocatalytic Pyridyl Radical Reactions

We report the photoredox alkylation of halopyridines using functionalized alkene and alkyne building blocks. Selective single-electron reduction of the halogenated pyridines provides the corresponding heteroaryl radicals, which undergo anti-Markovnikov addition to the alkene substrates. The system is shown to be mild and tolerant of a variety of alkene and alkyne subtypes. A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.

Fluorescence enhancement of 1-napthol-5-sulfonate by forming inclusion complex with beta-cyclodextrin in aqueous solution

Fluorescence enhancement of 1-naphthol-5-sulfonate (1N5S) upon inclusion in beta-cyclodextrin is studied by spectrophotometry and spectrofluorimetery techniques. The spectral shifts were only observed in the emission spectra in different solvents and were found to be directly correlated to the solvent's hydrogen-bond donor strength (alpha). Spectral changes in the absorption spectra upon the addition of beta-cyclodextrin to 1N5S in aqueous medium were too small to allow for the determination of the binding constant. On the other hand; fluorescence measurements show that the emission of both the anionic and neutral forms of 1N5S increase upon the addition of beta-cyclodextrin with an increase in the quantum yield by about 60%. Fluorescence measurements show 1:1 inclusion of 1N5S in the beta-cyclodextrin cavity with an association constant of 88+/-10M(-1). (1)H NMR studies are used to confirm the inclusion and to provide information on the geometry of 1N5S inside the cavity of beta-cyclodextrin.

Binding of the Environmental Pollutant Naphthol to Bovine Serum Albumin

The interactions between naphthol and bovine serum albumin (BSA) were investigated by spectroscopy. Our results prove the formation of complex between naphthol and BSA. Hydrophobic interaction dominates in the association reaction. The isomers stack with the aromatic residues in their binding sites with different geometries. Effects of BSA on the excited-state proton transfer and fluorescence spectra of the isomers indicate the different characters of their binding sites. 1-Naphthol inserts deeply into a hydrophobic cavity whereas 2-naphthol is in a basic environment on the surface of BSA. Naphthol statically quenches the fluorescence of BSA in a concentration-dependent manner positively deviating from the linear Stern-Volmer equation. Naphthol binds near Trp-134 in the subdomain IA of the native BSA and is accessible to Trp-212 when BSA is unfolded by naphthol. The folding pattern of the main chain is altered at high naphthol concentration as revealed by the change in the secondary structure. The binding of 1-naphthol is more cooperative than that of 2-naphthol. The extent of cooperativity was estimated by the Hill equation.

Spectroscopic studies on the inclusion complex of 2-naphthol-6-sulfonate with beta-cyclodextrin

The photophysical properties of 2-naphthol-6-sulfonate (2-NOH-6-S) in various solvents and in aqueous beta-cyclodextrin solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.20+/-0.02, while in water the yield is higher. The fluorescence quantum yield in water was found to depend on the pH value of the medium and increases as the pH increase up to a pH value of 4.0 where it comes to be constant. Absorption and fluorescence measurements show 1:1 inclusion of 2-NOH-6-S in the beta-cyclodextrin cavity. The association constant of 2-NOH-6-S-beta-cyclodextrin complex based on fluorescence measurements was calculated using Benesi-Hildbrand relationship and found to be 330+/-30M(-1). (1)H NMR studies are used to confirm the inclusion and to provide information on the geometry of 2-NOH-6-S inside the cavity of beta-cyclodextrin.

Proton Antenna Effect of the γ-Cyclodextrin Outer Surface, Measured by Excited State Proton Transfer

The reversible proton dissociation and geminate recombination of the common photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (pyranine), either in dilute aqueous solution or when forming a complex with gamma-cyclodextrin (gamma-CD), has been studied by time-resolved fluorescence spectroscopy and supplemented by molecular modeling and dynamics simulations. We find that the dissociation rate of the proton from the excited molecule was decreased to about approximately 50% of its value in water, while the rate of recombination was doubled. These observations were evaluated by molecular modeling of the reactants at atomic resolution. The combination of the two methodologies indicates that the pyranine in the complex can assume more than one level of interaction with the solvent. The polysugar torus surrounding the pyranine perturbs the hydrogen bond in the dye's immediate vicinity and deforms the electrostatic potential inside the Coulomb cage, causing major deviations from a simple spheric symmetry. These observations can account for the special kinetic features measured for the complex. We suggest that this system can be used as a basic model for evaluating the mechanism of proton transfer in non-homogeneous systems, such as the surface of proteins or biomembranes.

Time-resolved dynamics of proton transfer in proteinous systems

The dynamics of proton dissociation from an acidic moiety and its subsequent dispersion in the bulk is regulated by the physical chemical properties of the solvent. The solvent has to provide a potential well to accommodate the discharged proton, screen it from the negative charge of the conjugated base, and provide an efficient mode for the diffusion of the proton to the bulk. On measuring the dynamics of proton dissociation in the time-resolved domain, the kinetic analysis of the reaction can quantitate the properties of the immediate environment. In this review we implement the kinetic analysis for evaluating the properties of small cavities in proteins and the diffusion of protons within narrow channels. On the basis of this analysis,we discuss how the clustering of proton-binding sites on a surface can endow the surface with enhanced capacity to attract protons and to funnel them toward a specific site.

The complex between 9-(n-decanyl)acridone and Bovine Serum Albumin. Part 2. What do fluorescence probes probe?

Factor analysis indicates that the fluorescence spectrum of 9-(n-decanyl)acridone (NDA), when bound to Bovine Serum Albumin (BSA), can be described quite adequately as the sum of two spectra, attributed to a "free" and a "bound" species. Kinetic evidence indicates that upon electronic excitation the system undergoes a net increase in free NDA, relative to the equilibrium distribution in the ground state, which would be consistent with Lewis acid sites on BSA being responsible for the binding. The system does not attain a position of equilibrium during the duration of the excited singlet state. This permits the determination of excited state rate constants for binding and unbinding of NDA on BSA, as well as the decay constants for the two forms of the probe.

Effect of beta-cyclodextrin on the excited state proton transfer in 1-naphthol-2-sulfonate

The photophysical properties of 1-naphthol-2-sulfonate (1-NOH-2-S) in various solvents and in aqueous beta-cyclodextrin (CD) solution have been investigated. The fluorescence quantum yields in non-aqueous solvents are approximately 0.5, while in water the fluorescence quantum yield is 0.1. The fluorescence quantum yield doubled on the addition of beta-CD. In aqueous solution, proton transfer to water takes place efficiently leading to the formation of the anion form with its longer wavelength emission broad band at about 460 nm. Any environmental changes have been found to affect the rate of deprotonation and subsequently the band intensity at 460 nm. In non-aqueous solution the anion emission band disappears completely. Upon the addition of beta-CD to the aqueous solution of 1-NOH-2-S, the anion emission decreases with an increase in the intensity of the neutral form at 362 nm. Fluorescence measurements show 1:1 inclusion of 1-NOH-2-S in the beta-CD cavity with an association constant of 1915 M(-1) using Benesi-Heldbrand treatment. 1H NMR studies are used to confirm the inclusion and to provide information on the orientation of 1-NOH-2-S inside the cavity of beta-CD.