Dynamics of small molecules within the F127 PEO-PPO-PEO triblock copolymer gel and sol phases studied at the molecular scale

The dynamics of naphthalene derivatives with different hydrophobicities bound to F127 polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO) micelles in the gel and sol phases were studied using a quenching methodology for the triplet excited states of the naphthalenes. Studies with triplet excited states probe a larger reaction volume than the volumes accessible when using fluorescent singlet excited states. The use of triplet excited states enables the determination of the dynamics between different compartments of a supramolecular system, which in the case of F127 micelles are the micellar core, the micellar corona and the aqueous phase. This report includes laser flash photolysis studies for the four naphthalene derivatives in the F127 gel and sol phases. The triplet excited states were quenched using the nitrite anion as the quenchers. The association and dissociation rate constants of the naphthalenes from the micelles and the quenching rate constants for the naphthalenes bound to the micelles were determines from the curved quenching plot (observed decay rate constant vs. nitrite concentration).

Investigation of OH radical kinetics with glycine, alanine, serine and threonine in the aqueous phase

<p>Amino acids are key substances in biological activities and can be emitted into the atmosphere as constituents of primary aerosols. Understanding the radical kinetics of amino acids is necessary to evaluate their atmospheric effects. In the present study, the hydroxyl radical (OH) reaction kinetics of glycine, alanine, serine and threonine were investigated in the aqueous phase. The temperature and pH dependent rate constants were measured by a laser flash photolysis-long path absorption setup using the competition kinetics method. Based on the measurements and speciation calculations, the OH radical reaction rate constants of the fully protonated (H<sub>2</sub>A<sup>+</sup>) and neutral (HA<sup>±</sup>) form were determined. The following T-dependent Arrhenius expressions were derived for the OH radical reactions with glycine, <em>k</em>(<em>T</em>, H<sub>2</sub>A<sup>+</sup>) = (9.1 ± 0.3) × 10<sup>9</sup> × exp[(-2360 ± 230 K)/<em>T</em>], <em>k</em>(<em>T</em>, HA<sup>±</sup>) = (1.3 ± 0.1) × 10<sup>10</sup> × exp[(-2040 ± 240 K)/<em>T</em>]; alanine, <em>k</em>(<em>T</em>, H<sub>2</sub>A<sup>+</sup>) = (1.0 ± 0.1) × 10<sup>9</sup> × exp[(-1030 ± 340 K)/<em>T</em>], <em>k</em>(<em>T</em>, HA<sup>±</sup>) = (6.8 ± 0.4) × 10<sup>10</sup> × exp[(-2020 ± 370 K)/<em>T</em>]; serine, <em>k</em>(<em>T</em>, H<sub>2</sub>A<sup>+</sup>) = (1.1 ± 0.1) × 10<sup>9</sup> × exp[(-470 ± 150 K)/<em>T</em>], <em>k</em>(<em>T</em>, HA<sup>±</sup>) = (3.9 ± 0.1) × 10<sup>9</sup> × exp[(-720 ± 130 K)/<em>T</em>]; and threonine, <em>k</em>(<em>T</em>, H<sub>2</sub>A<sup>+</sup>) = (5.0 ± 0.1) × 10<sup>10</sup> × exp[(-1500 ± 100 K)/<em>T</em>], <em>k</em>(<em>T</em>, HA<sup>±</sup>) = (3.3 ± 0.1) × 10<sup>10</sup> × exp[(-1320 ± 90 K)/<em>T</em>] (in units of L mol<sup>-1</sup> s<sup>-1</sup>).</p> <p>The density functional theory calculation was performed using GAUSSIAN to simulate the energy barriers (<em>E<sub>Barrier</sub></em>) of OH radical induced H-atom abstraction. According to the simulated results, amino and carboxyl group increase the <em>E<sub>Barrier</sub></em> at the adjacent C‑atom and thus reduce the OH radical reactivity. Hydroxide and methyl group decrease the <em>E<sub>Barrier</sub></em> at the adjacent C-atom, leading to an increase in the OH radical rate constant.</p>

Azobenzene/Tetraethyl Ammonium Photochromic Potassium Channel Blockers: Scope and Limitations for Design of Para-Substituted Derivatives with Specific Absorption Band Maxima and Thermal Isomerization Rate

Azobenzene/tetraethyl ammonium photochromic ligands (ATPLs) are photoactive compounds with a large variety of photopharmacological applications such as nociception control or vision restoration. Absorption band maximum and lifetime of the less stable isomer are important characteristics that determine the applicability of ATPLs. Substituents allow to adjust these characteristics in a range limited by the azobenzene/tetraethyl ammonium scaffold. The aim of the current study is to find the scope and limitations for the design of ATPLs with specific spectral and kinetic properties by introducing para substituents with different electronic effects. To perform this task we synthesized ATPLs with various electron acceptor and electron donor functional groups and studied their spectral and kinetic properties using flash photolysis and conventional spectroscopy techniques as well as quantum chemical modeling. As a result, we obtained diagrams that describe correlations between spectral and kinetic properties of ATPLs (absorption maxima of E and Z isomers of ATPLs, the thermal lifetime of their Z form) and both the electronic effect of substituents described by Hammett constants and structural parameters obtained from quantum chemical calculations. The provided results can be used for the design of ATPLs with properties that are optimal for photopharmacological applications.

Kinetics and Energetics of Intramolecular Electron Transfer in Single-Point Labeled TUPS-Cytochrome c Derivatives

Electron transfer within and between proteins is a fundamental biological phenomenon, in which efficiency depends on several physical parameters. We have engineered a number of horse heart cytochrome c single-point mutants with cysteine substitutions at various positions of the protein surface. To these cysteines, as well as to several native lysine side chains, the photoinduced redox label 8-thiouredopyrene-1,3,6-trisulfonate (TUPS) was covalently attached. The long-lived, low potential triplet excited state of TUPS, generated with high quantum efficiency, serves as an electron donor to the oxidized heme c. The rates of the forward (from the label to the heme) and the reverse (from the reduced heme back to the oxidized label) electron transfer reactions were obtained from multichannel and single wavelength flash photolysis absorption kinetic experiments. The electronic coupling term and the reorganization energy for electron transfer in this system were estimated from temperature-dependent experiments and compared with calculated parameters using the crystal and the solution NMR structure of the protein. These results together with the observation of multiexponential kinetics strongly support earlier conclusions that the flexible arm connecting TUPS to the protein allows several shortcut routes for the electron involving through space jumps between the label and the protein surface.

Unravelling the Photoprotection Capacity of Resveratrol on Histidine Oxidation

Exposure to sun radiation causes great oxidative stress and activates a numerous of defense mechanisms in living systems, such as the synthesis of antioxidants. Resveratrol (RSV), a naturally occurring stilbene molecule, has antioxidant properties and is synthesized in large amounts when plants are under high oxidative stress. Likewise, under UV and visible radiation, biomolecules are oxidized, losing their physiological properties and, therefore, avoiding the harmful effects of solar radiation is crucial in order to preserve the functionality of cellular components. In proteins, one essential component that is often susceptible to degradation is the amino acid histidine (His), which can be modified via several oxidizing mechanisms. In this article, we evaluate the photoprotection capacity of RSV in photosensitized oxidation of His, which is initiated with a one-electron transfer reaction, yielding the His radical cation (His•+). The photoprotective properties of RSV are evaluated using kinetics analysis during steady-state irradiation and laser flash photolysis experiments. The experimental results reveal that the presence of RSV in the solution causes an evident decrease of the His consumption initial rates as a result of a reaction between His•+ and RSV that recovers the amino acid. In addition, we conclude that during its antioxidant action, RSV is consumed being a sacrificial antioxidant.

Effective Singlet Oxygen Sensitizers Based on the Phenazine Skeleton as Efficient Light Absorbers in Dye Photoinitiating Systems for Radical Polymerization of Acrylates

A series of dyes based on the phenazine skeleton were synthesized. They differed in the number of conjugated double bonds, the arrangement of aromatic rings (linear and/or angular system), as well as the number and position of nitrogen atoms in the molecule. These compounds were investigated as potential singlet oxygen sensitizers and visible light absorbers in dye photoinitiating systems for radical polymerization. The quantum yield of the singlet oxygen formation was determined by the comparative method based on the 1H NMR spectra recorded for the tested dyes in the presence of 2,3-diphenyl-p-dioxene before and after irradiation. The quantum yield of the triplet state formation was estimated based on the transient absorption spectra recorded using the nanosecond flash photolysis technique. The effectiveness of the dye photoinitiating system was characterized by the initial rate of trimethylolpropane triacrylate (TMPTA) polymerization. In the investigated photoinitiating systems, the sensitizer was an electron acceptor, whereas the co-initiator was an electron donor. The effectiveness of TMPTA photoinitiated polymerization clearly depended on the arrangement of aromatic rings and the number of nitrogen atoms in the modified phenazine structure as well as the quantum yield of the triplet state formation of the photosensitizer in the visible light region.

Iron(III) hydroxocomplex - methyl viologen dication system as a prospective tool for determination of hydroxyl radical reaction rate constants with environmental pollutants

Reactivity of oxidative species with target pollutants is one of the crucial parameters for application of any system based on advanced oxidation processes (AOPs). This work presents new useful approach how to determine the hydroxyl radical reaction rate constants (kOH) using UVA laser flash photolysis technique. Fe(III) hydroxocomplex at pH 3 was applied as a standard source of hydroxyl radicals and methyl viologen dication (MV2+) was used as selective probe for •OH radical. Application of MV2+ allows to determine kOH values even for compounds which do not generate themselves optically detectable transient species in reaction with hydroxyl radicals. Validity of this approach was tested on a wide range of different persistent pesticides and its main advantages and drawbacks in comparison with existing steady-state and time-resolved techniques were discussed.