Intriguing Chloride: Involvement of Chloride Ions in Proton Transfers.

The proton transfer from carbon to a chloride ion and the proton transfer to a molecule of water promoted by chloride ions in the acid-catalyzed formation of hydroxamic acids from aldehydes and substituted nitrosobenzenes in mixed solvents have been proposed based on experimental and theoretical investigations. The formation of uncommon contact ion pairs consisting of the nitrosocarbinolic cation intermediate and a chloride anion, followed by the proton transfer from a C-H moiety of the cation intermediate, has been proposed. The influence of chloride on the proton transfer to a water molecule of the solvent-separated nitrosocarbinolic-cation-chloride ion pair was investigated too. The insights are based on the obtained kinetic and other evidence with regard to (1) influences of chloride anions on the observed reaction rates and primary kinetic isotope effects (PKIE) in the reaction; (2) the observed variation of the PKIE-s and rates of the reaction when perchlorate anions are present along with the chloride ions; and (3) the consideration of a model of the nitrosocarbinolic-cation-intermediate-chloride ion pair and transition structure for the proposed proton transfers based on the ab initio calculations.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔEa (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Hydrogen Tunnelling as a Probe of the Involvement of Water Vibrational Dynamics in Aqueous Chemistry?

Our study of tunnelling in proton-coupled electron transfer (PCET) oxidation of ascorbate with hexacyanoferrate(III) follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics that led to the proposal of the involvement of collective intermolecular excitonic vibrational water dynamics in aqueous chemistry. To test the proposal, the hydrogen tunnelling modulation observed in the PCET reaction studied in the presence of low concentrations of various partial hydrophobic solutes in the water reaction system has been analyzed in terms of the proposed involvement of the collective intermolecular vibrational water dynamics in activation process in the case. The strongly linear correlation between common tunnelling signatures, isotopic values of Arrhenius prefactor ratios ln AH/AD and isotopic differences in activation enthalpies ΔΔH‡ (H,D) observed in the process in fairly diluted water solutions containing various partial hydrophobic solutes (such as dioxane, acetonitrile, ethanol, and quaternary ammonium ions) points to the common physical origin of the phenomenon in all the cases. It is suggested that the phenomenon can be rooted in an interplay of delocalized collective intermolecular vibrational dynamics of water correlated with vibrations of the coupled transition configuration, where the donor-acceptor oscillations, the motions being to some degree along the reaction coordinate, lead to modulation of hydrogen tunnelling in the reaction.

Solvent dependence of the kinetic isotope effect in the reaction of ascorbate with the 2,2,6,6-tetramethylpiperidine-1-oxyl radical: tunnelling in a small molecule reaction.

The oxidation of ascorbate with the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in water and water-dioxane mixed solvent has been demonstrated to be a proton-coupled electron transfer (PCET) process, involving hydrogen tunnelling at room temperature. The magnitude of the kinetic isotope effect (KIE) k(H)/k(D) in the reaction increases with decrease of the solvent polarity. The evidence comprise: (a) the spectroscopic and kinetic evidence for the interaction of ascorbate and TEMPO; (b) the observation of KIEs k(H)/k(D) of 24.2(0.6) in water and 31.1(1.1) in 1:1 v/v water-diox. (diox = dioxane), at 298 K; (c) the observation of isotope effect on the Arrhenius prefactor, A(H)/A(D) of 0.6(0.2) in the reaction in water and 1.2(0.2) in 1:1 v/v water-diox solvent; (d) the observation of isotope differences in the enthalpies of activation in water and D(2)O, Delta(r)H(double dagger) (in H(2)O) = 31.0(0.4) kJ/mol, Delta(r)H(double dagger) (in D(2)O) = 40.0 (0.5) kJ/mol; in 1:1 v/v water-diox and 1:1 v/v D(2)O-diox, Delta(r)H(double dagger) (in H(2)O/diox) = 23.9(0.2) kJ/mol, Delta(r)H(double dagger) (in D(2)O/diox) = 32.1(0.3) kJ/mol; (e) the temperature dependence of the KIEs in water and 1:1 v/v water-dioxane; these KIEs range from 27.3 at 285.4 K to 19.1 at 317.4 K in water and from 34.3 to 24.6 at the corresponding temperatures in 1:1 v/v water-diox, respectively; (f) the observation of an increase of the KIE in 10-40% v/v dioxane-water solvents relative to the KIE in water alone. There is a weak solvent dependence of the rate constant on going from water to 1:1 v/v water-diox. solvent, from 2.20(0.03) mol(-1) dm(3) s(-1) to 5.50(0.14) mol(-1) dm(3) s(-1), respectively, which originates from the mutual compensation of the enthalpy and entropy of activation.

Antique Traditional Practice: Phenolic Profile of Virgin Olive Oil Obtained from Fruits Stored in Seawater.

Virgin olive oil (VOO) is a functional food specific to the Mediterranean diet and related to human health, especially as a protector of cardiovascular health, in the prevention of several types of cancers, and in modification of immune and inflammatory response. Phenolic compounds have central importance for these extraordinary health benefits. In the production of high-quality olive oils, it is very important to process freshly picked olives and avoid any storage of fruits. However, in Croatia there is a very traditional and environmentally friendly method of olive oil production, where olive fruits are stored in seawater for some time prior to processing. This practice is also notable nowadays since there are people who prefer the characteristic flavor of the "seawater olive oil", although some people argue against its quality and biomedical relevance. In this study, the phenolic contents of VOO prepared from the immediately processed fresh olives and olives processed after storage in seawater were compared with the use of high-performance liquid chromatography-mass spectrometry (HPLC-MS) and spectrophotometric analysis. The results suggest that "seawater olive oil" should be considered as a safe food of biomedical relevance, as it still contains a significant proportion of important phenolics like hydroxytyrosol, tyrosol and oleacein (e.g., 63.2% of total phenols in comparison to VOO).

Synthesis, X-ray crystal structure study, and cytostatic and antiviral evaluation of the novel cycloalkyl-N-aryl-hydroxamic acids.

In vitro evaluation of the novel cycloalkyl-N-(4-chlorophenyl)-hydroxamic acids (2a-g) demonstrated that 2b,d,e exhibited rather marked inhibitory activity (IC50 = 7-10 microM) against pancreatic carcinoma, 2b-d against colon carcinoma, 2d against laryngeal carcinoma, and 2b,d against breast carcinoma. 2e showed the most pronounced anti-cytomegalovirus activity (EC50 = 1.5 and 0.8 microg mL(-1)) only at > or = 5-fold lower than the cytotoxic concentration. 2d and 2f showed modest, albeit selective, activity against cytomegalovirus (2d, EC50 = 7.3-8.9 microg mL(-1), selectivity index 7-10; 2f, EC50 = 7-13 microg mL(-1), selectivity index 10).