Real-time observation of the charge-transfer-to-solvent dynamics

Intermolecular electron-transfer reactions have a crucial role in biology, solution chemistry and electrochemistry. The first step of such reactions is the expulsion of the electron to the solvent, whose mechanism is determined by the structure and dynamical response of the latter. Here we visualize the electron transfer to water using ultrafast fluorescence spectroscopy with polychromatic detection from the ultraviolet to the visible region, upon photo-excitation of the so-called charge transfer to solvent states of aqueous iodide. The initial emission is short lived (~60 fs) and it relaxes to a broad distribution of lower-energy charge transfer to solvent states upon rearrangement of the solvent cage. This distribution reflects the inhomogeneous character of the solvent cage around iodide. Electron ejection occurs from the relaxed charge transfer to solvent states with lifetimes of 100–400 fs that increase with decreasing emission energy.

Understanding the chemistry of dental erosion

The mineral in our teeth is composed of a calcium-deficient carbonated hydroxyapatite (Ca10-xNax(PO4)6-y(CO3)z(OH)2-uFu). These substitutions in the mineral crystal lattice, especially carbonate, renders tooth mineral more acid soluble than hydroxyapatite. During erosion by acid and/or chelators, these agents interact with the surface of the mineral crystals, but only after they diffuse through the plaque, the pellicle, and the protein/lipid coating of the individual crystals themselves. The effect of direct attack by the hydrogen ion is to combine with the carbonate and/or phosphate releasing all of the ions from that region of the crystal surface leading to direct surface etching. Acids such as citric acid have a more complex interaction. In water they exist as a mixture of hydrogen ions, acid anions (e.g. citrate) and undissociated acid molecules, with the amounts of each determined by the acid dissociation constant (pKa) and the pH of the solution. Above the effect of the hydrogen ion, the citrate ion can complex with calcium also removing it from the crystal surface and/or from saliva. Values of the strength of acid (pKa) and for the anion-calcium interaction and the mechanisms of interaction with the tooth mineral on the surface and underneath are described in detail.

Conformational preferences of natural and C3-modified epothilones in aqueous solution

The conformational properties of the microtubule-stabilizing agent epothilone A ( 1a) and its 3-deoxy and 3-deoxy-2,3-didehydro derivatives 2 and 3 have been investigated in aqueous solution by a combination of NMR spectroscopic methods, Monte Carlo conformational searches, and NAMFIS calculations. The tubulin-bound conformation of epothilone A ( 1a), as previously proposed on the basis of solution NMR data, was found to represent a significant fraction of the ensemble of conformations present for the free ligands in aqueous solution.

Ultrafast Relaxation Dynamics of Osmium-Polypyridine Complexes in Solution

We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp)3 (Os1) and Os(bpy)2(dpp) (Os2) in ethanol, where dmbp is 4,4′-dimethyl-2,2′-biypridine, bpy is 2,2′-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest 3MLCT state, whose quantum yield we estimate to be ≤5.0 × 10–3. For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafast studies revealed a short-lived (≤100 fs) fluorescence, which stems from the lowest singlet metal-to-ligand-charge-transfer (1MLCT) state and decays by intersystem crossing to the manifold of 3MLCT states. In addition, Os1 exhibits a 50 ps lived emission from an intermediate triplet state at an energy 2000 cm–1 above that of the long-lived (25 ns) phosphorescence. In Os2, the 1MLCT–3MLCT intersystem crossing is faster than that in Os1, and no emission from triplet states is observed other than the lowest one. These observations are attributed to a higher density of states or a smaller energy spacing between them compared with Os1. They highlight the importance of the energetics on the rate of intersystem crossing.

Understanding the chemistry of dental erosion.

Dental erosion is caused by repeated short episodes of exposure to acids. Dental minerals are calcium-deficient, carbonated hydroxyapatites containing impurity ions such as Na(+), Mg(2+) and Cl(-). The rate of dissolution, which is crucial to the progression of erosion, is influenced by solubility and also by other factors. After outlining principles of solubility and acid dissolution, this chapter describes the factors related to the dental tissues on the one hand and to the erosive solution on the other. The impurities in the dental mineral introduce crystal strain and increase solubility, so dentine mineral is more soluble than enamel mineral and both are more soluble than hydroxyapatite. The considerable differences in structure and porosity between dentine and enamel influence interactions of the tissues with acid solutions, so the relative rates of dissolution do not necessarily reflect the respective solubilities. The rate of dissolution is further influenced strongly by physical factors (temperature, flow rate) and chemical factors (degree of saturation, presence of inhibitors, buffering, pH, fluoride). Temperature and flow rate, as determined by the method of consumption of a product, strongly influence erosion in vivo. The net effect of the solution factors determines the overall erosive potential of different products. Prospects for remineralization of erosive lesions are evaluated.