Investigation of etch pits on KDP crystals with porous sol–gel coatings

We have investigated etch-pit formation on potassium dihydrogen phosphate (KDP) crystals with porous anti-reflective coatings. Etch pits develop beneath the sol–gel coatings after exposure to ambient humidity. The etch pits are homogeneously distributed with a density and an average size governed by the relative humidity and the coating thickness. Furthermore, the etch pits are self-similar in shape and possess facets corresponding to low-energy planes of KDP. Results from optical microscopy, light scattering, and atomic force microscopy support the following model for etch-pit formation in this system. Water adsorbed from the environment into the porous sol–gel coating contacts the crystal surface, causing etch-pit nucleation at high undersaturation. The plume of KDP rising from an etch pit slowly diffuses laterally, reducing the undersaturation and shutting off nucleation in the surrounding area. Because surface kinetics are rapid compared to mass transport through the sol–gel, etch pits continually undergo equilibration to maintain a low-energy geometry and generate an average spacing. Growth continues until the reservoir of water in the sol–gel is saturated with KDP. Coarsening is only observed in high relative humidity environments. Received: 13 Juni 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

Range distributions for 25–200 keV 14N+ ions

Many features involved in the problem of heat flow through a semifinite metal sample irradiated with nanosecond laser pulses are discussed with reference both to the underlying physics and to the numerical solution of the heat diffusion equation.

Analytical expressions for several quantities, such as average velocities for the liquid-solid interface motion, maximum melted depth, and maximum surface temperature are presented. Experimental impurity profiles detected in Al samples after laser irradiation are analyzed on the basis of a diffusion-segregation hypothesis. The effects of ruby laser irradiation on virgin and ion-implanted Al single crystals, studied by using Rutherford backscattering and channelling are discussed in terms of epitaxial lattice regrowth, defect formation and metastable solid solutions.     

Carotenoid triplet state lifetimes

Carotene and xanthophyll triplet lifetimes are found to depend on the concentration of the parent molecule. These results account for some of the variations in carotenoid triplet lifetimes reported previously. The rate constants obtained for ground state quenching correlate with the number of conjugated double bonds, the longer chain systems having higher quenching rate constants.     

Warum interferieren die Wahrscheinlichkeiten?

The wavelike interference law connecting various transition probabilities, usually taken as the expression of a basic and irreducible quantum dualism, is derived as a consequence of the postulate that a general connection law between the probabilities exists at all, together with requirements of symmetry, invariance, and other features of a non-wavelike nature. Duality is thus divested of its fundamental character, and a way is found to understand quantum mechanics on a non-quantal basis.     

Applications of pulse radiolysis and flash photolysis to some problems related to the chemistry of cancer therapy

The semireduced form (Adr) of the anti-tumour agent adriamycin is implicated in cardiotoxic side effects. Adr, prepared by reduction with e _aq ^– and CO ₂ ^– , has pK values at 2.9 and 9.2. In the pH range 6–11, Adr is relatively stable, existing in equilibrium with Adr and the hydroquinone, the latter subsequently losing the sugar within 100 ms. The E ₇ ¹ of Adrais –328 mV, so at equilibrium the reaction O ₂ ^– + AdrO₂ + Adr lies well over to the left. The relative yields of O₂(¹g) formed via energy transfer from triplet excited components of haematoporphyrin derivative, the drug used in photodynamic therapy, have been measured by observations of the phosphorescence emission at 1270 nm. The measurements suggest that the most active component, dihaematoporphyrin ether or ester, is comparatively ineffective itself photodynamically, but may serve as a pool for the more effective haematoporphyrin and hydroxyethylvinylporphyrin to act inside the cancer cell. One-electron oxidation of dopa, through N ₃ ^. or photoionization, leads to the melanin precursor dopachrome via dopasemiquinone and dopaquinone. The latter reacts with cysteine to form cysteinyldopa, a marker for malignant melanoma metastasis. N ₃ ^. —initiated oxidation of cysteinyldopa proceeds via a different mechanism involving the corresponding semiquinone, quinone and a quinone-imine which rearranges to a more stable benzothiazole.     

Chemical, pulse radiolysis and density functional studies of a new, labile 5,6-indolequinone and its semiquinone

The chemical and spectroscopic characterization of 5,6-indolequinones and their semiquinones, key transient intermediates in the oxidative conversion of 5,6-dihydroxyindoles to eumelanin biopolymers, is a most challenging task. In the present paper, we report the characterization of a novel, relatively long-lived 5,6-indolequinone along with its semiquinone using an integrated chemical, pulse radiolytic, and computational approach. The quinone was obtained by oxidation of 5,6-dihydroxy-3-iodoindole (1a) with o-chloranil in cold ethyl acetate or aqueous buffer: it displayed electronic absorption bands around 400 and 600 nm, was reduced to 1a with Na2S2O4, and reacted with o-phenylenediamine to give small amounts of 3-iodo-1H-pyrrolo[2,3-b]phenazine (2). The semiquinone exhibited absorption maxima at 380 nm (sh) and 520 nm and was detected as the initial species produced by pulse radiolytic oxidation of 1a at pH 7.0. DFT investigations indicated the 6-phenoxyl radical and the N-protonated radical anion as the most stable tautomers for the neutral and anion forms of the semiquinone, respectively. Calculated absorption spectra in water gave bands at 350 (sh) and 500 nm for the neutral form and at 310 and 360 (sh) nm for the anion. Disproportionation of the semiquinone with fast second-order kinetics (2k = 1.1 x 1010 M-1 s-1) gave a chromophore with absorption bands resembling those of chemically generated 1a quinone. Computational analysis predicted 1a quinone to exist in vacuo as the quinone-methide tautomer, displaying low energy transitions at 380 and 710 nm, and in water as the o-quinone, with calculated absorption bands around 400 and 820 nm. A strong participation of a p orbital on the iodine atom in the 360-380 nm electronic transitions of the o-quinone and quinone-methide was highlighted. The satisfactory agreement between computational and experimental electronic absorption data would suggest partitioning of 1a quinone between the o-quinone and quinone-methide tautomers depending on the medium.     

The Classical Coulomb Problem in Pre-Maxwell Electrodynamics

We explore certain difficulties in the covariant classical mechanics associated with off-shell electrodynamics, through an examination of the classical Coulomb problem. We present a straightforward solution of the classical equations of motion for a test event traversing the field induced by a fixed event (an event moving uniformly along the time axis at a fixed point in space). This solution reveals the essential difficulties in the formalism at the classical level. We then offer a new model of the particle, as a certain distribution of events on the worldline, which eliminates these difficulties and permits comparison of classical off-shell electrodynamics with the standard Maxwell theory In this model, the fixed event induces a Yukawa-type potential, permitting a semiclassical identification of the pre-Maxwell time scale with the inverse mass of the intervening photon. Numerical solutions to the equations of motion are compared with the standard Maxwell solutions—they are seen to coincide when > 10^–6 sec, providing an initial estimate of this parameter.     

Chromobacterium violaceum ω-transaminase variant Trp60Cys shows increased specificity for (S)-1-phenylethylamine and 4'-substituted acetophenones, and follows Swain-Lupton parameterisation

For biocatalytic production of pharmaceutically important chiral amines the ω-transaminase enzymes have proven useful. Engineering of these enzymes has to some extent been accomplished by rational design, but mostly by directed evolution. By use of a homology model a key point mutation in Chromobacterium violaceum ω-transaminase was found upon comparison with engineered variants from homologous enzymes. The variant Trp60Cys gave increased specificity for (S)-1-phenylethylamine (29-fold) and 4'-substituted acetophenones (～5-fold). To further study the effect of the mutation the reaction rates were Swain-Lupton parameterised. On comparison with the wild type, reactions of the variant showed increased resonance dependence; this observation together with changed pH optimum and cofactor dependence suggests an altered reaction mechanism.