Intramolecular energy transfer in compounds with two 1-pyrenoate groups separated by methylene spacers

Steady-state fluorescence anisotropy (r) and fluorescence lifetime () measurements have been used to study the efficiency of nonradiative singlet energy transfer as a function of alkane size in 1-pyrenecarboxylic acid alkanediyl esters (as a function ofm in Py-COO-(CH₂) _m -OOC-Py, where Py denotes pyrene substituted in the 1-position, andm=2–6). Experiments were performed in media of different viscosity, , obtained by changing the temperature (from –20 to 40°C) of dilute solutions in ethylene glycol and by examination of the compounds in a solid matrix of poly(methyl methacrylate) (PMMA) at ambient temperature. The Py-COO-(CH₂)_m-OOC-Py exhibit intramolecular excimer emission in ethylene glycol at these temperatures, but the intensity of this emission is much lower than when these compounds are placed in common solvents of lower . The values of indicate that excitation hopping or intramolecular energy transfer takes place between the chromophores attached to the ends of the alkane bridges. Values ofr obtained by the extrapolationsT/0 orT/0 in ethylene glycol, as well as the values obtained in the rigid matrix of PMMA, show very little dependence onm. A theoretical conformational analysis, using the rotational isomeric state (RIS) model, was also performed. The combination of the experimental results forr in the media of high with the theoretical (RIS) analysis produces an estimated value of 21 ±2 Å for the Förster radius in Py-COO-(CH₂)_m-OOC-Py.     

Ion-molecule reactions of 2-methoxyethanol with some amines, carboxylic acids and amino acids under chemical ionization conditions

2-Methoxyethanol chemical ionization of amines, carboxylic acids and amino acids has been found to produce numerous adduct ions. The most intense adduct ions for amines are [M + H](+) and [M + 77](+), for carboxylic acids [M + 27](+), [M + 59](+) and [M + 77](+), and for amino acids [M + H](+), [M + 13](+), [M + 27](+) and [M + 77](+). Either the adduct ion [M + H](+) or [M + 77](+) was the most abundant ion found for amino acids. The proton affinities of amino acids are noticed to control the formation of the [M + H](+) and [M + 77](+) ions. The relative abundance of [M + 13](+) and [M + 27](+) ions varied for different amino acids being most intense for phenylalanine and aspartic acid. Copyright 1999 John Wiley & Sons, Ltd.     

Status of electroweak phase transition and baryogenesis

I review recent progress on the electroweak phase transition and baryogenesis, focusing on the minimal supersymmetric Standard Model as the source of new physics.     

Defect induced asymmetric pit formation on hydroxyapatite

Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.