Yoshimasa Hama

Various types of nonbridging oxygen hole center in high‐purity silica glass

Optical absorption measurements of the 2.0‐eV band and photoluminescence measurements of the 1.9‐eV emission, excited by various excitation bands, were carried out on high‐purity silica glasses subjected to γ‐ray irradiation. Two, and possibly three, different forms of nonbridging oxygen hole centers were deconvoluted from the results of the isochronal annealing experiments. The difference in the peak wavelength of the 2.0‐eV absorption and 1.9‐eV luminescence bands among various forms of nonbridging oxygen hole centers is reported.

Defects and optical absorption bands induced by surplus oxygen in high-purity synthetic silica

The nature of excess oxygen in as‐manufactured and γ‐irradiated high‐purity synthetic silicas is investigated. Electron‐spin‐resonance measurements suggest that peroxy radicals ( 3/4 SiOO⋅) could be produced either by the cleavage of peroxy linkages ( 3/4 SiOOSi 3/4 ) or by the reaction of E’centers ( 3/4 Si⋅) with oxygen molecules. The excess oxygen is found to exist in the glass in two forms: as peroxy linkages and as interstitial molecular oxygen. The peroxy linkage is shown to be the cause of optical absorption at 3.8 eV. Heat treatment at 900–1000 °C results in the growth of the 3.8‐eV band, that is, the peroxy linkages, through the reaction of oxygen vacancies and interstitial dioxygen molecules. These results indicate that the 5.0‐ and 3.8‐eV bands (which are characteristic of ‘‘oxygen‐deficient’’ and ‘‘oxygen‐surplus’’ silica, respectively) can coexist in a glass, depending on the synthesis conditions.

Relation between the 1.9 eV luminescence and 4.8 eV absorption bands in high‐purity silica glass

Photoluminescence measurements of the 1.9 eV emission were carried out on high‐purity silica glasses subjected to γ‐ray irradiation. The time decay of the luminescence, when excited by the 4.8 eV band, indicates that the 4.8 eV absorption and the 1.9 eV luminescence are caused by two different defects, and that an energy transfer occurs between the two defects. Comparison with electron spin resonance observations shows that both the nonbridging oxygen hole center (responsible for the 1.9 eV luminescence) and another undetermined defect (responsible for the 4.8 eV absorption) must be present in the glass before the 1.9 eV luminescence band can be excited by 4.8 eV photons.

Transporter Database, TP-Search: A Web-Accessible Comprehensive Database for Research in Pharmacokinetics of Drugs

This Letter to the Editor informs the readers of TPSearch, a unique comprehensive database for membrane transporter proteins that we have constructed to facilitate the study of drug transporters on a broad scale in the world and to provide a research tool for optimization of pharmacokinetic properties in terms of transporters during the early stage of drug development in pharmaceutical companies. During the past decade, there has been a significant increase in the molecular characterization of transporter proteins in animals and humans (1). With newer information on the genetic/genomic studies, this has led to a better understanding of the importance of such transporter proteins as one of the main determinant factors to play a key role in drug disposition; that is, absorption, distribution, and excretion (ADE) of drugs (2–4). Because the amount of available data is rapidly increasing, a need for a publicly accessible database with comprehensive information about all of the known membrane transporters becomes increasingly important. We have constructed TP-Search, a Web-accessible relational database on ADE-associated transporter proteins (http://www.tp-search.jp/), enabling users to search dynamically transporter-related information by chemical structures/ names of substrate/inhibitor/inducers, gene expression, functions, drug-drug interaction involving transporters, and so on. The other databases on transporters, which are currently available, are http://nutrigen.4t.com/humanabc.htm (database on ABC transporters by M. Müller), http://www.med.rug.nl/ mdl/ (database of University Hospital Groningen), http:// www.gene.ucl.ac.uk/nomenclature/genefamily/abc.html (HGNC gene family nomenclature ABC transporters), http:// lab.digibench.net/transporter/ (human membrane transporter database), http://xin.cz3.nus.edu.sg/group/adment/adment.asp (ADME-associated proteins database), and http:// www.mhc.com/PGP/index.html (P-glycoprotein interaction). These have appeared to provide only certain aspects of specific class or group of membrane transporter proteins, whereas TP-Search aims at providing a comprehensive database on drug-transporters. Among those databases, the human membrane transporter database has been intended to support pharmacogenomic studies and so provides much information on sequence variants, altered functions caused by polymorphisms/mutations, and (patho)physiological role and associated diseases (5). The ADME-associated protein database provides comprehensive information on ADMEassociated proteins, which include not only membrane transporter proteins involved in drug disposition, but also other proteins, such as plasma proteins, intracellular binding proteins, and drug metabolizing enzymes (6). Other transporter databases listed above are mainly focused on the information for ABC-transporters. Our methodology was as follows. Membrane transporter proteins were selected from a comprehensive search of available literature consisting of research papers, review articles, pharmacology textbooks, and other relevant publications (via PubMed; http://www.ncbi.nlm.gov/PubMed/), resulting in approximately 1,940 articles published from 1968 to 2002. The system is a typical Web application built on Application server, Web server, and Relational Database Management System (RDMS) to provide the services via the Internet. The user connects with the URL at http://www.tp-search.jp/ by a Web browser such as Netscape Navigator or Microsoft Internet Explorer. The database, TP-Search, contains information on more than 75 membrane transporters (Table I), including cDNA and amino acid sequences, gene family, putative membrane topology, driving force, transport direction, substrate/ inhibitor/inducer (chemical structures and kinetic data, i.e., Km/Ki), and tissue distribution in humans as well as in mice and rats, and drug-drug interactions involving transporters. All information available in this database is linked to the original references in PubMed, which ensures the users can confirm the validity of data and to obtain more detailed information available in the original references. Terminology regarding genes often causes confusion. In our database, we use primarily the “Nomenclature of Mammalian Transporter Genes,” (http://www.gene.ucl.ac.uk/cgibin/nomenclature/searchgenes.pl/), such as the solute carrier superfamily (SLC) and ATP-binding cassette transporters (ABC). These standardized gene names, accompanied by conventional names, are both given in this database. The sequential information for transporters posted in the database was available through “Locus-Link” (http://www.ncbi.nlm. nih.gov/LocusLink/), and names and structures of the compounds, such as substrates/inhibitors/inducers, were searched through “Chem-Link” and “Japanese Accepted Names for Pharmaceuticals” (http://moldb.nihs.go.jp/jan/). TP-Search is searchable by transporter name, tissue name (liver, kidney, intestine, brain, and expression in cell line; Fig. 1), substrate/inhibitor/inducer name, and drug-drug interaction. Implemented as a relational database, searches involving any combination of these options or selection field are also supported. Because drug transporters have demonstrated a broad substrate specificity, drug-drug interaction involving these transporters is considered very likely. Approximately 1,200 Fig. 1. Example of search results on transporters expressed in human and rat kidney. Pharmaceutical Research, Vol. 21, No. 11, November 2004 (© 2004) Letter to the Editor

Effect of implanted ion species on the decay kinetics of 2.7 eV photoluminescence in thermal SiO2 films

Decay kinetics of photoluminescence (PL) existing around 2.7 eV has been studied in various ion‐implanted thermal SiO2 films as a function of implantation conditions. The PL observed in many samples shows decay constants shorter than 10 ms, which is a well‐observed decay constant for silica glass. The change in the decay constant and that in the PL intensity have been found to be systematically related with the mass and the dose of the implanted ions. Therefore, despite the short decay constant, the present 2.7 eV PL is attributable to a triplet‐to‐singlet transition of oxygen deficient centers, as in the case of silica glass. The rapid decay is interpreted as the increase in spin‐orbit coupling interaction due to structural deformations by ion implantation such as the formation of paramagnetic defects and/or densification.

Studies on the recombination of cation—electron pairs by long-range tunneling, as studied by ITL measurement in irradiated polymers

Abstract Measurement of isothermal luminescence (ITL) of irradiated polyethylene terephthalate (PET) and polyethylene adipate (PEA) was carried out at low temperature over a long period of time after irradiation. The ITL decay obeys the law I ( t ) = I 0 /(1 + at ) m as a function of time. All of the parameters, I 0 , a and m , depend on irradiation time and dose rate under which the sample was irradiated. These parameter values for PET are determined in detail. The ITL process could be interpreted as due to recombination of cation—electron pairs through electron tunneling to the cation. The distribution of the separation distances of cation—electron pairs could be obtained by Laplace inverse transformation of the ITL decay function based on an electron tunneling model.

Gamma-Ray Induced 2 eV Optical Absorption Band in Pure-Silica Core Fibers

The mechanism of optical absorption near 2 eV induced by γ-irradiation in pure silica glass has been studied by many workers1–6. It appears that non-bridging oxygen hole centers (NBOHC), which were revealed by Friebele et al. through the study of electron spin resonance (ESR) spectra1, are the most probable factor causing the optical absorption2–5. However, it is also clear that the 2 eV band cannot be ascribed to only one kind of defect center, because, as the OH-group content of the sample increases, the peak wavelength shifts from 630 nm to 600 nm3,5 and the oscillator strength decreases3. Friebele et al. thus assumed that the 2 eV band is caused mainly by NBOHC in high-OH silica (i.e., silica which contains a high amount of OH groups) and by a non-paramagnetic center such as ≡ Si:- in low-OH silica3. Contrary to the above assumption, the present authors thought that the difference in the 2 eV band induced in high-OH silica and in low-OH silica is caused by hydrogen bonds between the NBOHC and an OH group near the NBOHC, and that the 2 eV band in both low- and high-OH silica is caused only by NBOHC2. In this paper, experimental results are presented followed by discussion of the model derived from these results.

Si—O—Si strained bond and paramagnetic defect centers induced by mechanical fracturing in amorphous SiO2

Mechanical stress is applied to high‐purity amorphous SiO2 samples by means of fracturing. The electron‐spin‐resonance spectra suggest that the formation of a majority of paramagnetic defects is from cleavage of Si—O—Si bridges in the glass network, but there are some sample‐to‐sample variances in fracture‐induced paramagnetic defects, suggesting cleavage of differing chemical bonding states in the samples. Nonstoichiometric bonds, ≡Si—Si≡ and ≡Si—O—O—Si≡, are assumed to be one reason for the sample dependency. Formation of Si—O—Si strained bonds from mechanical fracturing is confirmed from sequential γ‐ray irradiation and heat annealing experiments. The Si—O—Si strained bond is approximately annealed at about 300 °C. By comparing the fracture‐induced defects for glass preforms and optical fibers, the change in chemical bonding state can be analyzed. Analysis of mechanical‐fracture‐induced defects is a strong technique for elucidation of the chemical bonding state of silica glass.

Compact soft x-ray source using Thomson scattering

A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2m2. An infrared laser beam (λ0=1047nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.

Photoreduction of iron(III) tetraphenylporphyrin in ethanol studied by laser flash photolysis: effects of concentration on quantum yields

Chloroiron(III) tetraphenylporphyrin, CIFeIIITPP, in deaerated ethanol is photoreduced to yield iron(II) tetraphenylporphyrin, FeIITPP, upon irradiation with UV light. The 355 nm laser photolysis studies have confirmed that the initial yields for the formation of FeIITPP are markedly decreased with an increase in the concentration of CIFeIIITPP: ϕ= 0.015 at 5.35 × 10–6 mol dm–3 CIFeIIITPP and ϕ= 0.01 at 1.0 × 10–4 mol dm–3 CIFeIIITPP. The effects of the concentration on the quantum yields were interpreted in terms of the formation of photoreactive iron(III) porphyrin at low concentrations of CIFeIIITPP.