Theory of the structural fluctuation of Au55 clusters

Stabilities and structural fluctuations of both neutral and charged Au₅₅ clusters are examined and discussed in relation to recent experimental observations of small gold particles with an electron microscope. Transition probabilities between the icosahedral and cuboctahedral structures are calculated according to the transition state theory using a model potential consisting of attractive many-body, repulsive pairwise and Coulomb parts. It is shown that for a neutral cluster the cuboctahedral structure has too short life time to be observed around room temperature and that, on the other hand, for more than 6-fold multiply charged clusters, both structures have life times of the order of 0.1 s around room temperature and, therefore, the transition between them can be observed.     

One-step analysis of testis steroidogenesis from neonatal exposure to synthetic estrogen by normal-phase HPLC

Neonatal exposure to synthetic estrogen endocrine disruptors or estrogen-receptor inhibitors induces developmental abnormalities in the male reproductive system. To investigate whether neonatal exposure affects spermatogenesis in juvenile and pubertal testis, Sprague-Dawley rat pups were given synthetic estrogen endocrine disruptors or estrogen-receptor inhibitors by a single injection on the day of birth at concentrations ranging between 2 to 40 mm, and sacrificed on day 21 (juvenile), 35 (prepuberty) or 50 (puberty). The testes were weighed and examined histologically at each stage. Further, the metabolites of steroidogenesis were analyzed using normal-phase high performance liquid chromatography. Neonatal exposure significantly reduced testis weights and steroidogenesis to one- fifth to one-half of that of the juvenile control, and further suppressed irreversible steroidogenesis and spermatogenesis during puberty.     

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.     

Lipid-soluble and water-soluble arsenic compounds in blubber of ringed seal (Pusa hispida)

High concentrations of arsenic were observed in the blubber of ringed seals (Pusa hispida) in our previous study. To better understand the arsenic accumulation in blubber of marine mammals, arsenicals in the blubber of ringed seal were characterized using high performance liquid chromatography–inductively coupled plasma mass spectrometry (HPLC–ICPMS). The most predominant water-soluble arsenical in the blubber was dimethylarsinic acid (DMA), in spite of the predominance of arsenobetaine in other tissues. Lipid-soluble fraction was hydrolyzed under mild (tetraethylammonium hydroxide (TEAH) hydrolysis) and strong (NaOH hydrolysis) conditions, and then an aliquot of hydrolysate was injected onto HPLC–ICPMS. Both TEAH-labile and TEAH-stable/NaOH-labile lipid-soluble fractions contained precursors of DMA. These results suggest that the blubber might be the pool of DMA and DMA-containing precursors in ringed seals.