No abstract

The relation between grain size and grain‐boundary microcracking during cooling in aluminum titanate ceramics was studied. Microcracking temperature was determined by the measurement of thermal contraction and expansion, which was accompanied by acoustic emission. When the ceramics were cooled at a rate of 6°C/min, stress relaxation did not occur below the sintering temperature of 1500°C. The relation between the temperature difference from the sintering temperature to the microcracking temperature and the grain size showed good agreement with the prediction based on the energy criterion of grain‐boundary microcracking.

No abstract

Quartz grains in porcelain bodies cause cracking. Under the present conditions, acoustic emission (AE) has shown that the cracking occurred in a temperature range of 900°–800°C during cooling from firing at a temperature of 1200°C. This cracking can be explained by a large thermal expansion mismatch that was due to the negative thermal expansion of quartz at temperatures >1000°C and no stress relaxation of the glass phase. At a temperature of 573°C, which is the transition temperature of quartz, AE was not detected by the measuring system that was used, although there were many peripheral cracks around the large quartz grains. The energy release rate of the peripheral cracks at a temperature of 573°C was too low to be detected by the equipment that was used.

No abstract

Bending strength and Young's modulus of aluminum titanate ceramics at room temperature to 1300°C were examined. Bending strength increased from 62 MPa at room temperature to 280 MPa at 1100°C. Young's modulus also increased, to 99 GPa at 1100°C. These increments were caused by crack healing. In particular, crack cylinderization occurring at 1000° to 1100°C markedly increased the mechanical strength. The thermal‐hysteresis curves also showed healing of grain‐boundary cracks.

A novel acyl-CoA : cholesterol acyltransferase (ACAT) inhibitor, designated epi-cochlioquinone A has been isolated from the fermentation broth of Stachybotrys bisbyi SANK17777. The molecular formula, physicochemical properties, NMRspectroscopic analysis and X-ray crystallographic analysis revealed that this compoundwas a stereoisomer of cochlioquinone A, which has been previously reported as a nematocidal agent. It inhibited ACATactivity in an enzyme assay using rat liver microsomes with an IC50 value of 1.7pu. However, it showed about 10-fold less potent inhibitory effect on plasma lecithin cholesterol acyltransferase (LCAT) than on ACAT. In addition, it inhibited in vivo cholesterol absorption in rats by 50% at 75mg/kg.

Trehalamine, (3a^,4i?,55',65,6a5')-2-amino-4-(hydroxymethyl)-3a,5,6,6a-tetrahydro-4//-cyclopent[J]oxazole-4,5,6-triol (1) and D-glucose were obtained by acid hydrolysis of trehazolin (3), a trehalase inhibitor produced by actinomycetes. More vigorous hydrolytic treatment of trehazolin afforded an aminocyclitol, ( 1 R,2S,3R,4S,5R)-5-amino-l -(hydroxymethyl)cyclopentane-1 ,2,3,4-tetraol (2). Trehalamine, the aglycon of trehazolin, was also found in the culture broths of two trehazolin producing strains, Micromonospora sp. SANK62390 and Amycolatopsis sp. SANK60791. These trehazolin related compounds trehalamine (1) and 2 were poor inhibitors of trehalase (1; IC501.8 x 10~4 m, 2; > 5.0 x 10~4 m). On the other hand they inhibited more potently rat intestinal sucrase (1; IC50 6.8 x 10~5m) and sweet almond /?-glucosidase (2; IC50 5.6 x 10"6m) than trehazolin.In the previous paper, we reported the isolation and characterization of trehazolin (3), an inhibitor of trehalase produced by Micromonospora sp. SANK623901}. Trehazolin showed a specific and potent inhibitory activity against trehalase and was shown to have a glucosyl moiety and a unique aglycon part in its structure. Since the aglycon portion of trehazolin seems to account for the trehalase inhibitory activity, it will be informative to assess the enzyme inhibitory properties of the aglycon to understand the nature of the activity of trehazolin. Furthermore, in respect to the microbial biosynthesis of the unique metabolite, it is of interest if the aglycon itself is present in the broths of the trehazolin producing microorganisms. Therefore, we attempted to characterize the aglycon and related compounds by hydrolysis of trehazolin and further to discover them from culture broths of trehazolin producers.On the first attempt of acidic hydrolysis of trehazolin at 100°C, the sole product was a 5-membered Fig. 1. The structures of trehalamine (1), 2, and trehazolin (3).