Observations on chemical and enzymatic approaches to α-2,3-sialylated octyl β-lactoside

A comparison of chemical and chemo-enzymatic syntheses of alpha-2,3-sialylated octyl lactoside is reported. the chemical approach, starting from lactose and sialic acid, required 14 steps and proceeded in 5% overall yield; poor a-selectivity in the sialylation step necessitated a difficult and low yielding separation of anomers. A chemoenzymatic approach, employing recombinant Trypanosoma cruzi trans-sialidase to effect the key sialylation reaction, required 10 steps and gave a similar overall yield. Whereas the chemo-enzymatic synthesis required only three chromatographic purification steps overall, the chemical synthesis required at least nine. (C) 2002 Elsevier B.V. All rights reserved.Univ St Andrews, Sch Chem, St Andrews KY16 9ST, Fife, ScotlandUniv St Andrews, Ctr Biomol Sci, St Andrews KY16 9ST, Fife, ScotlandEscola Paulista Med, Disciplina Biol Celular, BR-04023062 São Paulo, BrazilEscola Paulista Med, Disciplina Biol Celular, BR-04023062 São Paulo, BrazilWeb of Scienc

Effects of Growth Rates and Compositions on Dendrite Arm Spacings in Directionally Solidified Al-Zn Alloys

Dendritic spacing can affect microsegregation profiles and also the formation of secondary phases within interdendritic regions, which influences the mechanical properties of cast structures. To understand dendritic spacings, it is important to understand the effects of growth rate and composition on primary dendrite arm spacing (lambda(1)) and secondary dendrite arm spacing (lambda(2)). In this study, aluminum alloys with concentrations of (1, 3, and 5 wt pct) Zn were directionally solidified upwards using a Bridgman-type directional solidification apparatus under a constant temperature gradient (10.3 K/mm), resulting in a wide range of growth rates (8.3-165.0 mu m/s). Microstructural parameters, lambda(1) and lambda(2) were measured and expressed as functions of growth rate and composition using a linear regression analysis method. The values of lambda(1) and lambda(2) decreased with increasing growth rates. However, the values of lambda(1) increased with increasing concentration of Zn in the Al-Zn alloy, but the values of lambda(2) decreased systematically with an increased Zn concentration. In addition, a transition from a cellular to a dendritic structure was observed at a relatively low growth rate (16.5 mu m/s) in this study of binary alloys. The experimental results were compared with predictive theoretical models as well as experimental works for dendritic spacing. (C) The Minerals, Metals & Materials Society and ASM International 201