Glycine, extreme conditions

Enzymes do have their disadvantages. Their solvent incompatibility and instability restrict their industrial use. However, researchers strive to find means to overcome such drawbacks to make use of the catalytic efficiency of enzymes. Aims Biologies (1997) has developed cross-linked enzyme crystals (CLECs) to increase the versatility of enzymes in organic reactions. CLECs exhibit a high level of stability in extreme conditions of temperature and pH and in exposure to both aqueous and organic solvents. The synthesis of the antibiotic cephalexin was carried out using CLECs (see Fig. 3.21). The N-protection step of methyl phenyl glycinate in the classical synthesis was eliminated. 

Under proper conditions, Cu(Il) Ion selective electrodes are well suited to extreme complexation conditions. Our glycine complexation experiments at low pH and at high pH produced very similar results In spite of extremely different extents of Cu(II) complexation between the two types of experiments. 

Best dissociation conditions vary with the antibodies (Section 8.2). Most techniques depend on the deformation of the interacting surfaces of the antibody and antigen, obtained with chaotropic ions (3.5 M potassium thiocyanate in 100 mM phosphate buffer, pH 6.6), organic acids with low surface tension (propionic acid, acetic acid. Section 8.2), denaturants (8 M urea, 7 M guanidine-HCl), and pH extremes (100 mM glycine-HCl, pH 2.5). 

On notera, en particulier, la conversion assez active de la glycine et de la lysine, aminoacides dont la dissimilation par le tissu hepatique in vitro est extremement lente sous des conditions experimentales differentes. 

Another clever approach by Masamune et al. is an asymmetric aldol reaction of the ketene silyl acetal 32. This reaction is effectively promoted by 20 mol % of chiral borane complexes 33, prepared from BH3 -THF and a,a-disubstituted glycine tosylamide 34, undo-reaction conditions in which aldehydes are slowly added to the reaction mixture (Scheme 8B1.20, Table 8B1.12). The catalysts and the reaction conditions have been designed according to the proposed catalytic cycle shown in Scheme 8B1.21. Thus, the use of a geminally disubstituted catalyst accelerates the ring closure of intermediate 35 as expected from the Thorpe-Ingold effect, and the slow addition of the aldehyde reduces the accumulation of 35, which might catalyze the aldol reaction with low enantioselectivity. Extremely... 

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