Supports isoelectric point

To an extent the surface charges are determined by the pH of the solution, and by the isoelectric point of the oxide, i.e. the pH at which the oxide surface is neutral. The surface is negative at pH values below the isoelectric point and positive above it. Obviously, the charged state of the surface enables one to bind catalyst precursors of opposite charge to the ionic sites of the support. 

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

This picture, although conceptually useful, is too simple. Kim et al. [43] showed that it is not the overall pH of the solution that dictates which species deposit on the surface, but the local pH at the support interface. The latter depends on the isoelectric point of the support, the coverage of molybdenum species and the number of NH4+ or H+ counter ions of the negative complexes. The reader is referred to [43] for details. 

Several factors are involved in the wide variation in tumor inhibitory activity of asparaginases from different sources (98). One obvious possibility is the rate of clearance of the enzyme from the circulation of the host animal, and Broome (26) was the first to obtain evidence implicating half-life as a factor in antitumor effectiveness. Guinea pig serum asparaginase, for example, has a half-life time of 11-19 hr, while a partially purified yeast asparaginase preparation without antilymphoma activity is almost completely cleared within 30 min. Differences in half-life alone cannot explain the differences in antitumor activity in all cases, however and the question still remains as to what structural features are responsible for rapid or slow clearance. Mashburn and Landin (85) have suggested that differences in half-life time may be related to the isoelectric point of the enzyme, and evidence also exists to support the idea that the tumor inhibitory activity of some asparaginases is related to their K, values (59, 67). 

In addition to anion and cation exchangers as enzyme carriers it has been demonstrated that mixed ion exchange supports can be used for binding enzymes with both acid and amino groups at pH values close to the isoelectric point, such as penidUin G acylase from Escherichia coli (Figure 2.4) ]49]. 

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