Fred van Rantwijk (1943) studied organic chemistry at the Delft University of Technology, where he remained as a staff member. He received his Ph.D. in 1980, for work under the guidance of Professor H. van Bekkum. Since the late 1980s he has been working on the application of enzymes in organic synthesis. His particular research interests are the use of enzymes in nonnatural reactions and nonnatural media, enzyme immobilization, and transformations using multi-enzyme systems.Roger Sheldon (1942) received a Ph.D. in organic chemistry from the University of Leicester (U.K.) in 1967. This was followed by postdoctoral studies with Prof. Jay Kochi in the U.S. From 1969 to 1980 he was with Shell Research in Amsterdam, and from 1980 to 1990 he was R&D Director of DSM Andeno. In 1991 he moved to his present position as Professor of biocatalysis and organic chemistry at the Delft University of Technology (The Netherlands). His primary research interests are in the application of catalytic methodologiesshomogeneous, heterogeneous, and enzymaticsto organic synthesis, particularly in relation to fine chemicals production. He developed the concepts of E factors and atom utilization for assessing the environmental impact of chemical processes.

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The broad applicability of the cross-linking of enzyme aggregates to the effective immobilisation of enzymes is demonstrated and the influence of many parameters on the properties of the resulting CLEAs is determined. The relative simplicity of the operation ideally lends itself to high-throughput methodologies. The aggregation method was improved up to 100% activity yield for any enzyme. For the first time, the physical structures of CLEAs are elucidated.

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Cross-linked enzyme aggregates (CLEAs) were prepared from several enzymes (penicillin G acylase, hydroxynitrile lyase, alcohol dehydrogenase, and two different nitrilases) by precipitation and subsequent cross-linking using dextran polyaldehyde. In most cases, higher immobilization yields were obtained using the latter cross-linker as compared with the commonly used glutaraldehyde. Active site titration of penicillin acylase CLEAs showed that the higher activity originated from a significantly lower loss in active sites using dextran polyaldehyde as a cross-linking agent. It is proposed that macromolecular cross-linkers are too large to penetrate the protein active site and react with catalytically essential amino acid residues.

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