The refolding kinetic behavior of denatured-reduced lysozyme in the presence of folding aids (acetamide, acetone, thiourea, l-arginine or glycerol) was studied utilizing a simplified model describing the competition between first-order folding reaction and third-order aggregation. It was found that the protein folding aids could be categorized into two groups. One of them at proper concentrations, such as acetamide, acetone, thiourea and l-arginine, stabilized unfolded protein or folding intermediates. In the presence of these additives, the folding rate decreased with increasing their concentration, and there existed a concentration where the aggregation rate constant was minimized. So, there was an optimum concentration for the folding aids to produce a high yield. The other group was protein stabilizers such as glycerol. In the presence of this kind of folding aids, both the refolding rate and yield were enhanced by increasing their concentration to a proper value. Moreover, their effect on improving protein refolding was additive to those of the first group. So the cooperative application of the two kinds of folding aids could result in favorable refolding rate and yield of protein.

The simplified kinetic model that assumes competition between first-order folding and third-order aggregation was used to model the fed-batch refolding of denatured-reduced lysozyme. It was found that the model was able to describe the process at limited concentration ranges, i.e., 1-2 and 5-7mg mL-1, respectively, at extensive guanidinium chloride (GdmCl) concentrations and controlled concentrations of oxidizing and reducing agents. The folding or aggregation rate constant was different at the two protein concentration ranges and strongly dependent on the denaturant concentration. As a result, both rate constants at the two concentration ranges were expressed as functions of GdmCl concentration. The rate constants determined by fed-batch experiments could be employed for the prediction of the fed-batch process but were not able to be extended to a batch refolding by direct dilution. Computer simulations show that the denaturant concentration and fed-batch flow rate are important factors influencing the refolding yield. Prolonged fed-batch time is beneficial to keep the transient intermediate concentration at a low level and to increase the yield of correctly folded protein. This is of importance when the denaturant concentration in refolding buffer solution is low. Thus, at a low denaturant concentration, fed-batch time should be sufficiently long, whereas at an appropriately high GdmCl concentration, a short fed-batch time or a high feed rate of the denatured protein is effective to give a high refolding yield.

An efficient procedure for the selection of high affinity clones from a heptapeptide phage display library was developed. Lysozyme was used as a model protein to demonstrate the selection strategy. Effect of bovine serum albumin (BSA) concentration on screening the phage library was discussed and proper BSA concentration on plate blocking was determined. The elution procedure was improved by alternatingly eluting the bound phages with glycine-HCl buffer (pH 2.2) and high-concentration target protein solution. The modified method was compared with others including the conventional protocol, and the results confirmed that the modified procedure could yield high affinity phages that might be lost by other screening methods. Through comparison of the DNA sequences of foreign peptides of the clones showing specificity to lysozyme molecules, the HWWW motif was found to be the necessary amino acid sequence for the affinity. The electrostatic and hydrophobic interactions are considered to contribute to the affinity for the protein. Moreover, protein chromatography with the immobilized HWWWPAS on Sepharose gel indicated the strong binding affinity of the peptide for lysozyme.