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.

Interest in producing large quantities of plasmid DNA has recently increased as a result of the rapid evolution of gene therapy and DNA vaccines. Hydrophobic chromatography is a popular technique in the downstream processing of plasmid DNA. However, the low capacity and the high mass transfer resistance of most commercially available packings for bio-macromolecules limit their application in a large-scale process. In this work, a hydrophobic absorbent with wide pores was synthesized by the solid porogenic method. Analyses by scanning electron microscopy and mercury intrusion porosimetry revealed that the matrix contained two families of pores, i.e., micropores smaller than 100 nm and superpores of 500-7300 nm. The superpores provided not only convective flow channels for the mobile phase, but also a large surface for biomolecules binding. So the chromatographic process can be operated at high flow rate with high column efficiency and low backpressure as identified on a 2-mL column (5mm i.d., 2cm length). With a loading up to 2.6 mg of 5.4 kb plasmid (pcDNA3) in 8mL feedstock and operated at a flow rate as high as 20 cm/min, nearly 100% of plasmid was recovered with a purity of 100%. The results indicate that the hydrophobic medium is promising for high-speed purification of plasmid DNA.

Lysozyme adsorption and purification by expanded bed chromatography of a customized Nd-Fe-B alloy-densified agarose (NFBA) gel modified with Cibacron Blue 3GA (CB) were investigated, and the results were compared with those obtained with CB-modified Streamline gel (CB-Streamline). The NFBA gel had a mean size of 102m and a mean density of 1.88g/ml. The breakthrough behavior of lysozyme was modeled considering the dispersion of the liquid and solid phases and the diffusive mass transport of protein to the solid phase. Using independently determined parameters, the model prediction agreed reasonably well to the experimental data. Although the two dye-ligand adsorbents showed nearly the same static capacity, the dynamic binding capacity of the CB-NFBA gel was nearly twice that of the CB-Streamline gel. Moreover, lysozyme was purified from chicken egg white solution by the expanded beds with the two adsorbents, and the expanded bed with the CB-NFBA gel produced much larger purification factor than that with the CB-Streamline gel.All the results indicated that the small-sized dense medium CB-NFBA gel was more efficient as an expanded bed adsorbent.

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.

Sorbitan trioleate (Span 85) modified with Cibacron Blue F-3GA (CB) was used as an affinity surfactant (CB-Span 85) to form affinity-based reversed micelles in n-hexane. It was found that the addition of hexanol to the reversed micellar system resulted in a significant increase in water content and hydrodynamic radius of the affinity-based reversed micelles. Moreover, the reversed micelles with hexanol revealed broader aggregation number distribution and larger average aggregation number than the reversed micelles without hexanol addition. This is considered to be due to the decreases in the micellar curvature and rigidity of the micellar interfacial layer and the increase in the micellar interfacial fluidity. Consequently, the solubilization capacity of lysozyme increased about 70% in the reversed micellar solution with 3 vol% hexanol. On the other hand, the capacity of BSA was only 30% increased under the same conditions due to its larger molecular size than lysozyme. Kinetic analysis revealed that the increase in the micellar interfacial fluidity in the presence of hexanol resulted in faster release of lysozyme from the micelles, thus leading to an increase of the overall volumetric mass transfer coefficient in the back extraction.

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.

This article reports oxidative protein refolding assisted by artificial chaperones in reverse micelles formed by nonionic surfactant of sorbitan trioleate modified with Cibacron Blue F-3GA in n-hexane. The denatured/reduced lysozymewas used as a model protein and cetyltrimethylammonium bromide (CTAB) and-cyclodextrin as the artificial chaperones. The use of the artificial chaperones has proved to increase the refolding efficiency of denatured-reduced lysozyme at the concentration range studied (3.5-5.9 mg/mL). Moreover, the artificial chaperones increased the refolding yield in a wide range of urea concentrations. However, the optimal urea concentration range was little affected by the presence of the artificial chaperones.

A heptapeptide phage display library was screened with insulin to find its ligands for affinity chromatography. The peptide was synthesized and coupled to EAH Sepharose 4B (5.4umol mL-1 bed). Then, insulin chromatography was carried out with mobile phases of different pH values and by the addition of urea and ethylene glycol. It was found that electrostatic interactions were predominant for the affinity binding, and hydrogen bonding might also contribute somewhat to the affinity. Finally, frontal analysis was performed and the dynamic binding capacity of the affinity column for insulin at 50% breakthrough was estimated at 60.6mg mL-1 bed, which was about two times higher than the theoretical binding capacity of the monomeric insulin. The result suggests that insulin was bound in dimer state in a stoichiometric relationship with the coupled peptide, indicating the high binding efficiency of the peptide ligand for insulin.

Sorbitan trioleate was modified with Cibacron Blue F-3GA (CB) to create an affinity surfactant and to form affinity-based reverse micelles in nhexane. The partitioning equilibria and the extraction kinetics of lysozyme and bovine serum albumin (BSA) were then examined. The solubilization capacity of the reverse micellar system for lysozyme increased linearly with increasing the CB concentration from 0.1 to 0.5 mmol L−1. In contrast, the capacity for BSA at 0.5 mmol L-1 of coupled CB was only about one-fifth that for lysozyme. It indicates a strong steric hindrance effect of the micelles for the high molecular mass protein. The overall volumetric mass transfer coefficient of lysozyme in the forward extraction increased from 0.43×10−3 to 1.25×10−3 s−1 with increasing CB concentration from 0.1 to 0.5 mmol L−1. Due to the high molecular mass of BSA, its volumetric mass transfer coefficient in the forward extraction was only one-sixth that of lysozyme. The ratio of the coefficient in the back extraction to that in the forward extraction was less than 0.03, much lower than those in other micellar systems. It indicates that the interfacial resistance in this system was severer than in others.

利用多角度激光光散射仪和圆二色谱仪研究小分子水溶性一元醇和多元醇对溶菌酶分子的稳定性以及对变性溶菌酶的空间构象和折叠复性的影响。发现在一元醇和二元醇溶液中溶菌酶分子的稳定性均随着醇浓度的增大而提高，而变性酶分子在醇溶液中形成过多的α螺旋和β折叠等二级结构不利于酶的折叠复性；只有较低浓度的多元醇和甲醇有辅助变性溶菌酶复性的作用，其中10%甲醇、20%甘油和30%乙二醇有利于促进酶的复性。