Future Trends

The progress of inorganic matrices such as silica and CPG as adsorbents in the downstream processing of biological molecules is hampered by their instability toward alkaline solutions. Sodium hydroxide solution up to 0.5 M concentration is the standard cleaning and sanitizing reagent for chromatography adsorbents used in the separation processes of biomolecules. Furthermore, certain chemistries used in the surface modification of matrices and some purification protocols require alkaline conditions.

Alkaline-resistant inorganic matrices are under development using different approaches. Development of zirco-nia and titania matrices by sol-gel techniques have already been mentioned. Surface coating of silica and CPG with an inert metal oxide such as zirconium oxide and preparation of glass-ceramic matrices may result in suitable products that satisfy all the requirements of an ideal matrix for the purification of biomolecules.

Methods for coating silica matrices with zirconium oxide were studied by Meijers et al. (38) and applied to CPG at Bioprocessing. Great improvement in the stability of the

Table 5. Application of Surface-Modified CPG (PROSEP®) in Separation Processes

Product

Trade name

Application

Protein A adsorbent Protein G adsorbent Heparin adsorbent Lysine adsorbent Metal-chelating adsorbent

Thiophilic adsorbent Immobilized antibodies

PROSEP®-A High Capacity

PROSEP®-G

PROSEP®-Heparin

PROSEP®-Lysine

PROSEP®-Chelating

PROSEP®-Thiosorb

Purification of igG from all species and subclasses except rat igG Purification of igG from all species and subclasses Purification of antithrombin iii and blood clothing factors Purification of plasminogen and tPA

A metal-binding adsorbent used in purification of metal-binding proteins and metal removals from dilute solutions Purification of antibodies

Purification of specific antigens and second antibodies

Note: IgG, immunoglobulin G; tPA, tissue-type plasminogen activator.

Note: IgG, immunoglobulin G; tPA, tissue-type plasminogen activator.

coated CPG to alkaline solutions were observed; however, total stability has not yet been achieved.

A preferred approach for making alkaline-stable inorganic matrices is the glass-ceramic route. Porous glass-ceramic with a skeleton of CaTi4(PO4)6 crystals has been prepared by controlled crystallization of glasses in the CaO-TiO2-P2O5-Na2O system and subsequent acid leaching of the resulting dense glass-ceramic (39). The medium pore diameter has been reported to be controllable in the range of 40 to 150 nm by changing the temperature of the heat treatment. Such an approach is being examined at Bioprocessing in collaboration with a glass-ceramic research center for developing alkaline stable inorganic matrices suitable for downstream processing ofbiological molecules.

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