NAMS 2002 Workshop - ICOM 2008

Membrane Modeling III - Process Simulations – 1 – Keynote
Wednesday July 16, 9:30 AM-10:15 AM, O’ahu/Waialua
Biopolymer Transport in Ultrafiltration: Role of Molecular Flexibility
A. Zydney (Speaker), The Pennsylvania State University, University Park, Pennsylvania, USA -
zydney@engr.psu.edu
J. Molek, The Pennsylvania State University, University Park, Pennsylvania, USA
D. Latulippe, The Pennsylvania State University, University Park, Pennsylvania, USA
Ultrafiltration is used extensively for the purification and concentration of a wide
range of biomolecules including natural proteins, enzymes, diagnostic antibodies,
and therapeutic proteins. These proteins typically have a dense hydrophobic
core, giving them a highly globular structure with relatively little molecular
flexibility. Consequently, the transport characteristics of these biomolecules are
traditionally described using a hard sphere analysis accounting for the steric,
hydrodynamic, and long-range (electrostatic) interactions in the membrane
pores. In contrast, polymer transport in membrane systems has typically been
described using models that account for the flow-induced elongation of the
flexible polymer chain. There is growing interest in second generation
biotherapeutics including PEGylated proteins, in which one or more long
polyethylene glycol (PEG) chains are covalently attached to a therapeutic
protein, as well as plasmid DNA, with the latter of interest in both gene therapy
applications and for DNA-based vaccines. These molecules have more complex,
and potentially flexible, morphologies. The objective of this study was to examine
the role of molecular flexibility in the transport of these novel biomolecules
through semipermeable ultrafiltration membranes.
PEGylated alpha-lactalbumin was produced by covalent attachment of an
activated polyethylene glycol, having molecular weight of 5, 10, or 20 kDa. A 3.0
kilobase pair plasmid was obtained from Stratagene and prepared by Aldevron.
Ultrafiltration experiments were performed in a stirred cell using composite
regenerated cellulose membranes provided by Millipore. Biomolecule
transmission was evaluated as a function of both filtrate flux and stirring speed to
independently control the degree of concentration polarization and flow-induced
elongation. Data were analyzed using both hydrodynamic models for hardsphere
solutes and flow-induced elongation models for flexible polymers.
The extent of plasmid transmission was a very strong function of the filtrate flux,
with minimal transmission below a critical value of the flux. This critical flux was
in good agreement with theoretical models accounting for the flow-induced
plasmid elongation, suggesting that these large plasmids behave as nearly
infinitely flexible polymers. In contrast, transmission of the PEGylated proteins at
low flux was dominated by hard-sphere interactions, with the polyethylene glycol