Separation of PEGylated variants of ribonuclease A and apo-¿-lactalbumin via reversed phase chromatography
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The covalent attachment of polyethylene glycol (PEG) molecules to pharmaceutical proteins, "PEGylation", often results in a population of conjugate species that includes differing numbers and locations of attached PEG chains. As some portion of this population may be biologically inactive, a challenging separation problem arises. An interesting alternative to the size-based resolution of these conjugates involves the use of reversed phase chromatography (RPC), treating the PEG moieties as hydrophobic purification tags. The use of RPC raises concerns about protein denaturation in the mobile and on the stationary phase. Here, the potential dual role of conjugated PEG chains as both group-specific separation tags and as steric or structural stabilizers in RPC was explored. In this work, RPC with C18-based media was used to resolve PEGylation number variants of ribonuclease A (RNase A) and apo-¿-lactalbumin (apo-¿Lac) in a neutral pH mobile phase. While the attachment of 20. kDa PEG molecules did not modify the structures of RNase A and apo-¿Lac, as confirmed by structural analysis using circular dichroism, exposure to the mobile phase modifier, acetonitrile, and to the C18 media during separation resulted in perturbations to both the secondary and tertiary structures of all species studied. RNase A experienced small perturbations that were mediated to some extent by PEGylation; these results were consistent with activity assays which showed that PEGylated RNase A species retained native-like activity after RPC separation. Apo-¿Lac, a more hydrophobic and less stable protein than RNase A, experienced extensive structural perturbations regardless of PEGylation state. The temperature of the mobile phase was found to strongly influence chromatographic separation of PEG-conjugates with conjugate species becoming more strongly retained with increasing temperature. This work shows that it is feasible to employ RPC with neutral pH mobile phases to resolve PEG conjugate number heterogeneity. © 2014 Elsevier B.V.