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Abstract: Many therapeutic biologics are formulated with excipients, including the protein excipient human serum albumin (HSA), to increase stability and prevent protein aggregation and adsorption onto glass vials. One biologic formulated with albumin is interferon alpha-2b (IFN α-2b). As is the case with other therapeutic biologics, the increased structural complexity of IFN α-2b compared to a small molecule drug requires that both the correct chemical structure (amino acid sequence) and also the correct secondary and tertiary structures (3 dimensional fold) be verified to assure safety and efficacy. Although numerous techniques are available to assess a biologic''s primary, secondary and tertiary structures, difficulties arise when assessing higher order structure in the presence of protein excipients. In these studies far UV circular dichroism spectropolarimetry (far UV-CD) was used to determine the secondary structure of IFN α-2b in the presence of a protein excipient (bovine serum albumin, BSA). We demonstrated that the secondary structure of IFN α-2b remains mostly unchanged at a variety of BSA to IFN α-2b protein ratios. A significant difference in alpha helix and beta sheet content was noted when the BSA to IFN α-2b ratio was 5:1 (w/w), suggesting a potential conformational change in IFN α-2b secondary structure when BSA is in molar excess. [Copyright &y& Elsevier]