CD spectroscopy analysis reveals that Vif mutant complexes are structurally different from wild-type. Purified Vif complexes including 6X-His-CBF-β (residues 1–182) and Elo B/C were purified by nickel affinity and size exclusion chromatography. Each complex was analyzed by circular dichroism spectroscopy. A) CD spectra for Vif wild-type and mutant complexes showed a distinction in the minima at 208 and 222 for mutant complexes that do not bind Cul5, suggesting these mutants have more alpha helical structure. B) Spectra analysis reveals differences between wild-type and mutant complex secondary structure and confirms that the mutants that do not bind Cul5 have a higher percentage of alpha helical structures; however, the percentage of beta-sheet structures is reduced.