Secondary structure of interleukin‐2(Alal25) in unfolded state*

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Abstract
Secondary structure of interleukin‐2(Alal25) in unfolded state was examined by circular dichroism (CD). Unfolding of tertiary structure of the protein, as determined by CD, was observed when the solvent pH was decreased below 3.0 or the disulfide bond was reduced. Consistent with the CD results, a stronger fluorescence enhancement of 1‐anilinonaphthalene‐8‐sulfonic acid was observed on acidification or reduction of interleukin‐2(Alal25) relative to that of the native protein, indicating a larger hydrophobic surface exposed to solvent. However, the secondary structure was fully retained in 5% acetic acid or aqueous HCl, pH 3.0. It seemed that α‐helical content of the protein is even greater at pH 2.0. Reduced protein showed a far u.v. CD spectrum indistinguishable from the oxidized one at pH 4.0. These results suggest that the secondary structure of interleukin‐2(Alal25) does not require tertiary structure.