Conformation of insulin and its fragments in surfactant solutions

The conformation of insulin, A and B chains and des(B23-30)octapeptide insulin (DOI) in surfactant solutions was studied by circular dichroism (CD). Addition of sodium dodecyl sulfate (NaDodSO4), dodecylammonium chloride (DodNH3Cl) or dodecyltrimethylammonium chloride (Dod(CH3)3NCl) slightly alters the CD spectrum of insulin, but not DOI. The CD of insulin in NaDodSO4 solution resembles that of DOI without the surfactant, suggesting the dissociation of the COOH-terminal antiparallel beta-form of insulin oligomers (DOI lacks B23-30 and exists as monomers). Reduced insulin and DOI lose their ordered structure in water, but their helical conformation is restored in surfactant solutions. Reduced DOI in NaDodSO4 solution appears to be more helical than DOI with the disulfide bonds intact as judged from their ellipticities. Oxidized A and B chains of insulin are unordered in aqueous solution. A chain that lacks cationic side groups remains unordered in a NaDodSO4 solution. It shows a small amount of helix in a Dod(CH3)3NCl solution and forms the interchain beta-form in a DodNH3Cl solution. In contrast, B chain becomes partially helical in all three surfactant solutions.