The importance of coulombic interactions for the induction of β structure in lysine oligomers by sodium dodecyl sulfate

Abstract Circular dichroism spectra have been obtained for tri(L‐lysine), tetra(L‐lysine), and penta(L‐lysine) in aqueous sodium dodecyl sulfate at 25°C. None of the oligomers are affected significantly by sodium dodecyl sulfate at detergent concentrations exceeding 0.01 M. Literature results show that the high‐molecular‐weight polymer forms a β strucure under these conditions. At detergent concentrations near 3.5 × 10−4 M the penta(L‐lysine), but not the smaller oligomers, undergoes a conformational change. Its circular dichroism under these conditions is essentially identical to that observed with poly(L‐lysine) when it forms a β structure in sodium dodecyl sulfate. Solutions of the penta(L‐lysine), which exhibit this modified circular dichroism, are also turbid, leading to the conclusion that the oligomer has formed an intermolecular β structure. When these experiments are conducted in the presence of 0.1 M sodium hydroxide, the sodium dodecyl sulfate produces neither turbidity nor a modified circular dichroism spectrum. These observations provide compelling evidence that Coulombic interaction between the anionic detergent head and the cationic lysyl amino groups is essential for the conformational change induced in penta(L‐lysine) by sodium dodecyl sulfate.