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Conformation of sequential and random copolypeptides of lysine and alanine in sodium dodecyl sulfate solution

A series of sequential polypeptides, (Lysi‐Alaj)n, and random copolypeptides, (Lysx, Alay)n, were synthesized. The competitive effect of Ala, a helix former, and Lys, whose homopolymer has a β‐form in neutral NaDodSO4 solution, was determined by CD and absorption spectroscopy. All the polypeptides studied were unordered in neutral solution without the surfactant. Of the six sequential polypeptides only (Lys‐Ala)n adopted a stable β‐form in NaDodSO4 solution. Most striking is the difference between this polypeptide, (Lys2‐Ala2)n and (Lysx, Alay)n, even though they all have equimolar Lys and Ala. (Lys2‐Ala2)n was partially helical in 2.5–5 mM NaDodSO4 but approached the unordered form in 50 mM NaDodSO4, whereas (Lys50, Ala50)n was completely helical in all NaDodSO4 concentrations. Even Lysrich (Lys2‐Ala)n and (Lys3‐Ala)n formed a partial helix and a trace of the β‐form, respectively, in low NaDodSO4 concentrations; both reverted to the unordered form in high NaDodSO4 concentrations. These results can be explained by Pauling‐Corey's model for β‐pleated sheets. Only (Lys‐Ala)n has all DodSO  4− ‐bound Lys+ residues on one side and Ala residues on the other side of the polypeptide chain. They can nestle quiet efficiently in a β‐sheet and between neighboring β‐sheets. Our results further imply that random copolypeptides are not completely random; they comprise varying segments of (Lysk‐Alam), where k and m could vary from zero to a small integer.