A series of sequential polypeptides, (Lys,-Ala,),, and random copolypeptides, (Lysx, Alay),, were synthesized. The competitive effect of Ala, a helix former, and Lys, whose homopolymer has a 0-form in neutral NaDodS04 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), adopted a stable 0-form in Na-DodSO4 solution. Most striking is the difference between this polypeptide, (Lysz-Alaz), and (Lys", Alay),, even though they all have equimolar Lys and Ala. (Lysz-Alaz), was partially helical in 2.5-5 mM NaDodS04 but approached the unordered form in 50 mM NaDodS04, whereas (Lys50, Ala50), was completely helical in all NaDodSO4 concentrations. Even Lysrich (Lysz-Ala), and (Lyss-Ala), formed a partial helix and a trace of the 0-form, respectively, in low NaDodS04 concentrations; both reverted to the unordered form in high NaDodSOc concentrations. These results can be explained by Pauling-Corey's model for @-pleated sheets. Only (Lys-Ala), has all DodS0;-bound Lys+ residues on one side and Ala residues on the other side of the polypeptide chain. They can nestle quite efficiently in a @sheet and between neighboring P-sheets. Our results further imply that random copolypeptides are not completely random; they comprise varying segments of (Lysk-Ala,), where k and m could vary from zero to a small integer.