An amphiphilic polypeptide, poly(sarcosine)-b-(L-Leu-Aib)(8) (SL16), was reported to self-assemble into vesicles. A GxxxG motif, which is known to induce helix dimerization, is incorporated into the hydrophobic helical block of SL16 to synthesize poly(sarcosine)-b-(L-Leu-Aib)(2)-(Gly-Aib-L-Leu-Aib-Gly-Aib)-(L-Leu-Aib)(3) (SG16). SG16 shows helix association in ethanol at a high concentration and low temperatures, which is not observed with SL16. SG16 self-assembles into vesicles, but are found to be more susceptible to rupture by the addition of Triton X-100 than SL16 vesicles. A mixture of SL16 and SG16 self-assembles into small sheets and micelles likely because of mismatch of the modes of helix association arising from sterical accommodation of iso-butyl groups at the helix-helix interface.