We are reporting end-capping of the amino groups of a polypeptide block copolymer by hydrophobic alkyl groups including methyl, ethyl, and propyl group. The end-capped polymer aqueous Solution undergoes sol-to-gel transition as the temperature increases in a physiologically important temperature range of 10-40 degrees C, whereas the unmodified polymer does not show the transition. In addition, secondary structure of the polypeptide changed from random coils to P-sheets and the assembled pattern changed from spherical micelles into cylindrical nanofibrous structures in water by the hydrophobic end-capping of the polymer. The enhanced hydrophobic interactions by the alkyl end groups recruit the hydrophobic polypeptides in a more organized way to drive such a conformational change of polypeptides.