Scheme 1

DPP design. The positive charges in hydrophilic DPP1 were averaged to obtain DPP2. Trp and Leu in DPP1 and DPP6 were substituted with hydroxyl containing Ser and Thr to obtain DPP4 and DPP5, respectively. The positive charges in amphipathic DPP2 were aggregated on one side to obtain amphipathic DPP3, and two Lys residues in the positive arms were substituted with two His residues to yield DPP7. The positive charge was reduced to obtain DPP8 and DPP10. The number of His residues in DPP7, 8 and 10 were increased to four to obtain DPP11, DPP9 and DPP12, respectively. In addition, a linear DPP (L-DPP) with the same sequence as DPP7 was also synthesized as controls