Investigating the effect of pH on the aggregation of two surfactant-like octapeptides.

The aggregation of two de novo designed surfactant-like peptides, with sequences Ac-IIKKEENN-OH (P1) and Ac-IIEENNDD-OH (P2), has been studied in aqueous solution at various pH values using titration, Circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). It was found that aggregates of P1 and P2 both display 5 pK(a)'s, some of which differ considerably from the tabulated values for those amino acids. In particular, a lysine of P1 titrated to 4.939 +/- 0.066. CD spectra of P1 were insensitive to pH, while CD spectra of P2 demonstrated a random coil-to-beta-sheet transition as pH was decreased. AFM images confirmed that P1 aggregates were spheres at all pH values and ranged in size from 3 to 20 nm. On the other hand, P2 aggregates were twisted ribbons below pH 4 but spheres less than 10 nm above pH 4. In addition, AFM images demonstrated the partial breakup of these twisted ribbons upon elevating the pH from 3.03 to 3.46 and the virtual disappearance of ribbons at pH 3.82. FTIR spectra of P2 indicate a structural transition from random coil to beta-sheet as pH was decreased. The role of backbone hydrogen bonding as well as charge is discussed.