Balancing the intermolecular forces in peptide amphiphiles for controlling self-assembly transitions

Abstract

While the influence of alkyl chain length and headgroup size on self-assembly behaviour has been well-established for simple surfactants, the rational control over the pH- and concentration-dependent self-assembly behaviour in stimuli responsive peptides remains an elusive goal. Here, we show that different amphiphilic peptides can have similar self-assembly phase diagrams, providing the relative strengths of the attractive and repulsive forces are balanced. Using palmitoyl-YYAAEEEEK(DO3A:Gd)-NH₂ and palmitoyl-YAAEEEEK(DO3A:Gd)-NH₂ as controls, we show that reducing hydrophobic attractive forces through fewer methylene groups in the alkyl chain will lead to a similar self-assembly phase diagram as increasing the electrostatic repulsive forces via the addition of a glutamic acid residue. These changes allow creation of self-assembled MRI vehicles with slightly different micelle and nanofiber diameters but with minimal changes in the spin-lattice T₁ relaxivity. These findings reveal a powerful strategy to design self-assembled vehicles with different sizes but with similar self-assembly profiles.

Fig. 1

(a) CD trace of 10 μM PA1 showing β-sheet nanofiber morphology at pH values ranging from 5.02–11.04. (b, c) TEM of 500 μM solutions of PA 1 at (b) pH = 5 and (c) pH = 9. In both CD and TEM, the PA was dissolved in 150 mM NaCl and 2.2 mM CaCl₂.

Fig. 2

(a) CD of PA2 at 50 μM dissolved in 150 mM NaCl, and 2.2 mM CaCl₂ at various pH values showing a transition at a pH of 5.75. (b) Ratio of pyrene luminescence at 376 vs. 392 nm for PA2 at pH 7.06 (red) and pH 9.02 (blue). The dotted lines correspond to the CAC at these pH values.

Fig. 3

(a) Concentration-pH phase diagrams of PA2 (blue) and PA3 (red) determined by CD (diamonds) and CAC (squares). (b–e) TEM images of PAs: 500 μM PA2 at (b) pH 5 (c) pH 9 and 100 μM PA3 at (d) pH 5 and (e) pH 9. All measurements were performed in 150 mM NaCl and 2.2 mM CaCl₂.

Fig. 4

DLS trace of 100 μM solution of PAs 2–6, dissolved in 150 mM NaCl and 2.2 mM CaCl₂, at pH = 9.0.

Fig. 5

(a) pH- and concentration-dependent phase diagram of PA 4 (blue) PA 5 (red) and PA 6 (green). (b–d) TEM of 500 μM solutions of PA 4, 5, 6 respectively, at pH 5, in a solution of 150 mM NaCl and 2.2 mM CaCl₂.

Chart 1

Structure of PA1, palmitoyl-YYAAEEEEK(DO3A:Gd)-NH₂.