Inter-relation of surface tension and optical turbidity in self-assembled peptide amphiphiles

Self-associating amphiphilic peptides FEFKFEFK (EFK8-I) and FEFRFEFR (EFR8-I) with an alternating pattern of polar and non-polar side chains were studied to investigate the influence of physicochemical factors that govern aggregation pathways. Changes in aggregation mechanism were determined using a combination of turbidimetric-potentiometric titrations and interfacial tension measurements. The key parameters investigated were pH and NaCl concentration. Turbidity results showed that EFK8-I and EFR8-I displayed reversible aggregation pathway as a function of pH and NaCl concentration. The pathway was depend strongly on the pH history of the solution, but was weakly influenced by changes in salt concentration. That is, the effect of changing the peptide protonation state was greater than the effect of changing the screening length by raising the NaCl concentration. The results indicated that the pK_as of peptide side groups were shifted during titration where the titratable groups may be over-stabilised by the formation of salt bridges. These observations suggest a plausible aggregation mechanism in terms of monomers, pre-aggregates and aggregates. Formation of larger aggregates (when the solution had high initial pH) was mainly stabilised by hydrophobic interactions between smaller pre-aggregates. Equilibrium surface tension results well correlated with solution turbidities suggest that the surface-active species were pre-aggregates. These pre-aggregates were irreversibly formed, as indicated by the distinct degree of conformation formation that depended on pH history. Larger aggregates may unfold into a large number of smaller pre-aggregates at the air–water interface, leading to lower equilibrium surface tensions. This study ultimately provides a better understanding of controlling aggregation and predicting surface functional modification of self-assembled polypeptides.