Effect of tryptophan oligopeptides on the size distribution of POPC liposomes: a dynamic light scattering and turbidimetric study.

A chemical regulation of POPC liposome size distribution was investigated, based on the affinity of indole-containing compounds for phosphocholine membranes. In particular, tryptophan oligopeptides have shown interesting properties of size regulation, both when liposomes were formed in their presence and when the peptides were added to a preformed liposome suspension. Combining dynamic light scattering (DLS) and turbidimetric data, it was possible to show how such peptides had an influence on the size distribution of spontaneously formed liposomes prepared by the thin film hydration, reverse-phase evaporation and ethanol (or methanol) injection methods. In the presence of Trp-Trp or Trp-Trp-Trp, a disappearance of large vesicle aggregates was observed, as suggested also by light microscopy analysis. On the contrary, no effect was detected using extruded vesicles. Turbidimetric titration allowed the determination of the relative efficacy of the size regulators, Trp-Trp-Trp being about 20 times more powerful than the dimer, while the monomer had no effect. In addition, other indole-containing compounds and the antimicrobial peptide indolicidin were tested, showing similar behaviours. Discussing the results according to the current knowledge about the preference of Trp residues for interfacial regions in lecithin bilayers, this study confirms the relevant role of tryptophan in the biomembrane binding properties of many peptides and introduces a new behavior in the field of liposomes-peptides interactions.