Tryptophan as a Probe to Study the Anticancer Mechanism of Action and Specificity of α-Helical Anticancer Peptides

In the present study, a single tryptophan, as a fluorescence probe, was shifted from the N-terminus to the middle and to the C-terminus of a 26-residue α-helical anticancer peptide sequence to study the mechanism of action and specificity. The hydrophobicity of peptides, as well as peptide helicity and self-associating ability, were slightly influenced by the position change of tryptophan in the peptide sequence, while the hemolytic activity and anticancer activity of the peptide analogs remained the same. The tryptophan fluorescence experiment demonstrated that peptide analogs were more selective against LUVs mimicking cancer cell membranes than LUVs mimicking normal cell membranes. During the interaction with target membranes, the N-terminus of an anticancer peptide may be inserted vertically or tilted into the hydrophobic components of the phospholipid bilayer first. The thermodynamic parameters of the peptides PNW and PCW, when interacting with zwitterionic DMPC or negatively charged DMPS, were determined by ITC. DSC experiments showed that peptide analogs significantly altered the phase transition profiles of DMPC, but did not dramatically modify the phase transition of DMPS. It is demonstrated that hydrophobic interactions are the main driving force for peptides interacting with normal cell membranes, whilst, electrostatic interactions dominate the interactions between peptides and cancer cell membranes. Utilizing tryptophan as a fluorescence probe molecule appears to be a practicable approach to determine the interaction of peptides with phospholipid bilayers.