Biophysical Characterization of the Membrane Activity of Crotalicidin and Its C-Terminal Fragment Ctn[15–34] on Bacterial Lipid Models: An Infrared Spectroscopy Study

Antimicrobial peptides (AMPs) interact with bacterial membranes through mechanisms influenced by both peptide properties and membrane lipid composition. This study investigated the interactions of crotalicidin (Ctn) and its C-terminal fragment (Ctn[15–34]) with model membranes mimicking Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus using Fourier-transform infrared spectroscopy. Both peptides showed membrane-dependent conformational changes, with the extent of α-helical formation varying with lipid composition. Ctn showed the greatest increase in α-helical content in the S. aureus model, whereas Ctn[15–34] exhibited lower α-helical propensity in the Gram-negative membrane models. We assessed membrane thermotropic behavior by monitoring temperature-dependent changes in the CH2 and C=O stretching bands. Ctn markedly increased the main phase transition temperature (Tm) of the E. coli and K. pneumoniae membrane models, while its effect on the S. aureus model was limited. Similarly, Ctn[15–34] increased Tm in the E. coli model, with a smaller increase observed in K. pneumoniae. In contrast, a slight decrease in Tm was observed in the S. aureus model. Analysis of individual lipids revealed that dimirystoylphosphatidylglycerol (DMPG) was the most strongly affected, with Ctn[15–34] decreasing its Tm by up to 6.59 °C. Overall, these findings suggest that Ctn and Ctn[15–34] interactions with bacterial membrane models are influenced by peptide physicochemical properties and membrane lipid composition, highlighting that α-helical formation alone does not account for the observed membrane responses.