The study shows that the tiny tail at the end of the human antimicrobial peptide LL‑37 (the VPRTES sequence) changes how the peptide sticks to bacterial‑like membranes and how well it kills microbes. Shorter versions of LL‑37 lose this tail and become less effective, while a slightly shorter version (LL‑32) can be even more powerful at high doses. The peptide’s flexibility and the exposed charged parts are key to its killing action, but the tail also helps control how the molecules group together on the membrane.

Cathelicidins are a family of host defense antimicrobial peptides in mammalian species. Among them, LL-37 is the only peptide of this family found in humans. Although LL-37 has been intensively investigated in the past, the mode of exerting its bactericidal activity through the specific interactions with bacterial membranes remains elusive. In this work, we combined microbiological and computational approaches with a tool box of experimental biophysical techniques, including conventional and surface-enhanced infrared absorption spectroscopy as well as fluorescence spectroscopy to characterize the structural and dynamic properties of LL-37 and shorter variants adsorbed on POPC/POPG (9:1) lipid bilayers as mimics of bacterial membranes. First, microbiological assays demonstrate that, while LL-32 and, in a lesser degree, LL-37 show hemolysis and antimicrobial activity, LL-20 remains practically inactive. Second, by comparing experimental and computational data of LL-37 with LL-20, we explained the bactericidal activity of the active peptide core as a consequence of an increased flexibility of the peptide structure, leading to reactive dangling charged side chains. Third, permeabilization assays showed a concentration-dependent membrane disruption activity of LL-37 and LL-32: at high peptide concentrations, LL-32 shows higher activity than LL-37, while, at low peptide concentrations, both peptides show similar activities. Responsible for this behavior is the C-terminal VPRTES tail (C_t-VPRTES tail), which, according to atomistic simulations, is able to promote the insertion of the peptide in the membrane and plays an essential role in controlling ordered peptide oligomerization on the surface of the membrane.