The short membrane-active
peptide BP100 [KKLFKKILKYL-NH2] is known as an effective antimicrobial and cell penetrating agent. For a functional
alanine scan each of the 11
amino acids was replaced with deuterated Ala-d3, one at a time. MIC assays showed that a substitution of Lys did not affect the antimicrobial activity, but it decreased when a hydrophobic residue was replaced. In most cases, a reduction in hydrophobicity led to a decrease in
hemolysis, and some
peptide analogues had an improved therapeutic index. Circular dichroism showed that BP100 folds as an amphiphilic α-helix in a bilayer. Its alignment was determined from (2)H NMR in oriented membranes of different composition. The azimuthal rotation angle was the same under all conditions, but the average helix tilt angle and the dynamical behavior of the
peptide varied in a systematic manner. In POPC/POPG bilayers, with a negative spontaneous curvature, the
peptide was found to lie flat on the bilayer surface, and with little wobble. In
DMPC/
DMPG, with a positive spontaneous curvature, BP100 at higher concentrations became tilted obliquely into the membrane, with the uncharged C-terminus inserted more deeply into the
lipid bilayer, experiencing significant fluctuations in tilt angle. In
DMPC/
DMPG/lyso-MPC, with a pronounced positive spontaneous curvature, the helix tilted even further and became even more mobile. The 11-mer BP100 is obviously too short to form transmembrane pores. We conclude that BP100 operates via a carpet mechanism, whereby the C-terminus gets inserted into the hydrophobic core of the bilayer, which leads to membrane perturbation and induces transient permeability.