The effect of pH on the structure, binding and model membrane lysis by cecropin B and analogs

Cecropins are a group of anti-bacterial, cationic peptides that have an amphipathic N-terminal segment, and a largely hydrophobic C-terminal segment and normally form a helix-hinge-helix structure. In this study, the ability of cecropin B (CB) and two analogs to lyse phospholipid bilayers, which have two levels of anionic content, has been examined by dye-leakage measurements over the pH range 2. 0-12.0. The two analogs differ from the natural peptide by having either two amphipathic segments (CB1) or two hydrophobic segments (CB3). All these peptides (except CB3 on low anionic content bilayers where it is not active) have maximal lytic activity on both types of bilayers at high pH. However, the pattern of secondary structure formation on these bilayers by the peptides, as measured by circular dichroism (CD), and the pattern of their ability to bind lipid monolayers, as measured using a biosensor, do not directly correlate with the pattern of their lytic ability. CB and CB1 with low anionic content bilayers have secondary structures as measured by CD with a similar pattern to membrane lysis, but binding is maximal near neutral, not high, pH. CB3 has some secondary structures on low anionic content bilayers at low pH and this becomes maximal over the basic range, but CB3 neither binds to nor lyses with these lipid layers. On high anionic content lipid layers, all peptides show high levels of secondary structures over most of the pH range and maximal binding at neutral pH (except for CB3, which does not bind). All three peptides lyse with high anionic content bilayers, but show no activity at neutral pH and reach maximal activity at very high pH. This work shows that pH is a major factor in the capability of antibacterial peptides to lyse with liposomes and that secondary structure and binding ability may not be the main determinants.