Conformational study of a custom antibacterial peptide cecropin B1: implications of the lytic activity

Cecropin B1 (CB1) with two amphipathic alpha-helical segments is a derivative of the natural antibacterial peptide, cecropin B. The assays of cell lysis show that, compared with cecropin A (CA), CB1 has a similar ability to lyse bacteria with a higher potency (two- to six-fold higher) in killing cancer cells. The difference may be due to the fact that the peptides possess different structures and sequences. In this study, the solution structure of CB1 in 20% hexafluoroisopropanol was determined by two-dimensional nuclear magnetic resonance (NMR) spectroscopy. The (1)H NMR resonances were assigned. A total of 350 inter-proton distances were used to calculate the solution structure of CB1. The final ensemble structures were well converged, showing the minimum root mean square deviation. The results indicate that CB1 has two stretches of helices spanning from residues 3 to 22 and from residues 26 to 33, which are connected by a hinge section formed by Gly-23 and Pro-24. Lys-25 is partially incorporated in the hinge region. The bent angle between two helical segments located in two planes was between 100 and 110 degrees. With comparisons of the known NMR structure of CA and its activities on bacteria and cancer cells, the structure-function relationship of the peptides is discussed.