A novel dendrimeric peptide with antimicrobial properties: in vitro characterization of SB056

To tap into the therapeutic potential of membrane-active antimicrobial peptides, it’s crucial to disclose which properties of the peptide/lipids are important for target selectivity, and to examine the peptide structure and its association with lipid bilayers. We structurally and dynamically characterized a novel semi-synthetic peptide with a dimeric scaffold, named SB056, and investigated its antimicrobial activity and modes of interaction with model membranes. SB056 was active against Gram-negative bacteria, but showed a limited activity against Gram-positive bacteria and was substantially inactive against Mycobacterium smegmatis and Candida spp. Both the potency and spectrum of antimicrobial activity of SB056 were comparable to those of colistin and polymyxin B. SB056 insertion into DMPC and DMPC/DMPG monolayers, to mimic neutral and anionic membranes, respectively, was measured, together with that of the peptide’s linear monomeric constituent. For both SB056 and the linear monomer monolayer activity was enhanced by the presence of DMPG, showing that electrostatic binding plays an important role in peptide- membrane interactions. Also, SB056 displayed a stronger intercalation activity with respect to the linear monomer. Given their alternating sequence of hydrophobic and charged/polar amino acids, both SB056 and its linear monomer are expected to form an amphiphilic beta-strand when in contact with a lipid bilayer. 1H NMR spectra of the linear SB056 and the dendrimeric SB056 peptides in water and 70:30 water:TFE mixture, showed that peptides are basically unfolded in both solutions, although a reduced mobility of the two peptides in the water/TFE mixture compared to pure water could be discerned, as well as a different conformational flexibility of the two peptides, with the dendrimer being more constrained. A simulated annealing procedure showed that for both peptides residues in the core part of the sequence lie in the beta part of the Ramachandran plot.