Topological effects on the designability and bactericidal potency of antimicrobial peptides.

New ideas and methods are being developed to generate highly designable small functional protein folds beyond the confines of natural structures, from secondary to quaternary level. Highly designable folds can have multiple sequence solutions, which are thermodynamically and kinetically stable. We have previously described how short syndiotactic helices can be exceptionally stable energetically, and how they can be used as a template for designing antibacterial agents. In this work, we have designed four syndiotactic, single turn, amphipathic; cationic 7-mer peptides which are the sequence and structural subset of earlier published 12-mer sequences. We examined the stability of the designed structures and its effects on the biological activity of such short peptide sequences. This was achieved by making objective comparisons between 12-mer and 7-mer sequences in terms of their antibacterial activity. Further, we investigated the mechanistic origins of clearly different bactericidal potency of single (7-mer) and double (12-mer) turn syndiotactic helices using molecular dynamics simulations. Our results suggest that conformationally constrained stable short double turn peptide scaffolds are highly designable, whereas single turn structures are more likely to be disordered. The stability of the designed peptide structure provides a platform for inclusion of multiple sequence variables and defined electrostatic fingerprints. Therefore, a stable peptide scaffold along with pre-defined electrostatic signatures can together be utilized for the design of novel antimicrobial peptides.