Biological consequences of improving the structural stability of hairpins that have antimicrobial activity.

Designing new antimicrobial peptides (AMPs) focuses heavily on the activity of the peptide and less on the elements that stabilize the secondary structure of these peptides. Studies have shown that improving the structure of naturally occurring AMPs can affect activity and so here we explore the relationship between structure and activity of two non-naturally occurring AMPs. We have used a backbone-cyclized peptide as a template and designed an uncyclized analogue of this peptide that has antimicrobial activity. We focused on beta-hairpin-like structuring features. Improvements to the structure of this peptide reduced the activity of the peptide against gram-negative, Escherichia coli but improved the activity against gram-positive, Corynebacterium glutamicum. Distinctions in structuring effects on gram-negative versus gram-positive activity were also seen in a second peptide system. Structural improvements resulted in a peptide that was more active than the native against gram-positive bacterium but less active against gram-negative bacterium. Our results show that there is not always a correlation between improved hairpin-structuring and activity. Other factors such as the type of bacteria being targeted as well as net positive charge can play a role in the potency of AMPs.