Probing Guanidino Pendant or Bridged Groups in Cyclic Antimicrobial Peptides Derived from Temporin L: A Strategy to Improve Efficacy against Gram-Negative Bacteria

The rise of antibiotic resistance underscores the urgent
need for
new antimicrobial agents. Antimicrobial peptides (AMPs), such as temporins,
offer broad-spectrum activity through unique mechanisms but are often
limited by cytotoxicity and poor stability. To improve the Gram-negative
activity-to-toxicity ratio, we designed a focused library of cyclic
Temporin L (TL) analogues bearing an additional positively charged
guanidino group, introduced either as a side-chain pendant or as a
bridged functionality. While guanidino modifications have been studied
in side-chain peptide contexts, this represents the first application
of guanidino-based bridging in AMP design. Among the synthesized compounds,
four (e.g., 2, 6, 7 and 12) were selected based
on their combined antimicrobial potency and low cytotoxicity toward
human keratinocytes, emerging as the most structurally representative
candidates of the introduced modifications. Further characterization
provided an integrated view of their biological properties, highlighting
guanidino-based cyclic temporins as attractive agents and a framework
for developing next-generation therapeutics against resistant and
biofilm-associated infections.