Effects of Arginine and Arginine Mimics on Antimicrobial Peptide Behavior

Antimicrobial peptides are naturally occurring components of the innate immune system which serve as a first line of defense against bacterial infection. While hundreds of antimicrobial peptides have been identified from natural sources, there is little in the way of consensus sequences that can identify peptides as antimicrobial. Instead, these peptides are more easily defined by the chemical properties of amphiphilicity, cationic net charge, and short length (10-30 aa). In an attempt to further elucidate the chemical mechanisms that play a role in antimicrobial activity, we compared two template peptides (C18G derived from human platelet factor and Ponericin L1 derived from ponerine ant venom) which contain arginine as the cationic moieties to the analgous pair which contain 2 - amino - 3 - guanidinopropionic acid. This effectively shortens the side chain length without modifying the cationic identity. We characterized these systems using a combination of CD spectroscopy for structure formation, fluorescence spectroscopy and quenching for lipid binding affinity, and microbiological techniques to investigate antibacterial efficacy. We found that the arginine analogs behaved similar to the parent peptides with respect to secondary structure formation in the presence of lipid bilayers, ability to permeabilize bacterial membranes, and ability to bind to anionic bilayers with high affinity. The same experiments were carried out on the 2 - amino - 3 - guanidinopropionic acid analogs. Overall this approach allows for SAR-like investigations into peptide structure/function relationships using natural and non-natural amino acids.