Lysine is more effective than arginine in two tryptophan-rich antimicrobial peptides

We compare biophysical and microbiological results to link structure and function in determining the mechanism of bacterial killing by two antimicrobial peptides (AMPs). Two 14-mer peptides were compared, each containing 6 tryptophans, in addition to 8 lysines (W6K8) or eight arginines (W6R8). MIC values show W6K8 is more effective at killing bacteria, and less toxic to red blood cells than is W6R8. Small angle X-ray scattering (SAXS) shows W6K8 is more efficient at fusing unilamellar vesicles (ULVs) that mimic bacterial lipid model membranes (LMMs), and W6R8 is more efficient at fusing ULVs that mimic eukaryotic cells, suggesting that membrane destabilization is a requirement for both activities. Circular dichroism (CD) exhibits excitonic coupling suggesting that AMP aggregation is involved in the killing mechanism, while secondary structure is mostly random coil and beta-sheet. Differential interference contrast (DIC) microscopy confirms that W6K8 aggregates GUVs to a greater extent than does W6R8 in bacterial LMMs. X-ray diffuse scattering (XDS) indicates that W6K8 penetrates more deeply than W6R8 into bacterial LMMs compared to W6R8. XDS also reports that W6K8 condenses the lipid area more than W6R8 in bacterial LMMS, but that the opposite is true in eukaryotic LMMs. FTIR indicates that W6K8 destabilizes the gel phase of DPPG, while W6R8 does not. These five biophysical methods indicate differences that could be responsible for the different microbiological results.