- The amphipathic α-helix is a common motif found in many cell lytic peptides including antimicrobial peptides. We have recently shown that significantly altering the amphipathic structure of a lytic peptide by reshuffling its sequence and/or replacing a few l-amino acids with their d-enantiomers did not significantly affect the antimicrobial activity of the peptides nor their ability to bind and permeate negatively charged (PE/PG) membranes. However, a pronounced effect was observed regarding their hemolytic activity and their ability to bind and permeate zwitterionic (PC/Cho) membranes. To shed light on these findings, here we used surface plasmon resonance (SPR) with mono- and bilayer membranes. We found that the l-amino acid (aa) peptides bound 10−25-fold stronger to PC/Cho bilayers compared with monolayers, whereas the diastereomers bound similarly to both membranes. A two-state reaction model analysis of the data indicated that this difference is due to the insertion of the l-aa peptides into the PC/Cho bilayers, whereas the diastereomers are surface-localized. In contrast, only an ∼2-fold difference was found with negatively charged membranes. Changes in the amphipathicity markedly affected only the insertion of the l-aa peptides into PC/Cho bilayers. Furthermore, whereas the all-l-aa peptides bound similarly to the PC/Cho and PE/PG membranes, the diastereomers bound ∼100-fold better to PE/PG compared with PC/Cho membranes, and selectivity was determined only in the first binding step. The effect of the peptides on the lipid order determined by using ATR-FTIR studies supported these findings. Besides shedding light on the mode of action of these peptides, the present study demonstrates SPR as a powerful tool to differentiate between non-cell-selective compared with bacteria-selective peptides, based on differences in their membrane binding behavior.