We have designed a series of cationic peptides with different helix repeats that can kill bacteria but remain benign to mammalian host cells. The selectivity to cell types is well correlated to their responses to different model lipid monolayers mimicking the out membranes of different cell types. We propose to use neutron reflection to explore the basic structural features of different interactions between these small peptides and different model lipid monolayers bearing charge and structural characteristics. In particular, this work focus on assessing the impacts of unnatural cationic amino acids in bacterial killing by determining the amount and location of membrane associated peptides bearing these unnatural amino acids.