Protamines are cationic antimicrobial peptides found commonly in the sperm cells of fish such as herring and salmon that are particularly active against food-borne Gram-negative organisms, although the precise mechanisms are unclear. Our experiments aim to gain more insight as to the mechanism of action of a protamine (clupeine from herring) by changing the peptide structure with: (i) addition of fatty acids (palmitoyl and myristoyl chloride) to make it more hydrophobic and (ii) quantitative exposure of the peptide to 1,2-cyclohexanedione to reduce the overall charge on the peptide. Using the native and modified clupeine samples in neutron reflectivity experiments, we aim to investigate the protein-membrane interactions in a Gram-negative model membrane system based on E. coli to better understand the initial steps involved in the interaction.