The crystal structure of the Fab' fragment of Mab 2F5, a potent neutralizer of both laboratory strains and primary clinical isolates of most clades of HIV-1, both uncompleted and complexed with the largely conserved peptide sequence ELDKWAS of the viral envelope protein gp41, has been elucidated and the characteristics of peptide-protein interactions determined. Having regard to such determination, the peptide-mimetics are constrained in the three-dimensional structure to provide an increased immunogenicity to the epitope sequence.