Structures of the bicyclic peptide library, hit #4, and peptide 7. Amino acid residues in peptide 7 are numbered from N- to C-terminus. The CPP sequence is shown in red, whereas the residues modified during optimization are shown in blue color. B, β-alanine; CPP, cell-penetrating peptide; Hmb, hydroxylmethylbenzoyl; Pra, propargylglycine; Δ, l-2,3-diaminopropionic acid (Dap).