s N-oxy succinimide, used to conjugate C2 of the linker to the ??-amino group of the C-terminal lysine residue of the second peptide monomer; and the tertiary amide linker is chemically bonded to the spacer moiety, Y???NH???(CH2)4???C4H???NH???Y where: C3 of X is covalently bonded to C4 of the spacer; and Y is a suitable protecting group, such as a t-butyloxycarbonyl (Boc) protecting group.