with the proviso that at least one of the following is true: (1) when Z is a hydrogen atom, R1 or R3 is chosen from His, Glu, and Asp; (2) when R1 is a neutral or negatively charged amino acid residue, R3 is His; or (3) when Z is a hydrogen atom, R14 is chosen from His, Asp and Glu; and with the further proviso that when in formula I R3, R3 in combination with R21, or R3 in combination with R4 dif