d ???OH; Z is chosen from a hydrogen atom and a peptide having 1 to 4 genetically encodable amino acids, comprising 1 to 4 histidines (His); with the proviso that (1) when Z is a hydrogen atom, R1 or R3 is chosen from His, Glu, and Asp, and when R1 is a neutral or negatively charged amino acid, R3 is His; or (2) when Z is a hydrogen atom, R14 is chosen from His, Asp and Glu; and with the further p