The Influence of Amino Group Position on Aryl Moiety of SarAr on Metal Complexation and Protein Labelling

New hexaazamacrobicyclic cage bi-functional chelators (BFCs), 1-N-(3-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane-1,8-diamine (m-SarAr) and 1-N-(2-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane-1,8-diamine (o-SarAr), were synthesised. Their complexation with selected transitions metal ions i.e. Cu^II, Co^II, and Cd^II was investigated over a range of pH at micromolar concentrations. Cu^II was complexed by m-SarAr and o-SarAr rapidly within 5min in pH range of 5-9 at ambient temperature. In contrast, the complexation of Co^II and Cd^II by these ligands was slower. The conjugation efficiencies of p-SarAr, m-SarAr, and o-SarAr to bovine serum albumin (BSA) were compared under various reactions. Conditions were optimised to a molar ratio of BSA/N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC)/BFC of 1:250:50 in pH 5 buffer for 30min at ambient temperature. Under these conditions, the average number of p-SarAr, m-SarAr, or o-SarAr attached to BSA were determined to be 2.21±0.16, 4.90×10^-1±2.48×10^-2, and 2.67×10^-2±2.67×10^-3, respectively. This fundamental study clearly demonstrates that the position of the amine on the phenyl ring has a significant effect on the metal complexation and conjugation reactions with BSA.