Tripodal Pseudopeptides with Three Histidine or Cysteine Donors: Synthesis and Zinc Complexation

Peptide coupling of benzene-1,3,5-carboxylic acid with 3 equiv of histidine ethyl ester or cysteine ethyl ester has yielded the tripodal pseudopeptide ligands THB and H(3)TCB. Likewise, the combination of tris(carboxyethyl)nitromethane with 3 equiv of cysteine ethyl ester gave the tripod H(3)TCM. With zinc salts, the pseudopeptides form the insoluble compounds (THB)(2)Zn(5)Cl(10), Zn(3)(TCB)(2), and Zn(3)(TCM)(2) which are likely to be coordination polymers. Solution studies of THB with potentiometric methods have identified the complex species [(THB)(2)Zn](2+), [(THB)Zn-OH(2)](2+), and [(THB)Zn-OH](+). The pK(a) of the zinc-bound water molecule is 6.2, making the (THB)Zn complex a viable model of carbonic anhydrase.