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 H3TCB. Likewise, the combination of tris(carboxyethyl)nitromethane with 3 equiv of cysteine ethyl ester gave the tripod H3TCM. With zinc salts, the pseudopeptides form the insoluble compounds (THB)(2)Zn5Cl10, 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-OH2](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.