Copper(II) Transport from a Leucine-containing Tripeptide to Cysteine. Relationship between the Position of Isobutyl Side Chain in the Peptide and the Rate of Ligand-exchange

Abstract

Transport of Cu (II)1 from leucine-containing tripeptides, L-leucylglycylglycine, glycyl-L-leucylglycine and glycylglycyl-L-leucine, to cysteine was examined by a stopped-flow spectrophotometric method. The reaction was observed by changes in the intensity of S→ Cu (II) charge transfer bands observed on coordinating Cu (II) with cysteine. Initially formed was an intermediary complex, which was the mixed-ligand complex consisting from Cu (II), the tripeptide and cysteine. The transient subsequently reacted with another cysteine undergoing prompt ligand-exchange to yield Cu (II)biscysteinate. Both the transient and the binary complexs showed the charge transfer bands; the former at 330-335 nm and the latter at 335 nm and 385 nm. The isobutyl side chain in the second position from the amino terminal and that at the carboxylate-end reduced the rate of formation of the transient complex. On the contrary, the side chain at the amino terminal reduced the rate of formation of the binary complex, while the chain in the second position accelerated the reaction.