The metal complexes of peptides and related compounds. 6. The copper (II) complexes of 6-aminohexanoic acid

Abstract

A model compound for protein lysine side chains, 6-aminohexanoic acid, forms copper(II) complexes in solution only with its dipolar ionic form (HA). This is shown by emf titrations at 25° and 3.0 M NaClO 4 medium, using glass and copper amalgam electrodes. The results found are consistent assuming the following main equilibria: ▪ The data are first treated by graphical methods and then by Letagrop, a general least squares computer program. Deviations of the data might be ascribed to small analytical errors but may also indicate some minor species; if so, they are Cu 2(HA) 3 4+ and Cu(HA) 4 2+. Although copper(II) ions can be bound to the ϵ-amino group of 6-aminohexanoic acid, after first being reduced and then reoxidized, the compositions of the present complexes indicate that in solution the carboxylate rather than the ϵ-amino group interacts directly with Cu(II) ions.