Microcalorimetric Studies: Enthalpies of Formation of Amino-acid and Peptide Complexes of Coppcr(ll) and Nickel(II)

Abstract

Measurements of the enthalpies of decomposition by acid at 24–25°C of a number of amino-acid and peptide complexes of CuII and NiII in solution (µ= 0.100 M) have been made using a Beckman microcalorimeter. The enthalpies of formation (from ions in solution) are derived for glycyl-glycinato(2-)copper(II), (glycylglycinato)(glycylglycinato(2-))copper(II) anion, leucylglycinato(2-)-copper(II), glycylglycinatonickel(II) cation, di(glycylglycinato)nickel(II), tri(glycylglycinato)nickel(II) anion, (glycylglycinato)(glycylglycinato(2-))nickel(II) anion, di(glycylglycinato(2-))nickel(II) dianion, glycinamidocopper(II) dication, di(glycinamido)copper(II) dication and di(glycinamido(1-))-copper(II). Equilibrium formation constants for certain of these complexes were measured from pH titration measurements. The bond strength of N→CuII in solution is greater than N→NiII whether the N atom belongs to amino or to peptide groupings.