Exploring the boundaries of direct detection and characterization of labile isomers – a case study of copper(ii)–dipeptide systems

Abstract

The investigation of the linkage isomers of biologically essential and kinetically labile metal complexes in aqueous solutions poses a challenge, as these microspecies cannot be separately studied. Therefore, derivatives are commonly used to initially determine the stability or spectral characteristics of at least one of the isomers. Here we directly detect the isomers, describe the metal ion coordination sphere, speciation and thermodynamic parameters by a synergistic application of temperature dependent EPR and CD spectroscopic measurements in copper(ii)-dipeptide systems including His-Gly and His-Ala ligands. The ΔH = (-23 ± 4) kJ mol value of the standard enthalpy change corresponding to the peptide-type to histamine-type isomerisation equilibrium of the [CuL] complex was corroborated by several techniques. The preferential coordination of the side-chains was observed at lower temperatures, whereas, metal-binding of the backbone atoms became favourable upon increasing temperature. This study exemplifies the necessity of using temperature dependent multiple methodologies for a reliable description of similar systems for upstream applications.

The Angiotensin Metabolite His-Leu Is a Strong Copper Chelator Forming Highly Redox Active Species

“…Cu(II)–Cu(II) dimer formation was treated on the assumption that the formation of a dimeric species enabling Cu(II)–Cu(II) spin coupling would lead to a decrease in EPR signal intensity, as indicated in the literature . The rt-EPR approach was used instead of a more standard frozen solution spectra, because of a recent finding that bis-complexes may be significantly overrepresented in frozen solutions , and because of a strong temperature dependence shown for Cu(II) complexes of other His-1 dipeptides . The dependence of the rt-EPR spectral intensity at pH 7.4 and a Cu(II):His-Gly ratio near 1, both optimal for putative Cu(II)/His-Gly dimerization, are presented in Figure S7.…”

“… 38 The rt-EPR approach was used instead of a more standard frozen solution spectra, because of a recent finding that bis-complexes may be significantly overrepresented in frozen solutions 23 , 61 and because of a strong temperature dependence shown for Cu(II) complexes of other His-1 dipeptides. 31 The dependence of the rt-EPR spectral intensity at pH 7.4 and a Cu(II):His-Gly ratio near 1, both optimal for putative Cu(II)/His-Gly dimerization, are presented in Figure S7 . The EPR signal shape remained unaltered in the measured Cu(II) concentration range of 0.5–10 mM.…”

“… 63 The parameters of the spectra are in fair agreement with those published previously for the Cu(II)/His-Gly complexes. 31 , 38 The analysis of EPR spectra revealed the systematic evolution of the g iso and A Cu parameters with the strength of the ligand field exerted by nitrogen ligands coordinated to the Cu(II) ion. The assignment of coordination modes was assisted by the A N patterns obtained from spectral simulations, where the highest value of 1.7–1.8 mT could be clearly assigned to the imidazole nitrogen coordination, the lowest value of 0.9–1.1 mT to the amine nitrogen, and the middle value of 1.6 mT to amide nitrogen binding.…”

“…More than a dozen of studies were devoted to Cu­(II) binding by the His-1-containing His-Xaa dipeptides, where Xaa was Gly, − Ala, , Val, Met, Phe, Tyr, and Lys. , Potentiometric titrations were used as the main experimental method in most of these papers, with auxiliary spectroscopic studies, except of the electronic paramagnetic resonance (EPR) study of the Cu­(II)/His-Gly , and Cu­(II)/His-Lys systems. , Most researchers detected mono- and bis-complexes with various degrees of deprotonation, depending on the pH. In many cases, the 2:2 dimers were also indicated at higher pH values.…”

“…More than a dozen of studies were devoted to Cu(II) binding by the His-1-containing His-Xaa dipeptides, where Xaa was Gly, 29 − 39 Ala, 31 , 40 Val, 40 Met, 41 Phe, 42 Tyr, 42 and Lys. 43 , 44 Potentiometric titrations were used as the main experimental method in most of these papers, with auxiliary spectroscopic studies, except of the electronic paramagnetic resonance (EPR) study of the Cu(II)/His-Gly 31 , 38 and Cu(II)/His-Lys systems. 43 , 44 Most researchers detected mono- and bis-complexes with various degrees of deprotonation, depending on the pH.…”