Jean-Jacques Delpuech

Multinuclear NMR and potentiometric study on tautomerism during protonation and zinc(II) complex formation of some imidazole-containing peptide derivatives

The acid–base properties and zinc(II) complexes of glycylhistamine, sarcosylhistamine, carcinine and carnosine have been studied by potentiometric, 13C and 14N NMR methods. Macroscopic species for the three states of protonation (LH22+, LH+, L) and the corresponding microspecies involving three protonation sites (terminal amino, N-1 and -3 imidazole nitrogens) are quantitatively estimated for the metal-free ligands. Zinc(II) complexation is shown to reverse the tautomeric preference between 1- and 3-H tautomeric forms of the imidazole ring (in LH+ and L), as compared to the free ligands where the 1-H tautomer is predominant.

Complexes of copper(II) ion with histamine-containing dipeptides: formation constants and structure

The acid–base properties and copper(II) complexes of glycyl- and sarcosyl-histamine (histamine = imidazole-4-ethanamine) have been studied by pH-metric, spectrophotometric and EPR methods, and compared to those of analogous histidine-containing dipeptides on the one hand and of carcinine (β-alanylhistamine) on the other. At pH 4–9 the predominant species is the 3N-co-ordinated complex CuLH–1(charges omitted) together with minor quantities of CuLH and CuL. The pK for metal ion-promoted deprotonation of the peptidic nitrogen is exceptionally low (pK= 3.20 and 3.66, respectively). The bis complexes CuL2 and CuL2H–1 also form in the presence of ligand in excess. Around pH 9–11 the monomeric 3N-co-ordinated hydroxo-complex CuLH–1(OH) and a polynuclear 4N-co-ordinated species are in equilibrium. The latter is assumed to be tetrameric Cu4L4H–8, with the imidazole rings as bridging bidentate units through co-ordination to both N3 and N1-pyrrolic nitrogens. At high pH (≈ 11) a further deprotonation results in the production of the monomeric 3N-co-ordinated hydroxo-complex CuLH–2(OH) with a pendant deprotonated N1-pyrrolic nitrogen.

Transition-metal complexes of carcinine, a peptide-type derivative of histamine

The acid–base properties and copper(II), nickel(II) and cobalt(II) complexes of carcinine {3-amino-N-[2-(imidazol-4-yl)ethyl]propanamide} and of N-tert-butoxycarbonylcarcinine have been studied at 25 °C by pH-metric, spectrophotometric and, in part, 1H NMR and EPR methods, and compared to those of histidine-containing dipeptides. Complexes of the type M(HL), ML and ML2(charges omitted) are found in the cobalt(II) and nickel(II) systems, with the additional presence of ML2H–1 for the latter system. Copper(II) forms CuLn(n⩽ 4) complexes with N-tert-butoxycarbonylcarcinine molecules acting as monodentate ligands. The copper(II)–carcinine system in the range pH 3–9 shows the expected series of 1 : 1 complexes, more or less deprotonated depending on the pH, of type Cu(HL), CuL and CuLH–1, and, for an excess of ligand, the unusual 1 : 4 complex CuL4H2 involving four N(3) inidazole nitrogens equatorially co-ordinated. At higher pH (> 9) the monomeric CuLH–2 and polynuclear Cu4L4H–8 complexes are formed depending on the total concentration. The tetrameric species probably involves co-ordination of deprotonated N(1)-pyrrole nitrogens, with consequent formation of imidazolate bridges. The co-ordination ability of the peptide nitrogen in histidyl- and histamine-peptides is shown to depend on the size of the chelate rings formed and the existence of a neighbouring carboxyl group in the ligand.

Solvent-exchange kinetics in nickel(II) solutions of aqueous tris(dimethylamino)phosphine oxide studied by pulsed phosphorus-31 nuclear magnetic resonance spectroscopy

When small quantities of water are progressively added to solutions of nickel(II) perchlorate in anhydrous P(NMe2)3O electronic and n.m.r. spectroscopy provide evidence for the presence of the six-co-ordinate mixed complex [Ni{P(NMe2)3O}2(OH2)4]2+ at a [H2O] : [Ni2+] mol ratio of ca. 8 : 1. Phosphorus-31 relaxation times of such solutions have been measured at two frequencies (8 and 14 MHz) and five temperatures (8–45 °C). These data yield the 31P hyperfine coupling constant (A)= 17.5 MHz, the electronic relaxation times (T1e and T2e)= 3.91 × 10–12 and 1.86 × 10–12 s (at 25 °C and 14 MHz), the trace of the square of the zero-field splitting tensor (Δ)= 1.96 cm–1, the relevant correlation time (τv)= 6.3 × 10–12 s, and the kinetic parameters for the exchange bound P(NMe2)3O [graphic omitted] bulk P(NMe2)3O, i.e. kM(25 °C)=(2.8 ± 0.4)× 106 s–1, ΔH‡= 25.5 ± 6.3 kJ mol–1, and ΔS‡=–35.5 ± 16.7 J K–1 mol–1. The mechanism of ligand substitution is dissociative. Rates are higher than for identically substituted nickel(II) complexes by about three orders of magnitude. This result is compared with literature data, and accounted for by a lower activation enthalpy, by ca. 25 kJ mol–1, as qualitatively expected from crystal-field theory.