Solution properties of bis(dipeptide)nickelate(III) complexes and kinetics of their decomposition in acid

Nickel Ion Complexes of Amino Acids and Peptides

“…The EPR spectrum for the [Ni III (H Ϫ1 Aib 2 ) 2 ] Ϫ complex shows g ʈ Ͼ g ⊥ , in contrast to typical EPR spectra of Ni(III)-tripeptide complexes with a tetragonally elongated octahedral geometry with g ⊥ Ͼ g ʈ . This spectrum is similar to that obtained for [Ni III (H Ϫ1 G 2 ) 2 ] Ϫ [6].…”

“…The potentiometric and spectroscopic results [153] indicate that the N-terminal tripeptide motif Arg-Thr-His is the exclusive binding site for nickel ions at the metal to HP2 1-15 molar ratio not higher than 1. A solution structure of the Ni(II) complex with the N-terminal pentadecapeptide of human protamine HP2 (HP2 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] ) was elucidated with the use of one-and two-dimensional 1 H NMR techniques and molecular modeling [163].…”

“…Oxidation of blue [bis(dipeptide)Ni(II)] yields violet-black solutions of Ni(III) complexes which are long-lived species in neutral solution. The acid-catalyzed conversion of the violet-black complex (A) to the yellow species (B) is followed by the acid-independent redox reaction to give diamagnetic products (C) (Scheme 4; reproduced by permission from [80]) [6]. The studies of the redox decomposition of Cu(III) and Ni(III) tripeptide complexes have shown that amino acid residues with methyl groups on the α-carbon tend to stabilize the trivalent oxidation state.…”

“…This fi ve-nitrogen complex is relatively stable from pH 6 to 11. EPR studies frequently are used to propose the structure of Ni(III) species because Ni(III)-peptide complexes are too unstable to obtain crystals for X-ray crystal structure determinations [6,193,235,247,248]. For [Ni III (H Ϫ2 G 3 )(H Ϫ1 G 3 )] 2Ϫ three peaks in the g ʈ region indicate one axial nitrogen coordinated to the metal ion.…”

“…In a tetragonal ligand fi eld the ground confi guration will be (d xz d yz ) 4 (d xy ) 2 (d z 2) 1 (d x 2 Ϫy 2) 0 for a square planar or elongated octahedral complex and (d xz d yz ) 4 (d xy ) 2 (d x 2 Ϫy 2) 1 (d z 2) 0 for a compressed octahedron. In the fi rst case the EPR spectra will show g ⊥ Ͼ g ʈ Ϸ 2.00 while in the latter situation the pattern g ʈ Ͻ g ⊥ Ͼ 2.00 will be observed [5,6].…”