The role of terminal amino group and histidine at the fourth position in the metal ion binding of oligopeptides revisited: Copper(II) and nickel(II) complexes of glycyl-glycyl-glycyl-histamine and its N-Boc protected derivative.
Copper(II) and nickel(II) binding properties of two pseudo tetrapeptides, N-Boc-Gly-Gly-Gly-Histamine (BGGGHa) and Gly-Gly-Gly-Histamine (GGGHa) have been investigated by pH-potentiometric titrations, UV-visible-, EPR-, NMR- and ESI-HRMS (electrospray ionization high resolution MS) spectroscopies, in order to compare the role of N-terminal amino group and imidazole moiety at the fourth position in the complex formation processes. Substantially higher stabilities were determined for the ML complexes of GGGHa, compared to those of BGGGHa, supporting the coordination of the terminal amino group and the histamine imidazole of the non-protected ligand. A dimeric Cu(2)H(-2)L(2) species, formed through the deprotonation of peptide groups of the ligands, was found in the GGGHa-copper(II) system. Deprotonation and coordination of further amide nitrogens led to CuH(-2)L and, above pH~10, CuH(-3)L. Experimental data supports a {NH(2), 2 × N(amide),N(im)} macrochelate structure in CuH(-2)L whereas a {NH(2), 3 × N(amide)} coordination environment in CuH(-3)L. The first two amide deprotonation processes were found to be strongly cooperative with nickel(II) and spectroscopic studies proved the transformation of the octahedral parent complexes to square planar, yellow, diamagnetic species, NiH(-2)L and above pH~9, NiH(-3)L. In the basic pH-range deprotonation and coordination of the amide groups also took place in the BGGGHa containing systems, leading to complexes with a {3 × N(amide),N(im)} donor set, and in parallel the re-dissolving of precipitate. Above pH~11, a further proton release from the pyrrolic NH group of the imidazole ring of BGGGHa occurred providing an additional proof for the different binding modes of the two ligands.
Department of Inorganic and Analytical Chemistry, University of Szeged, P.O. Box 440, H-6701 Szeged, Hungary. jancso@chem.u-szeged.hu
Attila
A
Jancsó
Katalin
K
Selmeczi
Patrick
P
Gizzi
Nóra V
NV
Nagy
Tamás
T
Gajda
Bernard
B
Henry
D003300:Copper / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009842:Oligopeptides / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*
D009842:Oligopeptides; D009942:Organometallic Compounds; D006639:Histidine; D003300:Copper; D009532:Nickel