title: A macrocyclic ligand able to bind gallium(III) by preorganized pendant arms; coordination and kinetic studies.
abstract: The equilibria and kinetics of the binding of gallium(III) to 4-(N),10-(N)-bis[2-(3-hydroxo-2-oxo-2-H-pyridine-1-y1)acetamido]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane (L) were investigated in acidic medium at ionic strength 1 M (NaClO4). Spectrophotometric titrations in the UV region revealed that L is able to bind Ga3+ also at high H+ concentration. The kinetic (stopped-flow) experiments are interpreted on the basis of three parallel reaction paths (i) M3+ + H2L2+ = M(H2L)5+ where M(H2L)5+ is in a steady state, (ii) M(OH)2+ + H2L2+ = M(HL)4+ + H2O and (iii) M(OH)2+ + HL+ = ML3+ + H2O. The first-order rate constants for conversion of the outer-sphere into the inner-sphere complexes are similar to those of the Ga(III)/tropolone system which is known to react according to the dissociative Id mechanism and to the relevant rate constants for water exchange at the metal ion. The effects of pH on the UV-Vis absorption, fluorescence emission properties and NMR spectral features on the Ga(III)/L system were also investigated. Spectrophotometric titrations in the UV region reveal that, in acid medium the prevailing species is M(HL)4+ whereas the chelate ML3+ prevails for [H+] < 0.01 M. The results indicate metal coordination at the oxygen atoms of the 3-hydroxo-2-oxopyridine residues.
authors.affiliation: Institute of Chemical Sciences, University of Urbino, P.za Rinascimento 6, I-61029, Urbino, Italy. vieri@chim.uniurb.it.
authors.forename: Gianluca
authors.initials: G
authors.lastname: Ambrosi
authors.forename: Alessia
authors.initials: A
authors.lastname: Boggioni
authors.forename: Mauro
authors.initials: M
authors.lastname: Formica
authors.forename: Vieri
authors.initials: V
authors.lastname: Fusi
authors.forename: Luca
authors.initials: L
authors.lastname: Giorgi
authors.forename: Simone
authors.initials: S
authors.lastname: Lucarini
authors.forename: Mauro
authors.initials: M
authors.lastname: Micheloni
authors.forename: Fernando
authors.initials: F
authors.lastname: Secco
authors.forename: Marcella
authors.initials: M
authors.lastname: Venturini
authors.forename: Giovanni
authors.initials: G
authors.lastname: Zappia
mesh_terms: D001665:Binding Sites; D018360:Crystallography, X-Ray; D005453:Fluorescence; D005708:Gallium / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D047028:Macrocyclic Compounds / Q000737:chemistry / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D010100:Oxygen / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D008024:Ligands; D047028:Macrocyclic Compounds; D005708:Gallium; D010100:Oxygen