title: Thermodynamics, kinetics, and mechanism of the stepwise dissociation and formation of Tris(L-lysinehydroxamato)iron(III) in aqueous acid.
abstract: pK(a) values for the hydroxamic acid, alpha-NH(3)(+), and epsilon-NH(3)(+) groups of L-lysinehydroxamic acid (LyHA, H(3)L(2+)) were found to be 6.87, 8.89, and 10.76, respectively, in aqueous solution (I = 0.1 M, NaClO(4)) at 25 degrees C. O,O coordination to Fe(III) by LyHA is supported by H(+) stoichiometry, UV-vis spectral shifts, and a shift in nu(CO) from 1648 to 1592 cm(-1) upon formation of mono(L-lysinehydroxamato)tetra(aquo)iron(III) (Fe(H(2)L)(H(2)O)(4)(4+)). The stepwise formation of tris(L-lysinehydroxamato)iron(III) from Fe(H(2)O)(6)(3+) and H(3)L(2+) was characterized by spectrophotometric titration, and the values for log beta(1), log beta(2), and log beta(3) are 6.80(9), 12.4(2), and 16.1(2), respectively, at 25 degrees C and I = 2.0 M (NaClO(4)). Stopped-flow spectrophotometry was used to study the proton-driven stepwise ligand dissociation kinetics of tris(L-lysinehydroxamato)iron(III) at 25 degrees C and I = 2.0 M (HClO(4)/NaClO(4)). Defining k(n) and k(-n) as the stepwise ligand dissociation and association rate constants and n as the number of bound LyHA ligands, k(3), k(-3), k(2), k(-2), k(1), and k(-1) are 3.0 x 10(4), 2.4 x 10(1), 3.9 x 10(2), 1.9 x 10(1), 1.4 x 10(-1), and 1.2 x 10(-1) M(-1) s(-1), respectively. These rate and equilibrium constants are compared with corresponding constants for Fe(III) complexes of acetohydroxamic acid (AHA) and N-methylacetohydroxamic acid (NMAHA) in the form of a linear free energy relationship. The role of electrostatics in these complexation reactions to form the highly charged Fe(LyHA)(3)(6+) species is discussed, and an interchange mechanism mediated by charge repulsion is presented. The reduction potential for tris(L-lysinehydroxamato)iron(III) is -214 mV (vs. NHE), and a comparison to other hydroxamic acid complexes of Fe(III) is made through a correlation between E(1/2) and pFe.
authors.affiliation: Department of Chemistry, Duke University, Box 90346, Durham, North Carolina 27708-0346, USA.
authors.forename: Joseph I
authors.initials: JI
authors.lastname: Wirgau
authors.forename: Ivan
authors.initials: I
authors.lastname: Spasojević
authors.forename: Hakim
authors.initials: H
authors.lastname: Boukhalfa
authors.forename: Ines
authors.initials: I
authors.lastname: Batinić-Haberle
authors.forename: Alvin L
authors.initials: AL
authors.lastname: Crumbliss
mesh_terms: D000588:Amines / Q000737:chemistry; D001665:Binding Sites; D005290:Ferric Compounds / Q000737:chemistry*; D006877:Hydroxamic Acids / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008239:Lysine / Q000031:analogs & derivatives* / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D000588:Amines; D005290:Ferric Compounds; D006877:Hydroxamic Acids; D008024:Ligands; C453319:aminohydroxamic acid lysinehydroxamic acid; C039763:lysine hydroxamate; D008239:Lysine