Interaction between Mo(VI) and siderophore models in aqueous solution Etelka Farkas a, * , Katalin Megyeri a ,L aszl  o Soms ak b , Lajos Kov acs b a Department of Inorganic and Analytical Chemistry, Lajos Kossuth University, 4010 Debrecen, Hungary b Department of Organic Chemistry, Lajos Kossuth University, Debrecen, Hungary Received 3 June 1997; received in revised form 5 December 1997; accepted 8 December 1997 Abstract A series of dihydroxamic acids (HORNOC-(CH 2 ) n -CONROH, where if R  H± then n  2; 5±7 and if R  CH 3 ± then n  4; 5) and two new dihydroxamate-based siderophore models, hexanedioic acid bis (3-hydroxycarbamoyl-methyl)amide (DhaI) and hex- anedioic acid bis(3-hydroxycarbamoyl-propyl)amide (DhaII) have been characterized in terms of chelating properties toward mo- lybdenum(VI). For comparison, the molybdenum(VI)±acetohydroxamic acid (Aha) and molybdenum(VI)±aminohydroxamic acid systems have also been studied. Potentiometric and spectrophotometric studies at ionic strength of 0.2 mol/dm 3 (KCl) and at 25  C have been performed and the equilibrium constants have been determined. It has been found, that of the dihydroxamic acids, only the DhaI and DhaII form water soluble complexes with molybdenum(VI). Polynuclear complexes most probably precipitate with the other dihydroxamic acids. Complexes are formed up to ca. the neutral pH in all systems. Above this pH MoO 2 4 and the free ligands exist. Although, very stable complexes are formed especially with DhaII, none of the studied ligands form a single bis(hydroxamato)dioxomolybdenum(VI) species. Mono(hydroxamato)trioxomolybdenum(VI) species are also formed, containing the uncoordinated moiety of the DhaI or DhaII in its protonated form. Out of aminohydroxamic acids, the b-alaninehydroxamic acid (b-Alaha) shows ``Aha-like'' coordination properties as the glutamic acid-c-hydroxamic acid (Glu-c-ha) does. The small dier- ences with this latter ligand are possibly due to weak coordination of the carboxylate which makes the mono(hydroxamato)trioxo- molybdenum(VI) species more stable and the uncoordinated carboxylates in bis(hydroxamato) dioxomolybdenum(VI) can protonate below pH 3. The tridentate coordination mode of aspartic acid-b-hydroxamic acid (Asp-b-ha) via the hydroxamate and carboxylate oxygens results in the formation of a dinuclear complex, [Mo 2 O 5 (LH) 2 ] 2 in addition to [MoO 3 (LH)]  (the protons are on the amino groups) in the pH-range 2.5±7.0. Ó 1998 Elsevier Science Inc. All rights reserved. Keywords: Molybdenum (VI) complexes; Hydroxamic acid; Aminohydroxamic acid; Equilibrium studies; Stability constants 1. Introduction Siderophores are known as speci®c iron-binding compounds synthetized by microbes to facilitate iron uptake by cells [1]. However, as it has been suggested [2], the siderophores in nitrogen-®xing bacteria are in- volved not only in the uptake of iron but also in the transport of molybdenum. In recent studies [2,3] on catecholate type siderophore analogues, the formation of a species containing a MoO 2 2 core coordinated by two catecholate chelates was found. Bis(hydroxamato) dioxomolybdenum(VI) species were synthetized and characterized with a series of benzohydroxamic acid de- rivatives [4]. Studies performed on N-substituted mono- and dihydroxamic acids with molybdenum(VI) showed that following oxygen abstraction from ligands and ox- idation of the metal ion, bis(hydroxamato)dioxomolyb- denum(VI) together with the corresponding amides and diamides were formed [5]. The interaction between the hydroxamate type natu- ral siderophore, desferrioxamine B (DFA) and molybde- num(VI) has been studied in our recent work [6]. A single species was found in the pH range 2±6 with the composition [MoO 2 (H 2 DFA)]  . Two hydroxamate Journal of Inorganic Biochemistry 70 (1998) 41±47 Abbreviations: Acetohydroxamic acid: Aha; Aspartic acid-b-hydro- xamic acid: Asp-b-ha; Glutamic acid-c-hydroxamic acid: Glu-c-ha; b- Alanine: b-Ala; b-Alaninehydroxamic acid: b-Alaha; Hexanedioic acid bis[(3-hydroxycarbamoyl-methyl)amide]: DhaI; Hexanedioic acid bis[(3-hydroxycarbamoyl-propyl)amide]: DhaII * Corresponding author. Tel.: +36 52 316 666; fax: +36 52 329 667; e- mail: farkase@tigris.klte.hu. 0162-0134/98/$19.00 Ó 1998 Elsevier Science Inc. All rights reserved. PII: S 0 1 6 2 - 0 1 3 4 ( 9 8 ) 0 0 0 1 1 - 7