ELSEVIER Inorganica Chimica Acta 241 (1996) 31-38 dgrl Synthesis, reactions and structure of a hydroxo-bridged dinuclear Zn(II) complex: modeling the hydrolytic zinc enzymes Yilma Gultneh a.., Allwar a, Bijan Ahvazi a, Die Blaise a, Ray J. Butcher a, John Jasinski b, Jerry Jasinski b a Chemistry Department, Howard University, 525 College St. N.W., Washington, DC 20059, USA b Chemistry Department, Keene State College, Keene, NH 03431, USA Received 2 February 1995 Abstract The dinuclear complex [Zn2L(/z-OH) ] (CIO4) 3"CH3CN (L = the dinucleating ligand a,a'-bis(bis(2-pyridylethyl)amino)-m-xylene) (1) has been prepared and characterized structurally by X-ray diffraction crystallography. The complex crystallizes in the monoclinic crystal system, spacegroupP2~/n, celldimensionsa= 13.109(2),b-- 13.339(2),c=24.317(5) ,~ andZ=4. The cation [Zn2L(/x-OH) ] 3+ contains a dinuclear zinc complex with hydroxide bridging, showing a Zn-/z-OH-Zn angle of 140.2 °, and Zn. • •Zn distance of 3.625 A. Each zinc ion is in a distorted tetrahedral coordination environment, but with slightly different bond angles and Zn-OH distances. Thus the two Zn ions in the dinuclear unit are slightly different and invequivalent. In aqueous solution the pKa of the Zn-coordinated H20 (the conjugate acid form of 1) is 7.55 :t: 0.05, which is one of the lowest values reported for zinc complexes modeling the hydrolytic enzymes. The rate of hydrolysis of bis-p-nitrophenylphosphate catalyzed by complex 1 is shown to increase from pH 7 to 8 but to decline at pH 9. Complex 1 reacts reversibly in acetonitrile solution with CO2 as shown by the IR peak observed at 1613 cm- ~assigned to the coordinated bicarbonate ion. Keywords: Dinuclear complexes; Hydroxo-bridged complexes; Zn(lI) complexes; Hydrolytic zinc enzymes 1. Introduction More than 200 known metalloenzymes of a wide variety of types, including anhydrases, hydrolases, esterases, trans- ferases, isomerases, ligases, oxidoreductases and lyases have been shown to contain zinc [ 1 ]. A variety of enzymes that catalyze the hydrolysis of phosphate esters (such as nucleo- tides), polypeptides, esters and amides, as well as the hydra- tion of biomolecules such as CO2 and acetaldehyde, have zinc centers at their active sites. Enzymes containing both mononuclear and dinuclear Zn(II) active sites are known. In the structurally well characterized zinc enzymes carbonic anhydrase [ 2] (containing zinc coordinated to three histidyl imidazoles of the protein and a water molecule) and carboxy- peptidase [ 3] (containing zinc coordinated to two histidyl imidazoles and one glutamate carboxylate group) the zinc atom is known to be in a four-coordinate environment with either H20 or OH- as the fourth ligand. In the alkaline phos- phatase from E. coli, there are two four-coordinate zinc ions both with either a H20 or OH- group as the fourth coordi- * Corresponding author. 0020-1693/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10020-1693(95)04791-6 nating ligand [4]. Five-coordination results [5] when an anion inhibitor binds to Zn(II) which is also likely the site of substrate coordination. In the alkaline phosphatase from E. coli containing two Zn(II) and one Mg(II) ions at each active site, the two zinc ions are found at a distance of 3.94 ~, (2.0 ~, resolution) [ 6]. Two enzymes have recently been structurally characterized which show the presence of two bridged Zn(II) ions at close distances. In bovine lens leucine aminopeptidase [7] two zinc ions are bridged by two aspartate carboxylate groups at a Zn... Zn distance of 2.88 ,~, while in phospholipase C from bacillus cereus two carboxylates and an OH- bridge two Zn (II) centers at a Zn. • •Zn distance of 3.3 A [ 8 ]. It is now recognized that the Zn-bound hydroxyl ion (in the conjugate base form, L3Zn-OH) of the active site com- plexes performs the crucial role of the strong nucleophile in the catalysis of the hydrolysis and hydrations of substrates in enzymes such as carboxypeptidase [9], carbonic anhydrase [ 10] and alkaline phosphatase [ 11 ]. The coordinated water is shown to have a pKa of about 7 in carbonic anhydrase [ 2] and about 6 in carboxypeptidase [ 12].