Trisimidazolylphosphine: M(II); Models for the metal-binding site in carbonic anhydrase

Abstract

The apparently simple processes of CO 2 hydration and HCO − 3 dehydration (eqn. 1) play a key role in several diverse physiological processes such as gas balance, photosynthesis, shell formation and pH control [1]. So important is this reaction to living systems that Nature has provided virtually all CO 2 + H 2O ⇄ H + + HCO 3 (1) organisms with an enzyme whose only known physiological role is to facilitate the interconversion of CO 2 and HCO − 3. According to X-ray crystallographic determinations [2] the active site of carbonic anhydrase consists of an essential Zn(II) ion held in the protein by three histidine imidazole units in a distorted tetrahedral fashion: the remaining Zn(II) ligand positions are said to be occupied by H 2O and/or OH −, these being important for the catalytic events. Although many studies with enzymes isolated from human and bovine erythrocytes have been undertaken, the mechanism by which CA catalyses the process in eqn. 1 remains elusive [1]. As an alternative approach to studying the catalysis of CO 2 hydration and HCO 3 − dehydration, we have initiated a program of synthesizing and evaluating simple tris-imidazolyl containing phosphines as approximations for the metal-binding sites in CA. Two of these ( I and II) have been shown to have several features in common with CA including low coordination numbers, for the Zn(II) and Co(II) complexes, ▪ pH-dependent visible absorption spectra of the Co(II) complexes and anion dependent Co(II) absorption spectra [3]. Importantly, the Zn(II) complexes of I and II show modest catalytic activity toward the interconversion of CO 2 and HCO − 3. As well, monovalent anions appear to inhibit the catalysis similar to the situation in the enzyme [3]. Although these previous indicate that small chelates such as I:M(II) or II:M(II) offer an effective approach to understanding various facets of the native enzyme, neither of these displays the phenomenal catalytic prowess of CA. We believe that when bound to Zn(II) in a tridentate fashion, the isopropyl groups of I encapsulate the metal in a restrictive way such that at most one additional ligand can easily be bound. If, during the catalytic sequence, the metal is required to be 5-coordinate to accommodate both CO 2 and an activated H 2O (OH −), then perhaps the modest catalytic ability of I:Zn(II) is related to steric encumbrance of this 5-coordinate state. With this premise in mind, the new work to be presented will focus on the synthesis and physical studies of complexes of III. While III still provides the requisite tridentate coordination of M(II), two sets of isopropyl groups protect the metal ion from 2:1 L:M(II) chelation but offer a greater accessibility of reactants to the metal surface. Groups Rand R′ are appended in such a way as to provide approximations of other groups in the active site of CA (like the threonine OH group) which are believed to be important in catalysis.