The 3^rddegree of biomimetism: associating the cavity effect, Zn^IIcoordination and internal base assistance for guest binding and activation

Abstract

A resorcinarene-based Zn(ii) complex presenting a hemilabile imidazole arm allows endo-coordination of acidic guests and catalyzes of acetonitrile hydration.

Submerging a Biomimetic Metallo‐Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study

“…35 and À2.20 ppm for WRim 4 Zn II and MRim 4 Zn II ,r espectively,c onsistent with the inclusion of a methyl group in the resorcinarene cavity.T he NMR signatures and complexation-induced shift (CIS) values are very similart o those previously reported for Rim 4 Zn II in MeCN in the presence of acetylacetonea nd triethylamine ( Table 2). [20] This indicates that, in water at pH 7, as in MeCN, acetylacetone is strongly bound to the metal iona sar esult of its deprotonation, which leads to the bidentate anionic ligand, acetylacetonate, and concomitant displacement of one imidazole arm. In this complex, one guest methyl group resides in the endo positiona nd the other resides in an exo position, exposed to the solvent (see the structures displayed in Figure6).…”

“…[18] This has been attributed to electrostaticr epulsion by the oxygen-rich narrow rim as as econd coordination sphere.I nc ontrast, rigid bowlshaped resorcinarene-based metal complexes,o fferingav ery differentsecond coordination sphereand shape, display strong affinity towards small anionic guests. [19] Our most recently described bowl complex,b ased on the new ligand Rim 4 , [20] is depicted in Scheme 1. In this system, the resorcinarene is functionalized at its large rim with four imidazole arms,t hree of which firmly hold the Zn II ion at the entrance of the bowl-shaped cavity,w hereast he fourth is hemilabile.…”

Oxygen‐Depleted Calixarenes as Ligands for Molecular Models of Galactose Oxidase

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Ac alix [4]arene ligand, in which two of the phenolf unctions are replaced by pyrazole units has been employed to mimic the His 2 -Tyr 2 (His:h istidine, Tyr: tyrosine) ligand sphere within the active site of the galactose oxidase( GO). The calixarene backbone forcest he corresponding copper(II) complex into as ee-saw-type structure, whichi sh itherto unprecedented in GO modelling chemistry.I tundergoes ao ne-electron oxidation that is centered at the phenolate donorl eading to ac opper-coordinated phenoxyl radical like in the GO.

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“…Hence, it is not surprisingt hat calixarenes, so far,h ave rarely been employed in bioinorganic chemistry.O nly modified forms in which the phenol units are functionalized by pendant N-donors were investigated, for instance,i nb iomimetic copper or zinc chemistry. [4] From the structural point of view,t he introductiono ft he pendant groups makes the metal complex more flexible. Even though not necessarily detrimental to the desired activity,t his flexibility eliminates one attractive feature of the calixarene platform,n amely its potential to direct donors to aw ell-defined binding pocket, which rendersc alixarenes appealing as three-dimensional analogueso fr igid multidentates ystems, such as salenso rp orphyrins.…”

“…We were interested in exploiting calixarene-shaped binding sites also in bioinorganic investigations,w hichr equired the introduction of soft donor atoms directly in place of one or more oxygena tomso ft he calixarene lower rim. Recently, some of us have developed the oxygen-depleted calixarene bispyrazolyl-tert-butyl-calix [4]arene ([H 2 (bpzCal)],F igure1)f eaturingt wo pyrazole moieties besidet wo phenolicd onors, [5] whichi sr eminiscent of aH is 2 -Tyr 2 coordination sphere found in certain enzymes. Oneo ft hose is the galactose oxidase( GO) and hence we decided to test the potentialo ft he [bpzCal] 2À ligandt oc onstruct molecular modelso ft he GO.…”