Thomas N Sorrell | PubMed Health

PubMed2006

A modular approach toward regulating the secondary coordination sphere of metal ions: differential dioxygen activation assisted by intramolecular hydrogen bonds.

Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure-function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea-carboxamide ligands that provide varying intramolecular hydrogen bond (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N'-tert-butylureayl)-N-ethyl]-bis(N' '-R-carbamoylmethyl)amine (H(4)1R) and bis[(N'-tert-butylureayl)-N-ethyl]-(N' '-R-carbamoylmethyl)amine (H(5)2R), where R=isopropyl, cyclopentyl, and (S)-(-)-alpha-methylbenzyl. The ligands with isopropyl groups H(4)1iPr and H(5)2iPr were combined with tris[(N'-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H(3)0iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH(2)2iPr]- (two H-bond donors), [CoIIH1iPr]- (one H-bond donor), and [CoII0iPr]- (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S=3/2 ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (Epa) with [CoIIH(2)2iPr]- being the most negative at -0.385 V vs [Cp2Fe]+/[Cp2Fe]. The properties of [CoIIH3buea]- (H3buea, tris[(N'-tert-butylureaylato)-N-ethyl]aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea]- and [CoIIH(2)2iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH(2)2iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: [CoIIIH3buea(16OH)]- has a -(O-H) band at 3589 cm-1 that shifts to 3579 cm-1 in [CoIIIH3buea(18OH)]-; [CoIIIH(2)2iPr(OH)]- has -(16O-H)=3661 and -(18O-H)=3650 cm-1. [CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating [CoIIH1iPr]- with excess dioxygen initially produces a species with an X-band EPR signal at g=2.0 that is assigned to a Co-O2 adduct, which is not stable and converts to a species having properties similar to those of the CoIII-OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (kint=2.5x10-7 M min-1). Moreover, the stability of the CoIII-OH complexes is correlated with the number of H-bond donors within the secondary coordination sphere; [CoIIIH3buea(OH)]- is stable in solution for days, whereas [CoIIIH(2)2iPr(OH)]- decays with a kint=5.9x10-8 M min-1. The system without any intramolecular H-bond donors [CoII0iPr]- does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H-bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the CoIII-OH complexes with [CoIIIH3buea(OH)]- being the most stable. We suggest that the greater number of intramolecular H-bonds involving the hydroxo ligand reduces the nucleophilicity of the CoIII-OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.

Amides↗

PubMed2006

Effects of coordinating metal ions on the mediated inhibition of trypsin by bis(benzimidazoles) and related compounds.

The presence of the Zn2+ ion dramatically enhances the inhibition of trypsin and tryptase by amidine-modified benzimidazole inhibitors via coordination to both the catalytically active Ser195 hydroxyl and His57 imidazole residues of the enzyme and the nitrogens of the amidine-modified benzimidazole inhibitor (Janc, J. W.; Clark, J. M.; Warne, R. L.; Elrod, K. C.; Katz, B. A.; Moore, W. R. Biochemistry 2000, 39, 4792-4800). Some new 5-amidino-2-substituted benzimidazoles were synthesized and compared to known related molecules to explore systematically the metal-mediated inhibition of bovine trypsin as a function of coordinating groups and metal ions. These compounds take advantage of the favorable interaction between the amidine group on one side of the inhibitor and the Asp189 carboxylate in the binding pocket of the enzyme. The 5-amidino-2-substituted benzimidazoles all demonstrated similar inhibition constants (Ki) of 20-50 microM in the absence of metal ions. In the presence of Zn2+, inhibition increased to varying extents, depending upon the group substituted at the 2 position of the benzimidazole. The largest increase in inhibition in the presence of Zn2+ was seen with (5-amidino-2-benzimidazolyl)-2-benzimidazolylmethane with an apparent inhibition constant (Ki') of 0.37 +/- 0.06 nM, giving a 59,000-fold increase in inhibition when Zn2+ is present. Other metal ions, including Mn2+, Sc3+, and Hg2+, also increased the inhibition by several of the benzimidazole derivatives synthesized. The compound bis(2-benzimidazolyl)methane (BBIM) was also examined because it lacks the amidine group that provides a favorable hydrogen-bonding interaction with Asp189 in the binding pocket of trypsin. In the absence of metal ions, BBIM did not have a detectable affinity for trypsin; however, in the presence of Zn2+, a Ki' of 127 +/- 3 nM was observed. This result demonstrates that an affinity for the enzyme in the absence of metal ions is not required for potent metal-mediated inhibition, greatly expanding the possibilities for metal mediation of nonmetalloenzymes.

Animals↗

PubMed2006

Pyrazolate-bridging dinucleating ligands containing hydrogen-bond donors: synthesis and structure of their cobalt analogues.

Convergent preparative routes to new urea-pyrazolate dinucleating ligands are described. Metal complexes of these ligands have hydrogen bond donors that are proximal to the metal centers that interact with other coordinated species. This is exemplified by Co(II) dimers with Co(II)-mu-Cl-Co(II) motifs, in which the chloro ligand is involved in four intramolecular hydrogen bonds. These noncovalent interactions appear to influence the Co(II)-Cl bonds, which are unusually long, having lengths greater than 2.5 A.

Bridged-Ring Compounds↗