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K Hegetschweiler

Publications and source records attributed to K Hegetschweiler.

8 recordsLinked to original sources
PubMed2001

Highly efficient phosphodiester hydrolysis promoted by a dinuclear copper(II) complex.

The interaction of Cu(II) with the ligand tdci (1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol) was studied both in the solid state and in solution. The complexes that were formed were also tested for phosphoesterase activity. The pentanuclear complex [Cu(5)(tdciH(-2))(tdci)(2)(OH)(2)(NO(3))(2)](NO(3))(4).6H(2)O consists of two dinuclear units and one trinuclear unit, having two shared copper(II) ions. The metal centers within the pentanuclear structure have three distinct coordination environments. All five copper(II) ions are linked by hydroxo/alkoxo bridges forming a Cu(5)O(6) cage. The Cu-Cu separations of the bridged centers are between 2.916 and 3.782 A, while those of the nonbridged metal ions are 5.455-5.712 A. The solution equilibria in the Cu(II)-tdci system proved to be extremely complicated. Depending on the pH and metal-to-ligand ratio, several differently deprotonated mono-, di-, and trinuclear complexes are formed. Their presence in solution was supported by mass, CW, and pulse EPR spectroscopic study, too. In these complexes, the metal ions are presumed to occupy tridentate [O(ax),N(eq),O(ax)] coordination sites and the O-donors of tdci may serve as bridging units between two metal ions. Additionally, deprotonation of the metal-bound water molecules may occur. The dinuclear Cu(2)LH(-3) species, formed around pH 8.5, provides outstanding rate acceleration for the hydrolysis of the activated phosphodiester bis(4-nitrophenyl)phosphate (BNPP). The second-order rate constant of BNPP hydrolysis promoted by the dinuclear complex (T = 298 K) is 0.95 M(-1) s(-1), which is ca. 47600-fold higher than that of the hydroxide ion catalyzed hydrolysis (k(OH)). Its activity is selective for the phosphodiester, and the hydrolysis was proved to be catalytic. The proposed bifunctional mechanism of the hydrolysis includes double Lewis acid activation and intramolecular nucleophilic catalysis.

Catalysis↗
PubMed2001

Complex formation of Ni(II), Cu(II), Pd(II), and Co(III) with 1,2,3,4-tetraaminobutane.

Complex formation of the two tetraamine ligands (2S,3S)-1,2,3,4-tetraaminobutane (threo-tetraaminobutane, ttab) and (2R,3S)-1,2,3,4-tetraaminobutane (erythro-tetraaminobutane, etab) with Co(III), Ni(II), Cu(II), and Pd(II) was investigated in aqueous solution and in the solid state. For Ni(II) and Cu(II), the pH-dependent formation of a variety of species [Mn(II)xLyHz](2x+z)+ was established by potentiometric titrations and UV/Vis spectroscopy. In sufficiently acidic solutions the divalent cations formed a mononuclear complex with the doubly protonated ligand of composition [M(H2L)]4+. An example of such a complex was characterized in the crystal structure of [Pd(H2ttab)Cl2]Cl2.H2O. If the metal cation was present in excess, increase of pH resulted in the formation of dinuclear complexes [M2L]4+. Such a species was found in the crystal structure of [Cu2(ttab)Br4].H2O. Excess ligand, on the other hand, lead to the formation of a series of mononuclear bis-complexes [Mq(HxL)(HyL)](q+x+y)+. The crystal structure of [Co(Hetab)2][ZnCl4]2Cl. H2O with the inert, trivalent Co(III) center served as a model to illustrate the structural features of this class of complexes. By using an approximately equimolar ratio of the ligand and the metal cation, a variety of polymeric aggregates both in dilute aqueous solution and in the solid state were observed. The crystal structure of Cu2(ttab)3Br4, which exhibits a two-dimensional, infinite network, and that of [Ni8(ttab)12]Br16.17.5H2O, which contains discrete chiral [Ni8(ttab)12]16+ cubes with approximate T symmetry, are representative examples of such polymers. The energy of different diastereomeric forms of such complexes with the two tetraamine ligands were analyzed by means of molecular mechanics calculations, and the implications of these calculations for the different structures are discussed.

Journal Article↗
PubMed2000

Facially coordinating cyclic triamines, part I. The coordination chemistry of cis-3,5-diaminopiperidine and substituted derivatives

An efficient and convenient method for the preparation of cis-3,5-diaminopiperidine (dapi) has been established and the coordination chemistry of this ligand with CoII, CoIII, NiII, CuII, ZnII, and CdII has been investigated in the solid state and in aqueous solution. Potentiometric measurements revealed a generally high stability for the bis complexes of the divalent cations with maximum stability for NiII (log beta2 = 21.2, beta2 = [M(dapi)2][M](-1)[dapi](-2), 25 degrees C, mu = 0.1 mol dm(-3)). Cyclic voltammetry established quasi-reversible formation of [Ni(dapi)2]3+ with a redox potential of 0.91 V (versus NHE) for the Ni(II/III) couple. [Co(dapi)2]3+ was prepared by aerial oxidation of the corresponding CoII precursor. The two isomers trans-[Co(dapi)2]3+ (1(3+), 26%) and cis-[Co(dapi)2]3+ (2(3+), 74%), have been separated and isolated as solid Cl- and CF3SO3- salts. In a non-aqueous medium 1(3+) and 2(3+) reacted with paraformaldehyde and NEt3 to give the methylidene-imino derivatives 3(3+) and 4(3+), in which the two piperidine rings are bridged by two or one N-CH2-O-CH2-N bridges, respectively. Crystal structure analyses were performed for H3dapi[ZnCl4]Cl, 1Cl3 x 2H2O, 2Cl3 x H2O, 3[ZnCl4]Cl, 4[ZnCl4]Cl, [Ni(dapi)2]Cl2 x H2O, [Cu(dapi)2](NO3)2, [Cu(dapi)Cl2], [(dapi)ClCd-(mu2-Cl)2-CdCl(dapi)], and [Co(dapi)(NO2)(CO3)]. The stability of [M(II)(dapi)]2+ and [M(II)(dapi)2]2+ complexes in aqueous solution, particularly the remarkably high tendency of [M(dapi)]2+ to undergo coordinative disproportionation is discussed in terms of the specific steric requirements of this ligand. Molecular mechanics calculations have been performed to analyze the different types of strain in these complexes. A variety of alkylated derivatives of dapi have been prepared by reductive alkylation with formaldehyde, benzaldehyde, salicylaldehyde, and pyridine-2-carbaldehyde. The NiII complexes of the pentadentate N3,N5-bis(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py2dapi) and the hexadentate N3,N5,1-tris(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py3dapi) have been isolated as crystalline ClO4- salts [Ni(py2dapi)Cl]ClO4 and [Ni(py3dapi)](ClO4)2 x H2O and characterized by crystal structure analyses.

Journal Article↗
PubMed1999

4-

An exceptionally stable 1:2 complex [FeL(2)](3-) is formed from the ligand H(3)L and Fe(III). In contrast, the affinity of this ligand for other biometals is relatively small. These properties make H(3)L a highly promising candidate for medical applications (e.g. for the treatment of iron overload).

Journal Article↗
PubMed1987

Kinetics and mechanisms of the oxidation of myoglobin by Fe(III) and Cu(II) complexes.

Two distinct mechanisms by which sperm whale myoglobin reduces, respectively, complexes of Fe(III) and Cu(II) and, in turn, is oxidized to metmyoglobin have been characterized. For both mechanisms, deoxymyoglobin is the active reductant. An outer sphere electron transfer, probably at the edge of the heme, is involved for Fe(III)NTA (NTA is nitrilotriacetic acid). This pathway does not involve ionic binding of the Fe(III) complex to the protein. The most reactive species of Fe(III)NTA is uncharged. No inhibition is observed with Ni(II) or Zn(II). An outer sphere site specific electron transfer is operative for reduction of Cu(II) complexes. The site has been characterized using NMR spectroscopy and involves one or more histidines. There is an initial binding of the Cu(II) chelate. The ternary complex of chelator-Cu(II)-deoxymyoglobin is a mandatory intermediate. Ni(II) and Zn(II) compete with Cu(II) for the binding site. A scheme for the participation of either or both of these mechanisms in reduction reactions of heme proteins is proposed. Both the overall redox potential, delta E0, and the stability constant for the ternary complex, K, govern the pathway and the reaction rate.

Binding Sites↗
PubMedDate not supplied

Derivatives of 1,3,5-triamino-1,3,5-trideoxy-cis-inositol as versatile pentadentate ligands for protein labeling with Re-186/188. prelabeling, biodistribution, and X-ray structural studies.

The pentadentate H3bhci [1,3,5-trideoxy-1, 3-bis((2-hydroxybenzyl)amino)-cis-inositol] and its bifunctionalized analogue H3bhci-glu-H [1,3,5-trideoxy-1, 3-bis((2-hydroxybenzyl)amino)-5-glutaramido-cis-inositol] were synthesized, and their coordination chemistry was investigated with inactive rhenium, with no carrier added Re-188 and with carrier added Re-186. The neutral Re(V) complexes [ReO(bhci)] and [ReO(bhci-glu-H)] are formed in good yields starting from [ReOCl3(P(C6H5)3)2] or in quantitative yield directly from [186/188ReO4]- in aqueous solution by reduction with Sn(II) or Sn(0). The X-ray structures of [ReO(bhci)] and [ReO(bhci-glu-H)] were elucidated revealing pentadentate "side on" coordination of the ligands to the "Re=O" core. The basic cyclohexane frame adopts a chair form in the case of [ReO(bhci)] and a twisted boat form in the case of [ReO(bhci-glu-H)]. [ReO(bhci)] crystallizes in the monoclinic space group C2/c with a = 27.425(3), b = 14.185(1), c = 19.047(2) A, and beta = 103.64(2) degrees and [ReO(bhci-glu-H)] in the monoclinic space group P21/c with a = 13.056(3), b = 10.180(1), c = 22.378(5) A, and beta = 98.205(9) degrees. Both 188Re complexes are stable in human serum for at least 3 days without decomposition. After injection into mice, [ReO(bhci-glu)]- is readily excreted through the intestines, while [ReO(bhci)] is excreted by intestines, liver, and the kidneys. TLC investigations of the urine showed exclusively the complexes [ReO(bhci-glu-H)] and [ReO(bhci)], respectively, and no decomposition products. For derivatization of antibodies, the carboxylic group of [ReO(bhci-glu-H)] was activated with N-hydroxysuccinimide, which required unusually vigorous reaction conditions (heating). The anti colon cancer antibody mAb-35 [IgG and F(ab')2 fragment] was labeled with [186/188ReO(bhci-glu)] to a specific activity of up to 1.5 mCi/mg (55 MBq/mg) with full retention of immunoreactivity. Labeling yields followed pseudo-first-order kinetics in antibody concentration with the ratio of rates between aminolysis and hydrolysis being about 2. Biodistributions of 186Re-labeled intact mAb-35 as well as of its F(ab')2 fragment in tumor-bearing nude mice revealed good uptake by the tumor with only low accumulation of radioactivity in normal tissue.

Animals↗