Carol Viragh

Short, strong hydrogen bonds on enzymes: NMR and mechanistic studies

The lengths of short, strong hydrogen bonds (SSHBs) on enzymes have been determined with high precision (^0.05 A ˚ ) from the chemical shifts (d ), and independently from the D/H fractionation factors (f ) of the highly deshielded protons involved. These H-bond lengths agree well with each other and with those found by protein X-ray crystallography, within the larger errors of the latter method (^ 0.2 to ^ 0.8 A ˚ ) (Proteins 35 (1999) 275). A model dihydroxynaphthalene compound shows a SSHB of 2:54 ^ 0:04 Abased on d ¼ 17:7 ppm and f ¼ 0:56 ^ 0:04; in agreement with the high resolution X-ray distance of 2:55 ^ 0:06 A ˚ . On ketosteroid isomerase, a SSHB is found ð2:50 ^ 0:02 AÞ; based on d ¼ 18:2 ppm and f ¼ 0:34; from Tyr- 14 to the 3-O 2 of estradiol, an analog of the enolate intermediate. Its strength is , 7 kcal/mol. On triosephosphate isomerase, SSHBs are found from Glu-165 to the 1-NOH of phosphoglycolohydroxamic acid (PGH), an analog of the enolic intermediate ð2:55 ^ 0:05 AÞ; and from His-95 to the enolic-O 2 of PGH ð2:62 ^ 0:02 AÞ: In the methylglyoxal synthase - PGH complex, a SSHB ð2:51 ^ 0:02 AÞ forms between Asp-71 and the NOH of PGH with a strength of

Synthesis and characterization of manganese(II) and iron(III) d5 tripodal imidazole complexes. Effect of oxidation state, protonation state and ligand conformation on coordination number and spin state

4.7 kcal/mol. When serine proteases bind mechanism-based inhibitors which form tetrahedral Ser-adducts analogous to the tetrahedral intermediates in catalysis, the Asp· · ·His H-bond of the catalytic triad becomes a SSHB (Proc. Natl Acad. Sci. USA 95 (1998) 14664), 2.49- 2.63 Ain length. Similarly, on the serine-esterase, butyrylcholinesterase complexed with the mechanism-based inhibitor m-(N,N,N- trimethylammonio)-2,2,2-trifluoroacetophenone, a SSHB forms between Glu-327 and His-438 of the catalytic triad, 2:61 ^ 0:04 Ain length, based on d ¼ 18:1 ppm and f ¼ 0:65 ^ 0:10: Very similar results are obtained with (human) acetylcholinesterase. The strength of this SSHB is at least 4.9 kcal/mol. q 2002 Elsevier Science B.V. All rights reserved.

Synthesis and characterization of a spin crossover iron(II)–iron(III) mixed valence supramolecular pseudo-dimer exhibiting chiral recognition, hydrogen bonding, and π–π interactions

The 1 : 3 Schiff base condensates of tris(2-aminoethyl)amine (tren) or tris(3-aminopropyl)amine (trpn) with 4-methyl-5-imidazolecarboxaldehyde, H3L1 and H3L2, respectively, were generated in situ and used to prepare complexes with manganese(II) and iron(III). The resultant complexes, [MnH3L1](ClO4)2, [MnH3L1](ClO4)2.EtOH.H2O, [MnH3L2](ClO4)2, [FeH3L1](ClO4)3.1.5(EtOH) and [FeHL1](I3) (0.525)(I)(0.475).2.625H2O, have been characterized by EA, IR, ES MS, variable temperature magnetic susceptibility, X-ray crystallography, and Mössbauer spectroscopy for the iron complexes. The three manganese(II) complexes are high spin with [MnH3L2](ClO4)2 exhibiting coordination number seven while the others are six coordinate. [FeH3L1](ClO4)3.1.5(EtOH) has two iron sites, a seven coordinate and a pseudo seven coordinate site. The complex is high spin at room temperature but exhibits a magnetic moment that decreases with temperature corresponding to conversion of one of the sites to low spin. [FeHL1](I3) (0.525)(I)(0.475).2.625H2O is low spin even at room temperature. In the present complexes the apical nitrogen atom, N(ap), of the tripodal ligand is pyramidal and directed toward the metal atom. The data show that the M-N(ap) distance decreases as the oxidation state of the metal increases, as the number of bound imidazole protons on the ligand increases, and as the number of carbon atoms in the backbone of the ligand (tren vs. trpn) increases. In a limiting sense, short M-N(ap) distances result in high spin seven coordinate mono capped octahedral complexes and long M-N(ap) distances result in low spin six coordinate octahedral complexes.

Small molecular products of dealkylation in soman-inhibited electric eel acetylcholinesterase.

Reaction of iron(II) and the 3 : 1 Schiff base condensate of 5-methylpyrazole-3-carboxaldehyde and tris(2-aminoethyl)amine in air gives a pseudo-dimer complex with a triple helix structure made of Δ–Δ and Λ–Λ pairings of spin crossover iron(II) and low spin iron(III) cations that are held together by three π–π and hydrogen bonding interactions.

Supramolecular assemblies prepared from an iron(II) tripodal complex, tetrafluoroborate, and alkali metal cations. The effect of cation size on coordination number, anion disorder and hydrogen bonding

Product analysis of dealkylation in P(S)C(S)-soman-inhibited electric eel acetylcholinesterase (AChE) by GC-MS using the selected ion monitoring mode has been carried out. The instrument was calibrated with pure standards of 2,3-dimethyl-1-butene and 2, 3-dimethyl-2-butene in the gas phase and methylene chloride extracts of 2,3-dimethyl-2-butanol and 3,3-dimethyl-2-butanol from the aqueous phase. The dealkylation in soman-inhibited AChE at pH 5.0 +/- 0.2 and 25 degrees C produces close to 40% alkenes and 50-60% 2, 3-dimethyl-2-butanol. No 3,3-dimethyl-2-butanol could be detected to provide direct evidence of the intervention of a secondary carbenium ion in the reaction path. All the products of the reaction originate from a tertiary carbenium ion. These findings are in good agreement with the results of Michel et al. [(1967) Arch. Biochem. Biophys. 121, 29], which were obtained by countercurrent distribution of tritium-labeled products and their identification by scintillation counting. The early experiments were performed with the mixture of the four soman diastereomers, all labeled with tritium in Calpha.

Composition and particle size of superparamagnetic corrosion products in tap water

The iron(II) complex cation, 12+, of the 1 : 3 Schiff base condensate of tris(2-aminoethyl)amine (tren) with imidazole-2-carboxaldehyde (H3L1) was reacted with MBF4 (M = Na, K, Rb, Cs and NH4). The products were double salts of the formula {[FeH3L1](BF4)2}·MBF4, 1[M(BF4)3]. The complexes have been characterized by EA, IR, X-ray crystallography, and Mossbauer spectroscopy. The resulting complexes crystallize in P-3 and the cations are located at the origin (0, 0, 0) and c/2 (0, 0, 0.5) on the c axis. Six tetrafluoroborate anions surround the cation sites and each binds to the sodium in a monodentate fashion and to the other cations in a bidentate fashion resulting in distorted octahedral and icosahedral complexes, respectively. Disorder is observed in the tetrafluoroborate anion of both 1[Na(BF4)3] and 1[Cs(BF4)3], resulting in two sets of fluorine atom positions (F set A and F set B), but is absent in the ammonium, potassium, and rubidium double salts. The disorder is attributed to coordination number preference and poor size match, respectively. The tetrafluoroborate anions are involved in extensive hydrogen bonding with the imidazole NH and imine CH of the iron complex. The hydrogen bonding arrangements observed with potassium, rubidium, cesium (F set A) and ammonium are almost identical with one another, but are altered in 1[Na(BF4)3] and 1[Cs(BF4)3] (F set B) complexes. These effects are explained on the basis of cation size. The formation of the double salt results in a shift in the spin equilibrium of the parent iron(II) complex from 62% high spin: 38% low spin at 295 K to pure low spin in the double salts.

Elucidation of the Fe(III) Gallate Structure in Historical Iron Gall Ink

Chemical analyses, magnetization, Mössbauer spectrum, and x-ray diffraction measurements were made on solids removed from tap water by means of membrane filters. The taps from which this water was obtained had previously been unused for prolonged periods of time. When these taps were reactivated and water was first drawn, it was observed that the quantity of coarse solids in the water gradually decreased with flow, while at the same time the quantity of fine solids gradually increased. The magnetization, Mössbauer spectra, and x-ray diffraction patterns of the solids showed the presence of a significant number of superparamagnetic particles of magnetite. In the temperature range of our measurements (77 K<T<300 K), paramagnetic iron-based species, particularly lepidocrocite, were also present in the solids. Contaminants such as Pb, Cu, and As were observed to be present in significant amounts, and it is shown that these are adsorbed to the magnetic nanoparticles. It was observed that almost all of the solid particles could be removed by means of 5-microm filters. This removal process can be explained by means of a model which assumes that initial deposition of coarse aggregates of corrosion products on the filters forms a coating, rich in extremely fine iron oxides. The coating has a high capacity for sorption of very small individual particles.

Synthesis and characterization of seven-coordinate tripodal imidazole complexes of iron(II) and manganese(II)

Synthetic, structural, spectroscopic and aging studies conclusively show that the main colorant of historical iron gall ink (IGI) is an amorphous form of Fe(III) gallate·xH2O (x = ∼1.5-3.2). Comparisons between experimental samples and historical documents, including an 18th century hand-written manuscript by George Washington, by IR and Raman spectroscopy, XRD, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy confirm the relationship between the model and authentic samples. These studies settle controversy in the cultural heritage field, where an alternative structure for Fe(III) gallate has been commonly cited.

Synthesis of phenyl or 4-nitrophenyl methyl β-ketophosphonate esters

The iron(II) and manganese(II) complexes of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 1-methyl-2-imidazolecarbaldehyde and the manganese(II) complex of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 4-imidazolecarbaldehyde are high-spin mono capped octahedral seven-coordinate complexes with a short, approximately 2.44 è, metal to apical nitrogen bond.

Supramolecular assemblies prepared from an iron(II) tripodal imidazole complex. A molecular scaffolding for the self assembly of icosahedral complexes of K+, Rb+, Cs+ and NH4+ cations

Seven new aryl methyl g -ketophosphonate esters were synthesized. The hydrolytic rates of the compounds under physiological conditions were studied. Most of the compounds are effective inhibitors of acetylcholinesterase. The enzyme recovers on the 10-50 h time scale from its adducts with two of the inhibitors.