High Yielding, One-Pot Synthesis of Bis(1H-indazol-1-yl)methane Catalyzed by 3d-Metal Salts.

Synthetic access to poly(indazolyl)methanes has limited their study despite their structural similarity to the highly investigated chelating poly(pyrazolyl)methanes and their potentially important indazole moiety. Herein is presented a high yielding, one-pot synthesis for the 3d-metal catalyzed formation of bis(1H-indazol-1-yl)methane from 1H-indazole utilizing dimethylsulfoxide as the methylene source. Complete characterization of bis(1H-indazol-1-yl)methane is given with 1H and 13C NMR, UV/Vis, FTIR, high resolution mass spectrometry and for the first time, single crystal X-ray diffraction. This simple, inexpensive pathway to yield exclusively bis(1H-indazol-1-yl)methane provides synthetic access to further investigate the coordination and potential applications of the family of bis(indazolyl)methanes.

Synthesis and Characterization of trans-Dichlorotetrakis(imidazole)cobalt(III) Chloride: A New Cobalt(III) Coordination Complex with Potential Prodrug Properties.

Numerous therapies for the treatment of cancer have been explored with increasing evidence that the use of metal-containing compounds could prove advantageous as anticancer therapeutics. Previous works on Ru(III) complexes suggest that structurally similar Co(III) complexes may provide good alternative, low-cost, effective prodrugs. Herein, a new complex, trans-[Co(imidazole)4Cl2]Cl (2), has been synthesized in high yields utilizing ligand exchange under refluxing conditions. The structure of 2 has been characterized by elemental analysis, 1H and 13C·NMR, ESI-MS, CV, and UV-Vis. The ability of 2 to become reduced in the presence of ascorbic acid was probed demonstrating the likely reduction of the Co(III) metal center to Co(II). In addition, preliminary cell line testing on 2 shows a lack of cytotoxicity.

Reinvestigation of a Ru(2)-incorporated polyoxometalate dioxygenase precatalyst, "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)": evidence for marginal,

A 1997 Nature paper (Nature 1997, 388, 353-355) and subsequent 1998 J. Am. Chem. Soc. paper (J. Am. Chem. Soc. 1998, 120, 11969-11976) reported that a putative Ru(2)-substituted polyoxoanion, "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1), is an all inorganic dioxygenase able to incorporate one O(2) into two adamantane CH bonds to yield 2 equiv of 1-adamantanol as the primary product. In a subsequent 2005 Inorg. Chem. publication (Inorg. Chem. 2005, 44, 4175-4188), strong evidence was provided that the putative dioxygenase chemistry is, instead, the result of classic autoxidation catalysis. That research raised the question of whether the reported Ru(2) precatalyst, 1, was pure or even if it contained two Ru atoms, since Ru is known to be difficult to substitute into polyoxoanion structures (Nomiya, K.; Torii, H.; Nomura, K.; Sato, Y. J. Chem. Soc. Dalton Trans. 2001, 1506-1521). After our research group had contact with three other groups who also had difficulties reproducing the reported synthesis and composition of 1, we decided to re-examine 1 in some detail. Herein we provide evidence that the claimed 1 actually appears to be the parent polyoxoanion [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) with small amounts of Ru (

Synthesis and characterization of V(V)(3,6-DBSQ)(3,6-DBCat)2, a d(0) metal complex with dioxygenase catalytic activity.

Transition-metal complexes containing redox-active quinoid ligands are of interest because of their catalytic capabilities in multielectron, substrate-activation reactions such as dioxygenase catalysis using O(2). The new catecholate complex V(V)(3,6-DBSQ)(3,6-DBCat)(2) (where 3,6-DBSQ = 3,6-di-tert-butylsemiquinone and 3,6-DBCat = 3,6-di-tert-butylcatecholate) was synthesized by combining VO(acac)(2) with 1 equiv of 3,6-DBBQ (where 3,6-DBBQ = 3,6-di-tert-butylbenzoquinone) and 2 equiv of H(2)(3,6-DBCat) in dry methanol under an inert atmosphere. The resultant complex was characterized by single-crystal X-ray diffraction, elemental analysis, near-IR, UV/vis, and electron paramagnetic resonance (EPR) spectroscopy. The crystallography as well as the near-IR and EPR studies suggest that the radical spin is localized on the 3,6-DBSQ ligand at room temperature, making V(V)(3,6-DBSQ)(3,6-DBCat)(2) a type 1 mixed-valence complex. Initial dioxygenase catalysis studies reveal that V(V)(3,6-DBSQ)(3,6-DBCat)(2) is a good dioxygenase precatalyst for the substrate H(2)(3,6-DBCat) with O(2) in ca. 600 total turnovers to >93% intra- and extradiol products with only 1-2% of the undesired benzoquinone autoxidation product.

Alpha-synuclein aggregation variable temperature and variable pH kinetic data: a re-analysis using the Finke-Watzky 2-step model of nucleation and autocatalytic growth.

The aggregation of proteins is believed to be intimately connected to many neurodegenerative disorders. We recently reported an "Ockham's razor"/minimalistic approach to analyze the kinetic data of protein aggregation using the Finke-Watzky (F-W) 2-step model of nucleation (A-->B, rate constant k(1)) and autocatalytic growth (A+B-->2B, rate constant k(2)). With that kinetic model we have analyzed 41 representative protein aggregation data sets in two recent publications, including amyloid beta, alpha-synuclein, polyglutamine, and prion proteins (Morris, A. M., et al. (2008) Biochemistry 47, 2413-2427; Watzky, M. A., et al. (2008) Biochemistry 47, 10790-10800). Herein we use the F-W model to reanalyze protein aggregation kinetic data obtained under the experimental conditions of variable temperature or pH 2.0 to 8.5. We provide the average nucleation (k(1)) and growth (k(2)) rate constants and correlations with variable temperature or varying pH for the protein alpha-synuclein. From the variable temperature data, activation parameters DeltaG(double dagger), DeltaH(double dagger), and DeltaS(double dagger) are provided for nucleation and growth, and those values are compared to the available parameters reported in the previous literature determined using an empirical method. Our activation parameters suggest that nucleation and growth are energetically similar for alpha-synuclein aggregation (DeltaG(double dagger)(nucleation)=23(3) kcal/mol; DeltaG(double dagger)(growth)=22(1) kcal/mol at 37 degrees C). From the variable pH data, the F-W analyses show a maximal k(1) value at pH approximately 3, as well as minimal k(1) near the isoelectric point (pI) of alpha-synuclein. Since solubility and net charge are minimized at the pI, either or both of these factors may be important in determining the kinetics of the nucleation step. On the other hand, the k(2) values increase with decreasing pH (i.e., do not appear to have a minimum or maximum near the pI) which, when combined with the k(1) vs. pH (and pI) data, suggest that solubility and charge are less important factors for growth, and that charge is important in the k(1), nucleation step of alpha-synuclein. The chemically well-defined nucleation (k(1)) rate constants obtained from the F-W analysis are, as expected, different than the 1/lag-time empirical constants previously obtained. However, k(2)x[A](0) (where k(2) is the rate constant for autocatalytic growth and [A](0) is the initial protein concentration) is related to the empirical constant, k(app) obtained previously. Overall, the average nucleation and average growth rate constants for alpha-synuclein aggregation as a function of pH and variable temperature have been quantitated. Those values support the previously suggested formation of a partially folded intermediate that promotes aggregation under high temperature or acidic conditions.

Protein aggregation kinetics, mechanism, and curve-fitting: a review of the literature.

Protein aggregation is an important phenomenon that alternatively is part of the normal functioning of nature or, central to this review, has negative consequences via its hypothesized central role in neurodegenerative diseases. A key to controlling protein aggregation is understanding the mechanism(s) of protein aggregation. Kinetic studies, data curve-fitting, and analysis are, in turn, keys to rigorous mechanistic studies. The main goal of this review is to analyze and report on the primary literature contributions to protein aggregation kinetics, mechanism, and curve-fitting. Following a brief introduction, the multiple different physical methods that have been employed to follow protein aggregation are presented and briefly discussed. Next, key information on the starting proteins and especially the products, and any detectable intermediates, involved in protein aggregation are presented. This is followed by tabulation (in the Supporting information) and discussion (in the main text), of the many approaches in the literature striving to determine the kinetics and mechanism of protein aggregation. It is found that these approaches can be broadly divided into three categories: (i) kinetic and thermodynamic, (ii) empirical, and (iii) other approaches. The first two approaches are the main focus of the present contribution, their goal being curve-fitting the available kinetic data and obtaining quantitative rate constants characterizing the nucleation, growth, and any other parts of the overall aggregation process. The large literature of protein aggregation is distilled down to five classes of postulated mechanisms: i) the subsequent monomer addition mechanism, ii) the reversible association mechanism, iii) prion aggregation mechanisms, iv) an "Ockham's razor"/minimalistic model first presented in 1997 and known as the Finke-Watzky 2-step model, and v) quantitative structure activity relationship models. These five classes of mechanisms are reviewed in detail in historical order; where possible corresponding kinetic equations, and fits to aggregation data via the proposed mechanisms, are analyzed and discussed. The five classes of mechanisms are then analyzed and discussed in terms of their similarities and differences to one another. Also included is a brief discussion of selected empirical approaches used to investigate protein aggregation. Three problem areas in the protein aggregation kinetic and mechanistic studies area are identified, and a Summary and Conclusions section is provided en route to moving the field forward towards the still unachieved goal of unequivocal elucidation of the mechanism(s) of protein aggregation.

Fitting yeast and mammalian prion aggregation kinetic data with the Finke-Watzky two-step model of nucleation and autocatalytic growth.

Recently, we reported 14 amyloid protein aggregation kinetic data sets that were fit using the "Ockham's razor"/minimalistic Finke-Watzky (F-W) two-step model of slow nucleation (A --> B, rate constant k 1) and fast autocatalytic growth (A + B --> 2B, rate constant k 2), yielding quantitative (average) rate constants for nucleation ( k 1) and growth ( k 2), where A is the monomeric protein and B is the polymeric protein [Morris, A. M., et al. (2008) Biochemistry 47, 2413-2427]. Herein, we apply the F-W model to 27 representative prion aggregation kinetic data sets obtained from the literature. Each prion data set was successfully fit with the F-W model, including three different yeast prion proteins (Sup35p, Ure2p, and Rnq1p) as well as mouse and human prions. These fits yield the first quantitative rate constants for the steps of nucleation and growth in prion aggregation. Examination of a Sup35p system shows that the same rate constants are obtained for nucleation and for growth within experimental error, regardless of which of six physical methods was used, a unique set of important control experiments in the protein aggregation literature. Also provided herein are analyses of several factors influencing the aggregation of prions such as glutamine/asparagine rich regions and the number of oligopeptide repeats in the prion domain. Where possible, verification or refutation of previous correlations to glutamine/asparagine regions, or the number of repeat sequences, in literature aggregation kinetics is given in light of the quantitative rate constants obtained herein for nucleation and growth during prion aggregation. The F-W model is then contrasted to four literature mechanisms that address the molecular picture of prion transmission and propagation. Key limitations of the F-W model are listed to prevent overinterpretation of the data being analyzed, limitations that derive ultimately from the model's simplicity. Finally, possible avenues of future research are suggested.

Fitting neurological protein aggregation kinetic data via a 2-step, minimal/"Ockham's razor" model: the Finke-Watzky mechanism of nucleation followed by autocatalytic surface growth.

The aggregation of proteins has been hypothesized to be an underlying cause of many neurological disorders including Alzheimer's, Parkinson's, and Huntington's diseases; protein aggregation is also important to normal life function in cases such as G to F-actin, glutamate dehydrogenase, and tubulin and flagella formation. For this reason, the underlying mechanism of protein aggregation, and accompanying kinetic models for protein nucleation and growth (growth also being called elongation, polymerization, or fibrillation in the literature), have been investigated for more than 50 years. As a way to concisely present the key prior literature in the protein aggregation area, Table 1 in the main text summarizes 23 papers by 10 groups of authors that provide 5 basic classes of mechanisms for protein aggregation over the period from 1959 to 2007. However, and despite this major prior effort, still lacking are both (i) anything approaching a consensus mechanism (or mechanisms), and (ii) a generally useful, and thus widely used, simplest/"Ockham's razor" kinetic model and associated equations that can be routinely employed to analyze a broader range of protein aggregation kinetic data. Herein we demonstrate that the 1997 Finke-Watzky (F-W) 2-step mechanism of slow continuous nucleation, A --> B (rate constant k1), followed by typically fast, autocatalytic surface growth, A + B --> 2B (rate constant k2), is able to quantitatively account for the kinetic curves from all 14 representative data sets of neurological protein aggregation found by a literature search (the prion literature was largely excluded for the purposes of this study in order provide some limit to the resultant literature that was covered). The F-W model is able to deconvolute the desired nucleation, k1, and growth, k2, rate constants from those 14 data sets obtained by four different physical methods, for three different proteins, and in nine different labs. The fits are generally good, and in many cases excellent, with R2 values >or=0.98 in all cases. As such, this contribution is the current record of the widest set of protein aggregation data best fit by what is also the simplest model offered to date. Also provided is the mathematical connection between the 1997 F-W 2-step mechanism and the 2000 3-step mechanism proposed by Saitô and co-workers. In particular, the kinetic equation for Saitô's 3-step mechanism is shown to be mathematically identical to the earlier, 1997 2-step F-W mechanism under the 3 simplifying assumptions Saitô and co-workers used to derive their kinetic equation. A list of the 3 main caveats/limitations of the F-W kinetic model is provided, followed by the main conclusions from this study as well as some needed future experiments.

Size discrimination in the coordination chemistry of an isoindoline pincer ligand with CdII and ZnII.

The reactions of Cd2+ and Zn2+ with the pyridine-arm isoindoline ligand 4'-MeLH = 1,3-bis[2-(4-methylpyridyl)imino]isoindoline produced the series of octahedrally coordinated complexes M(4'-MeL)2, [M(4'-MeLH)2]2+, and [M(4'-MeL)(4'-MeLH)]+. The complexes M(4'-MeL)2 resulted from reactions of the respective metal perchlorates with deprotonated ligand, whereas the complexes [M(4'-MeLH)2](ClO4)2 resulted from reactions with ligand in the absence of added base. The mixed-ligand complexes [M(4'-MeL)(4'-MeLH)]+ were generated in solution by reactions of equimolar quantities of M(4'-MeL)2 and [M(4'-MeLH)2]2+. Whereas [Cd(4'-MeL)(4'-MeLH)]+ is stable in solution, [Zn(4'-MeL)(4'-MeLH)]+ converts to and establishes equilibrium with the tetrahedrally coordinated, trinuclear complex [Zn3(4'-MeL)4]2+. The complexes Cd(4'-MeL)2 (1), Zn(4'-MeL)2 (2), and [Cd(4'-MeL)(4'-MeLH)]ClO4 (5) were characterized by single-crystal X-ray diffraction, with the latter complex being shown to contain 4'-MeLH coordinated as a protonated iminium zwitterionic ligand. The [M(4'-MeLH)2]2+ and [M(4'-MeL)(4'-MeLH)]+ complexes are tautomeric in solution because of the shuttling of the iminium protons between imine N atoms. The rate of prototropic tautomerism in [Cd(4'-MeLH)2]+ was followed by 1H NMR spectroscopy. Over the temperature range 276-312 K, a linear Eyring plot with the activation parameters DeltaG++ = 16.0 +/- 0.1 kcal/mol, DeltaH++ = 2.9 +/- 0.1 kcal/mol, and DeltaS++ = -44.0 +/- 0.3 cal/mol.K was obtained.

Cd(II), Zn(II), and Pd(II) complexes of an isoindoline pincer ligand: consequences of steric crowding.

The sterically crowded isoindoline pincer ligand, 6'-MeLH, prepared by condensation of 4-methyl-2-aminopyridine and phthalonitrile, exhibits very different reaction chemistry with Cd2+, Zn2+, and Pd2+. Three different ligand coordination modes are reported, each dependent upon choice of metal ion. This isoindoline binds to Cd2+ as a charge-neutral, zwitterionic, bidentate ligand using imine and pyridine nitrogen atoms to form the eight-coordinate fluxional complex, Cd(6'-MeLH)2(NO3)2. In the presence of Zn2+, however, loss of a pyridine arm occurs through solvolysis and tetrahedrally coordinated complexes are formed with coordination of pyrrole and pyridine nitrogen atoms. Reaction with Pd2+ produces the highly distorted, square planar complex Pd(6'-MeL)Cl in which a deprotonated isoindoline anion coordinates as a tridentate pyridinium NNC pincer ligand.