Overall Rate Of Formation Research Articles

Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W(NAr)(OSiPh3)2(C4H8) (Ar = 2,6-i-Pr2C6H3) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W(NAr)(OSiPh3)2(C3H6). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λmax). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W(NAr)(OSiPh3)2(αMeC3H5)), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W(NAr)(OSiPh3)2(C3H6). The W-Cα bond in an unobservable TBP W(NAr)(OSiPh3)2(C4H8) isomer in which the C4H8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W(NCPh3)(OSiPh3)2(C4H8) as for W(NAr)(OSiPh3)2(C4H8), except that the rate of consumption of W(NCPh3)(OSiPh3)2(C4H8) is about half that of W(NAr)(OSiPh3)2(C4H8). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W(NCPh3)(OSiPh3)2(C3H6) and propylene from W(NCPh3)(OSiPh3)2(C4H8) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

A Biochemical and Functional Protein Complex Involving Dopamine Synthesis and Transport into Synaptic Vesicles

Synaptic transmission depends on neurotransmitter pools stored within vesicles that undergo regulated exocytosis. In the brain, the vesicular monoamine transporter-2 (VMAT(2)) is responsible for the loading of dopamine (DA) and other monoamines into synaptic vesicles. Prior to storage within vesicles, DA synthesis occurs at the synaptic terminal in a two-step enzymatic process. First, the rate-limiting enzyme tyrosine hydroxylase (TH) converts tyrosine to di-OH-phenylalanine. Aromatic amino acid decarboxylase (AADC) then converts di-OH-phenylalanine into DA. Here, we provide evidence that VMAT(2) physically and functionally interacts with the enzymes responsible for DA synthesis. In rat striata, TH and AADC co-immunoprecipitate with VMAT(2), whereas in PC 12 cells, TH co-immunoprecipitates with the closely related VMAT(1) and with overexpressed VMAT(2). GST pull-down assays further identified three cytosolic domains of VMAT(2) involved in the interaction with TH and AADC. Furthermore, in vitro binding assays demonstrated that TH directly interacts with VMAT(2). Additionally, using fractionation and immunoisolation approaches, we demonstrate that TH and AADC associate with VMAT(2)-containing synaptic vesicles from rat brain. These vesicles exhibited specific TH activity. Finally, the coupling between synthesis and transport of DA into vesicles was impaired in the presence of fragments involved in the VMAT(2)/TH/AADC interaction. Taken together, our results indicate that DA synthesis can occur at the synaptic vesicle membrane, where it is physically and functionally coupled to VMAT(2)-mediated transport into vesicles.

Effects of Geometric Isomerism and Anions on the Kinetics and Mechanism of the Stepwise Formation of Long-Range DNA Interstrand Cross-Links by Dinuclear Platinum Antitumor Complexes

Reported herein is a detailed study of the kinetics and mechanism of formation of a 1,4-GG interstrand cross-link by the dinuclear platinum anticancer compound [15N][{cis-PtCl(NH3)2}2{mu-NH2(CH2)6NH2}]2+ (1,1/c,c (1)). The reaction of [15N]1 with 5'-{d(ATATGTACATAT)2} (I) has been studied by [1H,15N] HSQC NMR spectroscopy in the presence of different concentrations of phosphate. In contrast with the geometric trans isomer (1,1/t,t), there was no evidence for an electrostatic preassociation of 1,1/c,c with the polyanionic DNA surface, and the pseudo-first-order rate constant for the aquation of [(15)N]1 was actually slightly higher (rather than lower) than that in the absence of DNA. When phosphate is absent, the overall rate of formation of the cross-link is quite similar for the two geometric isomers, occurring slightly faster for 1,1/t,t. A major difference in the DNA binding pathways is the observation of phosphate-bound intermediates only in the case of 1,1/c,c. 15 mM phosphate causes a dramatic slowing in the overall rate of formation of DNA interstrand cross-links due to both the slow formation and slow closure of the phosphate-bound monofunctional adduct. A comparison of the molecular models of the bifunctional adducts of the two isomers shows that helical distortion is minimal and globally the structures of the 1,4 interstrand cross-links are quite similar. The effect of carrier ligand was investigated by similar studies of the ethylenediamine derivative [15N]1-en. A pKa value of 5.43 was determined for the [15N]1,1/c,c-en diaquated species. The rate of reaction of [15N]1-en with duplex I is similar to that of 1,1/c,c and the overall conformation of the final adduct appears to be similar. The significance of these results to the development of "second-generation" polynuclear platinum clinical candidates based on the 1,1/c,c chelate (dach) series is discussed.

Inorganic formation of apatite in brackish seawater from the Baltic Sea: an experimental approach

Long-term incubation experiments were conducted to study the formation of apatite in calcite-seeded brackish seawater from the Baltic Sea proper. Apatite failed to precipitate from solutions undersaturated with respect to the precursor octacalcium phosphate (OCP), despite a supersaturation with respect to hydroxyapatite (HAP), fluorapatite (FAP), and carbonate-fluorapatite (CFAP). It is suggested that the direct nucleation of apatite is blocked by the presence of dissolved magnesium in seawater. In fact, our marine solutions remained metastable throughout 700 days of incubation. However, formation of apatite took place in seawater supersaturated with respect to OCP, although dissolved magnesium slowed the overall rate of formation. Thus, the precipitation of OCP provides a kinetic boundary for the formation of apatite in natural seawater. These findings strongly suggest that the formation of authigenic apatite in the marine environment generally proceeds via a precursor pathway involving OCP rather than through direct nucleation. Reliable information on the kinetic boundary (even possible complete inhibition) is necessary for proper evaluation of sink estimates in budgets and models of the global phosphorus cycle.

The lipoamide dehydrogenase from Mycobacterium tuberculosis permits the direct observation of flavin intermediates in catalysis.

Lipoamide dehydrogenase catalyses the NAD(+)-dependent oxidation of the dihydrolipoyl cofactors that are covalently attached to the acyltransferase components of the pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and glycine reductase multienzyme complexes. It contains a tightly, but noncovalently, bound FAD and a redox-active disulfide, which cycle between the oxidized and reduced forms during catalysis. The mechanism of reduction of the Mycobacterium tuberculosis lipoamide dehydrogenase by NADH and [4S-(2)H]-NADH was studied anaerobically at 4 degrees C and pH 7.5 by stopped-flow spectrophotometry. Three phases of enzyme reduction were observed. The first phase, characterized by a decrease in absorbance at 400-500 nm and an increase in absorbance at 550-700 nm, was fast (k(for) = 1260 s(-)(1), k(rev) = 590 s(-)(1)) and represents the formation of FADH(2).NAD(+), an intermediate that has never been observed before in any wild-type lipoamide dehydrogenase. A primary deuterium kinetic isotope effect [(D)(k(for) + k(rev)) approximately 4.2] was observed on this phase. The second phase, characterized by regain of the absorbance at 400-500 nm, loss of the 550-700 nm absorbance, and gain of 500-550 nm absorbance, was slower (k(obs) = 200 s(-)(1)). This phase represents the intramolecular transfer of electrons from FADH(2) to the redox-active disulfide to generate the anaerobically stable two-electron reduced enzyme, EH(2). The third phase, characterized by a decrease in absorbance at 400-550 nm, represents the formation of the four-electron reduced form of the enzyme, EH(4). The observed rate constant for this phase showed a decreasing NADH concentration dependence, and results from the slow (k(for) = 57 s(-)(1), k(rev) = 128 s(-)(1)) isomerization of EH(2) or slow release of NAD(+) before rapid NADH binding and reaction to form EH(4). The mechanism of oxidation of EH(2) by NAD(+) was also investigated under the same conditions. The 530 nm charge-transfer absorbance of EH(2) shifted to 600 nm upon NAD(+) binding in the dead time of mixing of the stopped-flow instrument and represents formation of the EH(2).NAD(+) complex. This was followed by two phases. The first phase (k(obs) = 750 s(-)(1)), characterized by a small decrease in absorbance at 435 and 458 nm, probably represents limited accumulation of FADH(2).NAD(+). The second phase was characterized by an increase in absorbance at 435 and 458 nm and a decrease in absorbance at 530 and 670 nm. The observed rate constant that describes this phase of approximately 115 s(-)(1) probably represents the overall rate of formation of E(ox) and NADH from EH(2) and NAD(+), and is largely determined by the slower rates of the coupled sequence of reactions preceding flavin oxidation.

GroEL-mediated folding of structurally homologous dihydrofolate reductases

Using stopped-flow fluorescence techniques, we have examined both the refolding and unfolding reactions of four structurally homologous dihydrofolate reductases (murine DHFR, wild-type E. coli DHFR, and two E. coli DHFR mutants) in the presence and absence of the molecular chaperonin GroEL. We show that GroEL binds the unfolded conformation of each DHFR with second order rate constants greater than 3×107 M−1s−1 at 22°C. Once bound to GroEL, the proteins refold with rate constants similar to those for folding in the absence of GroEL. The overall rate of formation of native enzyme is decreased by the stability of the complex between GroEL and the last folding intermediate. For wild-type E. coli DHFR, complex formation is transient while for the others, a stable complex is formed. The stable complexes are the same regardless of whether they are formed from the unfolded or folded DHFR. When complex formation is initiated from the native conformation, GroEL binds to a pre-existing non-native conformation, presumably a late folding intermediate, rather than to the native state, thus shifting the conformational equilibrium toward the non-native species by mass action. The model presented here for the interaction of these four proteins with GroEL quantitatively describes the difference between the formation of a transient complex and a stable complex as defined by the rate constants for release and rebinding to GroEL relative to the rate constant for the last folding step. Due to this kinetic partitioning, three different mechanisms can be proposed for the formation of stable complexes between GroEL and either murine DHFR or the two E. coli DHFR mutants. These data show that productive folding of GroEL-bound proteins can occur in the absence of nucleotides or the co-chaperonin GroES and suggest that transient complex formation may be the functional role of GroEL under normal conditions.

Electrochemical copolymerisation of 5-substituted indoles

The electrochemical oxidation of mixtures of indole-5-carboxylic acid (ICA) and 5-cyanoindole (CI) produces a film containing free cyclic trimers and polymeric species consisting of linked cyclic trimer units. Trimers containing 3 ICA molecules and 3 CI molecules are present, together with cotrimers which contain 2 ICA and 1 CI molecule and 1 ICA and 2 CI molecules. A mechanism is proposed which explains the observed rate of trimer formation and the relative amounts of trimer and cotrimer formed. In this mechanism, trimer formation occurs via adsorbed ICA and CI radical cations, with the proportion of the trimers in the layer being governed by the coupling of a statistical distribution of these radical cations; the overall rate of formation of each trimer is then given by the combined coupling rate of the ICA and CI radical cations involved. As for the monosubstituted indoles, further linking of trimers and cotrimers occurs when forming the polymer species. The oxidation potentials of the cotrimers lie between those of the (3 ICA) trimer and the (3 CI) trimer, consistent with the proposal that the oxidation potential correlates with the combined electron-withdrawing/donating power of all three substituents in the trimer. Copolymerisation on an electrode surface by the electrochemical oxidation and coupling of preformed ICA and CI trimers is also possible.

Surface heterogeneity in the formation and decomposition of carbon surface oxides

The formation of surface oxides on exposure of carbon (Spherocarb) to oxygen (16–6560 Pa) at temperatures at ca. 700 K has been investigated experimentally using controlled atmosphere thermogravimetry and temperature-programmed desorption (TPD) with evolved gas analysis. Under the conditions of the study. Type B oxides are formed, with the gasification of one C atom (as CO or CO 2 ) for every O atom that is bound to the surface. The broad TPD spectrum obtained for the decomposition of Type B oxides formed on an initially clean surface is invariant over a wide range of initial oxide loadings, from 45 to 990 μmol g −1 . It is concluded that lateral interactions between adsorbed species do not affect the kinetics of decomposition of the surface oxides and that the broad TPD spectrum results entirely from the heterogeneous nature of the carbon surface and the oxides formed on it. Activation energy distribution energies for the decomposition of Type B oxides range from ca. 225 to 475 kJ mol −1 . The formation of oxides with a particular activation energy occurs purely statistically, with the fraction of new oxide, having decomposition activation energies in the range E and E+dE , being simply Ψ 0 ( E ) dE , where Ψ 0 ( E ) is a characteristic of the carbon itself and independent of the oxide loading. Therefore, the rate of formation of complexes having a particular activation energy for their thermal decomposition can be expressed simply in terms of the overall rate of formation of Type B complexes and the distribution Ψ 0 ( E ), which is essentially the same as the activation energy distribution function recovered from the TPD spectra.

Kinetics and mechanism of the peroxidase-catalyzed iodination of tyrosine

The kinetics of iodination of tyrosine by hydrogen peroxide and iodide, catalyzed by both horseradish peroxidase (HRP) and lactoperoxidase (LPO), were studied. The initial rates of formation of both molecular I2 and monoiodotyrosine (MIT) were measured with stopped flow techniques. The following reactions occur in both systems. Enzymatic: FeIII + H2O2-->Fev = O + H2O; Fev = O + I(-)-->FeIII-O-I-; FeIII-O-I- + H(+)-->FeIII + HOI; FeIII-O-I- + I- + H(+)-->FeIII + I2 + HO-. Iodine equilibria: I2 + I-<-->I3-; I2 + H2O<-->HOI + I- + H+. Nonenzymatic iodination, one or both of the following: Tyr + HOI-->MIT + H2O; Tyr + I2-->MIT + I- + H+, where FeIII is native peroxidase, Fev = O is compound I and Tyr is tyrosine. The big difference in the two systems is that the following reaction also occurs with LPO: FeIII-O-I- + Tyr-->MIT + FeIII + HO-, which is the dominant mechanism of iodination for the mammalian enzyme. The overall rate of formation of MIT is about 10 times faster for LPO compared to HRP under comparable conditions. A small decrease in rate occurs when D-tyrosine is substituted for L-tyrosine in the LPO reaction. Thus LPO has a tyrosine binding site near the heme. A kinetically controlled maximum is observed in I3- concentration. Once equilibrium is established, I2 is the dominant form of inorganic iodine in solution. However, hypoiodous acid may be the inorganic iodination reagent.

Effect of stabilising elements on formation of σ phase in experimental ferritic stainless steels containing 39%Cr

The effect of additions of the stabilising elements aluminium, niobium, titanium, and zirconium on the formation of σ phase in the experimental ferritic stainless steel Fe–39Cr–2Mo–2Ni (wt-%) has been studied. It was found that additions of titanium and zirconium increased the overall rate of formation of σ phase, whereas additions of aluminium and niobium decreased the rate. Precipitation of σ phase can be significantly retarded by making an addition of from 1 to 3%Al. The morphology of the σ phase formed in these alloys is discussed. Time–temperature–transformation curves for some of the alloys are presented.MST/1567

Effect of stabilising elements on formation of σ phase in experimental ferritic stainless steels containing 39%Cr

The effect of additions of the stabilising elements aluminium, niobium, titanium, and zirconium on the formation of σ phase in the experimental ferritic stainless steel Fe–39Cr–2Mo–2Ni (wt-%) has been studied. It was found that additions of titanium and zirconium increased the overall rate of formation of σ phase, whereas additions of aluminium and niobium decreased the rate. Precipitation of σ phase can be significantly retarded by making an addition of from 1 to 3%Al. The morphology of the σ phase formed in these alloys is discussed. Time–temperature–transformation curves for some of the alloys are presented.MST/1567

Multinuclear NMR studies of the reactions of (Me 3Al) 2 with Me 2AsNMe 2, MeAs(NMe 2) 2 and As(NMe 2) 3

The reactions of (Me 3Al) 2 with Me 2AsNMe 2, MeAs(NMe 2) 2 and As(NMe 2) 3 were studied as a function of time at room temperature and over the temperature range of −95 to 24°C by using 1H and 13C NMR spectroscopy. At −95°C, all the aminoarsines form the respective mono AlN bonded adducts. In addition, MeAs(NMe 2) 2 and As(NMe 2) 3 form bis AlN bonded adducts and As(NMe 2) 3 forms an AlAs bonded adduct. The bis adducts readily decompose and the AlAs bonded adduct dissociates at low temperature. All the mono AlN bonded adducts decompose by a pathway that transfers a methyl group from the aluminium to the arsenic atom, cleaves the AsN bond, and yields the next higher methyl homologue in the arsine series [MeAs(NMe 2) 2,Me 2AsNMe 2 or Me 3As]. The bis AlN bonded adducts decompose to the next higher methyl homologue in the mono adduct series [MeAs(NMe 2)NMe 2·AlMe 3 or Me 2AsNMe 2·AlMe 3]. The other product in both cases is the unstable Me 2AlNMe 2, which dimerizes or undergoes further reaction with excess (Me 3Al) 2 or aminoarsine. The role of numerous competing side reactions and adduct/aminoarsine exchange equilibria in affecting the rate of adduct decomposition, overall rate of formation of the final product arsine, and final product distribution was established for each aminoarsine/(Me 3Al) 2 system. The initial stoichiometry of the reaction greatly influences the relative importance of the various reactions that occur in solution. Only when the moles of available Me 3Al molecules equal those of Lewis base nitrogen sites, do the reactions proceed in a straightforward manner to give Me 3As and (Me 2AlNMe 2) 2. The results from the NMR studies were used to design a new, high yield synthetic route to Me 3As.

TRANSIENT RESPONSE ANALYSIS IN THE DETERMINATION OF ADSORPTION RATE COEFFICIENTS FOR PROPYLENE AND NITRIC OXIDE ON NiO/Al203 AEROGEL CATALYSTS

Nickel oxide on alumina aerogel catalysts are known to be active and selective for the formation of acrylonitrile from nitric oxide and propylene. During an effort to investigate the mechanism of this reaction some transient feed experiments were carried out. The results obtained from these transient experiments were used to calculate the adsorption coefficients of propylene and nitric oxide on a NiO/Al2O3 aerogel catalysts with 1:1 Ni to Al atomic ratio. Adsorption coefficients of nitric oxide and propylene was determined by independently flowing the respective gases over the catalyst in a stream of helium at 410° C. Adsorption coefficient of nitric oxide on the catalyst at an oxidized state has been found to be less than that of propylene. When the two reactant gases flowed across the catalyst together the rate of adsorption of nitric oxide increased as the catalyst was reduced by propylene adsorption. The increased rate of adsorption of nitric oxide does not, however, influence the overall rate of formation of acrylonitrile.

OPTIMUM DESIGN OF ZEOLITE/SILICA-ALUMINA CATALYSTS

An analysis of the rate of formation of the desired product and of the selectivity has been carried out for composite catalysts wherein small particles of zeolite are distributed nonuniformly within a large particle of the more porous silica-alumina. Two nonuniform distributions have been analyzed, namely, (i) the composite-center distribution, where a uniform dilute region of zeolite in silica-alumina is surrounded by a silica-alumina matrix, and (ii) the composite-surface distribution, where a uniform dilute region is located around the pure silica-alumina matrix. Two reaction schemes, A→B→C and A→C→D, which fairly approximate the catalytic cracking process over the Zeolite/silica-alumina catalyst have been examined. The reaction scheme A→B→C is employed for the composite-center distribution, while the reaction scheme A→C→D is considered for the composite-surface distribution, because the overall rate of formation of the desired product can be thus optimized. There are conditions under which the selecti...

Assembly of human adult and sickle hemoglobins from their oxygenated subunits. Differential rates of beta chain tetramer dissociation.

The rate of assembly of oxyhemoglobins A and S at pH 7 and 20 degrees C has been followed spectrophotometrically at 582.5 nm, a maximum of the difference spectrum of isolated oxygenated subunits and intact oxyhemoglobin. In 0.1 M potassium phosphate buffer, the resultant time courses following mixing of equivalent concentrations of alpha and beta chains were homogeneous and followed first order kinetics. These time courses were protein concentration independent over a range of 8.0 to 60 X 10(-6) M in heme after mixing and presumably reflect the rate of dissociation of the beta chain tetramer. This rate-limiting dissociation reaction was found to be nearly 2-fold slower for the beta S than the beta A chain tetramer exhibiting half-times of 27 and 15 min, respectively. The overall rate of formation of Hb S and Hb A appears to be significantly influenced by the rates of dissociation of their respective beta chains. These findings are in contrast to mixing experiments done in 0.01 M potassium phosphate buffer which revealed time courses which were heterogeneous and markedly dependent upon protein concentration. The stability of the oxygenated beta chain tetramer and, therefore, the overall kinetic profile of liganded hemoglobin reconstitution is acutely sensitive to buffer conditions.

The distribution and fate of 3H-glucose and 3H-galactose in oocytes of Oryzias latipes: Formation of the cortical alevoli.

The distribution and fate of 3H-glucose and 3H-galactose and the formation of the cortical alveoli in the oocytes of the teleost Oryzias latipes were investigated. Fish were given intraperitoneal injections of either labeled hexose, and the relative amounts of each were measured in autoradiographs prepared for light and electron microscopy. There were significant differences in the amounts of each hexose in the oocytes at all time intervals studied. During the earlier time periods, the oocyte incorporates more 3H-glucose than 3H-galactose in both the cytoplasmic and cortical alveoli compartments; however, the overall rate of formation of the cortical alevoli appears to be similar for the two tracers. Electron microscopic autoradiographs reveal the participation of cytoplasmic elements, e.g., endoplasmic reticulum and Golgi apparatus, in the formation of the cortical alveoli. The chorion does not incorporate significant amounts of either tracer, while the yolk granules contain small but significant amounts.

Kinetics and Mechanism of Transfer Action of Saccharifying α-Amylase of Bacillus subtilis

1. The formation of phenol (φ) was observed in the mixture of maltose (M) and phenyl α-glucoside (φG), which was catalyzed by saccharifying α-amylase (EC 3. 2. 1. 1) of Bacillus subtilis, although φG itself was not hydrolyzed either in the presence or absence of glucose. Thin layer chromatography (TLC) of the reaction mixture revealed the formation of a small amount of phenyl α-maltoside (φM). From these results, it was concluded that φM was produced from M and φG by glucosyl transfer catalyzed by the enzyme, and then φM was hydrolyzed into φ and M. Since the overall rate of formation of phenol, vφ, is limited by the rate of transfer, vr, the latter can be obtained directory from the measurement of the former. This system was utilized for studying the kinetics of transfer action of this enzyme. 2. The rate of transfer as a function of φG concentration at a fixed concentration of M proceeded according to the Michaelis-Menten kinetics within the range of concentrations of φG examined, and the apparent Michaelis constant KmφG for φG was 250 mM at a maltose concentration of 42 mM. On the other hand, the rate of transfer, vr, as a function of the concentration of M in a fixed concentration of φG showed a remarkable sigmoidal shape in the region of lower substrate concentrations and showed a maximum at around 100 mM, above which it decreased hyperbolically. 3. The ratio between the rate of transfer, vT, and that of hydrolysis of maltose, vW, vT/vW increased with the concentration of φG, showing that the hydrolysis of M is inhibited by φG. 4. Methyl alcohol was found to serve as an acceptor, judging from the formation of methyl α-glucoside detected by TLC. 5. The hydrolysis of M, and the transfer action of a glucosyl residue of M to φG and to M were explained by assuming essentially the same mechanism as that previously proposed for the hydrolysis of φM.

Precipitation of nickel dimethylglyoximate from homogeneous solution: Studies of the reactions of biacetyl, hydroxylamine and nickel II

The reaction between biacetyl and hydroxylamine to form dimethylglyoxime was found to follow simple kinetics in the absence of nickel. However, with nickel II present, the kinetic studies showed definite evidence for complex formation between biacetyl monoxime and nickel. This complex and others formed with nickel in the reaction mixture determines the overall rate of formation of nickel dimethyl-glyoximate. Supersaturation concentrations of several hundred times the equilibrium solubility of nickel dimethylglyoximate were attained in nucleation experiments. The kinetic and nucleation studies provide explanations for some previously anomalous experimental observations, e.g., the persistent supersaturation in this system provides an explanation for the inability to precipitate completely small amounts of nickel II with dimethylglyoxime. The yellow colour observed in the initial stage of the precipitation of nickel from homogeneous solution arises from dissolved nickel dimethylglyoximate.