Reversible ring opening in the hydrolysis of spiro ortho esters

1985 ◽  
Vol 63 (10) ◽  
pp. 2673-2678 ◽  
Author(s):  
Robert A. McClelland ◽  
Claude Moreau
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Ring Opening ◽  
Lone Pair ◽  
Order Rate Constant ◽  
Product Analysis ◽  
Ortho Ester ◽  
First Order ◽  
Order Rate ◽  
Ortho Esters

Hydrolysis kinetics are reported for four spiro ortho esters: 3,4-dihydro-6-methoxy-1H-2-benzopyran-1-spiro-2′-1′,3′-dioxolane (13), its 1′,3′-dioxane analog (14), and the 6-unsubstituted versions of each (11 and 12). For comparison, also included are the diethoxy analogs: 1,1-diethoxy-3,4-dihydro-6-methoxy-1H-2-benzopyran (10) and the 6-unsubstituted compound (9). Product analysis implicates an initial opening of the dioxolane or dioxane ring in the spiro ortho esters, as expected on the basis of stereoelectronic considerations. The intermediate dialkoxycarbocations can be observed in HCl solutions. A detailed analysis has been carried out for the 6-methoxy systems to provide the rate constants k1, the second-order rate constant for H+-catalyzed formation of the cation from the ortho ester, k2, the first-order rate constant for water addition to the cation, and k−1, the first-order rate constant for ring closing of the cation to reform the ortho ester. The two spiro ortho esters are shown in this analysis to undergo reversible ring opening in their hydrolysis, in that values of k−1, are greater than k2. The differences, however, are not large, k−1/k2 being 1.2 (dioxolane, 13) and 3.8 (dioxane, 14). Comparison with the diethoxy ortho ester also reveals that the ring opening process (k1, rate constants) is inherently more difficult with the dioxolane, although not with the dioxane. An argument involving lone pair orientation is advanced to explain this.

Comparison of Metal Alkoxide Initiators in the Ring-Opening Polymerization of Caprolactone

2012 ◽  
Vol 506 ◽  
pp. 142-145 ◽  
Author(s):  
W. Meelua ◽  
V. Bua-Own ◽  
Robert Molloy ◽  
Winita Punyodom
Keyword(s):  
Rate Constant ◽  
Ring Opening ◽  
Order Rate Constant ◽  
Ring Opening Polymerization ◽  
High Conversion ◽  
Metal Alkoxide ◽  
First Order ◽  
Order Rate

Four metal tert-butoxides, namely: aluminum (III) tert-butoxide, Al (Ot-C4H9)3, tin (II) tert-butoxide, Sn (Ot-C4H9)2, titanium (IV) tert-butoxide, Ti (Ot-C4H9)4, and lithium tert-butoxide, Li (Ot-C4H9), were used as initiators in the bulk ring-opening polymerization (ROP) of ε-caprolactone (CL). The polymerizations were carried out at 120 °C for 72 hrs. It was found that the Al (Ot-C4H9)3 only partially dissolved in the CL monomer and gave a low % conversion. The Li (Ot-C4H9) initiator did not dissolve and gave no polymerization. In contrast, the Sn (Ot-C4H9)2 initiator dissolved completely, albeit very slowly, and gave a high % conversion. The Ti (Ot-C4H9)4 initiator was by far the easiest and quickest to dissolve and also gave a high % conversion. Consequently, Ti (Ot-C4H9)4-initiated polymerization was studied further by dilatometry at 120°C. From the kinetic results, a first-order rate constant, kp, of 0.120 l mol-1 min-1 was obtained.

scholarly journals The pH-dependence of second-order rate constants of enzyme modification may provide free-reactant pKa values

1977 ◽  
Vol 167 (3) ◽  
pp. 859-862 ◽  
Author(s):  
K Brocklehurst ◽  
H B F Dixon
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Second Order ◽  
Quasi Equilibrium ◽  
Pka Values ◽  
First Order ◽  
Order Rate ◽  
Parallel Reactions

1. Reactions of enzymes with site-specific reagents may involve intermediate adsorptive complexes formed by parallel reactions in several protonic states. Accordingly, a profile of the apparent second-order rate constant for the modification reaction (Kobs., the observed rate constant under conditions where the reagent concentration is low enough for the reaction to be first-order in reagent) against pH can, in general, reflect free-reactant-state molecular pKa values only if a quasi-equilibrium condition exists around the reactive protonic state (EHR) of the adsorptive complex. 2. Usually the condition for quasi-equilibrium is expressed in terms of the rate constants around EHR: (formula: see text) i.e. k mod. less than k-2. This often cannot be assessed directly, particularly if it is not possible to determine kmod. 3. It is shown that kmod. must be much less than k-2, however, if kobs. (the pH-independent value of kobs.) less than k+2. 4. Since probable values of k+2 greater than 10(6)M-1.S-1 and since values of kobs. for many modification reactions less than 10(6)M-1.S-1, the equilibrium assumption should be valid, and kinetic study of such reactions should provide reactant-state pKa values. 5. This may not apply to catalyses, because for them the value of kcat./Km may exceed 5 X 10(5)M-1.S-1. 6. The conditions under which the formation of an intermediate complex by parallel pathways may come to quasi-equilibrium are discussed in the Appendix.

scholarly journals Compounds of 6, 13-Diamino-6, 13-Dimethyl-1, 4, 8, 11 - Tetraazacyclotetradecane

2021 ◽  
Author(s):  
◽  
Asokamali Siriwardena
Keyword(s):  
Rate Constants ◽  
Magnetic Measurements ◽  
Order Rate Constant ◽  
Acidic Solutions ◽  
First Order ◽  
Order Rate ◽  
Pendant Arm ◽  
Nickel Copper ◽  
13C Nuclear Magnetic Resonance ◽  
Kinetics Of

<p>The reaction of bis-(diaminoethane)nickel(II) chloride, ([Ni(en)2]Cl2 in methanol with formaldehyde and nitroethane in the presence of triethylamine proceeds readily to produce (6, 13-dimethyl-6, 13-dinitro-1, 4, 8, 11-tetraazacyclotetradecane)nickel(II) chloride, [Ni(dini)] - Cl2. Reduction of the nitro groups of this compound by catalytic hydrogenation yields three isomers of the pendant arm macrocyclic complex (6, 13-diamino-6, 13-dimethyl-1, 4, 8, 11-tetraazachyclotetradecane)nickel(II) chloride, designated a-, b- and c-[Ni(diam)]Cl2. These were separated by fractional crystallization. The aisomer was observed to isomerizes slowly in solution to the b- form. A parallel dissociation reaction of the a- isomer was also observed. The demetallation of a- and b- isomers of the diam complex of nickel by reaction with cyanide or concentrated acid at 140 degrees C produces the macrocycle meso-(6, 13-diamino-6, 13-dimethyl-1, 4, 8, 11-tetraazacyclotetra-decane), diam. A variety of hexamine, pentamine and tetramine complexes of diam with nickel(II), copper(II), cobalt(II) and (III), chromium(III), palladium(II), rhodium(III), zinc(II) and cadmium(II) were prepared. Hexamine and tetramine forms of labile metal complexes could be rapidly and reversibly interconverted by altering the pH. The hexamine cobalt(III) cation, [Co(diam)]3+ was by far the most inert of the prepared cobalt(III) complexes, remaining unaffected in hot acidic solutions. In contrast, a single pendant arm of the hexamine [Cr(diam)]3+ cation could be dissociated in acid. (Two possibly triamine complexes of lead were also prepared). These compounds were characterized by elemental analysis, magnetic measurements, electronic, infrared, 1H and 13C nuclear magnetic resonance spectra. The pendant arm protonation constants (log K) of diam and selected complexes of nickel, copper and palladium were calculated from potentiometric titration measurements at 25 degrees C. The log K values for diam at 25 degrees C (I = 0.1 M NaclO4) were 11.15, 9.7, 6.2 and 5.3. Kinetics of the parallel isomerization and dissociation of a-[Ni(dimH2)]4+ in HCl/NaCl solutions were monitored spectrophotometrically at 50 degrees C. The rate of reaction in acidic solutions showed a non-linear dependency on acid concentration. The observed first order rate constant (kobs) for disappearance of a-[Ni(diamH2)]4+ (by isomerization and dissociation) in 2.0 M HCl, 0.1 M NaOH and 2.0 M NaCl were 3.05 x 10-4, 2.0(3) x 10-2 and 5.0 x 10-5 s-1 respectively. The rate of the dissociation component of the reaction of a-[Ni(diamH2)]4+ in 2.0 M HCl at 50 degrees C was 1.82 x 10-7 s-1. Acid bydrolysis kinetics of (Cu[diamH2])(ClO4)4 in hydrochloric acid and perchloric acid at 50 and 70 degrees C were studied spectrophotometrically. The reactions were slow and the observed first order rate constants were to a first approximation independent of the particular acid or its concentration. The observed first order rate constants were 1 x 10-9 and 8 x 10-9 s-1 at 50 and 70 degrees C respectively. Questions about the nature of the reaction being followed have been raised.</p>

The Dephosphorylation of Adenosine 5′ -Triphosphate in a Binary and Ternary Zn2+ Complex

1974 ◽  
Vol 29 (11-12) ◽  
pp. 680-682 ◽  
Author(s):  
Peter Amsler ◽  
David Buisson ◽  
Helmut Sigel
Keyword(s):  
Rate Constant ◽  
Ternary Complex ◽  
Order Rate Constant ◽  
Reactive Species ◽  
Ph Optimum ◽  
First Order ◽  
Order Rate

The dephosphorylation of ATP was characterized by determining the dependence of the first-order rate constant on pH in the absence and presence of Zn2+ and together with Zn2+ and 2,2′-bipyridyl. The Zn2+-accelerated reaction passes through a pH optimum at about 8. The decrease in the rate at higher pH is due to the formation of Zn(ATP) (OH)3-; this species is relatively insensitive towards dephosphorylation. It is concluded that Zn(ATP)2- is the reactive species and that the interaction between N (7) and Zn2+ in this complex is crucial for its reactivity. In the presence of 2,2′-bipyridyl (Bipy) the ternary complex, Zn (Bipy) (ATP)2-, is formed which is rather stable towards dephosphorylation. It is suggested that the described effects of acceleration and inhibition are helpful for understanding the recycled processes in nature.

HUMAN MEIZOTHROMBIN 1, ITS ISOLATION AND PROPERTIES

1987 ◽  
Author(s):  
Zbigniew S Latallo ◽  
Craig M Jackson
Keyword(s):  
Rate Constants ◽  
Antithrombin Iii ◽  
Order Rate Constant ◽  
Red Cross ◽  
Apparent Lack ◽  
First Order ◽  
Order Rate ◽  
Matching Grant ◽  
Echis Carinatus

Meizothrombin (MT) and meizothrombin des Fragment 1 (MT1) are intermediates in the conversion of prothrombin to α-thrombin (αTH). Due to their transient character, properties of these enzymes are difficult to establish. Isolation of MT1 was achieved by affinity chromatography on D-Phe-Pro-Arginal (FPRal)immobilized on Affi-Gel 10 as originally employed for thrombin purification (Patel et al. Biochim.Biophys. Acta 748,321 (1983)). Human prethrombin 1 was activated with the purified activator from Echis carinatus venom in the presence of Ca++;, benzamidine and FPRal gel at pH 7.8. After exhaustive washing the MT1 was eluted with 0.1 M hydroxylamine in 0.15 M Na acetate buffer, pH 5.5. Under these conditions the MT1 is stable and can bestored at -70°C. Upon changing the pH of the preparation to 8.0, complete conversion into aTH occurred atroom temperature within 48 hours. Homogeneity of both preparations wasdemonstrated by PAGE. The Km and ke, values for MT1 measured on Tos-Gly-Pro-Arg pNA(0.1 M NaCl, 0.01 M TRIS, 0.01 M HEPES, 0.1% PEG, pH 7.8, 25°C) were 15.7 /iM and 126 s-1. The kinetic con stants for the aTH resulting from autocatalytic degradation of MT1 were indistinguishable from those previously established forαTH obtained by Xa activation i.e. 4.77 /μM and 126 s-1. Clotting activity of MT1 was found to be only one fifth as high as that of the resulting μTH(746 u/mg vs. 3900 u/mg as tested using the NIH standard) .Inhibitionof MTl by antithrombin III was alsomuch less rapid than αTH andmost importantly, it was not affected by high affinity heparin( Mr20,300). Under conditions of the experiment (0.3 M NaCl, 0.0rl M TRIS, 0.01 M HEPES, 2.5 mM EDTA, 0.1% PEG, pH 7.8, 25°C; [ATIII] 100 nM, [E] 10 nM), the pseudo first order rate constants in the absence of heparin were 4.04 × 10-3V1 (MTl) and 1.13 × 10-3V1 (αTH), giving apparent second order rate constants of 4.04 × 103 and 1.13 × 10-4M-1s-1. In the presence of 4.5 nM of heparin the observed first order rate constant for MTl remained unchanged whereas it increased to 6.241 × 10-3s-1 (5.5 fold) for αTH. This apparent lack of an effect of heparin may be of significance in vivo.Supported by a Matching Grant from the American National Red Cross and by the Southeastern Michigan Blood Service.

Kinetics of reconstitution of apotyrosinase by copper

1990 ◽  
Vol 68 (2) ◽  
pp. 476-479
Author(s):  
Donald C. Wigfield ◽  
Douglas M. Goltz
Keyword(s):  
Rate Constant ◽  
Copper Concentration ◽  
Copper Ions ◽  
Copper Ion ◽  
Order Rate Constant ◽  
First Order ◽  
Order Rate ◽  
Pseudo First Order ◽  
Rate Limiting ◽  
Kinetics Of

The kinetics of the reconstitution reaction of apotyrosinase with copper (II) ions are reported. The reaction is pseudo first order with respect to apoenzyme and the values of these pseudo first order rate constants are reported as a function of copper (II) concentration. Two copper ions bind to apoenzyme, and if the second one is rate limiting, the kinetically relevant copper concentration is the copper originally added minus the amount used in binding the first copper ion to enzyme. This modified copper concentration is linearly related to the magnitude of the pseudo first order rate constant, up to a copper concentration of 1.25 × 10−4 M (10-fold excess), giving a second order rate constant of 7.67 × 102 ± 0.93 × 102 M−1∙s−1.Key words: apotyrosinase, copper, tyrosinase.

scholarly journals Inactivation of the endogenous argininosuccinate lyase activity of duck δ-crystallin by modification of an essential histidine residue with diethyl pyrocarbonate

1993 ◽  
Vol 293 (2) ◽  
pp. 537-544 ◽  
Author(s):  
H J Lee ◽  
S H Chiou ◽  
G G Chang
Keyword(s):  
Enzyme Activity ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Histidine Residue ◽  
Argininosuccinate Lyase ◽  
First Order ◽  
Diethyl Pyrocarbonate ◽  
Order Rate ◽  
Lyase Activity ◽  
Pseudo First Order

The argininosuccinate lyase activity of duck delta-crystallin was inactivated by diethyl pyrocarbonate at 0 degrees C and pH 7.5. The inactivation followed pseudo-first-order kinetics after appropriate correction for the decomposition of the reagent during the modification period. The plot of the observed pseudo-first-order rate constant versus diethyl pyrocarbonate concentration in the range of 0.17-1.7 mM was linear and went through the origin with a second-order rate constant of 1.45 +/- 0.1 M-1.s-1. The double-logarithmic plot was also linear, with slope of 1.13, which suggested a 1:1 stoichiometry for the reaction between diethyl pyrocarbonate and delta-crystallin. L-Arginine, L-norvaline or L-citrulline protected the argininosuccinate lyase activity of delta-crystallin from diethyl pyrocarbonate inactivation. The dissociation constants for the delta-crystallin-L-arginine and delta-crystallin-L-citrulline binary complexes, determined by the protection experiments, were 4.2 +/- 0.2 and 0.12 +/- 0.04 mM respectively. Fumarate alone had no protective effect. However, fumarate plus L-arginine gave synergistic protection with a ligand binding interacting factor of 0.12 +/- 0.02. The double-protection data conformed to a random Uni Bi kinetic mechanism. Fluorescence-quenching studies indicated that the modified delta-crystallin had minimum, if any, conformational changes as compared with the native delta-crystallin. Inactivation of the enzyme activity was accompanied by an increasing absorbance at 240 nm of the protein. The absorption near 280 nm did not change. Treatment of the modified protein with hydroxylamine regenerated the enzyme activity to the original level. These results strongly indicated the modification of an essential histidine residue. Calculation from the 240 nm absorption changes indicated that only one histidine residue per subunit was modified by the reagent. This super-active histidine residue has a pKa value of approximately 6.8 and acts as a general acid-base catalyst in the enzyme reaction mechanism. Our experimental data are compatible with an E1cB mechanism [Raushel (1984) Arch. Biochem. Biophys. 232, 520-525] for the argininosuccinate lyase with the essential histidine residue close to the arginine-binding domain of delta-crystallin. L-Citrulline, after binding to this domain, might form an extra hydrogen bond with the essential histidine residue.

The Kinetics of Isotope Effects in a Model Solvolysis System Involving One Optically Active Ion Pair Intermediate

1974 ◽  
Vol 52 (10) ◽  
pp. 1937-1941 ◽  
Author(s):  
P. Christian Vogel
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Isotope Effects ◽  
Kinetic Scheme ◽  
Ion Pair ◽  
Rate Equations ◽  
Differential Analysis ◽  
First Order ◽  
Order Rate ◽  
Special Cases

The derivation of the observed first-order rate constants from the "exact" integrated rate equations for the kinetic scheme of reaction 1 is presented. It is shown that the solvolytic exponential first-order rate constant is a special case of the polarimetric rate constant and that the optical activity of the product is determined by a multiplicative ratio of rate constants for the optically important reactions of the ion pair intermediate. A form of the integrated first-order polarimetric rate equation with a linearly independent parameter set is presented. The functions for the first-order rate constants derived using the steady state approximation are special cases of the functions derived from the exact equations, as are the functions for the first-order rate constants for two systems which involve pre-equilibria followed by a slow product forming step. These functions cannot all be derived one from the other. A differential analysis of observed isotope effects as functions of isotope effects on the rate constants for reactions involving the intermediates is presented.

The Initial Stages of the Pyrolysis of Dimethyl Ether

1975 ◽  
Vol 53 (18) ◽  
pp. 2742-2747 ◽  
Author(s):  
Philip D. Pacey
Keyword(s):  
Dimethyl Ether ◽  
Rate Constant ◽  
Arrhenius Plot ◽  
Flow System ◽  
Order Rate Constant ◽  
Chain Termination ◽  
Rate Coefficients ◽  
First Order ◽  
Order Rate ◽  
Initiation Reaction

Dimethyl ether was pyrolized in a flow system at 782–936 K and 25–395 Torr with conversions from 0.2–10%. Product analyses were consistent with a simple Rice–Herzfeld mechanism with most chain termination by the recombination of CH3 radicals. The rate coefficients for both the initiation and termination reactions appeared to be slightly pressure dependent. The first-order rate constant for the initiation reaction,[Formula: see text]calculated from the rate of C2H6 formation, was k1 = 1015.0±0.5exp (−318 ± 8 kJ mol−1/RT) s−1, corresponding to ΔHf0(CH3O) = −5 ± 8 kJmol−1. Comparison of CH4 and C2H6 yields enabled calculation of the rate constant for the reaction of CH3 with dimethyl ether. From 373−936 K, the Arrhenius plot for this reaction is a curve.

Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

1984 ◽  
Vol 62 (3) ◽  
pp. 580-585 ◽  
Author(s):  
Parminder S. Surdhar ◽  
Rizwan Ahmad ◽  
David A. Armstrong
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Order Rate Constant ◽  
Second Order ◽  
Radical Species ◽  
Spectral Changes ◽  
Order Rate ◽  
Visible Spectra ◽  
Uv Visible ◽  
Presence And Absence

Spectral changes and rates of reaction of flavins and several radical species have been investigated at pH 7, 9, and 10 in the presence and absence of β-mercaptoethanol. The radicals •CO2−, eaq−, and [Formula: see text] reacted with FAD at pH 10 to give a spectrum of FAD •Fl− with rate constants of 7 ± 1 × 108 and 4 ± 1 × 108 M−1 s−1 for •CO2− and [Formula: see text] respectively. At pH 7 only •FlH was observed and at pH 9 a mixture of •FIH and •Fl−.Interactions between flavin radicals and sulphydryl at 10−4 M concentration did not cause perturbations in the uv–visible spectra until either the radical and/or the sulphydryl were ionized. With FAD at pH 9 or 10 and LFl at pH 10 the 370 nm peak of •Fl− was enhanced by about 15% and a second larger growth occurred near 450 nm in the presence of 10−4 to 10−2 M sulphydryl. We attribute this to the formation of labile intermediate RSHFl•−, which must also be involved in the reduction of Fl by [Formula: see text] at pH 9 or 10.The second order rate constant k13 for reaction of [Formula: see text] with FAD at pH 9 and 10 was found to be 4.2 ± 0.5 × 108 M−1 s−1 and 2.0 ± 0.4 × 108 M−1 s−1 respectively. The rate constant for the reaction between [Formula: see text] and LFl at pH 10 was slightly faster, 7 ± 1 × 108 M−1 s−1, probably reflecting the fact that LFl lacks the bulky negatively charged adenine dinucleotide group of FAD.

Sign in / Sign up

Export Citation Format

Share Document