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1.
Inoue T Suzuki S Nishio N Yamaguchi K Kataoka K Tobari J Yong X Hamanaka S Matsumura H Kai Y 《Journal of molecular biology》2003,333(1):117-124
The obligate methylotroph Methylomonas sp. strain J produces two azurins (Az-iso1 and Az-iso2) as candidates for electron acceptor from methylamine dehydrogenase (MADH) in the electron-transfer process involving the oxidation of methylamine to formaldehyde and ammonia. The X-ray crystallographic study indicated that Az-iso2 gives two types of crystals (form I and form II) with polyethylene glycol (PEG4000) and ammonium sulfate as the precipitants, respectively. Comparison between the two Az-iso2 structures in forms I and II reveals the remarkable structural changes at the top surface of the molecule around the copper atom. Az-iso2 possesses Gly43 instead of Val43 or Ala43, which is unique among all other azurins around the copper ligand His46, inducing the remarkable structural change in the loop region from Gly37 to Gly43. When the structure of Az-iso2 is superimposed on that of amicyanin in the ternary complex composed of MADH, amicyanin, and cytochrome c(551), the loop of Az-iso2 deeply overlaps with the light subunit of MADH. However, the Az-iso2 molecule is probably able to avoid any steric hindrance with the cognate MADH to form the complex for intermolecular electron-transfer reaction, since the loop containing Gly43 is flexible. We discuss why the electron-transfer activity of Az-iso2 is fivefold higher than that of Az-iso1. 相似文献
2.
The intramolecular electron-transfer rate constant for the Cu(II)–topaNH2⇌ Cu(I)–topaSQ equilibrium in methylamine oxidase has been measured by temperature-jump relaxation techniques. At pH 7.0 the estimated kobs = 150±30 s–1 for both methylamine and benzylamine; assuming the equilibrium constant is ≈0.7–1 at pH 7.0 and 296 K, this would correspond
to a forward electron-transfer rate constant kET≈ 60–75 s–1. Although substantially slower than the previously determined kET≈ 20 000 s–1 for pea seedling amine oxidase [5] steady-state kinetics measurements established that kET > kcat≈ 4–10 s–1. Thus the Cu(I)-semiquinone state is a viable intermediate in methylamine oxidase turnover.
Received: 16 August 1995 / Accepted: 21 December 1995 相似文献
3.
Alexander F. Arendsen Marcel de Vocht Yvonne B. M. Bulsink W. R. Hagen 《Journal of biological inorganic chemistry》1996,1(4):292-296
Aldehyde:ferredoxin oxidoreductase (AOR) from the hyperthermophilic archaeon Pyrococcus furiosus is a homodimeric protein. Each subunit carries one [4Fe-4S] cubane and a novel tungsten cofactor containing two pterins.
A single iron atom bridges between the subunits. AOR has previously been studied with EPR spectroscopy in an inactive form
known as the red tungsten protein (RTP): reduced RTP exhibits complex EPR interaction signals. We have now investigated the
active enzyme AOR with EPR, and we have found an S = 1/2 plus S = 3/2 spin mixture from a non-interacting [4Fe-4S]1+ cluster in the reduced enzyme. Oxidized AOR affords EPR signals typical for W(V) with g–values of 1.982, 1.953, and 1.885. The W(V) signals disappear at a reduction potential E
m,7.5 of +180 mV. This unexpectedly high value indicates that the active-site redox chemistry is based on the pterin part of the
cofactor.
Received: 18 December 1995 / Accepted: 26 March 1996 相似文献
4.
ETA subtype selective antagonists constitute a novel and potentially important class of agents for the treatment of pulmonary
hypertension, heart failure, and other pathological conditions. In this paper, 60 benzodiazepine derivatives displaying potent
activities against ETA and ETB subtypes of endothelin receptor were selected to establish the 3D-QSAR models using CoMFA and CoMSIA approaches. These models
show excellent internal predictability and consistency, external validation using test-set 19 compounds yields a good predictive
power for antagonistic potency. Statistical parameters of models were obtained with CoMFA-ETA (q
2 = 0.787, r
2 = 0.935, r
2
pred
= 0.901), CoMFA-ETB (q
2 = 0.842, r
2 = 0.984, r
2
pred
= 0.941), CoMSIA-ETA (q
2 = 0.762, r
2 = 0.971, r
2
pred
= 0.958) and CoMSIA-ETB (q
2 = 0.771, r
2 = 0.974, r
2
pred
= 0.953) respectively. Field contour maps (CoMFA and CoMSIA) corresponding to the ETA and ETB subtypes reflects the characteristic similarities and differences between these types. The results of this paper provide
valuable information to facilitate structural modifications of the title compounds to increase the inhibitory potency and
subtype selectivity of endothelin receptor. 相似文献
5.
P. C. Tiburcio F. C. F. Galvez L. J. Cruz V. C. Gavino 《Journal of applied phycology》2007,19(6):727-731
Gamma linolenic acid (GLA) degradation in Spirulina followed first-order reaction kinetics. At an accelerated temperature range of 45 to 55°C, the degradation rate constants
(k
r) of GLA obtained were 4.0 × 10−2 to 8.8 × 10−2 day−1. The energy of activation (E
a) was 16.53 kcal mol−1, and the Q10 was 2.22. Based on 20% GLA degradation, the shelf life of sun-dried Spirulina at 30°C is 263 days or 8.6 months using the Arrhenius plot, and 258 days or 8.5 months using the Q
10 approach.
Presented at the 6th Meeting of the Asia Pacific Society of Applied Phycology, Manila, Philippines. 相似文献
6.
Ish K. Dhawan Roopali Roy Brian P. Koehler Swaranalatha Mukund Michael W. W. Adams Michael K. Johnson 《Journal of biological inorganic chemistry》2000,5(3):313-327
Thermococcus litoralis (Tl) have been investigated by using the combination of EPR and variable-temperature magnetic circular dichroism (VTMCD)
spectroscopies. The results reveal a [Fe4S4]2+,+ cluster (E
m=−368 mV) that undergoes redox cycling between an oxidized form with an S=0 ground state and a reduced form that exists as a pH- and medium-dependent mixture of S=3/2 (g=5.4; E/D=0.33) and S=1/2 (g=2.03, 1.93, 1.86) ground states, with the former dominating in the presence of 50% (v/v) glycerol. Three distinct types of
W(V) EPR signals have been observed during dye-mediated redox titration of as-isolated Tl FOR. The initial resonance observed
upon oxidation, termed the “low-potential” W(V) species (g=1.977, 1.898, 1.843), corresponds to approximately 25–30% of the total W and undergoes redox cycling between W(IV)/W(V) and
W(V)/W(VI) states at physiologically relevant potentials (E
m=−335 and −280 mV, respectively). At higher potentials a minor “mid-potential” W(V) species, g=1.983, 1.956, 1.932, accounting for less than 5% of the total W, appears with a midpoint potential of −34 mV and persists
up to at least +300 mV. At potentials above 0 mV, a major “high-potential” W(V) signal, g=1.981, 1.956, 1.883, accounting for 30–40% of the total W, appears at a midpoint potential of +184 mV. As-isolated samples
of Tl FOR were found to undergo an approximately 8-fold enhancement in activity on incubation with excess Na2S under reducing conditions and the sulfide-activated Tl FOR was partially inactivated by cyanide. The spectroscopic and redox
properties of the sulfide-activated Tl FOR are quite distinct from those of the as-isolated enzyme, with loss of the low-potential
species and changes in both the mid-potential W(V) species (g=1.981, 1.950, 1.931; E
m=−265 mV) and high-potential W(V) species (g=1.981, 1.952, 1.895; E
m=+65 mV). Taken together, the W(V) species in sulfide-activated samples of Tl FOR maximally account for only 15% of the total
W. Both types of high-potential W(V) species were lost upon incubation with cyanide and the sulfide-activated high-potential
species is converted into the as-isolated high-potential species upon exposure to air. Structural models are proposed for
each of the observed W(V) species and both types of mid-potential and high-potential species are proposed to be artifacts
of ligand-based oxidation of W(VI) species. A W(VI) species with terminal sulfido or thiol ligands is proposed to be responsible
for the catalytic activity in sulfide-activated samples of Tl FOR.
Received: 9 September 1999 / Accepted: 17 February 2000 相似文献
7.
Kinetics of the steady-state oxidation of n–alkylferrocenes (alkyl = H, Me, Et, Bu and C5H11) by H2O2 to form the corresponding ferricenium cations catalyzed by horseradish peroxidase has been studied in micellar systems of
Triton X-100, CTAB, and SDS, mostly at pH 6.0 and 25 °C. The rate of oxidation of ferrocenes with longer alkyl radicals is
too slow to be measured. The reaction obeying the [RFc]:[H2O2] = 2 : 1 stoichiometry is strictly first-order in both HRP and RFc in a wide concentration range. The corresponding observed
second-order rate constants k, which refer to the interaction of the peroxidase compound II (HRP-II) with RFc, decrease with the elongation of the alkyl
substituent R, and this in turn is accompanied by an increase in the formal redox potentials E°′ in the same medium. Increasing the surfactant concentration lowers the rate constants k, the effect being due to the nonproductive binding of RFc to micelles rather than to enzyme inactivation. The micellar effects
are accounted for in terms of the Berezin pseudo-phase model of micellar catalysis applied to the interaction of enzyme with
organometallic substrates. The oxidation was found to occur primarily in the aqueous pseudo-phase and the calculated intrinsic
second-order rate constants k
w are (1.9 ± 0.5)×105, (2.7 ± 0.1)×104, and (5.9 ± 0.6)×103 M–1 s–1 for HFc, EtFc, and n–BuFc, respectively. The data obtained were used for estimating the self-exchange rate constants for the HRP-II/HRP couple
in terms of the Marcus formalism.
Received: 15 July 1996 / Accepted: 15 November 1996 相似文献
8.
C. F. Shaw III. Libin He Amalia Muñoz M. Meral Savas Susan Chi Cynthia L. Fink Tong Gan David H. Petering 《Journal of biological inorganic chemistry》1997,2(1):65-73
The model alkylating agent N-ethylmaleimide (NEM) reacts reversibly at the metal-bound thiolates of Zn7MT and Cd7MT. An unprecedented feature of this reaction is that it approaches equilibrium and requires a large excess of NEM (>1 mM
for 3 μM protein) to drive it to completion. The complex kinetics of the reaction can be followed by monitoring the release
of bound metal ions using the metallochromic dyes Zincon (ZI) for Zn7MT and pyridylazoresorcinol for Cd7MT. An initial lag phase is followed by more rapid release of zinc ions. The observed pseudo-first-order rate constants for
the two phases are independent of the ZI and Zn7MT concentrations. The complex NEM concentration dependence of each phase,
k
f, obs=k
f
1+k
f
2 [NEM] and k
s, obs=k
s
1+k
s
2 [NEM],
demonstrates that the forward reactions are second order and the reverse reactions are first order. The alkylation can be
reversed using 2-mercaptoethanol to compete for the protein-bound NEM and regenerate the Zn-binding capability of alkylated
MT. An explanation of these observations, based on the reversibility of cysteine alkylation by NEM, was developed and tested.
The reactions of Cd7MT are less complete than those of Zn7MT and occur more slowly. 111Cd-NMR studies of the partially alkylated 111Cd7MT reveal that reaction with only four equivalents of NEM completely alters the cluster structure and eliminates the spectral
signatures of the α and β clusters, although very little cadmium has been removed from the protein. This finding substantiates
the proposed kinetic intermediate, a partially alkylated MT with complete or nearly complete retention of the metal ions,
and rules out the possibility of cooperative reactions at either cluster.
Received: 5 August 1996 / Accepted: 24 October 1996 相似文献
9.
Modification of the electron-transfer sites of Pseudomonas aeruginosa azurin by site-directed mutagenesis 总被引:1,自引:0,他引:1
Site-directed mutagenesis of the structural gene for azurin from Pseudomonas aeruginosa has been used to prepare azurins in which amino acid residues in two separate electron-transfer sites have been changed: His-35-Lys and Glu-91-Gln at one site and Phe-114-Ala at the other. The charge-transfer band and the EPR spectrum are the same as in the wild-type protein in the first two mutants, whereas in the Phe-114-Ala azurin, the optical band is shifted downwards by 7 nm and the copper hyperfine splitting is decreased by 4.10(-4)/cm. This protein also shows an increase of 20-40 mV in the reduction potential compared to the other azurins. The potentials of all four azurins decrease with increasing pH in phosphate but not in zwitterionic buffers with high ionic strength. The rate constant for electron exchange with cytochrome c551 is unchanged compared to the wild-type protein in the Phe-114-Ala azurin, but is increased in the other two mutant proteins. The results suggest that Glu-91 is not important for the interaction with cytochrome c551 and that His-35 plays no critical role in the electron transfer to the copper site. 相似文献
10.
Short-term binding of fibroblasts to fibronectin: optical tweezers experiments and probabilistic analysis 总被引:5,自引:0,他引:5
The biophysical properties of the interaction between fibronectin and its membrane receptor were inferred from adhesion tests
on living cells. Individual fibroblasts were maintained on fibronectin-coated glass for short time periods (1–16 s) using
optical tweezers. After contact, the trap was removed quickly, leading to either adhesion or detachment of the fibroblast.
Through a stochastic analysis of bond kinetics, we derived equations of adhesion probability versus time, which fit the experimental
data well and were used to compute association and dissociation rates (k
+=0.3–1.4 s−1 and k
off=0.05–0.25 s−1, respectively). The bond distribution is binomial, with an average bond number ≤10 at these time scales. Increasing the fibronectin
density (100–3000 molecules/μm2) raised k
+ in a diffusion-dependent manner, leaving k
off relatively unchanged. Increasing the temperature (23–37 °C) raised both k
+ and k
off, allowing calculation of the activation energy of the chemical reaction (around 20 k
B
T). Increasing the compressive force on the cell during contact (up to 60 pN) raised k
+ in a logarithmic manner, probably through an increase in the contact area, whereas k
off was unaffected. Finally, by varying the pulling force to detach the cell, we could distinguish between two adhesive regimes,
one corresponding to one bond, the other to at least two bonds. This transition occurred at a force around 20 pN, interpreted
as the strength of a single bond.
Received: 2 November 1999 / Revised version: 6 March 2000 / Accepted: 19 April 2000 相似文献
11.
Tiecheng Qiao Robert Witkowski Robin Henderson G. McLendon 《Journal of biological inorganic chemistry》1996,1(5):432-438
The kinetics of methemoglobin reduction by cytochrome b
5 has been studied by stopped-flow and saturation transfer NMR. A forward rate constant k
f = 2.44×104 M–1 s–1 and a reverse rate constant k
b = 540 M–1s–1 have been observed at 10 mm, pH 6.20, 25 °C. The ratio k
f/k
b = k
eq = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b
5 and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b
5 to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic strengths supports such
collisional mechanism. It is also found that the reaction rate strongly depends on the conformations of methemoglobin.
Received: 20 February 1996 / Accepted: 4 June 1996 相似文献
12.
Joo-Yeon Han Astrid Gräslund Lars Thelander A. G. Sykes 《Journal of biological inorganic chemistry》1997,2(3):287-294
Four reductions of the R2 subunit of mouse ribonucleotide reductase have been studied and found to exhibit different behaviour
from that of Escherichia coli R2. An important difference is that there is no stable met-R2 (Fe2
II I) form of mouse R2. With hydroxyurea, hydrazine and hydroxylamine uniphasic kinetics are observed for the combined reduction
of radical Tyr ˙ and Fe2
II I components to tyrosine and Fe2
II respectively. The rate constants, determined at 370 nm (emphasising FeIII decay) and 417 nm (emphasising Tyr ˙ decay), differ by factors of 2–3, allowing some mechanistic features to be defined. The studies with hydrazine are particularly
important. In the case of E. coli R2, a first phase corresponding to two-equivalent reduction of the met-R2 component has been observed [18]. It is likely
that the four times slower second phase reaction of active E. coli R2 also corresponds to the Fe2
II I → Fe2
II change and is followed by fast intramolecular Fe2
II reduction of the higher potential Tyr ˙. The latter changes are believed to hold also for (active) mouse R2. The FeIIFeIII semi forms have been detected at low levels by EPR for mouse R2 (9%) and E. coli (∼5%) in previous studies. Further substrate reduction of FeIIFeIII occurs at a comparable rate to account for the transient behaviour of FeIIFeIII. For mouse R2 the combined FeIII decay processes (which we are unable to separate) give smaller uniphasic rate constants at 370 nm than at 417 nm. A fitted-base-line
(FBL) treatment of absorbance changes at 417 nm targets more closely the Tyr ˙ decay as a means of monitoring the rate-determining step. The FBL method gives rate constants k (M–1 s–1) at 25 °C and pH 7.5 for hydroxyurea (1.46), hydrazine (0.163) and hydroxylamine (4.4). Surprisingly, phenylhydrazine, with
a less strong reduction potential (0.25 V), gives a substantially faster reduction of the Tyr ˙ as the only redox step (rate constant 27 M–1 s–1). In this case a slower second phase at 370 nm is independent of reductant and corresponds to rate-controlling release of
FeIII. Overall the results indicate a more reactive redox centre for mouse R2 and help develop further an understanding of factors
affecting the reactivity of R2.
Received: 11 October 1996 / Accepted: 11 February 1997 相似文献
13.
S. C. J. Meskers C. Dennison Gerard W. Canters Harry P. J. M. Dekkers 《Journal of biological inorganic chemistry》1998,3(6):663-670
The dynamic quenching of the luminescence of racemic Eu(III)(pyridine-2,6-dicarboxylate=dpa)3
3– by the title proteins is investigated and the enantioselectivity of the proteins in the quenching of the Δ and Λ enantiomers
of Eu(dpa)3
3– is determined. The two diastereomeric quenching rate constants pertaining to azurin (k
q
Δ=3.3×106, k
q
Λ=2.7×106 M–1 s–1, pH 7.2, ionic strength I=22 mM) are lower than for its Met→44Lys mutant (k
q
Δ=1.9×107, k
q
Λ=1.4×107 M–1 s–1, same pH and I), indicating that energy transfer occurs from Eu(dpa)3
3– to the Cu(II) centre when the luminophore is bound to the hydrophobic patch of the protein near residue 44. The enantioselectivity
remains unaltered by the mutation: k
q
Δ/k
q
Λ=1.27±0.04, so Lys44 is probably not in direct contact with the Eu chelate. The I and pH dependence of k
q indicate that the lysine residue interacts electrostatically with Eu(dpa)3
3–. For plastocyanin the quenching rates are of the order of 106 M–1 s–1; for amicyanin they are two orders of magnitude larger (k
q
Δ=12×107, k
q
Λ=11×107 M–1 s–1, pH 7.2, I=22 mM). The variation of k
q is attributed to differences in the charge distribution on the proteins, which influences the binding of the luminophore
to the protein surface. For amicyanin the anion binding site near Lys59 and Lys60 may be involved in the energy transfer.
Received: 16 June 1998 / Accepted: 18 September 1998 相似文献
14.
Juozas Kulys Kastis Krikstopaitis Arturas Ziemys 《Journal of biological inorganic chemistry》2000,5(3):333-340
N -substituted phenothiazines (PTs) and phenoxazines (POs) catalyzed by fungal Coprinus cinereus peroxidase and Polyporus pinsitus laccase were investigated at pH 4–10. In the case of peroxidase, an apparent bimolecular rate constant (expressed as k
cat/K
m) varied from 1 ×107 M−1 s−1to 2.6×108 M−1 s−1 at pH 7.0. The constants for PO oxidation were higher in comparison to PT. pH dependence revealed two or three ionizable
groups with pK
a values of 4.9–5.7 and 7.7–9.7 that significantly affected the activity of peroxidase. Single-turnover experiments showed
that the limiting step of PT oxidation was reduction of compound II and second-order rate constants were obtained which were
consistent with the constants at steady-state conditions. Laccase-catalyzed PT and PO oxidation rates were lower; apparent
bimolecular rate constants varied from 1.8×105 M−1 s−1 to 2.0×107 M−1 s−1 at pH 5.3. PO constants were higher in comparison to PT, as was the case with peroxidase. The dependence of the apparent
bimolecular constants of compound II or copper type 1 reduction, in the case of peroxidase or laccase, respectively, was analyzed
in the framework of the Marcus outer-sphere electron-transfer theory. Peroxidase-catalyzed reactions with PT, as well as PO,
fitted the same hyperbolic dependence with a maximal oxidation rate of 1.6×108 M−1 s−1 and a reorganization energy of 0.30 eV. The respective parameters for laccase were 5.0×107 M−1 s−1 and 0.29 eV.
Received: 20 September 1999 / Accepted: 24 February 2000 相似文献
15.
Andreas Erkens Klaus Schneider A. Müller 《Journal of biological inorganic chemistry》1996,1(2):99-110
In this study we confirmed the previous observation that the cytoplasmic NAD-linked hydrogenase of Alcaligenes eutrophus H16 is EPR-silent in the oxidized state. We also demonstrated the presence of significant Ni-EPR signals when the enzyme
was either reduced with the natural electron carrier NADH (5–10 mM) or carefully titrated with sodium dithionite to an intermediate,
narrow redox potential range (–280 to –350 mV). Reduction with NADH under argon atmosphere led to a complex EPR spectrum at
80 K with g values at 2.28, 2.20, 2.14, 2.10, 2.05, 2.01 and 2.00. This spectrum could be differentiated by special light/dark treatments
into three distinct signals: (1) the "classical" Ni-C signal with g values at 2.20, 2.14 and 2.01, observed with many hydrogenases in the reduced, active state; (2) the light-induced signal
(Ni-L) with g values at 2.28, 2.10 and 2.05 and (3) a flavin radical (FMN semiquinone) signal at g = 2.00. The assignment of the Ni-EPR signal was clearly confirmed by EPR spectra of hydrogenase labeled with 61Ni (nuclear spin I = 3/2) yielding a broadening of the Ni spectra at all g values and a resolved 61Ni hyperfine splitting into four lines of the low field edge in the case of the light-induced Ni-EPR signal. The redox potentials
determined at pH 7.0 for the described redox components were: for FMN –170 mV (midpoint potential, Em, for appearance), –200 mV (EPR signal intensity maximum) and –230 mV (Em for disappearance); for the Ni centre (Ni-C), –290 mV (Em for appearance), –305 mV (signal intensity maximum) and –325 mV (Em for disappearance). Exposure of the NADH-reduced hydrogenase to carbon monoxide led to an apparent Ni-CO species indicated
by a novel rhombic EPR signal with g values at 2.35, 2.08 and 2.01.
Received: 19 July 1995 / Accepted: 10 September 1995 相似文献
16.
DNA binding by trans-[(H2O)(Pyr)(NH3)4RuII]2+ (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, 4-bnpy) is highly selective for G7 with K
G=1.1×104 to 2.8×104, with the more hydrophobic Pyr ligands exhibiting slightly higher binding. A strong dependence on ionic strength indicates
that ion-pairing with DNA occurs prior to binding. At μ=0.05, d[RuII-DNA]/dt=k[RuII][DNA], where k=0.17–0.21 M–1 s–1 with the various Pyr ligands. The air oxidation of [(py)(NH3)4RuII]
n
-DNA to [(py)(NH3)4RuIII]
n
-DNA at pH 6 occurs with a pseudo-first-order rate constant of k
obs=5.6×10–4 s–1 at μ=0.1, T=25 °C. Strand cleavage of plasmid DNA appears to occur by both Fenton/Haber-Weiss chemistry and by base-catalyzed routes,
some of which are independent of oxygen. Base-catalyzed cleavage is more efficient than O2 activation at neutral pH and involves the disproportionation of covalently bound RuIII and, in the presence of O2, Ru-facilitated autoxidation to 8-oxoguanine. Disproportionation of [py(NH3)4RuIII]
n
-DNA occurs according to the rate law: d[RuII–GDNA]/dt=k
0[RuIII–GDNA]+k
1[RuIII–GDNA][OH–], where k
0=5.4×10–4 s–1 and k
1=8.8 M–1 s–1 at 25 °C, μ=0.1. The appearance of [(Gua)(py)(NH3)4RuIII] under argon, which occurs according to the rate law: d[RuIII–G]/dt=k
0[RuIII–GDNA]+k
1[OH–][RuIII–GDNA] (k
0=5.74×10–5 s–1, k
1=1.93×10–2 M–1 s–1 at T=25 °C, μ=0.1), is consistent with lysis of the N-glycosidic bond by RuIV-induced general acid hydrolysis. In air, the ratio of [Ru-8-OG]/[Ru-G] and their net rates of appearance are 1.7 at pH 11,
25 °C. Small amounts of phosphate glycolate indicate a minor oxidative pathway involving C4′ of the sugar. In air, a dynamic
steady-state system arises in which reduction of RuIV produces additional RuII.
Received: 11 November 1998 / Accepted: 3 March 1999 相似文献
17.
Correlation of anaerobic biodegradability and the electrochemical
characteristic of azo dyes 总被引:3,自引:0,他引:3
Some experiments were conducted to study some electrochemical factors affecting the bacterial reduction (cleavage) of azo dyes, knowledge of which will be useful in the wastewater treatments of azo dyes. A common mixed culture was used as a test organism and the reductions of Acid Yellow 4, 11, 17 and Acid Yellow BIS were studied. It was found that the azo dyes were reduced at different rates, which could be correlated with the reduction potential of the azo compounds in cyclic voltammetric experiments. Acid Yellow BIS (E
r − 616.75 mV) was reduced at the highest rate of 0.0284 mol g dry cell weight−1 h−1, Acid Yellow 11 (E
r − 593.25 mV) at 0.0245 mol g dry cell weight−1 h−1 and Acid Yellow 4 (E
r − 513 mV) at 0.0178 mol g dry cell weight−1 h−1. At the same time, the decolourization rate of Acid Yellow 17 (E
r − 627.5 mV) was 0.0238 mol g dry cell weight−1 h−1, which was affected by the nature of chlorine substituent. Reduction of these azo dyes did not occur under aeration conditions. These studies with a common mixed culture indicate that the reduction of azo dyes may be influenced by the chemical nature of the azo compound. The reduction potential is a preliminary tool to predict the decolourization capacity of oxidative and reductive biocatalysts. 相似文献
18.
This article reports rate constants for thiol–thioester exchange (k
ex), and for acid-mediated (k
a), base-mediated (k
b), and pH-independent (k
w) hydrolysis of S-methyl thioacetate and S-phenyl 5-dimethylamino-5-oxo-thiopentanoate—model alkyl and aryl thioalkanoates, respectively—in water. Reactions such as
thiol–thioester exchange or aminolysis could have generated molecular complexity on early Earth, but for thioesters to have
played important roles in the origin of life, constructive reactions would have needed to compete effectively with hydrolysis
under prebiotic conditions. Knowledge of the kinetics of competition between exchange and hydrolysis is also useful in the
optimization of systems where exchange is used in applications such as self-assembly or reversible binding. For the alkyl
thioester S-methyl thioacetate, which has been synthesized in simulated prebiotic hydrothermal vents, k
a = 1.5 × 10−5 M−1 s−1, k
b = 1.6 × 10−1 M−1 s−1, and k
w = 3.6 × 10−8 s−1. At pH 7 and 23°C, the half-life for hydrolysis is 155 days. The second-order rate constant for thiol–thioester exchange
between S-methyl thioacetate and 2-sulfonatoethanethiolate is k
ex = 1.7 M−1 s−1. At pH 7 and 23°C, with [R″S(H)] = 1 mM, the half-life of the exchange reaction is 38 h. These results confirm that conditions
(pH, temperature, pK
a of the thiol) exist where prebiotically relevant thioesters can survive hydrolysis in water for long periods of time and
rates of thiol–thioester exchange exceed those of hydrolysis by several orders of magnitude. 相似文献
19.
D. Habedank I. Reindl G. Vietzke U. Bauer A. Sperfeld S. Gl?ser K. D. Wernecke F. X. Kleber 《European journal of applied physiology and occupational physiology》1998,77(5):421-426
The ventilatory equivalent for CO2 defines ventilatory efficiency largely independent of metabolism. An impairment of ventilatory efficiency may be caused by
an increase in either anatomical or physiological dead space, the latter being the most important mechanism in the hyperpnoea
of heart failure, pulmonary embolism, pulmonary hypertension and the former in restrictive lung disease. However, normal values
for ventilatory efficiency have not yet been established. We investigated 101 (56 men) healthy volunteers, aged 16–75 years,
measuring ventilation and gas exchange at rest (n = 64) and on exercise (modified Naughton protocol, n = 101). Age and sex dependent normal values for ventilatory efficiency at rest defined as the ratio ventilation:carbon dioxide
output (V˙
E:V˙CO2), exercise ventilatory efficiency during exercise, defined as the slope of the linear relationship between ventilation and
carbon dioxide output (V˙
E vs V˙CO2 slope), oxygen uptake at the anaerobic threshold and at maximum (V˙O2AT,V˙O2max, respectively) and breathing reserve were established. Ventilatory efficiency at rest was largely independent of age, but
was smaller in the men than in the women [V˙
E:V˙CO2 50.5 (SD 8.8) vs 57.6 (SD 12.6) P<0.05]. Ventilatory efficiency during exercise declined significantly with age and was smaller in the men than in the women
(men: (V˙
E vs V˙CO2 slope = 0.13 × age + 19.9; women: V˙
E vs V˙CO2 slope = 0.12 × age + 24.4). The V˙O2AT and V˙O2max were 23 (SD 5) and 39 (SD 7) ml O2 · kg · min−1 in the men and 18 (SD 4) and 32 (SD 7) in the women, respectively, and declined significantly with age. The V˙O2AT was reached at 58 (SD 9)% V˙O2max. Breathing reserve at the end of exercise was 41% and was independent of sex and age. It was concluded from this study that
ventilatory efficiency as well as peak oxygen uptake are age and sex dependent in adults.
Accepted: 11 June 1997 相似文献
20.
João P. André H. R. Maecke Éva Tóth A. A. Merbach 《Journal of biological inorganic chemistry》1999,4(3):341-347
The ligand DOTASA was designed and synthesized in the aim of obtaining a kinetically and thermodynamically stable Gd(III)
chelate which, through its uncoordinated carboxylate function, will provide an efficient pathway to couple the complex to
bio- or macromolecules without affecting the coordination pattern of DOTA. Furthermore, it allows us to study the influence
of an extra carboxylate arm on the parameters determining proton relaxivity in comparison to the commercial agent [Gd(DOTA)(H2O)]–. A combined variable-temperature 17O NMR, EPR and nuclear magnetic relaxation dispersion study on the Gd(III) chelate resulted in k
298
ex=(6.3±0.2)×106 s–1 for the water exchange rate and τ298
R=125±2 ps for the rotational correlation time. The slight increase in both k
298
ex and τ298
R, as compared to those for [Gd(DOTA)(H2O)]–, is attributed to the presence of the extra negative charge. The longer rotational correlation time results in a proton relaxivity
of 5.03 mM–1 s–1 for [Gd(DOTASA)(H2O)]2–, which is approximately 30% higher than that for [Gd(DOTA)(H2O)]–. The increased water exchange rate of [Gd(DOTASA)(H2O)]2– has no consequence for proton relaxivity since this latter is exclusively limited by fast rotation for both complexes. However,
for slowly rotating macromolecular agents, which contain a covalently coupled DOTASA unit instead of a coupled DOTA, this
increased exchange rate will have a significant positive effect.
Received: 31 December 1998 / Accepted: 4 March 1999 相似文献