首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 62 毫秒
1.
《Inorganica chimica acta》1988,149(2):259-264
The bis(N-alkylsalicylaldiminato)nickel(II) complexes Ni(R-sal)2 with R = CH(CH2OH)CH(OH)Ph (I), R = CH(CH3)CH(OH)Ph (II) and R = CH2CH2Ph (III; Ph = phenyl) were prepared and characterized. In the solid state I and II are paramagnetic (μ = 3.2 and 3.3 BM at 20 °C, respectively), whereas III is diamagnetic. It follows from the UV-Vis spectra that in acetone solution I is six-coordinate octahedral and III is four-coordinate planar, the spectrum of II showing characteristics of both modes of coordination. Vis spectrophotometry and stopped-flow spectrophotometry were applied to study the kinetics of ligand substitution in I–III by H2salen (= N,N′-disalicylidene-ethylenediamine) in the solvent acetone at different temperatures. The kinetics follow a second-order rate law, rate = k[H2-salen] [complex]. At 20 °C the sequence of rate constants is k(III):k(II):k(I) = 11 850:40.6:1. The activation parameters are ΔH(I) = 112, ΔH(II) = 40.7, ΔH(III) = 35.7 kJ mol−1 and ΔS(I) = 92, ΔS(II) = −103, ΔS(III) = −89 J K−1 mol−1. The enormous difference in rate between complexes I, II and III, which is less pronounced in methanol, is attributed to the existence of a fast equilibrium planar ⇌ octahedral, which is established in the case of I and II by intramolecular octahedral coordination through the hydroxyl groups present in the organic group R. An A-mechanism is suggested to control the substitution in the sense that the entering ligand attacks the four-coordinate planar complex, the octahedral complex being kinetically inert.  相似文献   

2.
Several experimental methods (circular dichroism, viscosity, intrinsic fluorescence, and fluorescence labeling) were used to study the conformational folding/unfolding transitions in a compact monomeric form of the Caf113-149 subunit under the action of guanidine hydrochloride in the temperature range 5–45°C. It has been shown that transitions always occur between two major states (unfolded and compact). This has made it possible to determine all the main thermodynamic functions that characterize the compact state of the Caf113-149 subunit: stability temperature T m, free energy of stabilization ΔG st, enthalpy ΔH tr, and heat capacity jump ΔC in collapse of the structure. These data have been confirmed by an independent experiment on melting of fluorescently labeled protein.  相似文献   

3.
The thermotropic properties of bovine blood coagulation Factors IX and X, as well as the activation intermediates and products of these proteins, have been investigated by differential scanning microcalorimetry in the presence and absence of Ca2+. Bovine Factor IX displays a single thermal-denaturation transition characterized by a temperature midpoint (TM) of 54.5 ± 0.5 °C and a calorimetric enthalpy (ΔHc) of 105 ± 15 kcal/mol, in the absence of Ca2+. In the presence of Ca2+ concentrations sufficient to saturate its sites on Factor IX, the Tm value is increased to 57.0 ± 0.5 °C and the ΔHc is virtually unchanged. When the activation intermediate, Factor IXα, is similarly analyzed in the absence of Ca2+, a broad, diffuse thermogram was obtained which did not lend itself to calculation of thermodynamic parameters. In the presence of Ca2+, Factor IXα displayed thermograms characterized by a TM of 51.0 ± 0.5 °C and a ΔHc of 109 ± 10 kcal/mol. The activated product, Factor IXaα, in the absence of Ca2+ (the values in the presence of saturating Ca2+ are given in parentheses), undergoes thermal denaturation with a TM of 54.5 ± 0.5 °C (57.0 ± 0.5 °C) and a ΔHc of 158 ±10 kcal/mol (156 ± 10 kcal/mol). Similarly, the terminal-activation product, Factor IXaβ, displays a TM of 51.5 ± 0.5 °C (54.0 ± 0.5 °C) and a ΔHc of 85 ± 5 kcal/mol (126 ± 10 kcal/mol). Bovine blood coagulation Factor X has been analyzed in this same fashion, and shows very similar thermal properties to Factor IX. The thermal denaturation of Factor X is represented by a TM of 54.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔHc of 102 ± 10 kcal/mol (118 ± 10 kcal/mol), whereas its activated form, Factor Xaβ, possesses a TM of 55.0 ± 0.5 °C (55.0 ± 0.5 °C) and a ΔHc of 92.0 ± 5 kcal/mol (136 ± 10 kcal/mol). These studies indicate that, for many of these proteins, Ca2+ induces a conformational alteration to a more thermally stable form, which also requires the absorption of greater amounts of heat for thermal denaturation.  相似文献   

4.
Histamine was immobilized on Sepharose CL‐6B (Sepharose) for use as a ligand of hydrophobic charge induction chromatography (HCIC) of proteins. Lysozyme adsorption onto Histamine‐Sepharose (HA‐S) was studied by adsorption equilibrium and calorimetry to uncover the thermodynamic mechanism of the protein binding. In both the experiments, the influence of salt (ammonium sulfate and sodium sulfate) was examined. Adsorption isotherms showed that HA‐S exhibited a high salt tolerance in lysozyme adsorption. This property was well explained by the combined contributions of hydrophobic interaction and aromatic stacking. The isotherms were well fitted to the Langmuir equation, and the equilibrium parameters for lysozyme adsorption were obtained. In addition, thermodynamic parameters (ΔHads, ΔSads, and ΔGads) for the adsorption were obtained by isothermal titration calorimetry by titrating lysozyme solutions into the adsorbent suspension. Furthermore, free histamine was titrated into lysozyme solution in the same salt‐buffers. Compared with the binding of lysozyme to free histamine, lysozyme adsorption onto HA‐S was characterized by a less favorable ΔGads and an unfavorable ΔSads because histamine was covalently attached to Sepharose via a three‐carbon‐chain spacer. Consequently, the immobilized histamine could only associate with the residues on the protein surface rather than those in the hydrophobic pocket, causing a less favorable orientation between histamine and lysozyme. Further comparison of thermodynamic parameters indicated that the unfavorable ΔSads was offset by a favorable ΔHads, thus exhibiting typical enthalpy‐entropy compensation. Moreover, thermodynamic analyses indicated the importance of the dehydration of lysozyme molecule and HA‐S during the adsorption and a substantial conformational change of the protein during adsorption. The results have provided clear insights into the adsorption mechanisms of lysozyme onto the new HCIC material. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010  相似文献   

5.
《Inorganica chimica acta》1988,149(1):151-154
The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)2·(H3O+·χH2O)2·UO2Cl42− (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (Kex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log Kex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol−1. The value of the entropy change, ΔS°, was calculated from ΔH° and Kex and equals −690 J K−1 mol−1, whereas ΔG° = −6.45 kJ mol−1. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K−1 mol−1, ΔH° = −101 kJ mol−1 and ΔG° = −8.37 kJ mol−1), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H5O2+ ions in it, in contrast to the H3O+ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H5O2+ ions is held in the crown ether cavity by four hydrogen bonds. The H5O2+ ion has a central bond. The uranium atom forms UO2Cl42− as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.  相似文献   

6.
Thermodynamics of the B to Z transition in poly(dGdC)   总被引:1,自引:0,他引:1  
The thermodynamics of the B to Z transition in poly(dGdC) was examined by differential scanning calorimetry, temperature-dependent absorbance spectroscopy, and CD spectroscopy. In a buffer containing 1 mM Na cacodylate, 1 mM MgCl2, pH 6.3, the B to Z transition is centered at 76.4°C, and is characterized by ΔHcal = 2.02 kcal (mol base pair)?1 and a cooperative unit of 150 base pairs (bp). The tm of this transition is independent of both polynucleotide and Mg2+ concentrations. A second transition, with ΔHcal = 2.90 cal (mol bp)?1, follows the B to Z conversion, the tm of which is dependent upon both the polynucleotide and the Mg2+ concentrations. Turbidity changes are concomitant with the second transition, indicative of DNA aggregation. CD spectra recorded at a temperature above the second transition are similar to those reported for ψ(–)-DNA. Both the B to Z transition and the aggregation reaction are fully and rapidly reversible in calorimetric experiments. The helix to coil transition under these solution conditions is centered at 126°C, and is characterized by ΔHcal = 12.4 kcal (mol bp)?1 and a cooperative unit of 290 bp. In 5 mM MgCl2, a single transition is seen centered at 75.5°C, characterized by ΔHcal = 2.82 kcal (mol bp)?1 and a cooperative unit of 430 bp. This transition is not readily reversible in calorimetric experiments. Changes in turbidity are coincident with the transition, and CD spectra at a temperature just above the transition are characteristic of ψ(–)-DNA. A transition at 124.9°C is seen under these solution conditions, with ΔHcal = 10.0 kcal (mol bp)?1 and which requires a complex three-step reaction mechanism to approximate the experimental excess heat capacity curve. Our results provide a direct measure of the thermodynamics of the B to Z transition, and indicate that Z-DNA is an intermediate in the formation of the ψ-(–) aggregate under these solution conditions.  相似文献   

7.
The successive enthalpy changes for the four steps of oxygen binding by diphosphoglycerate-free adult human hemoglobin have been measured by direct calorimetry at pH 7.4 and 6°. Average results in kcal/(mole O2) are: ΔH1 = ?25.1 ± 2.8; ΔH2 = ?12.6 ± 3.0, ΔH3 = ?12.5 ± 3.0, and ΔH4 = ?10.1 ± 1.4. These results imply a substantial temperature dependence for the cooperativity of O2 binding by the protein and generally resemble the van't Hoff results by Roughton et al. [Roy. Soc. of London Proc., B 144, 29 (1955)] for sheep hemoglobin at pH 9.1 and a temperature range of 2° to 19°.  相似文献   

8.
In this study, a 60.8-kDa dimeric lectin was isolated from the Phaseolus vulgaris cv. jade bean and characterized. The lectin was bound on Blue Sepharose 6 and Q Sepharose and was finally purified by size exclusion chromatography on Superdex 200. Its hemagglutinating activity toward rabbit erythrocytes was dependent on divalent cations, especially calcium ions. Various carbohydrates tested were devoid of any effect on the hemagglutinating activity. The lectin was stable at pH between 4.5 and 9.4 and temperatures between 30 and 70 °C. It did not exert antifungal activity toward Valsa mali, Setosphaeria turcica, Mycosphaerella arachidicola, Fusarium oxysporum and Bipolaris maydis. The IC50 of the antiproliferative activity of the lectin toward MCF-7 human breast cancer cells was 174 μM. It did not inhibit proliferation of WRL-68 human normal embryonic hepatocytes. The lectin was dependent on calcium ions for hemagglutinating activity and possessed a blocked N-terminus. These two characteristics make the lectin unique among Phaseolus lectins.  相似文献   

9.
The energetics of formation of diastereomeric electron-transfer complexes between L-dopa or L-adrenaline and iron(III) complex ions bound to poly(L-glutamate) or poly(D-glutamate) were measured by microcalorimetry at 25°C and pH 7. When the association of substrates by Fe-polypeptide systems was virtualy complete, ΔHll = 1.3 ± 0.1 and ΔHdl = 0.9 ± 0.1 kcal/mol with both substrates. A diastereomeric discrimination energy of 400 ± 100 cal/mol is thus observed, which compares satisfactorily with that directely obtained by differential microcalorimetric measurements. These results are fully consistent with previous findings indicating that thermodynamic effects are of minor importance in the overall stereoselectivity of the electron transfer reactions investigated.  相似文献   

10.
《Inorganica chimica acta》1988,147(1):127-130
The thermodynamic parameters (log β101, ΔH101, and ΔS101) for the formation of the 1:1 complexes between lanthanide cations and 3- and 4-hydroxybenzoate anions were determined by potentiometric and calorimetric titrations in aqueous solutions of 0.10 M (NaClO4) ionic strength at 25 °C.  相似文献   

11.
《Inorganica chimica acta》1987,128(2):169-173
The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL2+, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k1 (25.0°C) = (6.8 ±0.2)×105 mol−1 kg s−1, ΔH31 = 47.5 ± 4.9 kJ mol−1, ΔS31 = 26±16 J K−1 mol−1, and ΔV31 (30.0°C) = 27.2±1.5 cm3 mol−1; for the second step with k2 (25.0°C) = 26.8±0.8 mol−1 kg s−1, ΔH32 = 91.6± 0.8 kJ mol−1, ΔS32 = 90±3 J K−1 mol−1, and ΔV32 (35.0°C) = 21.8±0.9 cm3 mol−1. The large positive activation volumes strongly indicate a dissociative character of the activation process.  相似文献   

12.
We have investigated the binding of 2′-deoxyuridine 5′-monophosphate (2′-dUMP) to Leishmania major deoxyuridine 5′-triphosphate nucleotide hydrolase (dUTPase) by isothermal titration microcalorimetry under different experimental conditions. Binding to dimeric L. major dUTPase is a non-cooperative process, with a stoichiometry of 1 molecule of 2′-dUMP per subunit. The utilization of buffers with different ionization enthalpies has allowed us to conclude that the formation of the 2′-dUMP–dUTPase complex, at pH 7.5 and 30 °C, is accompanied by the uptake of 0.33±0.05 protons per dUTPase subunit from the buffer media. Moreover, 2′-dUMP shows a moderate affinity for the enzyme, and binding is enthalpically driven across the temperature range studied. Besides, whereas ΔG° remains practically invariant as a function of temperature, both ΔH and ΔS° decrease with increasing temperature. The TS and TH were 23.4 and 13.6 °C, respectively. The temperature dependence of the enthalpy change yields a heat capacity change of ΔCp°=?618.1±126.4 cal·mol?1·K?1, a value low enough to discard major conformational changes, in agreement with the fitting model. An interpretation of this value in terms of solvent-accessible surface areas is provided.  相似文献   

13.
The self-association of β-lactoglobulin C at low pH (ca. 2.5) in glycine buffers has been studied at four temperatures, 10, 16, 20, and 25 °C, by low- and high-speed sedimentation equilibrium experiments. One buffer had an ionic strength of 0.1 and the other an ionic strength of 0.2. With either buffer the concentration dependence of the apparent weight average molecular weight, Mwa, was characteristic of a nonideal self-association. Like its genetic variants, β-lactoglobulin A and B, the self-association of β-lactoglobulin C increased with decreasing temperature; however, at the same temperature the association was always stronger in the buffer having the higher ionic strength. Several models were used to test the self-association, and a monomer-dimer self-association seemed to describe the self-association best with either buffer. Values of the association equilibrium constant, K2, and the second virial coefficient, BM1, are reported at each temperature for both series of experiments. Values of the thermodynamic functions, ΔG °, ΔH °, and ΔS °, are also reported for these experiments.  相似文献   

14.
Y C Fu  H V Wart  H A Scheraga 《Biopolymers》1976,15(9):1795-1813
The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔHh° in poly(L -ornithine) in 0.1 N KCl has been determnined calorimetrically to be ?1530 ± 210 and ?1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH? ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D ,L -ornithine). Since it was discovered that poly(L -ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L -ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature Ttr and the quantity (?θh/?T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be ?33 cal/mol and ?5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10?2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.  相似文献   

15.
Adsorption of uranium from aqueous solution onto the magnetically modified yeast cell, Rhodotorula glutinis, was investigated in a batch system. Factors influencing sorption such as initial solution pH, biomass dosage, contact time, temperature, initial uranium concentration and other common cations were analyzed. Sorption isotherm, kinetic and thermodynamic studies of uranium on magnetically modified R. glutinis were also carried out. The temperature dependent equilibrium data agreed well with the Langmuir model. Kinetic data obtained at different temperatures were simulated using pseudo-first-order and pseudo-second-order kinetic models, the pseudo-second-order kinetic model was found to describe the data better with correlation coefficients near 1.0. The thermodynamic parameters, ΔH°, ΔS° and ΔG° were calculated from the sorption data gained at different temperatures. These thermodynamic parameters showed that the sorption process was endothermic and spontaneous. All results indicated that magnetically modified R. glutinis can be a potential sorbent for uranium wastewater treatment.  相似文献   

16.
The title compound, I, crystallizes in the monoclinic space group P21 with cell constants: a = 6.599(3), b = 11.121(2), c = 8.375(1) Å and β = 106.35(2)°; V = 589.74 Å3 and D(calc; Z = 2) = 1.974 g cm−3. The compound is isomorphous and isostructural with its Co analogue. A total of 2982 data were collected over the range of 4°  20  70°; of these, 2537 (independent and with I ⩾ 3σ(I)) were used in the structural analysis. Data were corrected for absorption (μ = 16.6 cm−1) and the relative transmission coefficients ranged from 1.000 to 0.9504. Refinement was carried out for both enantiomeric configurations and the crystal used was found to contain cations with Δ(λδ) absolute configuration. The final R(F) and Rw(F) residuals were, respectively 0.0220 and 0.0239 for (−−−; i.e.Δ(λδ)) and 0.0231 AND 0.0317 FOR (+++; i.e.Λ(δλ)). Thus, the former was selected as correct for our specimen.In the case of I, as well as in the Co derivative [cis-Co(en)2(NO2)2]Cl (II), the conformation of one of the rings is opposite that expected for the lowest energy conformation, which in the current case should be Δ(λλ)).The RhN(NO)2 distances are 2.020(2) and 2.010(2) Å, while the RhN(amine) distances, trans to the NO2 ligands are 2.085(2) and 2.093(1) Å, values distinctly longer than the other two RhN distances (2.064(1) and 2.068(1) Å). The latter are the RhN distances to the terminalNH2 ligands located trans to each other. Thus, we observe a trans effect, which is more pronounced in I than in II, and which is of comparable magnitude to that observed in the case of the trien derivative, [cis-α-Rh(trien)(NO2)2]Cl(III).Parallel with an increase in metalN distances in going from [cis-α-Co(trien)NO2)2]Cl·H2) (IV) to (III) is an increase in the torsional angles of the outer rings (NCCN) of about 10°. Comparison of these parameters in I and II reveal that this change is not so marked for this pair since in I they are −54.9° and 52.8° while in II they are 50.2° and −48.1°; i.e. a change of only 4°. This important difference between trien and en derivatives is caused by the presence of the central five-membered ring, which for compounds III and IV remains largely unchanged, except for the metalN distances.The NO bond lengths are 1.244(3), 1.220)(2), 1.237(2) and 1.211(2) Å, which are similar to those found for the analogous Co isomer. The CN bond lengths are 1.492(3), 1.474(2), 1.486(2) and 1.475(2) Å, while the CC bonds are 1.509(3) and 1.524(3) Å. These values are also comparable with those obtained for the Co isomer and, in fact, the pattern of the bonds is nearly identical in both, including the common feature of having a longer CC bond for the en ring with the conformation opposite that expected.As was the case with the Co analogue, the Cl anion is associated with the hydrogens of the secondary nitrogen (trans to the −NO2) ligands, the Cl…H7 distance being 2.18(3) Å and the <Cl…H7N2 = 163°.  相似文献   

17.
By means of differential scanning calorimetry, effects of systematic series of Group I and VII ions on the phase state of model multibilayer dimyristoylphosphatidylcholine (di(14:0)PC) membranes have been studied at a lipid/ion molar ratio of 3/1. The sign-changing correlations between the ionic radii of cations and temperature shifts of di(14:0)PC phase transition were obtained. For cosmotropic Li+ and Na+, the observed shifts were positive (LiCl: ΔT m = 0.6°C; ΔT p = 1.9°C), whereas chaotropic K+ and Rb+ presence resulted in negative shifts (RbCl: ΔT m = ?0.3°C; ΔT p = ?2.5°C). The anions (Cl?, Br?, I?) showed a similar effect increasing with the ion chaotropicity. An essentially weaker effect of Cs+ as compared to other alkali metal ions (CsCl: ΔT m ≈ 0°C; ΔT p = ?0.1°C) can be one of the reasons of its accumulation in living organisms. Generalization of all available data allowed us to specify some important factors of lipid-ion interactions that should be taken into account in further investigations in this field.  相似文献   

18.
Differential scanning calorimetry and temperature-dependent uv spectroscopy are used to thermodynamically characterize the double-strand to single-strand transition of the self-complementary deoxyribo-oligonucleotide ATGCAT. The calorimetric experiments provide a value of 33.6 kcal (mol of double strand)?1 for the transition between 10 and 90° C. In conjunction with available temperature-dependent nmr data (which reveals terminal base pair fraying), we attempt to define specifically those interactions to which the calorimetrically measured enthalpy change refers.Values of ΔHV.H. (van 't Hoff enthalpy change) are derived from the spectroscopic and calorimetric data and compared with the ΔH obtained directly from the calorimetric experiment. This comparison reveals that the part of the thermally-induced transition that occurs between 10 and 90°C is well represented by a two-state process. It is noted that in assessing the applicability of the two-state model it is best to compare the ΔHcal. with ΔHV.H. obtained from the calorimetric rather than the spectroscopic data.  相似文献   

19.
A kinetic study of the rate of pyruvate reduction by goldfish LDH-M4 (the homotetrameric form of lactate dehydrogenase which predominates in skeletal muscle) provided an analysis of the effects of pH and temperature on v (reaction velocity) and estimates of how temperature might affect catalysis in vivo, where the physiological pH regulation imposes a temperature coefficient of ?0.015 to ?0.020 pH unit/ °C. Consistent with published data for other LDHs, (i) V (maximum reaction velocity) was pH insensitive over a physiological pH range, (ii) the Km for pyruvate, KP, was sensitive to both pH and temperature, and (iii) the Km for NADH and the dissociation constant for NADH were both sensitive to temperature, but not to pH. V approximately doubled with each 10 °C (Ea = 11.7 kcal/mol). The effects of pH and temperature on KP were consistent with two enthalpic contributions, an ionization enthalpy (ΔHi°) of 7.2 kcal/mol (probably a histidine imidazole), and an enthalpy (ΔHSO) of 5.8 kcal/mol for the combination of pyruvate with the nonionized (pH ? pK′) LDH-NADH complex. When the pH was varied according to the physiological temperature coefficient, v was more sensitive to temperature than for conditions of constant pH, the usual design of kinetic experiments. This effect was due to the decreased temperature sensitivity of KP caused by partial concellation of the ΔHi° effect by the pH regulation: dpHdT ? dpK′dT. At constant pH, on the other hand, KP increased strongly with temperature and had the effect of offsetting (especially at higher pH values) the large increases in V. It was suggested that the magnitudes of ΔHi° and ΔHSO might have been important in the evolution of LDHs and other enzymes of cold-blooded animals.  相似文献   

20.
Enthalpic variations in the formation of adenine nucleotide · manganese complexes, as measured by microcalorimetry, are reported. All the results are obtained in the temperature range 6–30°C at I = 0.2 and pH values 7.00 or 7.50. All the reactions are endothermic and the ΔH values increase with the length of the phosphate chain and with temperature.The ΔH values are compared with those previously obtained for adenine nucleotide · magnesium complexes.The comparison between calarimetric and potentiometric ΔH values is made. The divergence observed between these results at low temperature leads us to assume the formation of nucleotide aggregates induced by the presence of manganese ions. This hypothesis is confirmed by differential ultraviolet spectra.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号