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1.
The antidepressant drug tetramezine [1,2‐bis‐(3,3‐dimethyldiaziridin‐1‐yl)ethane] consists of two bridged diaziridine moieties with four stereogenic nitrogen centers, which are stereolabile and, therefore, are prone to interconversion. The adjacent substituents at the nitrogen atoms of the diaziridines moieties exist only in an antiperiplanar conformation, which results in a coupled interconversion. Therefore, three stereoisomers exist (meso form and two enantiomeric forms), which epimerize when the diaziridine moieties are regarded as stereogenic units due to the coupled interconversion. Here, we have investigated the epimerization between the meso and enantiomeric forms by dynamic gas chromatography. Temperature‐dependent measurements were performed, and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers of the epimerization were found to be Δ G≠ = 100.7 kJ mol ?1 at 25°C and Δ G≠ = 104.5 kJ mol ?1 at 37°C, respectively. The activation enthalpy and entropy were determined to be Δ H≠ = 70.3 ± 0.4 kJ mol ?1 and Δ S≠ = ?102 ± 2 J mol ?1 K ?1. Chirality, 2011. © 2010 Wiley‐Liss, Inc. 相似文献
2.
N‐alkylated trans‐diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2‐di‐ n‐propyldiaziridine 1 , 1‐isopropyl‐2‐ n‐propyldiaziridine 2 , and 1,2‐diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil‐β‐Dex with high separation factors α (1‐isopropyl‐2‐ n‐propyldiaziridine: 1.18; 1, 2‐diisopropyldiaziridine: 1.24; 100°C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150°C, and peak coalescence at elevated temperatures. The enantiomerization barriers Δ G? and activation parameters Δ H? and Δ S? were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1‐isopropyl‐2‐ n‐propyldiaziridine and 1,2‐diisopropyldiaziridine exhibit similar high interconversion barriers Δ G? (100°C) of 128.3 ± 0.4 kJ mol ?1 and 129.8 ± 0.4 kJ mol ?1, respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process. Chirality, 2010. © 2009 Wiley‐Liss, Inc. 相似文献
4.
We investigated the stereodynamics of 5,5’‐substituted tropos BIPHEP ligands (2,2’‐bis(diphenylphosphino)‐biphenyls) by enantioselective dynamic high‐performance liquid chromatography (DHPLC) to elucidate the influence of the substitution pattern and electronics of the substituents (methyl, methoxy, and hydroxyl groups). By temperature‐dependent dynamic HPLC measurements the activation parameters Δ G╪, Δ H╪, and Δ S╪ could be determined with high precision, revealing that the activation barrier of these 5,5’‐substituted BIPHEP ligands ranges in a narrow band between 87.8 and 93.0 kJ mol –1, making them highly attractive as deracemizable dynamic chiral ligands in asymmetric catalysis. Interestingly, the activation parameters are highly influenced by a hydroxyl or methoxy group in the 5,5’‐position of the BIPHEP ligands. Chirality 25:126–132, 2013. © 2012 Wiley Periodicals, Inc. 相似文献
5.
The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L) 2·(H 3O +·χH 2O) 2·UO 2Cl 42− (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, Δ H°, was calculated from the slope and equals −212 kJ mol −1. The value of the entropy change, Δ S°, was calculated from Δ H° and Kex and equals −690 J K −1 mol −1, whereas Δ G° = −6.45 kJ mol −1. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (Δ S° = −314 J K −1 mol −1, Δ H° = −101 kJ mol −1 and Δ G° = −8.37 kJ mol −1), it can be seen that Δ H° and Δ S° 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 H 5O 2+ ions in it, in contrast to the H 3O + ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H 5O 2+ ions is held in the crown ether cavity by four hydrogen bonds. The H 5O 2+ ion has a central bond. The uranium atom forms UO 2Cl 42− 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.
The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal) 2H 2O] 2−(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps ( k1 and k2: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)] 2−. The rates for each step depended linearly on the concentrations of free oxalate species, Hox − and ox 2−. The second-order rate constants for the replacement by ox 2− were much larger in the k1 step than in the k2 step and the activation parameters were determined as follows: Δ H≠= 43.5 ± 5.6 kJ mol −1, Δ S±-53 ± 19 J K −1 mol −1 and Δ H≠= 43.6 ± 0.5 kJ mol −1, δ S≠ = -62 ± 2 J K −l mol −1 for the k1 and k2 steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm 3 mol −1 at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants. 相似文献
7.
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 FeL 2+, 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)×10 5 mol −1 kg s −1, ΔH 31 = 47.5 ± 4.9 kJ mol −1, Δ S31 = 26±16 J K −1 mol −1, and Δ V31 (30.0°C) = 27.2±1.5 cm 3 mol −1; for the second step with k2 (25.0°C) = 26.8±0.8 mol −1 kg s −1, Δ H32 = 91.6± 0.8 kJ mol −1, Δ S32 = 90±3 J K −1 mol −1, and Δ V32 (35.0°C) = 21.8±0.9 cm 3 mol −1. The large positive activation volumes strongly indicate a dissociative character of the activation process. 相似文献
8.
Nongelling solutions of structurally regular chain segments of agarose sulphate show disorder–order and order–disorder transitions (as monitored by the temperature dependence of optical rotation) that are closely similar to the conformational changes that accompany the sol–gel and gel–sol transitions of the unsegmented polymer. The transition midpoint temperature ( Tm) for formation of the ordered structure on cooling is ~25 K lower than Tm for melting. Salt-induced conformational ordering, monitored by polarimetric stopped-flow, occurs on a millisecond time scale, and follows the dynamics expected for the process 2 coil ? helix. The equilibrium constant for helix growth ( s) was calculated as a function of temperature from the calorimetric enthalpy change for helix formation (Δ Hcal = ?3.0 ± 0.3 kJ per mole of disaccharide pairs in the ordered state), measured by differential scanning calorimetry. The temperature dependence of the nucleation rate constant ( knuc), calculated from the observed second-order rate constant ( kobs) by the relationship kobs = knuc(1 ? 1/ s) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL ?1; 0.5 M Me 4NCl or KCl): Δ H* = 112 ± 5 kJ mol ?1; Δ S* = 262 ± 20 J mol ?1 K ?1; Δ G* 298 = 34 ± 6 kJ mol ?1; ( knuc) 298 = (7.5 ± 0.5) × 10 6 dm 3 mol ?1 s ?1. The endpoint of the fast relaxation process corresponds to the metastable optical rotation values observed on cooling from the fully disordered form. Subsequent slow relaxation to the true equilibrium values (i.e., coincident with those observed on heating from the fully ordered state) was monitored by conventional optical rotation measurements over several weeks and follows second-order kinetics, with rate constants of (2.25 ± 0.07) × 10 ?4 and (3.10 ± 0.10) × 10 ?4 dm 3 mol ?1 s ?1 at 293.7 and 296.2 K, respectively. This relaxation is attributed to the sequential aggregation processes helix + helix → dimer, helix + dimer → trimer, etc., with depletion of isolated helix driving the much faster coil–helix equilibrium to completion. Light-scattering measurements above and below the temperature range of the conformational transitions indicate an average aggregate size of 2–3 helices. 相似文献
9.
The reaction of Ru(XTPP)(DMF) 2, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O 2 and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M ?1 s ?1 at 25°C, those for the O 2 reaction from 0.132 to 0.840 M ?1 s ?1 at 25°C. Similar activation parameters (ΔH CO± = 87 ± 1 kJ mol ?1, ΔS CO± = 22 ± 4 JK ?1 mol ?1; ΔH O2± = 81 ± 1 kJ mol ?1, and ΔS O2± = 11 ± 5 JK ?1 mol ?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ ? following normal behavior; the O 2 reactions correlated with σ 8° indicating O 2 is π-bonded in the transition states. A dissociative mechanism is postulated for the process. 相似文献
10.
We have measured the thermodynamic parameters of the slow-fast tail-fiber reorientation transition on T2L bateriophage. Proportions of the virus in each form were determined from peak-height measurements in sedimention-velocity runs and from average diffusion coefficients obtained by quasielastic laser light scattering. Computer simulation of sedimentation confirmed that there were no undetected intermediates in the transition, which was analyzed as a two-state process. Van't Hoff-type plots of the apparent equilibrium constant and of the pH midpoint of the transition as function of reciprocal temperature led to the following estimates of the thermodynamic parameters for the transition at pH 6.0 and 20°C: Δ H° = ?139 ± 18Kcal mol ?1, ΔS° = ?247 ± 46 cal K ?1 mol ?1, and Δ G° = ?66 ± 22 kcal mol ?1. Per mole of protons taken up in the transition, the analogous quantities were ?15.9 ± 1.7 kcal mol ?1, ?26.3 ± 2.2 cal K ?1 mol ?1, and ?8.22 ± 1.8 kcal mol ?1. The net number of protons taken up was about 8.5 ± 1.5. The large values of the thermodynamic functions are consistent with a highly cooperative reaction and with multiple interactions between the fibres and the remainder of the phage. The negative entropy of the transition is probably due to immobilization of the fibres. 相似文献
11.
The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M −1 s −1 at 25 °C, Δ H# = 34 ± 2 kJ mol −1 and Δ S# = −108 ± 7 J K −1 mol −1. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants. 相似文献
12.
A profound influence of water has previously been detected in the complexation of the enantiomers of methyl 2‐chloropropanoate (MCP) and the chiral selector octakis(3‐ O‐butanoyl‐2,6‐di‐ O‐pentyl)‐γ‐cyclodextrin (Lipodex‐E) in NMR and sensor experiments. We therefore investigated the retention behavior of MCP enantiomers on Lipodex‐E by gas chromatography (GC) under hydrous conditions. Addition of water to the N 2 carrier gas modestly reduced the retention factors k of the enantiomers, notably for the second eluted enantiomer ( S)‐MCP. This resulted in an overall decrease of enantioselectivity ‐ΔS,R(ΔG) in the presence of water. The effect was fully reversible. Consequently, for a conditioned column in the absence of residual water, the determined thermodynamic data, i.e. ΔS,R(ΔH) = –12.64 ± 0.08 kJ mol ‐1 and ΔS,R(ΔS) = –28.18 ± 0.23 J K ‐1 mol ‐1, refer to a true 1:1 complexation process devoid of hydrophobic hydration. Chirality 28:124–131, 2015. © 2015 Wiley Periodicals, Inc. 相似文献
13.
This is the first study where the pyrolysis of the freshwater macroalga Chara vulgaris was explored to reveal its bioenergy potential. The suitability of C. vulgaris to bioenergy conversion via pyrolysis was accessed in terms of kinetic triplet and thermodynamic parameters. For this purpose, pyrolysis experiments under a thermogravimetric scale were conducted at multiple heating rates (5, 10, and 20 °C min?1) in an inert atmosphere. The mass-loss profiles of C. vulgaris pyrolysis showed that there are two dominant decomposition stages that are related to distinct chemical components inside its structure and that directly affect the calculated kinetic triplet and thermodynamics parameters. The average activation energy obtained using isoconversional methods of Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Starink, and Friedman was in the range of 52.87–72.91 kJ mol?1 for the first decomposition stage and 156.14–174.65 kJ mol?1 for the second decomposition stage. The pyrolytic conversion of C. vulgaris initially follows a second-order reaction model (first decomposition stage), while in second decomposition stage is controlled by a second-order Avrami-Erofeev reaction model. The kinetic results derived from the non-isothermal decomposition of C. vulgaris proved its suitable characteristics for pyrolysis. Additionally, multi-stage kinetic interpretation was successfully attained based on two kinetic triplets, where reconstructed pyrolysis behavior correlated well with experimental pyrolysis behavior. The changes in enthalpy, Gibbs free energy, and entropy for first decomposition stage were 67.58±0.25 kJ mol?1, 180.77±5.30 kJ mol?1, and ?176.65±0.41 J mol?1 K?1, and for the second decomposition stage the values were 166.70±0.09 kJ mol?1, 285.51±1.29 kJ mol?1, and ?124.29±0.09 J mol?1 K?1, respectively. Based on thermodynamic aspects, C. vulgaris is particularly interesting for use as a raw material to produce biofuels and bioenergy. 相似文献
14.
Racemic mer-[CoCl(en)(NH 2CH 2CHNCH 2CH 2NH 2)]ZnCl 4, which contains no dissymetric chelate rings, no asymmetric carbon centers and no asymmetric nitrogen centers, has been resolved using sodium arsenic(III)—(+)-tartrate. The chirality arises by virtue of the coordinated unsymmetric tridentate ligand and the less soluble diastereoisomeride is associated with the (+)-cation. The rate of base hydrolysis was measured spectrophotometrically using tris buffers. Kinetic parameters (25 °C, \3m; ∼ 0.04 M) are kOH = 1.28 X 10 3 M −3 s −1, Ea = 87.0 ± 0.7 kJ mol −1 and Δ S#; = +98 ± 1.4 J K −1 mol −1. Complete racemisation accompanies the base hydrolysis reaction and the rate of loss of optical activity is 0.5 times that of base hydrolysis. These data are interpreted in terms of the formation of a symmetrical trigonal bipyramid intermediate generated from the conjugate base. 相似文献
15.
In CD 3CN solutions the kinetic parameters characterising rotation about the CNMe 2 and CNH 2 bonds in [UO 2(1,1-DMU) 5] 2+ (1,1-DMU = 1,1- dimethylurea) were determined as: k(265 K) = 39.1 ± 0.4 and 2960 ± 60 s ?1, Δ H3 = 49.1 ± 0.76 and 61.1 ±0.5 kJ mol ?1, Δ S2 = ?28.3 ± 2.7 and 53.1 ± 2.2 J K ?1 mol ?1 respectively from 1H NMR studies. Resonances arising from the three isomeric 1,3-DMU (= 1,3-dimethylurea) ligands were observed for [UO 2(1,3-DMU) 5] 2+ in CD 3CN solution and the kinetic parameters characterising their isomerisations were also determined. The three isomers of 1,3-DMU have not previously been detected in solution and it appears that coordination of 1,3-DMU to UO 22+ increases the barrier to rotation about the carbon nitrogen bond, as is also shown to be the case for 1,1-DMU. 相似文献
16.
Many macromolecular interactions, including protein‐nucleic acid interactions, are accompanied by a substantial negative heat capacity change, the molecular origins of which have generated substantial interest. We have shown previously that temperature‐dependent unstacking of the bases within oligo(dA) upon binding to the Escherichia coli SSB tetramer dominates the binding enthalpy, ΔH obs, and accounts for as much as a half of the observed heat capacity change, ΔC p. However, there is still a substantial ΔC p associated with SSB binding to ssDNA, such as oligo(dT), that does not undergo substantial base stacking. In an attempt to determine the origins of this heat capacity change, we have examined by isothermal titration calorimetry (ITC) the equilibrium binding of dT(pT) 34 to SSB over a broad pH range (pH 5.0–10.0) at 0.02 M, 0.2 M NaCl and 1 M NaCl (25°C), and as a function of temperature at pH 8.1. A net protonation of the SSB protein occurs upon dT(pT) 34 binding over this entire pH range, with contributions from at least three sets of protonation sites (pK a1 = 5.9–6.6, pK a2 = 8.2–8.4, and pK a3 = 10.2–10.3) and these protonation equilibria contribute substantially to the observed ΔH and ΔC p for the SSB‐dT(pT) 34 interaction. The contribution of this coupled protonation (∼ −260 to −320 cal mol −1 K −1) accounts for as much as half of the total ΔC p. The values of the “intrinsic” ΔC p,0 range from −210 ± 33 cal mol −1 °K −1 to −237 ± 36 cal mol −1K −1, independent of [NaCl]. These results indicate that the coupling of a temperature‐dependent protonation equilibria to a macromolecular interaction can result in a large negative ΔC p, and this finding needs to be considered in interpretations of the molecular origins of heat capacity changes associated with ligand‐macromolecular interactions, as well as protein folding. Proteins 2000;Suppl 4:8–22. © 2000 Wiley‐Liss, Inc. 相似文献
17.
Thermodynamic parameters for the reduction of ferrioxamine E as calculated from redox potentials determined at four different temperatures were found to be Δ H ≠ =7.1±3.4 kJ mol ?1 and Δ S≠=?146 J mol ?1 K ?1. The negative entropy value is large, because the decrease in the charge at the metal center and an increase in its ionic radius force the structure of the complex to become less rigid and resemble the desferrisiderophore. The hydrophilic groups of the system are now (relatively more) available for solvent interaction. Thus, a large negative entropy change accompanies the reduction of the complex. Kinetics of reduction of ferrioxamine by V II, Cr II, Eu II, and dithionite were measured at different temperatures and by dithionite at different pH values. The Cr II and Eu II reactions proceed by an inner‐sphere mechanism and have second‐order rate constants at 25° of 1.37×10 4 and 1.23×10 5 M ?1 s ?1, respectively. For the V II reduction, the corresponding rate constant was 1.89×10 3 M ?1 s ?1. The activation parameters for the V II reduction were Δ H≠ = 8.3 kJ mol ?1; Δ S≠ = ?154 J mol ?1 K ?1. These values are indicative of an outer‐sphere mechanism for V II reduction. The reduction by dithionite is half order in dithionite concentration indicating that SO . is the sole reducing species. log of reduction rate constants of different trihydroxamates by this reductant were correlated with their respective redox potentials, and the variation was found to be in approximate correspondence with the expectations of Marcus relationship. 相似文献
18.
AbstractThe protease from Aspergillus tamarii Kita UCP1279 extraction by aqueous two-phase PEG-Citrate (ATPS) systems, using a factorial design 2 4, was investigated. Then, the variables studied were polyethylene glycol (PEG) molar mass (M PEG), concentrations of PEG (C PEG) and citrate (C CIT), and pH. The responses analyzed were the partition coefficient (K), activity yield (Y) and purification factor (PF). The thermodynamic parameters of the ATPS partition were estimated as a function of temperature. ATPS was able to pre-purify the protease (PF = 1.6) and obtained 84% activity yield. The thermodynamic parameters ΔG° m (?10.89?kJ mol ?1), ΔH m (?5.0?kJ?mol ?1) and partition ΔS m (19.74?J mol ?1 K ?1) showed that the preferential migration of almost all protein contaminants of the crude extract to the salt-rich phase, while the preferred protease was the PEG rich phase. The extracted enzyme presents optimum temperature and pH at range of 40–50?°C and 9.0–11.0, respectively. Moreover, the enzyme was identified as serine protease based on inhibition profile. ATPS showed the satisfactory performance as the first step for Aspergillus tamarii Kita UCP1279 protease pre-purification. 相似文献
19.
AbstractThis research is focussed on kinetic, thermodynamic and thermal inactivation of a novel thermostable recombinant α-amylase (Tp-AmyS) from Thermotoga petrophila. The amylase gene was cloned in pHIS-parallel1 expression vector and overexpressed in Escherichia coli. The steady-state kinetic parameters ( Vmax, Km, kcat and kcat/K m) for the hydrolysis of amylose (1.39?mg/min, 0.57?mg, 148.6?s ?1, 260.7), amylopectin (2.3?mg/min, 1.09?mg, 247.1?s ?1, 226.7), soluble starch (2.67?mg/min, 2.98?mg, 284.2?s ?1, 95.4) and raw starch (2.1?mg/min, 3.6?mg, 224.7?s ?1, 61.9) were determined. The activation energy ( Ea), free energy (ΔG), enthalpy (ΔH) and entropy of activation (ΔS) at 98?°C were 42.9?kJ mol ?1, 74?kJ mol ?1, 39.9?kJ mol ?1 and ?92.3 J mol ?1 K ?1, respectively, for soluble starch hydrolysis. While ΔG of substrate binding (ΔG E-S) and ΔG of transition state binding (ΔG E-T) were 3.38 and ?14.1?kJ mol ?1, respectively. Whereas, EaD, Gibbs free energy (ΔG*), increase in the enthalpy (ΔH*) and activation entropy (ΔS*) for activation of the unfolding of transition state were 108, 107, 105?kJ mol ?1 and ?4.1 J mol ?1 K ?1. The thermodynamics of irreversible thermal inactivation of Tp-AmyS revealed that at high temperature the process involves the aggregation of the protein. 相似文献
20.
The hypervalent muscle pigment ferrylmyoglobin, formed by activation of metmyoglobin by hydrogen peroxide, was found to be reduced in a second-order reaction by N-tert-butyl-α-phenylnitrone (PBN, often used as a spin trap). In acidic aqueous solution at ambient temperature, the reduction is relatively slow (δ H? = 65 ± 2 kJ · mol -1 and δ S? = -54 ± 7 J · mol -1. K -1 for pH = 5.6), but phase transitions during freezing of the buffered solutions accelerates the reaction between ferrylmyoglobin and PBN. In these heterogenous systems at low temperature (but not when ice-formation was inhibited by glycerol), a PBN-derived radical intermediate was detected by ESR-spectroscopy, identified as a nitroxyl radical by a parallel nitrogen hyperfine coupling constant of 31.8 G, and from microwave power saturation behavior concluded not to be located in the heme-cleft of the protein. The acceleration of the reaction is most likely caused by a lowering of the pH during the freezing of the buffered solutions whereby ferrylmyoglobin becomes more oxidizing. 相似文献
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