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1.
Jana Pospíšilová 《Biologia Plantarum》1975,17(6):392-399
The mutual relationship between the water potential (γ w ), its components, namely the osmotic potential (γ s ) and the pressure potential (γ p ), and the water saturation deficit (ΔW sat ) were determined in the leaves of different insertion levels. During the water stress development in kale plants induced by decreasing soil moisture theγ w decreased, parallely in all the leaves but the same decrease ofγ q was accompanied by the highest decrease of theγ p , probably due to the accumulation of osmotically active solutes, and the lowest decrease ofγ p in the upper leaves and with the lowest decrease ofγ s and the highest decrease ofγ p in the lower leaves. Also the corresponding values of the ΔW sat were always lower in the upper than in the middle and lower leaves. Thus the upper leaves wilted at more negative values ofγ w than the other leaves. On the contrary, during the wilting of the cut off leaves the relationship betweenγ w and ΔW sat in the upper, middle and lower leaves was practically the same. The very slightly higher decrease ofγ s in the upper leaves in comparison with the other leaves was compensated by a lower deerease of theirγ p . These changes in the ratios ofγ w ,γ s ,γ p and ΔW sat with the leaf insertion levels enabled the preference of the upper leaves in retaining the necessary water supply during the wilting of plantsin situ. 相似文献
2.
Evolution of enzyme catalytic power. Characteristics of optimal catalysis evaluated for the simplest plausible kinetic model 总被引:3,自引:3,他引:0
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Keith Brocklehurst 《The Biochemical journal》1977,163(1):111-116
1. Evolutionary changes in the structure of an enzyme that provide an increase in its Km value are considered. Provided that Km increases as a result of increases in the forward rate constants of the catalysis relative to the reverse rate constants, the enzyme catalyses the conversion of a fixed concentration of its substrate more rapidly when its structure provides that Km>[S] than when Km<[S]. 2. Catalytic efficiency of enzymes is discussed in terms of the simplest plausible model, the Haldane [(1930) Enzymes, Longmans, London] reversible three-step model: [Formula: see text] The rate equation for the forward reaction of this model (formation of P) may be written in the simple form: [Formula: see text] Keq. is the equilibrium constant (=[P]eq./[S]eq.), and kcat.=V/[E]T, where [E]T is the total enzyme concentration. 3. To assess the effectiveness of an enzyme, it is necessary only to determine the extent to which the constraints of a particular kinetic mechanism permit v2 (v when Km»[S]) to approach vd (the diffusion-limited rate). 4. The value of the optimal rate of catalysis (vopt., the maximal value of v2) is dictated by the equilibrium constant for the reaction, Keq.; v2=vd/a, where [Formula: see text] when k+1 is assumed equal to k−3, and vopt.=vd/amin.. When Keq.≥1, it is necessary that k+2»k−1 for a to take its minimum value, amin.; when Keq.«1, it is necessary only that k+2»Keq.·k−1, i.e. a can equal amin. even if k+2<k−1. When Keq.»1, vopt.=vd; when Keq.=1, vopt.=vd/2, and when Keq.«1, vopt.=Keq.·vd. 5. The analysis, together with predicted effects of evolutionary pressure, suggests that in practice the rates of the fastest enzyme-catalysed freely reversible reactions might be expected to be lower than the value of k+1[E]T[S] by about an order of magnitude, particularly if Keq.<1. 6. The existing literature suggests that, in general, appropriate values of Km have evolved for the provision of high rates of catalysis but that many values of kcat. are not large enough to provide optimal rates of catalysis unless the value of k+1 in vivo is lower than its value in free solution. 相似文献
3.
T. I. Muhiddinov 《Russian Journal of Genetics》2010,46(6):689-698
As a prerequisite to studying the genetics and breeding of chasmogamous and cleistogamous flowers, a preliminary experiment was performed to estimate the extent of cross-pollination in cotton varieties and hybrids. Vicinism estimates varied from 0.53 to 15.36%, i.e., the proportion of cross-pollination was relatively high, leading to a biological contamination. As a result of such contamination, genetic collection lines and varieties lose genetic homogeneity and become heterozygous and genetically heterogeneous. The genetic control of the flower type was studied in the Gossipium hirsutum L. × G. barbadense L. interspecific hybrids, and phenotypic segregation of the 3: 1 and 15: 1 types with monogenic (3: 1) and digenic (15: 1) differences of noncumulative polymerization was observed. The corresponding types of genotypic segregation were 1: 2: 1 (1Cg 1 Cg 1 cg 2 cg 2 : 2Cg 1 cg 1 cg 2 cg 2 : 1cg 1 cg 1 cg 2 cg 2 ) and 1: 2: 2: 4: 1: 2: 1: 2: 1 (1) Cg 1 Cg 1 Cg 2 Cg 2 -1; (2) Cg 1 Cg 1 cg 2 cg 2 -2; (3) Cg 1 cg 1 Cg 2 Cg 2 -2; (4) Cg 1 cg 1 Cg 2 cg 2-4; (5) Cg 1 Cg 1 cg 2 cg 2 -1; (6) Cg 1 cg 1 cg 2 cg 2 -2; (7) cg 1 cg 1 Cg 2 Cg 2-1; (8) cg 1 cg 1 Cg 2 cg 2 -2; (9) cg 1 cg 1 cg 2 cg 2 -1. Genotypes (1)–(8) had chasmogamous flowers, while double-recessive genotype (9) had cleistogamous flowers. Based on this, genotypes with individual phenotypic expression were identified in F2, and their correlation with the most important morphological, biological, and agricultural features was studied. Special attention was paid to the productivity of hybrid plants intended for use in breeding to obtain intensive varieties. The study made it possible to isolate forms, families, genetic collection lines, and varieties with isogenic or nonisogenic determination of these characters and chasmogamous and cleistogamous flowers of G. hirsutum L. and G. barbadense L. prototypes by using original methods to examine the two types of flowers; the methods do not have analogs in cotton breeding worldwide. 相似文献
4.
Phthalic anhydride has been detected spectrophotometrically in the hydrolysis of phthalamic acid and N-phenylphthalamic acid in solutions which were made up to 5 M with sodium perchlorate. In solutions of lower ionic strength the variation of kobs with acid concentration follows the equation, kobs=(k1 + k2 [H3O+])/(1 + Ka/[H3O+]), and the values of k1 and k2 are both enhanced. The p values for the variation of k1 and k2 with the aryl group for the hydrolysis of N-arylphthalamic acids are ?1.23 and ?0.94. A mechanism involving nucleophilic catalysis by the carboxyl group in which breakdown of the tetrahedral intermediate is rate-limiting was proposed. 相似文献
5.
1. In fresh chloroplasts, three b-type cytochromes exist. These are b-559HP (λmax, 559 nm; Em at pH 7, +370 mV; pH-independent Em), b-559LP (λmax, 559 nm; Em at pH 7, +20 mV; pH-independent Em) and b-563 (λmax, 563 nm; Em at pH 7, ?110 mV; pH-independent Em). b-559HP may be converted to a lower potential form (λmax, 559 nm; Em at pH 7, +110 mV; pH-independent Em).2. In catalytically active b-f particle preparations, three cytochromes exist. These are cytochrome f (λmax, 554 nm; Em at pH 7, +375 mV, pK on oxidised cytochrome at pH 9), b-563 (λmax, 563 nm; Em at pH 7, ?90 mV, small pH-dependence of Em) and a b-559 species (λmax, 559 nm, Em at pH 7, +85 mV; pH-independent Em).3. A positive method of demonstration and estimation of b-559LP in fresh chloroplasts is described which involves the use of menadiol as a selective reductant of b-559LP. 相似文献
6.
Yoshihiro Yamada 《Population Ecology》1995,37(1):9-21
A revised key-factor analysis was presented for analyzing the temporal changes in the ratio of insect absolute number to plant resource. Ten data sets for 5 insect species were then analyzed. In this key-factor analysis, the key factor is defined as the factor contributing highly to between-year variation inR r , the log rate of the inter-year change of the insect-plant ratio. The yearly change of plant resource was handled as a separate factor, expressed byr pl , log ratio of plant resource in yearn to plant resource in yearn+1. The following was revealed: 1) In 7 of the 10 data sets examined,r pl influenced variations ofR r ; in particular in 3 casesr pl was the main key factor. 2) Generation-to-generation fluctuations of absolute insect densities showed density dependence in 4 cases, while those of insect-plant ratios, in 8 cases. 3) The Royama model or a linear model, explained well the relationship between log insect-plant ratio (X r ) andR r and the relationship betweenX r and log yearly change rate of absolute insect density (R abs ). However, in the 7 cases in whichr pl was a critical factor for variations ofR r , with, increase ofX r ,R r showed a steeper, decrease around the equilibrium point (the point for whichR r is 0) thanR abs . This occurred becauser pl tended to be negatively correlated withX r . Consequently, in two casesX r fluctuated cyclicly or chaotically although without the changes in plant resource, fluctuations ofX r would be damped oscillations approaching equilibrium. 相似文献
7.
Sheng-Nan Wu Bing-Shuo Chen Tai-I Hsu Yung-Han Wu 《Journal of theoretical biology》2009,259(4):828-25
The rapidly inactivating (INaf) and noninactivating Na+ currents (INa(NI)) were characterized in NG108-15 neuronal cells differentiated with dibutyryl cyclic AMP in this study. Standard activation and inactivation protocols were used to evaluate the steady-state and kinetic properties of the INaf present in these cells. The voltage protocols with a slowly depolarizing ramp were implemented to examine the properties of INa(NI). Based on experimental data and computer simulations, a window component of the rapidly inactivating sodium current (INaf(W)) was also generated in response to the slowly depolarizing ramp. The INaf(W) was subtracted from INa(NI) to yield the persistent Na+ current (INa(P)). Our results demonstrate the presence of INa(P) in these cells. In addition to modifying the steady-state inactivation of INaf, ranolazine or riluzloe could be effective in blocking INaf(W) and INa(P). The ability of ranolazine and riluzole to suppress INa(P) was greater than their ability to inhibit INaf(W). In current-clamp recordings, current-induced voltage oscillations were applied to elicit action potentials (APs) through a gradual transition between spontaneous depolarization and upstroke. Ranolazine or riluzole at a concentration of 3 μM then effectively suppressed the AP firing generated by oscillatory changes in membrane current. The data suggest that a small rise in INa(NI) facilitates neuronal hyper-excitability due the decreased threshold of AP initiation. The underlying mechanism of the inhibitory actions of ranolazine or riluzole on membrane potential in neurons or neuroendocrine cells in vivo may thus be associated with their blocking of INa(NI). 相似文献
8.
Purification of characterization of a minor form of hepatic microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls 总被引:14,自引:0,他引:14
A minor form of hepatic microsomal cytochrome P-450 has been purified to apparent homogeneity from rats treated with the polychlorinated biphenyl mixture, Aroclor 1254. This newly isolated hemoprotein, cytochrome P-450e, is inducible in rat liver by Aroclor 1254 and phenobarbital, but not by 3-methylcholanthrene. Two other hemoproteins, cytochromes P-450b and P-450c, have also been highly purified during the isolation of cytochrome P-450e based on chromatographic differences among these proteins. By Ouchterlony double-diffusion analysis with antibody to cytochrome P-450b, highly purified cytochrome P-450e is immunochemically identical to cytochrome P-450b but does not cross-react with antibodies prepared against other rat liver cytochromes P-450 (P-450a, P-450c, P-450d) or epoxide hydrolase. Purified cytochrome P-450e is a single protein-staining band in sodium dodecyl sulfate-polyacrylamide gels with a minimum molecular weight (52,500) slightly greater than cytochromes P-450b or P-450d (52,000) but clearly distinct from cytochromes P-450a (48,000) and P-450c (56,000). The carbon monoxide-reduced difference spectral peak of cytochrome P-450e is at 450.6 nm, whereas the peak of cytochrome P-450b is at 450 nm. Ethyl isocyanide binds to ferrous cytochromes P-450e and P-450b to yield two spectral maxima at 455 and 430 nm. At pH 7.4, the 455:430 ratio is 0.7 and 1.4 for cytochromes P-450b and P-450e, respectively. Metyrapone binds to reduced cytochromes P-450e and P-450b (absorption maximum at 445–446 nm) but not cytochromes P-450a, P-450c, or P-450d. Metabolism of several substrates catalyzed by cytochrome P-450e or P-450b reconstituted with NADPH-cytochrome c reductase and dilauroylphosphatidylcholine was compared. The substrate specificity of cytochrome P-450e usually paralleled that of cytochrome P-450b except that the rate of metabolism of benzphetamine, benzo[a]pyrene, 7-ethoxycoumarin, hexobarbital, and testosterone at the 16α-position catalyzed by cytochrome P-450e was only 15–25% that of cytochrome P-450b. In contrast, cytochrome P-450e catalyzed the 2-hydroxylation of estradiol-17β more efficiently (threefold) than cytochrome P-450b. Cytochrome P-450d, however, catalyzed the metabolism of estradiol-17β at the greatest rate compared to cytochromes P-450a, P-450b, P-450c, or P-450e. The peptide fragments of cytochromes P-450e and P-450b, generated by either proteolytic or chemical digestion of the hemoproteins, were very similar but not identical, indicating that these two proteins show minor structural differences. 相似文献
9.
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11.
Michal Svoboda Monique Lambert Jean Christophe 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,675(1):46-61
(1) The kinetic parameters of rat pancreatic adenylate cyclase were evaluated, using GTP, p[NH]ppG or GTPγS as nucleotide activator, cholecystokinin as peptide hormone, and GDPβS and dibutyryl cyclic GMP as inhibitors of guanosine triphosphate and CCK-8, respectively. The time courses of activation and the degree of activation at steady state (EA/ETOT) were compatible with a simple two-state model of activation-deactivation based on a pseudo-monomolecular activation process (rate constant k+2, and a deactivation process (rate constant koff) that included, depending on the activating nucleotide, the hydrolysis of GTP (rate constant k2) and/or the dissociation of the intact nucleotide (rate constant k?1), so that EA/ETOT = k+1/(k+1 + k2 + k?a). (2) The hormone CCK-8 increased the value of k+1 with GTP dose-dependently, from 0.2 to 10.9 min?1. The value of k?1 increased 0.01 to 0.3 min?1 but the value of k2 was unaltered at 7 min?1, so that EA/ETOT increased 15-fold, from 4% to 61%. (3) A cholera toxin pretreatment at 30 μg/ml allowed also a large increase in EA/ETOT with GTP (up to 51%) but the underlying mechanism was different. It consisted of a 14-fold decrease in the koff value of the GTP-activated enzyme (from 7 min? to 0.5 min?1) that corresponded to a reduction in GTPase activity. When testing the system with p[NH]ppG, two added effects of the cholera toxin pretreatment were observed: a 4-fold increase in the value of k+1 (from 0.2 to 0.8 min?1) and the occurrence of a significant 0.3 min?1 value for k?1. 相似文献
12.
New Mutational Variants of Neurospora Nadp-Specific Glutamate Dehydrogenase 总被引:5,自引:3,他引:2
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The am locus of Neurospora codes for NADP-dependent glutamate dehydrogenase (GDH). Four new am mutants that produced mutationally altered GDH have been characterized. Mutant am119 is a CRM-negative, complementing mutant that maps between am2 and am1. The other three mutants are CRM formers that produce varieties of GDH that can be activated by glutamate or succinate. The GDH of am130 and am131 is similar in terms of activation properties to that of am3. The GDH of am122 requires very high concentrations of dicarboxylate for activity. The mutation in am130 maps between am14 and am2 and resulted in a replacement at residue 75 of the GDH (pro → ser). The mutation in am122 maps near am11 and apparently resulted in the replacement of the tryptophan residue at position 389 with an unknown amino acid. The mutation in am131 maps between am2 and am1. 相似文献
13.
The Mechanism of the Calorigenic Action of Thyroid Hormone : Stimulation of Na+ + K+-activated adenosinetriphosphatase activity 总被引:1,自引:0,他引:1
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In an earlier study, we proposed that thyroid hormone stimulation of energy utilization by the Na+ pump mediates the calorigenic response. In this study, the effects of triiodothyronine (T3) on total oxygen consumption (QOO2), the ouabain-sensitive oxygen consumption [QOO2(t)], and NaK-ATPase in liver, kidney, and cerebrum were measured. In liver, ~90% of the increase in QOO2 produced by T3 in either thyroidectomized or euthyroid rats was attributable to the increase in QOO2(t). In kidney, the increase in QOO2(t) accounted for 29% of the increase in QOO2 in thyroidectomized and 46% of the increase in QOO2 in euthyroid rats. There was no demonstrable effect of T3 in euthyroid rats on QOO2 or QOO2(t) of cerebral slices. The effects of T3 on NaK-ATPase activity in homogenates were as follows: In liver +81% from euthyroid rats and +54% from hypothyroid rats. In kidney, +21% from euthyroid rats and +69% from hypothyroid rats. T3 in euthyroid rats produced no significant changes in NaK-ATPase or Mg-ATPase activity of cerebral homogenates. Liver plasma membrane fractions showed a 69% increase in NaK-ATPase and no significant changes in either Mg-ATPase or 5'-nucleotidase activities after T3 injection. These results indicate that thyroid hormones stimulate NaK-ATPase activity differentially. This effect may account, at least in part, for the calorigenic effects of these hormones. 相似文献
14.
Toshihiro Saito Yoshihiko Sato Yutaka Sawamura Moriyuki Shoda Takeshi Takasaki-Yasuda Kazuo Kotobuki 《Tree Genetics & Genomes》2012,8(4):689-694
Most cultivars of Japanese pear (Pyrus pyrifolia Nakai) exhibit gametophytic self-incompatibility controlled by a single S-locus with multiple S-haplotypes. A self-compatible (SC) cultivar, ??Osanijisseiki?? (S 2 S 4 sm ), arising by a bud mutation of ??Nijisseiki?? (S 2 S 4 ), has a stylar-part mutant S 4 sm -haplotype, which lacks the pistil S 4 gene, which is the S 4 -RNase gene. To efficiently breed SC cultivars, we selected ??Nashi Chuukanbohon Nou 1 Gou?? (??NCN1??) harboring homozygous S 4 sm from a self-progeny of Osanijisseiki and crossed it with ??Okusankichi?? (S 5 S 7 ), ??Hakkou?? (S 4 S 5 ), or ??Ri-14?? (S 1 S 2 ). Fruit set (%) was compared after self-pollination of the trees in the three progenies. All trees derived from the three progenies were predicted to be SC, except for the S 4 S 4 sm trees in the progeny of NCN1 × Hakkou. However, S 1 S 4 sm trees in the progeny of NCN1 × Ri-14 proved to be self-incompatible (SI). The pollen from Osanijisseiki was incompatible with ??Doitsu?? (S 1 S 2 ), but that from Nijisseiki was compatible, suggesting a possibility that the S 4 sm pollen was rejected by S 1 -harboring pistils. This possibility was clarified by crossing the pollen from NCN1 (S 4 sm S 4 sm ) to Doitsu, ??Imamuraaki?? (S 1 S 6 ), or ??Hougetsu?? (S 1 S 7 ), all of which proved incompatible. On the other hand, S 4 sm pollen was accepted by pistils harboring the S 2 , S 3 , S 5 , S 6 , S 7 , S 9 , and S k haplotypes. The dual recognition of S 1 and S 4 pistils by S 4 sm pollen can be attributed to a mutation of the pollen S 4 gene(s). 相似文献
15.
Chao Gu Shao-Ling Zhang Shao-Xi Huang Wei Heng Qing-Zhong Liu Hua-Qing Wu Jun Wu 《Tree Genetics & Genomes》2010,6(4):579-590
Self-incompatibility has been studied extensively at the molecular level in Solanaceae, Rosaceae, and Scrophulariaceae, all of which exhibit gametophytic self-incompatibility. In the present study, we successfully isolated nine S-RNase alleles from cultivars of Chinese cherry by PCR amplification from genomic DNA and stylar cDNA combining with cleaved amplified polymorphic sequence marker. Analysis of amino acid sequences revealed five novel S-alleles, S 2 , S 4 , S 6 , S 8 , and S 9 , with respective accession numbers in the NCBI database of EF541168, EF541173, EF541172, FJ628598, and FJ628599. Results showed that “Dongtang” and “Yinzhu” contained six S-alleles (S 1 , S 3 , S 5 , S 7 , S 8 , and S 9 ); “Taishanganying” contained four S-alleles (S 1 , S 2 , S 4 , and S 6 ); “Daiba”, “Dayingzui”, and “Xiaomizi” contained four S-alleles (S 1 , S 2 , S 5 , and S 8 ); “Laiyangduanzhi”, “Shuangquanchangba”, and “Daqingye” contained three S-alleles (S 1 , S 2 , and S 8 ). It is interesting that different cultivars collected from the same place hold the same S-genotypes. Moreover, pollination tests and pollen tube growth assays showed that nine cultivars were self-compatible. Chinese cherry presented in this article are naturally polyploidy, which is a very useful material for the study of self-compatibility, and much of this information will be valuable for further work on self-(in)compatibility of fruit tree in Rosaceae. 相似文献
16.
Coiling of beta-pleated sheets 总被引:4,自引:0,他引:4
C Chothia 《Journal of molecular biology》1983,163(1):107-117
To form strongly twisted β-sheets, strands have to be coiled as well as twisted (Nishikawa &; Scherga, 1976). I show that strands coil in the appropriate right-handed direction if their main-chain torsion angles fulfil the following conditions: ψi ? ?φi + 1, ψi + 1 > ?φi + 2, ψi + 2 ? ?φi + 3, ψi + 3 > ?φi + 4…Lactate dehydrogenase, pancreatic trypsin inhibitor, thermolysin and concanavalin A contain strongly twisted β-sheets and in each case the strands are coiled by their φ, ψ values fulfilling these conditions. 相似文献
17.
Most methods for analyzing real-time quantitative polymerase chain reaction (qPCR) data for single experiments estimate the hypothetical cycle 0 signal y0 by first estimating the quantification cycle (Cq) and amplification efficiency (E) from least-squares fits of fluorescence intensity data for cycles near the onset of the growth phase. The resulting y0 values are statistically equivalent to the corresponding Cq if and only if E is taken to be error free. But uncertainty in E usually dominates the total uncertainty in y0, making the latter much degraded in precision compared with Cq. Bias in E can be an even greater source of error in y0. So-called mechanistic models achieve higher precision in estimating y0 by tacitly assuming E = 2 in the baseline region and so are subject to this bias error. When used in calibration, the mechanistic y0 is statistically comparable to Cq from the other methods. When a signal threshold yq is used to define Cq, best estimation precision is obtained by setting yq near the maximum signal in the range of fitted cycles, in conflict with common practice in the y0 estimation algorithms. 相似文献
18.
《Journal of Fermentation Technology》1987,65(3):291-294
The evaporation rate (NH2O) of liquid in a shaking flask was measured under various shaking conditions: temperature, humidity, flask shape, liquid volume in the flask (VL), length of the stopper in the flask neck (LC), rotational speed of the shaker (N), and wind velocity (VW). The rate was significantly affected by these factors, and the existence of a distribution of water vapor pressure was suggested inside and outside the flask. To predict the evaporation rate, the following empirical equation was derived by the least squares method: NH2O=1.26×10−2SF1.18LF−1.3 (ps–p)1.24VL0.11N0.05VW0.26LC−0.37 where, SF is the cross-sectional area of flask neck, LF is the length of flask neck, ps is the saturated partial pressure of water vapor at the temperature of the air surrounding the flask, and p is the partial pressure of water vapor in the flask. 相似文献
19.
《Inorganica chimica acta》1988,150(1):81-100
The (NH3)5CoOC(NH2)23+ ion is consumed in water according to the rate law k(obs.) = k1 + k2[OH−], where k1 = 4.0 × 10−5 s−1 and k2 = 14.2 M−1 s−1 (0–0.1 M [OH−];μ = 1.1 M, NaClO4, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH3)5CoOH23+ is observed, but in 0.1–1.0 M [OH−], 7% of the directly formed products is the urea-N complex (NH3)5CoNHCONH22+ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k1) and base-catalyzed (k2) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k2[OH−]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k1 + k2[OH−] = (ks + kON) + (kOH + k′ON) [OH−] for the reactions of the O-bonded isomers, where ks, kOH are the specific rates for hydrolysis, and kON, k′ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; kON = 1.3 × 10−5 s−1 and k′ON = 1.1 M−1 s−1 (μ = 1.0 M, NaClO4, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH2 capture (versus H2O), while −NHR and −NR2 do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H2O, Me2SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH3)5CoNHCONRR′](ClO4)2·xH2O (R,R′ = H, CH3). They are kinetically inert in neutral to basic solution but in acid they protonate (H2O, pK′a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H2O and Me2SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form kNO(obs.) = (k + kNO)[H+]/(K′a + [H+]) and the active species in the reaction is the protonated form; k, kNO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me2SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH3)5CoNH2CONH23+, diastereotopic exo-NH2 protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. 13C and 1H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′NO(obs.) = K′NO[H+]/[H+](K′a), where K′NO is the equilibrium constant = [(NH35Co(urea-O)3+]/[(NH3)5Co(urea-N)3+]. Values for K′NO(=kNO/kON = 260 and pK′a ≈ 3 for the NH2CONH2 system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′NO(obs.) = 2.6 × 10−2) and instability in acid solution (e.g. pH 1, K′NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH3)5Co(III) are contrasted with the result for the analogous Rh(III)NH2CONH2 system K′NO ≈ 1). 相似文献
20.
Identification of new S-RNase self-incompatibility alleles and characterization of natural mutations in Iranian almond cultivars 总被引:1,自引:0,他引:1
Akram Hafizi Behrouz Shiran Bahram Maleki Ali Imani Bojana Banović 《Trees - Structure and Function》2013,27(3):497-510
The two main objectives of this research were to identify new S-RNase alleles in Iranian almond cultivars and to characterize naturally occurring mutations in these alleles that may cause self-compatibility. We investigated S genotypes of 22 Iranian almond cultivars using stylar RNase electrophoresis, PCR and DNA sequencing. We report six previously unidentified P. dulcis S-RNase alleles (S 45 , S 46 , S 47 , S 48 , S 49 and S 50 ). Four of 12 tested S-RNases were found to be non-functional in vitro: S 49 , S 50 , S 24 /S na and S 25 /S 47 . Detected point mutations in the C3 coding region of S 49 - and S 50 -RNase, leading to the replacement of a highly conserved cysteine and histidine residues, are with the highest probability the reason of these S-RNases inactivity. Results also suggested that ten Iranian almond cultivars display unique S genotype. All presented data confirm Iranian cultivars as valuable almond sources which are of interest to almond breeding and conservation programs. 相似文献