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排序方式: 共有119条查询结果,搜索用时 31 毫秒
31.
Haloalkane dehalogenase from Rhodococcus rhodochrous was covalently immobilized onto a polyethyleneimine impregnated gamma-alumina support. The dehalogenating enzyme was found to retain greater than 40% of its original activity after immobilization, displaying an optimal loading (max. activity/supported protein) of 70 to 75 mg/g with an apparent maximum (max. protein/support) of 156 mg/g. The substrate, 1,2,3-trichloropropane, was found to favorably partition (adsorb) onto the inorganic alumina carrier (10 to 20 mg/g), thereby increasing the local reactant concentration with respect to the catalyst's environment, whereas the product, 2,3-dichloropropan-1-ol, demonstrated no affinity. Additionally, the inorganic alumina support exhibited no adverse effects because of solvent/component incompatibilities or deterioration due to pH variance (pH 7.0 to 10.5). As a result of the large surface area to volume ratio of the support matrix and the accessibility of the bound protein, the immobilized biocatalyst was not subject to internal mass transfer limitations. External diffusional restrictions could be eliminated with simple agitation (mixing speed: 50 rpm; flux: 4.22 cm/min). The pH-dependence of the immobilized dehalogenase was essentially the same as that for the native enzyme. Finally, both the thermostability and resistance toward inactivation by organic solvent were improved by more than an order of magnitude after immobilization. 相似文献
32.
Nagylaki T 《Journal of mathematical biology》2005,51(3):268-280
For many progressive chronic diseases, there exist useful prognostic indicators for the course of the disease and the survival of the patient. The evolution of such an indicator is modelled as a monotone transformation of a pure birth process with killing. Explicit formulas are derived for the probability distribution of this process at an arbitrary time, the distribution of the first-passage times, the joint distribution of the survival time and the maximum of the process, and the marginals of this joint distribution. In two examples, the general formulas are evaluated in closed form. 相似文献
33.
The level of an important carotenoid (-carotene) in the gut of Periplaneta americana depends on the content of the carotenoid in food: a carotenoid-fortified diet causes accumulation of -carotene up to 10 g/g wet weight, while on a carotenoid-deficient diet the level of this substance is low (0.7 g/g wet weight). In the eye, in contrast to the gut, a constant level of -carotene (1.3-1.4 g/g wet weight) is found regardless of the diet. This phenomenon remained unchanged over three years of feeding of the cockroaches with the carotenoid-deficient diet, suggesting that P. americana produces carotenoids by de novo biosynthesis. This suggestion was confirmed in experiments using intraperitoneal injection of the exogenous carotenoid biosynthesis precursor [14C]mevalonic acid pyrophosphate followed by extraction of carotenoid and chromatographic purification of the labeled product. Injection of 3.4 nmoles [14C]mevalonic acid pyrophosphate transiently increased the -carotene content in eyes on days 2 and 4 after injection of the label. Purification of radiolabeled carotenoids from eye and gut by the transfer of carotenoids into a less polar solvent, alkaline hydrolysis (saponification), and chromatography on alumina and cellulose columns decreased the specific radioactivity to a constant level that cannot be further decreased by repeated chromatography. The elution profile of these purified preparations of -carotene after chromatography is characterized by coincidence of symmetric peaks of count and absorption. We suggest that to create the optimal carotenoid concentration in the eye, P. americana uses two biochemical mechanism: 1) it accumulates carotenoids in reserve in the gut when abundant supplies of carotenoids are available in the diet; 2) it synthesizes carotenoids de novo when its food is deficient in these compounds. 相似文献
34.
HM0332和HM48—3菌株纯培养条件下解磷强度研究 总被引:6,自引:0,他引:6
室内研究结果说明,解磷细菌HM0332和HM48-3菌株以磷矿粉为磷源,在纯培养条件下产生有机酸,分解转化磷矿粉,有效磷转化率即解磷强度分别为8.28%和7.26%。 相似文献
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36.
Several studies have argued that under field conditions plant–soil feedback may be related to the local density of a plant species, but plant–soil feedback is often studied by comparing conspecific and heterospecific soils or by using mixed soil samples collected from different locations and plant densities. We examined whether the growth of the early successional species Jacobaea vulgaris in soil collected from the field is related to the local variation in plant density of this species. In a grassland restoration site, we selected eight 8 m × 8 m plots, four with high and four with low densities of J. vulgaris plants. In 16 subplots in each plot we recorded the density and size of J. vulgaris, and characteristics of the vegetation and the soil chemistry. Soil collected from each subplot was used in a greenhouse pot-experiment to study the growth of J. vulgaris, both in pure field soil and in sterile soil inoculated with a small part of field soil.In the field, flowering J. vulgaris plants were taller, the percentage of rosette plants was higher and seed density was larger in High- than in Low-density plots. In the pot experiment, J. vulgaris had a negative plant–soil feedback, but biomass was also lower in soil collected from High- than from Low-density plots, although only when growing in inoculated soil. Regression analyses showed that J. vulgaris biomass of plants growing in pure soil was related to soil nutrients, but also to J. vulgaris density in the field.We conclude that in the field there is local variation in the negative plant–soil feedback of J. vulgaris and that this variation can be explained by the local density of J. vulgaris, but also by other factors such as nutrient availability. 相似文献
37.
Chikao Hashimoto Kazuhiro Sugimoto Youhei Takahashi Mitsuo Kodomari 《Journal of peptide science》2013,19(10):659-662
In the synthesis of dipeptides (Boc‐AA1‐AA2‐OPac: AA1 and AA2 represent amino acids) protected by phenacyl (Pac) ester, amines and solid bases as the base for the conversion of the trifluoroacetic acid (TFA) salt of the amino component (TFA·H‐AA2‐OPac) into the corresponding free amino component (H‐AA2‐OPac) were examined. The synthesis of a dipeptide (Boc‐Ala‐Gly‐OPac) using amines for the conversion afforded an unsatisfactory yield with by‐products. On the other hand, the use of neutral alumina‐supported Na2CO3 (Na2CO3/n‐Al2O3) as a solid base for the conversion provided the dipeptide in a quantitative yield without by‐products. The application of Na2CO3/n‐Al2O3 to the synthesis of some dipeptides protected by Pac ester gave the desired peptides in excellent yields. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd. 相似文献
38.
39.
Jamison FW Foster TJ Barker JA Hills RD Guvench O 《Journal of molecular biology》2011,406(4):631-3180
The transmembrane protein CD44, which has been implicated in cancer biology and inflammation, mediates cell adhesion through multimeric interactions with the linear extracellular glycosaminoglycan hyaluronan (HA; in megadaltons). Affinity switching of CD44 from a low-affinity state to a high-affinity state is required for normal CD44 physiological function; crystal structures of the CD44 hyaluronan binding domain complexed with HA oligomers point to a conformational rearrangement at a binding site loop, leading to the formation of direct contact between the oligomer and an arginine side chain as a molecular basis for affinity switching. Here, all-atom explicit-solvent molecular dynamics simulations were used to characterize the dynamics and thermodynamics of oligomeric hyaluronan (oHA) and its two crystallographic complexes with the CD44 hyaluronan binding domain: the “A-form,” which lacks arginine-HA close contact, and the “B-form,” which has direct arginine side-chain-HA contact. From the simulations, the conformational properties of oHA are essentially unaltered in going from the unbound state to either the A-form or the B-form bound state, with the oligomer retaining its flexibility when bound and with only two of the eight monosaccharides in the oligomer maintaining uninterrupted contact with the protein. Biased simulations revealed that altering the backbone conformation of a tyrosine residue in the arginine loop can induce the A-form → B-form conformational transition and that a large free-energy barrier prevents ready interconversion between the two forms, thereby suggesting that the tyrosine backbone forms a molecular switch. 相似文献
40.
背景噪声对人感知声音时间信息的影响 总被引:1,自引:0,他引:1
对声音时间信息的分辨在人和动物感知声音信息的过程中至关重要.在自然声环境中,声音信息总处于一定的噪声背景下.文章以间隔探测阈值为指标测定了人对纯音和噪声的间隔探测阈值,以及持续噪声背景对间隔探测阈值的影响.声音信号采用1000~10000 Hz的纯音信号和白噪声信号,声音强度为70 dB SPL.背景噪声为持续白噪声,强度分别为45、55、65 dB SPL.结果表明,对纯音信号,随着背景噪声强度增加,间隔探测阈值有升高的趋势.对噪声信号来说,45、55 dB SPL的背景噪声对噪声信号的间隔探测阈值无显著影响,但65 dB SPL的背景噪声使间隔探测阈值显著升高.研究结果提示,背景噪声能够在一定程度上影响人对声音时间信息的感知,影响的程度与背景噪声的强度有关. 相似文献