全文获取类型
收费全文 | 592篇 |
免费 | 53篇 |
专业分类
645篇 |
出版年
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 9篇 |
2019年 | 7篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 9篇 |
2015年 | 23篇 |
2014年 | 31篇 |
2013年 | 31篇 |
2012年 | 24篇 |
2011年 | 33篇 |
2010年 | 18篇 |
2009年 | 28篇 |
2008年 | 26篇 |
2007年 | 32篇 |
2006年 | 28篇 |
2005年 | 28篇 |
2004年 | 19篇 |
2003年 | 21篇 |
2002年 | 30篇 |
2001年 | 16篇 |
2000年 | 22篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 9篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 3篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1927年 | 1篇 |
1922年 | 3篇 |
1921年 | 1篇 |
1920年 | 1篇 |
排序方式: 共有645条查询结果,搜索用时 15 毫秒
1.
Peak area matching and linear regression were used to quantify eight chlorinated pesticides and polychlorinated biphenyls (as Aroclor 1260) in human serum. There are no statistically significant differences in data obtained by these two quantifying techniques which were indicated by the paired t-test. For chlorinated pesticides, p = 0.053-0.62, and for polychlorinated biphenyls (as Aroclor 1260), p = 0.64. Analyte residues for the chlorinated pesticides ranged from 0.5 ppb for hexachlorobenzene (HCB) to 186 ppb for dichlorodiphenyldichloroethylene (DDE). Analyte residues for the polychlorinated biphenyls (as Aroclor 1260) ranged from 5-114 ppb. The absolute mean percent difference between the two quantifying techniques ranged from 0.06% for DDE to 8.06% for dieldrin (HEOD) among the chlorinated pesticides. The absolute mean percent difference between the two quantifying techniques for the polychlorinated biphenyls (as Aroclor 1260) was 3.4%. Peak area matching and linear regression were found to be comparable for quantifying these environmental residues in serum when the following conditions apply: 1) the concentration of the chlorinated pesticides is greater than or equal to 0.5 ppb (e.g., HCB, hexachlorocyclohexane (HCCH), oxychlordane (OC), heptachlor epoxide (HE), transnonachlor (TN), HEOD, and dichlorodiphenyltrichloroethane (DDT); 2) the concentration of the chlorinated pesticide is greater than or equal to 3 ppb (e.g., DDE); and 3) the total concentration of polychlorinated biphenyls (e.g., as Aroclor 1260) is greater than or equal to 5 ppb. 相似文献
2.
A. W. Bedo E. J. H. Head R. J. Conover E. P. W. Horne L. R. Harris 《Polar Biology》1990,10(8):561-570
Summary Zooplankton and water samples were collected at weekly intervals between April 25 and May 30, 1986 in Barrow Strait, N.W.T. (Canadian Arctic Archipelago). In tows from 0–30 m, the zooplankton community (>202 m) was dominated by Pseudocalanus. The population was apparently growing and developing as shown by an increase in the proportion of adults (stage VI) and decreases in the proportion of stages III, IV, and V as the season progressed. Respiration and excretion rates of the Pseudocalanus populations were probably linked, there being an immediate increase in excretion rate, accompanying an increase in feeding rate when chlorophyll concentrations increased, which was followed by a smaller increase in respiration rate after a time lag. Hence, there was a large decrease in the ON ratio. Increased metabolism coincided with changes in the population structure, as did protease and bodily protein, but could not be clearly linked to dietary acclimation. Only laminarinase activity could be statistically related to an identifiable fraction of potential nutritional value in the water, particulate soluble carbohydrate, but neither showed overall seasonal change. 相似文献
3.
Trimethyloxonium modification of single batrachotoxin-activated sodium channels in planar bilayers. Changes in unit conductance and in block by saxitoxin and calcium 总被引:13,自引:9,他引:4 下载免费PDF全文
Single batrachotoxin-activated sodium channels from rat brain were modified by trimethyloxonium (TMO) after incorporation in planar lipid bilayers. TMO modification eliminated saxitoxin (STX) sensitivity, reduced the single channel conductance by 37%, and reduced calcium block of inward sodium currents. These effects always occurred concomitantly, in an all-or-none fashion. Calcium and STX protected sodium channels from TMO modification with potencies similar to their affinities for block. Calcium inhibited STX binding to rat brain membrane vesicles and relieved toxin block of channels in bilayers, apparently by competing with STX for the toxin binding site. These results suggest that toxins, permeant cations, and blocking cations can interact with a common site on the sodium channel near the extracellular surface. It is likely that permeant cations transiently bind to this superficial site, as the first of several steps in passing inward through the channel. 相似文献
4.
5'-Nucleotidase is a member of a recently identified class of membrane proteins that is anchored via a phosphatidylinositol-containing glycolipid. The enzyme was readily solubilized with full retention of catalytic activity by nonionic and anionic detergents such as alkylthioglucosides, deoxycholate, and 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane-sulfonate (CHAPS), while the cationic detergent dodecyltrimethylammonium bromide (DTAB) caused loss of activity. 5'-Nucleotidase was released only at high detergent concentrations, suggesting that it is tightly associated with the membrane. DTAB and deoxycholate caused a loss of heat stability, while alkylthioglucosides had no effect. CHAPS produced a remarkable increase in the heat stability of the partially purified (glycoprotein fraction) and purified enzyme. Arrhenius plots of solubilized 5'-nucleotidase showed "break points" for all detergents in the temperature range 30-37 degrees C. SDS-PAGE of pure 5'-nucleotidase showed a single subunit of molecular mass 70 kilodaltons (kDa), while sucrose density gradient sedimentation gave a peak of activity corresponding to 132 kDa, indicating that the enzyme exists as a dimer. Gel filtration of the solubilized enzyme in several detergents showed apparent molecular masses between 200-630 kDa, suggesting that lymphocyte 5'-nucleotidase may be present in high molecular mass aggregates in its native state. 相似文献
5.
Regulation of expression of the 3β-hydroxysteroid dehydrogenases of human placenta and fetal adrenal
J.I. Mason K. Ushijima K.M. Doody K. Nagai D. Naville J.R. Head L. Milewich W.E. Rainey M.M. Ralph 《The Journal of steroid biochemistry and molecular biology》1993,47(1-6):151-159
The appropriate expression of 3β-hydroxysteroid dehydrogenase/Δ5→4-isomerase (3β-HSD) is vital for mammalian reproduction, fetal growth and life maintenance. Several isoforms of 3β-HSD, the products of separate genes, have been identified in various species including man. Current investigations are targeted toward defining the processes that regulate the levels of specific isoforms in various steroidogenic tissues of man. High levels of expression of 3β-HSD were observed in placental tissues. It has been generally considered that the multinucleated syncytiotrophoblastic cells are the principal sites of 3β-HSD expression and, moreover, that 3β-HSD expression is intimately associated with cyclic AMP-promoted formation of syncytia. Herein we report the presence of 3β-HSD immunoreactive and mRNA species in uninucleate cytotrophoblasts in the chorion laeve, similar to that in syncytia but not cytotrophoblast placenta. In vitro, 3β-HSD levels in chorion laeve cytotrophoblasts were not increased with time nor after treatment with adenylate cyclase activators, whereas villous cytotrophoblasts spontaneously demonstrated progressive, increased 3β-HSD expression. Moreover, 3β-HSD synthesis appeared to precede morphologic syncytial formation. Thus high steroidogenic enzyme expression in placenta is not necessarily closely linked to formation of syncytia. Both Western immunoblot and enzymic activity analyses also indicated that the 3β-HSD expressed in these cytotrophoblastic populations was the 3β-HSD type I gene product (Mr, 45K) and not 3β-HSD type II (Mr, 44K) expressed in fetal testis. In cultures of fetal zone and definitive zone cell of human fetal adrenal, 3β-HSD expression was not detected until ACTH was added. ACTH, likely acting in a cyclic AMP-dependent process, induced 3β-HSD type II activity and mRNA expression. The higher level of 3β-HSD mRNA in definitive zone compared with fetal zone cells was associated with parallel increases in cortisol secretion relative to dehydroepiandrosterone sulfate formation. 相似文献
6.
7.
8.
Denise D. Beusen Janusz Zabrocki Urszula Slomczynska Richard D. Head Jeff L.-F. Kao Garland R. Marshall 《Biopolymers》1995,36(2):181-200
Potent, cyclic hexapeptide analogues of somatostatin are generally believed to adopt some common secondary structural features: a II′ β turn at one end of the cycle, and a type VI turn with a cis amide bond at the other. A proposed cis amide surrogate, the 1,5-disubstituted tetrazole, has been placed into a cyclic hexapeptide analog of somatostatin in order to constrain the putative cis amide bond. The final cyclization was done by either chemical or enzymatic means. The product, cyclo(Ala6-Tyr7-D -Trp8-Lys9-Val10-Phe11-Ψ[CN4]), was found to have 83% of the activity of somatostatin. Solution nmr analysis in DMSO/water revealed that the backbone as well as side chain χ1 and χ2 were well ordered. Relaxation matrix methods were used to extract distance restraints from the nuclear Overhauser effect spectroscopy data set, and these were used in a systematic search of torsional space to identify structures consistent with the nmr data. Restrained minimizations of these structures using a number of different force fields produced structures having the expected βII′ turn at D -Trp8-Lys9 and αβVIa turn in the Phe11-Ψ[CN4]-Ala6 portion of the molecule. The similarity of the minimized structures to those previously reported for cyclic hexapeptide analogues of somatostatin confirms the similarity of the tetrazole geometry to that of the cis amide in solution. © 1995 John Wiley & Sons, Inc. 相似文献
9.
KOLOTKIN, RONETTE L, SUSAN HEAD, ALAN BROOKHART. Construct validity of the Impact of Weight on Quality of Life questionnaire. The Impact of Weight on Quality of Life questionnaire (IWQOL) is a 74-item self-report, condition-specific instrument that (1) assesses the effect of weight on quality of life in eight key areas, and (2) may be used as a treatment outcome measure and/or an evaluation tool for healthcare policy makers and third-party payers. This study explores IWQOL construct validity and provides new information on internal consistency, treatment effects, and differences between men and women. IWQOL total scores correlated highly with other measures of overall quality of life, and subscale scores correlated well with counterparts in the assessment battery. Internal consistency estimates for the IWQOL scales generally were high. For the women, 4-week participants, and the total sample, pretreatment-posttreatment differences were significant for all IWQOL scales and total score. For men, treatment differences were significant for the total score and all subscales except for Work and Mobility. Treatment differences for 2-week participants were significant for all scales except for Work. Consistent with previous IWQOL study results, the Comfort With Food scale scores reflected more discomfort at posttreatment as compared with pretreatment. The IWQOL, already translated into French and Italian, currently is demonstrating clinical and research utility as a quality-of-life outcome measure for clinical trials of antiobesity drugs and surgical treatments for patients with obesity. 相似文献
10.
Affinity-chromatographic isolation and some properties of troponin C from different muscle types. 总被引:8,自引:7,他引:1 下载免费PDF全文
1. The formation of a complex between troponin I and troponin C that is stable in 6M-urea and dependent on Ca2+ was demonstrated in extracts of vertebrate striated and smooth muscles. 2. A method using troponin I coupled to Sepharose is described for the rapid isolation of troponin C from striated and smooth muscles of vertebrates. 3. Troponin C of rabbit cardiac muscle differs significantly in amino acid composition from troponin C of skeletal muscle. The primary structures of troponin C of red and white skeletal muscle are very similar. 4. The troponin C-like protein isolated from rabbit uterus muscle has a slightly different amino acid composition, but possess many similar properties to the forms of troponin C isolated from other muscle types. 5. The electrophoretic mobilities of the I-troponin C complexes formed from components isolated from different muscle types are determined by the troponin I component. 相似文献