全文获取类型
收费全文 | 2118篇 |
免费 | 207篇 |
国内免费 | 2篇 |
专业分类
2327篇 |
出版年
2023年 | 17篇 |
2022年 | 28篇 |
2021年 | 49篇 |
2020年 | 24篇 |
2019年 | 42篇 |
2018年 | 51篇 |
2017年 | 42篇 |
2016年 | 69篇 |
2015年 | 90篇 |
2014年 | 90篇 |
2013年 | 116篇 |
2012年 | 133篇 |
2011年 | 132篇 |
2010年 | 72篇 |
2009年 | 69篇 |
2008年 | 109篇 |
2007年 | 99篇 |
2006年 | 75篇 |
2005年 | 91篇 |
2004年 | 87篇 |
2003年 | 66篇 |
2002年 | 67篇 |
2001年 | 43篇 |
2000年 | 34篇 |
1999年 | 38篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 19篇 |
1995年 | 17篇 |
1993年 | 17篇 |
1992年 | 25篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 23篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 19篇 |
1983年 | 24篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 23篇 |
1978年 | 13篇 |
1977年 | 15篇 |
1976年 | 16篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1973年 | 11篇 |
排序方式: 共有2327条查询结果,搜索用时 15 毫秒
1.
The alpha-(2----8)-linked sialic acid oligosaccharides (NeuAc)n exhibit an unusual degree of heterogeneity in the conformation of their linkages. This was diagnosed by observation in their 13C NMR spectra of an equivalent and unique heterogeneity in the chemical shifts of their anomeric carbons and subsequently confirmed by more comprehensive 1H and 13C NMR studies. In these studies both one-dimensional and two-dimensional experiments were carried out on the trisaccharide (NeuAc)3 and colominic acid. In addition to the unambiguous assignment of the signals in the spectra, these experiments demonstrated that both linkages of (NeuAc)3 differed in conformation from each other and from the inner linkages of colominic acid. The NMR data indicate that these conformational differences extend to both terminal disaccharides of oligosaccharides larger than (NeuAc)5, a result that has considerable physical and biological significance. In the context of the group B meningococcal polysaccharide, it provides an explanation for the conformational epitope of the group B meningococcal polysaccharide, which was proposed on the evidence that (NeuAc)10, larger than the optimum size of an antibody site, was the smallest oligosaccharide able to bind to group B polysaccharide specific antibodies. Because the two terminal disaccharides of (NeuAc)10 differ in conformation to its inner residues, the immunologically functional part of (NeuAc)10 resides in its inner six residues. This number of residues is now consistent with the maximum size of an antibody site. 相似文献
2.
3.
The orientation of washed Escherichia coli bacteria in suspension induced by applied a.c. electric fields has been monitored by observing the accompanying changes in the intensity of light scattered by the suspension. The data enable the anisotropy of electrical polarisability deltaalpha to be determined. Changes in deltaalpha due to the addition of various antibiotics to the suspension have been measured as a function of both the antibiotic concentration and the temperature of the suspension. The results are taken to indicate that there is an accumulation of the antibiotic molecules at the bacterial surface. 相似文献
4.
We evaluated the performance of Coptera haywardi (Ogloblin) (Diapriidae) and Pachycrepoideus vindemiae (Rondani) (Pteromalidae), both hymenopteran pupal parasitoids of Anastrepha spp. (Diptera: Tephritidae). Performance was studied by manipulating the following environmental conditions in the laboratory: (1) soil type, (2) soil moisture content, (3) soil compaction, and (4) depth at which pupae were buried in the soil. There were two experiments: in the first, exposure time of pupae was held constant and in the second, it varied. In the first experiment, C. haywardi was significantly more effective than P. vindemiae in parasitizing fly pupae. With exposure time held constant (36 h), only soil type and pupal burial depth were significantly related to parasitism rates. While P. vindemiae only parasitized pupae located on the soil surface, C. haywardi attacked pupae that were buried up to 5 cm deep, performing better in clayey than in loamy soil. In the second experiment, exposure time (24, 36, 48, and 72 h) had no significant effect on parasitism rates, but soil type did. P. vindemiae again only attacked pupae on the soil surface while C. haywardi was also able to parasitize pupae that were buried up to 5 cm deep. We conclude that C. haywardi represents a viable candidate to replace the environmentally unfriendly P. vindemiae in augmentative biological control programs against fruit flies. 相似文献
5.
M R Wessels V Pozsgay D L Kasper H J Jennings 《The Journal of biological chemistry》1987,262(17):8262-8267
We have derived oligosaccharides from the capsular polysaccharide of type III group B Streptococcus by enzymatic hydrolysis of a specific backbone glycosidic bond utilizing an endo-beta-galactosidase from Flavobacterium keratolyticus. Enzymatic digestion of the polysaccharide produced oligosaccharide fragments of one or more pentasaccharide repeating units. On the basis of 13C NMR, 1H NMR, and methylation analyses, it was established that the smallest digestion fragment was alpha-D-NeupNAc-(2----3)-beta-D-Galp-(1----4)-[beta-D-Glcp-(1----6 )]- beta-D-GlcpNAc-(1----3)-beta-D-Gal. The isolation of this oligosaccharide is consistent with the susceptibility of the beta-D-Galp-(1----4)-beta-D-Glcp linkage in the backbone of the type III group B streptococcal polysaccharide and confirms that the polysaccharide is composed of a pentasaccharide repeating unit. High resolution 13C NMR spectroscopic studies indicated that, as in the case of the pentasaccharide, the terminal sialic acid residues of the type III group B streptococcal polysaccharide were linked to O-3 and not to O-6 of its branch beta-D-galactopyranosyl residues as had been previously reported (Jennings, H. J., Rosell, K.-G., and Kasper, D. L. (1980) Can. J. Chem. 58, 112-120). This linkage was confirmed in an independent methylation analysis of the type III group B streptococcal polysaccharide. Thin layer chromatogram binding assay and radioactive antigen binding assays with radiolabeled oligosaccharides demonstrated the single repeating unit pentasaccharide oligosaccharide to be poorly antigenic. Increasing oligosaccharide size to a decasaccharide consisting of two repeating units resulted in an 8-fold increase in antigen binding in the direct radioactive antigen binding assay. The results suggest that a region of the immunodeterminant site critical for antibody binding is located in the backbone of the polysaccharide and involves the beta-D-galactopyranose-(1----4) beta-D-glucopyranose bond. 相似文献
6.
Margaret Nelson D. S. Nelson V. K. Kuchroo P. B. Spradbrow P. A. Jennings 《Cancer immunology, immunotherapy : CII》1987,24(3):231-236
Summary Tumours produce substances that inhibit the expression of cell-mediated immunity, in the form of delayed-type hypersensitivity in mice. Phenol-saline extracts of bovine ocular squamous cell carcinoma (BOSCC) which have immunotherapeutic activity in cattle were able to immunize mice against this depressive effect. Such immunization was effective against products of BOSCC, a spontaneous rat tumour, three of four human tumour cell lines and (in other experiments) mouse tumours. Phenol-saline extracts of mouse tumour cell lines were immunogenic (protective against depression of delayed-type hypersensitivity) in mice. Fractions of BOSCC phenol-saline extracts which were immunotherapeutically active in cattle were generally also protective in mice. The protective activity was lost after treatment with proteinase K, and was present in the supernatant after precipitation with 55% ammonium sulphate. It was not affected by treatment with RNase or DNase or by heating to 50 °C for 2 h. It was present in gel filtration fractions with an apparent molecular weight of 10,000–37,000 daltons. The immunogenic factor in mice and the immunotherapeutic factor in cattle may be related to each other. 相似文献
7.
The stability, toxicity and effectiveness of unmodified and phosphorothioate antisense oligodeoxynucleotides in Xenopus oocytes and embryos. 总被引:31,自引:16,他引:15 下载免费PDF全文
The properties of antisense phosphorothioate and unmodified oligodeoxynucleotides have been studied in Xenopus oocytes and embryos. We find that phosphorothioates, like unmodified oligodeoxynucleotides, can degrade Vg1 mRNA in oocytes via an endogenous RNase H-like activity. In oocytes, phosphorothioate oligodeoxynucleotides are more stable than unmodified oligodeoxynucleotides and are more effective in degrading Vg1 mRNA. In embryos, neither unmodified nor phosphorothioate deoxyoligonucleotides were effective in degrading Vg1 message at sub-toxic doses. 相似文献
8.
D. H. Jennings 《Plant, cell & environment》1988,11(2):147-148
Books reviewed in this article:
MINERAL NUTRITION IN HIGHER PLANTS By H orst M arschner
PLANT AND ANIMAL CELLS PROCESS POSSIBILITIES. Edited by C. W ebb & F. M ayituna 相似文献
MINERAL NUTRITION IN HIGHER PLANTS By H orst M arschner
PLANT AND ANIMAL CELLS PROCESS POSSIBILITIES. Edited by C. W ebb & F. M ayituna 相似文献
9.
A comparison of the lipid-lowering and intestinal morphological effects of cholestyramine, chitosan, and oat gum in rats 总被引:2,自引:0,他引:2
C D Jennings K Boleyn S R Bridges P J Wood J W Anderson 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1988,189(1):13-20
Cholestyramine, chitosan, and oat gum are lipid-lowering compounds. Cholestyramine use in humans may contribute to colonic adenocarcinoma; chitosan and oat gum are being studied in the rat to determine their potential for human use. To compare these compounds, we fed three groups of 10 male Sprague-Dawley rats one of the substances at 5% of diet with 1% cholesterol and 0.2% cholic acid; two other groups were fed cellulose with and without 1% cholesterol and 0.2% cholic acid. All groups had similar food intake and weight gains. Cholesterol feeding increased total liver lipids almost 3-fold and liver cholesterol concentration almost 10-fold. Cholestyramine, oat gum, and chitosan all significantly lowered liver cholesterol with cholestyramine feeding yielding levels identical to the noncholesterol-fed basal group. Chitosan and oat gum lowered liver cholesterol moderately. Cholestyramine and chitosan both significantly lowered serum cholesterol compared to the cellulose group. Oat gum was less effective. Hemoglobin and serum iron were similar in all groups except the oat gum group, which had decreased serum iron. Histological examination of small and large bowel with morphometry revealed statistically significant increases in both proximal and distal small bowel and distal large bowel mucosal thickness in the cholestyramine-fed group. No changes were noted in the proximal large bowel. Neither chitosan nor oat gum produced mucosal change other than an increase in the distal small bowel with the oat gum diet. Chitosan may have lipid-lowering effects similar to those of cholestyramine without the deleterious changes in intestinal mucosa. 相似文献
10.
Direct determination of homovanillic acid release from the human brain, an indicator of central dopaminergic activity 总被引:2,自引:0,他引:2
G W Lambert G Eisenhofer H S Cox M Horne V Kalff M Kelly G L Jennings M D Esler 《Life sciences》1991,49(15):1061-1072
The plasma concentration of the dopamine (DA) metabolite, homovanillic acid (HVA), is used as an indicator of central nervous system dopaminergic activity. Using percutaneously inserted catheters we were able to obtain blood samples simultaneously from the right and left internal jugular veins. Veno-arterial HVA plasma concentration differences combined with adjusted organ plasma flows were used, according to the Fick Principle, to determine the HVA overflow from the brain. The HVA overflow from the liver was also measured. HVA overflow from the brain represented 12% of the total body HVA production. A similar amount was released from the liver, illustrating the limited validity of peripheral plasma HVA measurements as an indicator of central dopaminergic activity. HVA release from the human brain displayed a degree of asymmetry, the overflow into the left internal jugular vein being 36% greater than that into the right. Cerebral venous blood flow scans indicated that cortical cerebral regions drained preferentially into the right internal jugular; by inference the higher HVA overflow on the left originated from dopamine-rich subcortical brain areas. Since HVA in plasma may arise from the metabolism of DA existing either as a neurotransmitter or a norepinephrine (NE) precursor we measured the internal jugular vein plasma concentrations of NE, and its metabolite dihydroxyphenylglycol (DHPG), to determine whether they displayed a similar pattern of release to HVA. The overflow of both NE and DHPG into the right internal jugular vein was approximately double that on the left. Since the overflow of HVA did not parallel that of NE and DHPG it may be inferred that the origin of much of the subcortically produced HVA is from dopaminergic neurons and not from the metabolism of precursor DA in noradrenergic neurones or cerebrovascular sympathetic nerves. 相似文献