全文获取类型
收费全文 | 749889篇 |
免费 | 80708篇 |
国内免费 | 498篇 |
出版年
2016年 | 7542篇 |
2015年 | 10502篇 |
2014年 | 12407篇 |
2013年 | 18336篇 |
2012年 | 19962篇 |
2011年 | 20546篇 |
2010年 | 13861篇 |
2009年 | 12839篇 |
2008年 | 18331篇 |
2007年 | 19352篇 |
2006年 | 18279篇 |
2005年 | 17651篇 |
2004年 | 17599篇 |
2003年 | 17063篇 |
2002年 | 16863篇 |
2001年 | 32106篇 |
2000年 | 32619篇 |
1999年 | 26065篇 |
1998年 | 8801篇 |
1997年 | 9288篇 |
1996年 | 8704篇 |
1995年 | 8523篇 |
1994年 | 8375篇 |
1993年 | 8375篇 |
1992年 | 22038篇 |
1991年 | 21563篇 |
1990年 | 21351篇 |
1989年 | 20822篇 |
1988年 | 19658篇 |
1987年 | 18556篇 |
1986年 | 17556篇 |
1985年 | 17949篇 |
1984年 | 14927篇 |
1983年 | 12848篇 |
1982年 | 10060篇 |
1981年 | 9172篇 |
1980年 | 8461篇 |
1979年 | 14395篇 |
1978年 | 11460篇 |
1977年 | 10638篇 |
1976年 | 10122篇 |
1975年 | 11176篇 |
1974年 | 12268篇 |
1973年 | 12081篇 |
1972年 | 11184篇 |
1971年 | 10155篇 |
1970年 | 8800篇 |
1969年 | 8809篇 |
1968年 | 8112篇 |
1967年 | 6926篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
T. A. Jaarsma E. Lohmanns H. Hendriks T. W. J. Gadella T. M. Malingré 《Plant Systematics and Evolution》1990,169(1-2):31-39
InS. tuberosum subspp.tuberosum andnodosum, S. grandiflorum andS. ibericum the presence of the pyrrolizidine alkaloids lycopsamine, echimidine and symphytine could be demonstrated. The taxonS. tuberosum contains an unknown compound that seems to be specific for this taxon. This compound is not the pyrrolizidine alkaloid anadoline which has previously been reported for this species. It is possibly represented by a peak on GC/MS with a molecular ion peak at m/z 623 (as TMS derivative) and can be used as a chemotaxonomic marker for the speciesS. tuberosum. The pyrrolizidine alkaloid pattern of the two subspecies ofS. tuberosum reinforces the close relationship. Fresh material ofS. tuberosum contained the triterpene isobauerenol, but in herbarium material isobauerenol was lacking. InS. grandiflorum, neither fresh nor dried material contains isobauerenol. In herbarium material ofS. ibericum also no isobauerenol could be found. More extensive chemotaxonomical research is necessary to support the view thatS. abchasicum is more closely related toS. ibericum than toS. grandiflorum. 相似文献
102.
F Albericio R van Abel G Barany 《International journal of peptide and protein research》1990,35(3):284-286
Attempts to anchor Fmoc-asparagine or glutamine as p-alkoxybenzyl esters for solid-phase peptide synthesis are fraught with difficulties. A convenient and effective method to prepare peptides with C-terminal asparagine or glutamine involves quantitative attachment of N alpha-Fmoc-C alpha-tert.-butyl aspartate or glutamate via the free omega-carboxyl groups to a tris(alkoxy)benzylamino (PAL) support. Chain elongation proceeds normally by standard Fmoc chemistry, and treatment with acid, e.g., CF3COOH--CH2Cl2, 90 min at 25 degrees, releases the desired peptides in greater than 95% yields without side reactions at the C-terminus. Feasibility of the approach has been demonstrated by the syntheses of the C-terminal octapeptide from human proinsulin, H-Leu-Ala-Leu-Glu-Gly-Ser-Leu-Gln-OH, and the serum thymic factor pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn-OH. 相似文献
103.
Cionin: a disulfotyrosyl hybrid of cholecystokinin and gastrin from the neural ganglion of the protochordate Ciona intestinalis 总被引:2,自引:0,他引:2
We have purified an acidic octapeptide from the neural ganglion of the protochordate Ciona intestinalis by a three-step procedure including C18 Sep-Pak fractionation, MonoQ ion-exchange chromatography, and C4 reversed-phase high-performance liquid chromatography. The purification was monitored by an immunoassay specific for the alpha-carboxyamidated COOH terminus common to the mammalian brain-gut hormones, cholecystokinin and gastrin. Automated Edman degradation revealed the sequence Asn-Tyr-Tyr-Gly-Trp-Met-Asp-Phe. In accordance with the high acidity of the peptide, amino acid analysis after cleavage with aminopeptidase M showed that both tyrosyl residues are sulfated. Hence, the structure is Asn-Tyr(SO3)-Tyr(SO3)-Gly-Trp-Met-Asp-Phe-NH2, as also confirmed by identity with the synthetic disulfated peptide in different chromatographic systems. The occurrence of two consecutively sulfated tyrosyl residues after a neutral residue challenges present concepts of consensus sites for tyrosyl sulfation. We conclude that the structure of the peptide, named cionin, suits that of a common ancestor for cholecystokinin and gastrin. 相似文献
104.
D Ayala P Lissoni C Archili S Barni G Roveda M Giacomelli A Bissi 《Journal of biological regulators and homeostatic agents》1990,4(4):150-152
The endocrine effects of ovariectomy need to be further investigated. The present study was carried out to evaluate the influence of the adjuvant ovariectomy on the mastectomy-induced changes in PRL response to TRH in breast cancer. The study included 34 patients with locally limited breast carcinoma, 18 of whom were treated with radical mastectomy, whereas the other 16 underwent mastectomy plus adjuvant ovariectomy. PRL secretion in response to TRH (200 mcg I.V. as bolus) was evaluated one day before and 7 days after surgery. In patients treated with mastectomy only, PRL increase after TRH was significantly higher after surgery than before. On the contrary, no difference was seen in patients treated with mastectomy plus ovariectomy. This study shows that the adjuvant ovariectomy may block the increase in PRL response to TRH induced by mastectomy in breast cancer. 相似文献
105.
106.
Determination of the amounts of the protein synthesis initiation and elongation factors in wheat germ 总被引:23,自引:0,他引:23
K S Browning J Humphreys W Hobbs G B Smith J M Ravel 《The Journal of biological chemistry》1990,265(29):17967-17973
Previous work by Browning et al. (Browning, K. S., Lax, S. R., Humphreys, J., Ravel, J. M., Jobling, S. A., and Gehrke, L. (1988) J. Biol. Chem. 263, 9630-9634) indicated that wheat germ extracts do not contain sufficient amounts of some of the protein synthesis initiation factors to obtain optimal translation of all mRNAs. In this investigation, a quantitative enzyme-linked immunosorbent assay was used to determine the amounts of eukaryotic initiation factors (eIF) 2, 3, 4A, 4F, and (iso)4F as well as the amounts of 40 S ribosomal subunits and elongation factors (EF) 1 alpha and 2 present in wheat germ extracts. EF-1 alpha is present in the highest amount (approximately 5% of the total protein), and eIF-4F is present in the lowest amount (approximately 0.03% of the total protein). The micromolar amounts of the factors and ribosomes are as follows: EF-1 alpha, 34; EF-2, 5.2; eIF-2, 1.5; eIF-3, 0.7; eIF-4A, 3.0, eIF-4F, 0.09; eIF-(iso)4F, 0.8; and 40 S ribosomal subunits, 3.2. The molar ratios of the factors to 40 S ribosomal subunits are approximately 11:1 for EF-1 alpha, 1.6:1 for EF-2, 0.45:1 for eIF-2, 0.2:1 for eIF-3, 0.9:1 for eIF-4A, 0.03:1 for eIF-4F, and 0.25:1 for eIF-(iso)4F. These findings strongly suggest that the concentrations of the initiation factors, particularly those factors required for the binding of mRNA to ribosomes, may play a major role in regulating the translation of mRNAs within the cell. 相似文献
107.
S T Chen L C Lo S H Wu K T Wang 《International journal of peptide and protein research》1990,35(1):52-54
2-Oxazolidone derivatives formed through an intramolecular reaction in the process of alkaline treatment of urethane-type N-protected peptides of which the N-terminal residues were Ser or Thr having unprotected hydroxyl groups. In oder to avoid this side reaction, the esters of these peptides could be cleaved by enzymatic hydrolyses instead of saponification. 相似文献
108.
109.
Lines of sugarbeet (Beta vulgaris L.) tolerant of multiple salts was accomplished by an in vitro multiple salt challenge. Petioles were placed on RV medium amended with 5 different salts along with Murashige and Skoog base salts for one month. Surviving shoots were cultured on RV medium to obtain petioles for subsequent challenges. During the first, second and third challenges, organogenically regenerated shoots developed from 5%, 46%, and 80% of the petioles, respectively. After the third multiple salt challenge, tolerant shoots were rooted and transplanted in soil. Salt was added to this soil at 1.0% by weight and plants were observed for 2 months. The ten most salt tolerant plants were vernalized to obtain seed. The R1 seed and controls were planted in soil containing 0%, 0.61% or 0.77% multiple salts per dry soil weight. Emergence of R1 seedlings was significantly greater than the controls under salt stress. Multiple salt tolerant R1 plants were maintained in salt amended soil to the 8–10 leaf stage and appeared as healthy and vigorous as the control growing in salt free soil.Contribution from Missouri Agricultural Experiment Station Journal Series No. 10948. University of Missouri, Columbia, MO, USA Mention of trade names does not constitute a guarantee or warranty of the product by University of Missouri of Holly Sugar Corporation and does not imply their approval to the exclusion of other products. 相似文献
110.
Exopolysaccharide production in Rhizobium and its role in invasion 总被引:13,自引:3,他引:10