全文获取类型
收费全文 | 713篇 |
免费 | 75篇 |
国内免费 | 2篇 |
出版年
2022年 | 6篇 |
2021年 | 14篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 11篇 |
2016年 | 17篇 |
2015年 | 27篇 |
2014年 | 31篇 |
2013年 | 33篇 |
2012年 | 56篇 |
2011年 | 34篇 |
2010年 | 21篇 |
2009年 | 34篇 |
2008年 | 41篇 |
2007年 | 39篇 |
2006年 | 29篇 |
2005年 | 34篇 |
2004年 | 28篇 |
2003年 | 29篇 |
2002年 | 26篇 |
2001年 | 26篇 |
2000年 | 20篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 5篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 5篇 |
1972年 | 4篇 |
1946年 | 3篇 |
1945年 | 3篇 |
排序方式: 共有790条查询结果,搜索用时 15 毫秒
1.
Both neonatal humans and mice are exquisitely susceptible to severe HSV infection. We have now documented a profound defect in the ability of neonatal C57BL/6 mice to produce anti-HSV ADCC antibody. This ability is acquired over the first 2 to 4 wk of life. Reconstitution of neonatal mice by i.p. injection of peritoneal cells from adult nonimmune syngeneic mice both affords dose-dependent protection against lethal HSV infection and reconstitutes the antibody-production defect. By cell-separation techniques (adherence, nylon wool column purification, B cell panning) and cell ablation techniques (silica treatment, irradiation, anti-T cell, anti-Ia, anti-Lyt-1.2 and anti-Lyt-2.2 monoclonal antibodies plus complement treatment) the subpopulations involved in the antibody production reconstitution of neonatal mice by adult cells were identified. These include both an Ia+, radioresistant, adherent, silica-sensitive macrophage population and a nylon wool column-purified, radiosensitive, anti-T, anti-Lyt-1.2-sensitive helper T cell population. The latter cell may be substituted for by concanavalin A-stimulated lymphokine-containing spleen cell supernatants or human recombinant IL 2. In addition to reconstitution of ADCC antibody production, the same cell populations, or cells plus lymphokine-containing supernatants or IL 2, protected the newborn mice from lethal HSV infection. Further characterization of this system and of soluble replacement factors has implications for therapy or immunoprophylaxis of human neonates with, or at risk of, HSV infection. 相似文献
2.
Indirect evidence suggests that legumes can adjust rapidly theresistance of their root nodules to O2 diffusion. Here we describeexperiments using O2 specific micro-electrodes and dark fieldmicroscopy to study directly the operation of this diffusionbarrier. The O2 concentration sensed by the electrode decreasedsharply in the region of the inner cortex and was less than1.0 mmol m3 throughout the infected tissue in nodulesof both pea (Pisum sativum) and french bean (Phaseolus vulgaris).In a number of experiments the ambient O2 concentration wasincreased to 40% while the electrode tip was just inside theinner cortex. In 13 out of 21 cases the O2 concentration atthis position either remained low and unchanged or increasedirreversibly to near ambient values. In the remaining casesthe O2 concentration increased after 1 to 2.5 min and then decreasedto its former value. These results are ascribed to an increasein resistance of the barrier in response to increased O2 fluxinto the nodule. It was shown microscopically that air spacesboth at the boundary between the infected zone and the innercortex, and within the infected zone started to disappear 3min after nodules were exposed to high ambient O2 concentrationsand had disappeared completely after 8 min. These spaces werenot changed by exposure of the nodule for 10 min to either N2or air. Key words: Oxygen, root nodules, air spaces 相似文献
3.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
相似文献
4.
根癌农杆菌Ti质粒的T区DNA带有致瘤基因,其基因1和基因2编码生长素吲哚乙酸生物合成途径中的两个酶。以pGV 354(pBR322质粒中插有Ti质粒C 58 T区DNA的HindⅢ15—HindⅢ22大片段)重组质粒出发,我们分离了基因1和基因2,并构建了带有卡那霉素抗性基因的重组质粒pBZ 692,通过基因载体pGV 3850,我们将基因1和基因2引入了高等植物。结果证明基因1和基因2能促使烟草、向日葵、土豆等转化组织分化长根,转化的根在MS_0培养基上能脱分化形成愈伤组织并自主生长,在转化的组织中有转化标记胭脂碱的存在。 相似文献
5.
G Le Fur E Zarifian T Phan H Cuche A Flamier F Bouchami M C Burgevin H Loo A Gérard A Uzan 《Life sciences》1983,32(3):249-255
[3H]spiroperidol binding to lymphocytes was measured in untreated paranoid or disorganized and treated paranoid schizophrenic patients. An increase in the Bmax was detected in untreated paranoid patients but a decrease was found in the disorganized patients. No difference was detected in the KD value. Neuroleptic treatment produced a decrease in the Bmax without affecting the KD value. Such results did not comply with the down regulation but might be explained by a change in membrane viscosity as [3H]spiroperidol binding sites on lymphocytes were coupled to phospholipid methylation. 相似文献
6.
Glucocorticoid and thyroid hormones inhibit proliferation of serum-free mouse embryo (SFME) cells 总被引:3,自引:0,他引:3
Mouse embryo cells derived in a serum-free medium formulation (SFME cells) do not exhibit growth crisis or chromosomal abnormalities and are nontumorigenic in vivo; these cells are also reversibly growth inhibited by serum or platelet-free plasma (Loo et al.; Science, 236:200-202, 1987). A portion of the inhibitory activity of serum could be extracted by charcoal, a procedure that removes steroid and thyroid hormones. Both L-3,5,3'-triiodothyronine (T3) and hydrocortisone inhibited growth of SFME cells in a reversible manner. The inhibitory activity of serum also was partially removed by treatment with anion exchange resin in a procedure designed to deplete serum of thyroid hormone. However, the effect of serum on untransformed SFME cells could not be prevented by addition of the antiglucocorticoid RU38486, and ras-transformed clones of SFME cells, which are capable of growing in serum-containing medium, retained inhibitory responses to glucocorticoid and, with some clonal variability, to T3. These results suggest that glucocorticoid or thyroid hormones may contribute to the inhibitory activity of serum on SFME cells, but additional factors are also involved. 相似文献
7.
K Maruyama D M Clarke J Fujii G Inesi T W Loo D H MacLennan 《The Journal of biological chemistry》1989,264(22):13038-13042
The sequence of 10 amino acids (ICSDKTGTLT357) at the site of phosphorylation of the rabbit fast twitch muscle Ca2+-ATPase is highly conserved in the family of cation-transporting ATPases. We changed each of the residues flanking Asp351, Lys352, and Thr353 to an amino acid differing in size or polarity and assayed the mutant for Ca2+ transport activity and autophosphorylation with ATP or P1. We found that conservative changes (Ile----Leu, Thr----Ser, Gly----Ala) or the alteration of Cys349 to alanine did not destroy Ca2+ transport activity or phosphoenzyme formation, whereas nonconservative changes (Ile----Thr, Leu----Ser) did disrupt function. These results indicate that very conservative changes in the amino acids flanking Asp351, Lys352, and Thr353 can be accommodated. A number of mutations were also introduced into amino acids predicted to be involved in nucleotide binding, in particular those in the conserved sequences KGAPE519, RDAGIRVIMITGDNK629, and KK713. Our results indicate that amino acids KGAPE519, Arg615, Gly618, Arg620, and Lys712-Lys713 are not essential for nucleotide binding, although changes to Lys515 diminished Ca2+ transport activity but not phosphoenzyme formation. Changes of Gly626 and Asp627 abolished phosphoenzyme formation with both ATP and Pi, indicating that these residues may contribute to the conformation of the catalytic center. 相似文献
8.
D M Clarke K Maruyama T W Loo E Leberer G Inesi D H MacLennan 《The Journal of biological chemistry》1989,264(19):11246-11251
Nucleotides encoding glutamate, glutamine, aspartate, or asparagine residues within the stalk sector of the sarcoplasmic reticulum Ca2+-ATPase were altered by oligonucleotide-directed site-specific mutagenesis. The mutant cDNAs were expressed in COS-1 cells, and mutant Ca2+-ATPases were assayed for Ca2+ transport function and phosphoenzyme formation. Multiple mutations introduced into stalks, 1, 2, and 3 resulted in partial loss of Ca2+ transport function. In most cases, subsequent mutation of individual amino acids in the cluster had no effect on Ca2+ transport activity. In one cluster, however, it was possible to assign the reduction in Ca2+ transport activity to alterations of Asn111 and Asn114. The mutant Asn114 to alanine retained about 50% activity, whereas the change Asn111 to alanine retained only 10% activity. None of the mutations affected phosphorylation of the enzyme by ATP in the presence of Ca2+ or by inorganic phosphate in the absence of Ca2+. The combined experiments suggest that the reduced Ca2+ uptake observed in the Asn111 and Asn114 mutants was not due to a defect in enzyme activation by Ca2+ or in formation of the phosphorylated enzyme intermediate but rather to incompetent handling of the bound Ca2+ following ATP utilization. These results demonstrate that the acidic and amidated residues within the stalk region do not constitute the high affinity Ca2+-binding sites whose occupancy is required for enzyme activation. They may, however, act to sequester cytoplasmic Ca2+ and to channel it to domains that are involved in enzyme activation and cation translocation. Simultaneous mutation of 4 glutamate residues to alanine in the lumenal loop between transmembrane sequences M1 and M2 did not affect Ca2+ transport activity, indicating that acidic residues in this lumenal loop do not play an essential role in Ca2+ transport. Similarly, mutation of Glu192 and Asp196 in the beta-strand domain between stalk helices 2 and 3 did not affect Ca2+ transport activity, although mutation of Asp196 did diminish expression of the protein. 相似文献
9.
Kalaiyarasi Ramalingam Scott R. Eaton Wayne L. Cody Joseph A. Loo Annette M. Doherty 《Letters in Peptide Science》1994,1(2):73-79
Summary A series of phosphopeptides Tyr(PO3H2)-Val-Pro-Xxx-Leu (Xxx=Met, Met(O), Nle, Dab or Cys), derived from the native platelet-derived growth factor- receptor (PDGF-) sequence, has been prepared to study their interaction with the src-homology 2 (SH2) domains of the p85 subunit of PI3 kinase. The phosphopeptides were synthesized using Fmoc methodology incorporating N-Boc dibenzyl-protected phosphotyrosine (Boc-Tyr[PO3(Bzl)2]) as the N-terminal amino acid, since the benzyl groups can be removed during resin cleavage with TFA. Only peptides containing methionine were found to exist partially as S-benzyl sulfonium salts after TFA cleavage from the resin. The desired peptide could be obtained from the S-benzyl sulfonium salt by hydrogenolysis. 相似文献
10.
Surfactant effects on protein structure examined by electrospray ionization mass spectrometry. 总被引:1,自引:0,他引:1 下载免费PDF全文
R. R. Loo N. Dales P. C. Andrews 《Protein science : a publication of the Protein Society》1994,3(11):1975-1983
Electrospray ionization mass spectrometry (ESI-MS) has proven to be a useful tool for examining noncovalent complexes between proteins and a variety of ligands. It has also been used to distinguish between denatured and refolded forms of proteins. Surfactants are frequently employed to enhance solubilization or to modify the tertiary or quaternary structure of proteins, but are usually considered incompatible with mass spectrometry. A broad range of ionic, nonionic, and zwitterionic surfactants was examined to characterize their effects on ESI-MS and on protein structure under ESI-MS conditions. Solution conditions studied include 4% acetic acid/50% acetonitrile/46% H2O and 100% aqueous. Of the surfactants examined, the nonionic saccharides, such as n-dodecyl-beta-D-glucopyranoside, at 0.1% to 0.01% (w/v) concentrations, performed best, with limited interference from chemical background and adduct formation. Under the experimental conditions used, ESI-MS performance in the presence of surfactants was found to be unrelated to critical micelle concentration. It is demonstrated that surfactants can affect both the tertiary and quaternary structures of proteins under conditions used for ESI-MS. However, several of the surfactants caused significant shifts in the charge-state distributions, which appeared to be independent of conformational effects. These observations suggest that surfactants, used in conjunction with ESI-MS, can be useful for protein structure studies, if care is used in the interpretation of the results. 相似文献