全文获取类型
收费全文 | 18947篇 |
免费 | 1556篇 |
国内免费 | 147篇 |
出版年
2023年 | 56篇 |
2022年 | 87篇 |
2021年 | 428篇 |
2020年 | 306篇 |
2019年 | 367篇 |
2018年 | 539篇 |
2017年 | 405篇 |
2016年 | 705篇 |
2015年 | 1099篇 |
2014年 | 1249篇 |
2013年 | 1317篇 |
2012年 | 1727篇 |
2011年 | 1591篇 |
2010年 | 991篇 |
2009年 | 836篇 |
2008年 | 1191篇 |
2007年 | 1036篇 |
2006年 | 930篇 |
2005年 | 861篇 |
2004年 | 784篇 |
2003年 | 611篇 |
2002年 | 560篇 |
2001年 | 426篇 |
2000年 | 355篇 |
1999年 | 269篇 |
1998年 | 123篇 |
1997年 | 96篇 |
1996年 | 86篇 |
1995年 | 91篇 |
1994年 | 84篇 |
1993年 | 64篇 |
1992年 | 124篇 |
1991年 | 100篇 |
1990年 | 110篇 |
1989年 | 90篇 |
1988年 | 75篇 |
1987年 | 77篇 |
1986年 | 57篇 |
1985年 | 66篇 |
1984年 | 52篇 |
1983年 | 48篇 |
1982年 | 43篇 |
1981年 | 46篇 |
1979年 | 42篇 |
1978年 | 37篇 |
1977年 | 37篇 |
1976年 | 38篇 |
1974年 | 37篇 |
1973年 | 33篇 |
1971年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
91.
The structure of the capsular polysaccharide from Klebsiella K79 was determined by the techniques of methylation, periodate oxidation, beta-elimination, chromic acid oxidation, and partial hydrolysis. N.m.r. spectroscopy (1H and 13C) was used extensively to establish the nature of the anomeric linkages of the polysaccharide and of oligosaccharides derived through degradative procedures. The polysaccharide was found to have the heptasaccharide, "5 + 2" repeating unit: (Formula: see text). 相似文献
92.
G-11 staining in Turner's syndrome with mos 45,X/46,X,r(?) 总被引:2,自引:0,他引:2
J C de Almeida J C Llerena D Molina Gomes R Rita Martins M Jung D F Reis A G Cunha 《Annales de génétique》1985,28(1):37-41
Mos 45,X/46,X,r(?) in 4 patients with Turner's syndrome and no signs of virilization, and in one pair of monozygotic twins, one of them with clitoral hypertrophy, was studied using combined cytogenetic techniques and specially G-11 staining for the characterization of the X or Y origin of the rings. In all 6 patients the ring was G-11 positive, attesting its Y origin. Both twins were operated and bilateral streak gonads with a bilateral nodule of testicular tissue were found. Similar small rings were also studied in one patient with mos 46,XX/46,X,r(X) and in one nonvirilized Turner's syndrome patient with a larger ring; in these two cases the ring was G-11 negative. It seems that the small rings occasionally found in Turner's syndrome are more frequently from Y origin and therefore prophylactic gonadectomy should be considered. 相似文献
93.
Survival of Bacteria and Fungi in Relation to Water Activity and the Solvent Properties of Water in Biopolymer Gels 总被引:9,自引:3,他引:6 下载免费PDF全文
Survival of bacteria (Rhizobium, Agrobacterium, and Arthrobacter spp.), fungal spores (Penicillium sp.), and yeasts (Saccharomyces sp.) was studied in relation to water activity (aw) and the presence of nutritive solutes. The cells were entrapped in polysaccharide gels, as is done to immobilize cells or enzymes, and then dehydrated. The number of living cells (1010 g of dry polymer−1) remained constant for periods of storage of >3 years at 28°C when the inocula were kept at an aw of <0.069. At aw values between 0.069 and 0.83 the number of survivors diminished more and more rapidly as the aw was raised. For a given aw and organism, there were large differences in survival rate as a function of the nutritive solutes used to culture the microorganisms. Low-molecular-weight compounds (with three or five carbon atoms) had a deleterious effect on survival, whereas compounds of higher molecular weight (C6 to C12) had a protecting effect. Thus, the aw alone was not a sufficient explanation for the deterioration of the inocula. Survival seemed to be more directly related to some properties of the water in the biopolymer. New concepts such as the discontinuity of properties of water and the point of mobilization of solutes, already proposed by Duckworth and Kelly (J. Food Technol. 8:105-113, 1973) and Seow (J. Sci. Food Agric., 26:535-536, 1975), have been taken into consideration to explain the interactions of water with the biopolymer and their specific effects on the microorganisms. 相似文献
94.
Z Damuni H Y Lim Tung L J Reed 《Biochemical and biophysical research communications》1985,133(3):878-883
A potent, heat-stable protein inhibitor of branched-chain alpha-keto acid dehydrogenase (BCKDH) phosphatase has been identified and purified to near homogeneity from bovine kidney mitochondria (Damuni, Z., Humphreys, J. S., and Reed, L. J., Proc. Natl. Acad. Sci. U.S.A., in press). This protein is a noncompetitive inhibitor of BCKDH phosphatase, with a Ki about 0.13 nM. By contrast, this protein inhibitor did not affect the activity of the cytosolic protein phosphatase-1 and phosphatase-2A or the mitochondrial pyruvate dehydrogenase (PDH) phosphatase at concentrations up to 10 nM. The cytosolic protein phosphatase inhibitor-1 and inhibitor-2 had no effect on the activity of BCKDH phosphatase or PDH phosphatase at concentrations up to 50 and 300 nM respectively. These results, together with previous evidence, demonstrate that BCKDH phosphatase and its inhibitor protein are distinct from the cytosolic protein phosphatase-1 and phosphatase-2A and from protein phosphatase inhibitor-1 and inhibitor-2, respectively. 相似文献
95.
R Zauner J Christner G Jung U Borchart W Machleidt A Videira S Werner 《European journal of biochemistry》1985,150(3):447-454
Two peptides, potentially representing antigenic determinants of a proposed gene product, were synthesized. The peptide sequences were deduced from the nucleotide sequence of the unidentified reading frame (URF)1 of the Neurospora crassa mitochondrial genome. Specific antisera to the synthetic peptides were produced. The antibodies recognized a single polypeptide species with an apparent relative molecular mass of about 30 000. The mitochondrial origin of this polypeptide was verified by in vivo labelling experiments in the presence of cycloheximide, as well as by in vitro translation using isolated mitochondria. The chemical identification of the protein was performed by partial radiosequencing of the N-terminal portion of the immunoprecipitated URF-1 product. The amount of URF-1 polypeptide present in N. crassa mitochondria is in the range of 1-2%. The protein is a constituent of the inner envelope of the organelle and probably part of a more complex membrane unit. 相似文献
96.
97.
The plant growth retarding activities of several dioxanylalkyl and dioxanylalkenyl triazoles were determined in seedlings of barley, rice, and oilseed rape. Out of these groups some substances proved to be among the most efficient growth retardants known. The compound 1-(4-trifluormethyl)-2-(1,2,4-triazolyl-(1))-3-(5-methyl-1,3-dioxan-5-yl)-propen-3-ol was investigated more closely. Shoot growth is reduced more intensively than root growth by this compound. At lower dosages root growth may even be stimulated. The action of this retardant can be antagonized by gibberellin A3 and byent-kaurenoic acid. It is suggested that its main biochemical action is to block the reactions that lead froment-kaurene toent-kaurenoic acid in the course of gibberellin biosynthesis. 相似文献
98.
Concordance of experimentally mapped or predicted Z-DNA sites with positions of selected alternating purine-pyrimidine tracts. 总被引:2,自引:0,他引:2 下载免费PDF全文
The recent electronmicroscopic and biochemical mapping of Z-DNA sites in phi X174, SV40, pBR322 and PM2 DNAs has been used to determine two sets of criteria for identification of potential Z-DNA sequences in natural DNA genomes. The prediction of potential Z-DNA tracts and corresponding statistical analysis of their occurrence have been made on a sample of 14 DNA genomes. Alternating purine and pyrimidine tracts longer than 5 base pairs in length and their clusters (quasi alternating fragments) in the 14 genomes studied are under-represented compared to the expectation from corresponding random sequences. The fragments [d(G X C)]n and [d(C X G)]n (n greater than or equal to 3) in general do not occur in circular DNA genomes and are under-represented in the linear DNAs of phages lambda and T7, whereas in linear genomes of adenoviruses they are strongly over-represented. With minor exceptions, potential Z-DNA sites are also under-represented compared to random sequences. In the 14 genomes studied, predicted Z-DNA tracts occur in non-coding as well as in protein coding regions. The predicted Z-DNA sites in phi X174, SV40, pBR322 and PM2 correspond well with those mapped experimentally. A complete listing together with a compact graphical representation of alternating purine-pyrimidine fragments and their Z-forming potential are presented. 相似文献
99.
Sulfmyoglobin. Resonance Raman spectroscopic evidence for an iron-chlorin prosthetic group 总被引:1,自引:0,他引:1
L A Andersson T M Loehr A R Lim A G Mauk 《The Journal of biological chemistry》1984,259(24):15340-15349
The green heme protein sulfmyoglobin (SMb) has been suggested to contain a sulfur-modified iron chlorin prosthetic group. To evaluate this hypothesis, we have obtained high-frequency (greater than 1000 cm-1) resonance Raman spectra of both oxidized and reduced SMb with 457.9-, 488.0-, 514.5-, 568.2-, and 647.1-nm excitation. The SMb spectra are compared to those of native met- and deoxymyoglobin (Mb). Vibrational frequencies for SMb are generally similar to those of Mb, suggesting a high-spin state for both the Fe(III) and Fe(II) SMb species, as is typical of native Mb. However, major differences between SMb and Mb occur both for patterns of relative spectral intensities and for depolarization ratios. In particular, all B1g-depolarized porphyrin modes in the Mb spectra have become polarized, totally symmetric vibrational modes in the SMb spectra. These contrasts reflect a dramatic lowering of the effective symmetry for the SMb prosthetic group. Several new bands are observed in SMb spectra that are not present in spectra of either native Mb or iron protoporphyrin IX complexes. The observation of additional polarized bands flanking the oxidation state marker, V4, is of particular interest. In a parallel study, we compared the resonance Raman spectral properties of iron protoporphyrin IX-derived chlorins and metallo-octaethylchlorins with those of the analogous porphyrins: the chlorin spectra exhibited altered intensity patterns, an increased number of totally symmetric (polarized) vibrational bands, and several new vibrational bands, including one or two in the region of the oxidation state marker, V4. Thus, the resonance Raman spectral characteristics of SMb and metallo-chlorins are complementary and strongly support a chlorin prosthetic group for SMb. Furthermore, they establish testable criteria for investigating the prosthetic group structures of other green heme proteins by resonance Raman spectroscopy. 相似文献
100.
Summary The sodium-dependentl-alanine transport across the plasma membrane of oocytes ofXenopus laevis was studied by means of [14C]-l-alanine,22Na+ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 21 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 21. The sodium-l-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess ofd-alanine overl-alanine caused about 60% inhibition. 相似文献