全文获取类型
收费全文 | 3414篇 |
免费 | 420篇 |
国内免费 | 1篇 |
出版年
2021年 | 31篇 |
2017年 | 27篇 |
2016年 | 35篇 |
2015年 | 60篇 |
2014年 | 81篇 |
2013年 | 117篇 |
2012年 | 140篇 |
2011年 | 156篇 |
2010年 | 90篇 |
2009年 | 100篇 |
2008年 | 132篇 |
2007年 | 147篇 |
2006年 | 131篇 |
2005年 | 142篇 |
2004年 | 138篇 |
2003年 | 127篇 |
2002年 | 129篇 |
2001年 | 78篇 |
2000年 | 100篇 |
1999年 | 83篇 |
1998年 | 53篇 |
1997年 | 43篇 |
1996年 | 39篇 |
1995年 | 53篇 |
1994年 | 45篇 |
1993年 | 37篇 |
1992年 | 63篇 |
1991年 | 72篇 |
1990年 | 42篇 |
1989年 | 51篇 |
1988年 | 51篇 |
1987年 | 48篇 |
1986年 | 48篇 |
1985年 | 62篇 |
1984年 | 56篇 |
1983年 | 60篇 |
1982年 | 58篇 |
1981年 | 41篇 |
1980年 | 41篇 |
1979年 | 45篇 |
1978年 | 37篇 |
1977年 | 36篇 |
1976年 | 45篇 |
1975年 | 33篇 |
1974年 | 37篇 |
1973年 | 44篇 |
1971年 | 33篇 |
1970年 | 32篇 |
1969年 | 31篇 |
1968年 | 39篇 |
排序方式: 共有3835条查询结果,搜索用时 15 毫秒
61.
Murine epidermal growth factor: heterogeneity on high resolution ion-exchange chromatography 总被引:4,自引:0,他引:4 下载免费PDF全文
We have shown that epidermal growth factor (EGF) purified either by the classical method of Savage and Cohen, or solely by h.p.l.c. techniques can be resolved into two species, EGF alpha and EGF beta. However, despite the apparent purity of such materials, as determined both chromatographically and by amino acid analysis, they failed to give homogeneous products on radioiodination. Analysis by isoelectric focusing on agarose gels followed by transfer to nitrocellulose and silver staining showed that EGF alpha could be further resolved into three sub-species which focused at pH 4.6, 4.3 and 4.1. EGF beta (which also focused at pH 4.6) contained very small amounts of the species with isoelectric points of 4.1 and 4.3, probably due to slight contamination of this preparation by EGF alpha. Preparative separation of the sub-species of EGF alpha was achieved by high performance anion-exchange chromatography at pH 6.5 on a Pharmacia Mono Q column. Radioiodination of these purified sub-species did not produce significant charge heterogeneity. However, two slightly different forms of [125I]EGF alpha 1 (pH 4.6 species) were separable by anion-exchange chromatography on the Mono Q column. All of the EGF species competed for binding to EGF receptors on A431 cells and were active mitogens for BALB/c 3T3 fibroblasts. 相似文献
62.
63.
New Class of Streptomycin-Resistant Mutants Incompatible with supX Suppressor Mutations in Salmonella typhimurium 总被引:1,自引:0,他引:1
Streptomycin-resistant colonies of Salmonella typhimurium appearing in platings of supX suppressors of strain leu-500 are less variegated in size than are those derived from strain leu-500 counterparts. Several of the streptomycin-resistant leu-500 clones, furthermore, yield suppressors and revertants of the leu-500 auxotrophy at unusually low rates, suggesting that they provide a genetic background inimicable to supX suppression. Two such "suppression-restrictive" leu-500 streptomycin-resistant (str) mutants, designated strains M(1) and M(4), were characterized as to their ability to receive the trp-supX-cysB linkage region by transduction. Coentry of a donor supX deletion mutation with the selected trp(+) marker was not observed even though these sites display more than 10% linkage in control experiments. This was demonstrably the result of nonviability of the combined supX mutant, M(1) or M(4) streptomycin-resistant genotype, rather than the lack of suppression of the leu-500 imparted auxotrophy. Both M(1)- and M(4)-type resistance was accompanied by pleiotropic effects resembling those caused by strB (nonribosomal)- rather than strA (ribosomal)-type resistance, but both restrictive mutants had a high upper limit of resistance corresponding to that of strA-type mutants. Transduction analyses indicated that the str character of neither the M(1) nor the M(4) strain was linked to the strA or the strB gene. These mutations define a previously undescribed locus, which we propose to designate strC, apparently related to streptomycin uptake rather than its intracellular action. Mutation at this locus is evidently incompatible with the inactivation or removal of the supX site, suggesting a functional association between products of the genes. 相似文献
64.
Richard T. Proffitt Lloyd L. Ingraham Gunter Blankenhorn 《Biochimica et Biophysica Acta (BBA)/General Subjects》1974,362(3):534-548
The present study was undertaken to investigate flavin-nicotinamide reactions and interactions. A series of novel flavin-nicotinamide biscoenzymes have been synthesized by a general three-step procedure. The structures of these compounds were confirmed by nuclear magnetic resonance spectra, absorption spectra and elemental analysis. These compounds consist of short linear hydrocarbon chains interconnecting the N-1 of nicotinamide and the N-10 of the 7,8-dimethyl-isoalloxazine ring. The compounds were reduced with sodium dithionite (Na2S2O4) and the flavin portion was reoxidized with ferricyanide. Re-reduction of the flavin portion by the nicotinamide portion of the molecule was followed anaerobically at 442 nm. When the interconnecting hydrocarbon chain was unsaturated, a second order reaction was observed with a rate equal to that of lumiflavin and 1-propyl-1,4-dihydronicotinamide (NprNicH2) under the same conditions. When the two halves of the biscoenzymes were connected by saturated three- and four-carbon chains, the expected unimolecular reaction was not observed. Instead, the reduced biscoenzyme, after separation from excess sodium dithionite, was shown to have a strong absorption at 298 nm. This absorption is characteristic of hydration of dihydronicotinamides at the 5,6-double bond.In further studies, the C3-biscoenzyme exhibited an absorption at 600 nm due to a complex between the reduced flavin and oxidized nicotinamide portions of the molecule. Absorbance at 600 nm increased linearly with the C3-biscoenzyme concentration, clearly indicating that this is an intramolecular complex. When the C3-biscoenzyme was at 0°C in 60–75% dimethylformamide buffer solution, no absorption at 600 nm was observed. When excess dithionite was removed, the spectrum under these conditions showed definite peaks at 297 and 357 nm. These respective peaks were attributed to hydrated dihydronicotinamide and dihydronicotinamide species present in the reaction mixture.The reduced flavin was postulated to be a catalyst for the hydration of dihydronicotinamide. This hypothesis was tested by incubating 1-propyl-1,4-dihydronicotinamide alone and with several concentrations of reduced riboflavin under basic anaerobic conditions. The results show that the reduced flavin increases the rate of disappearance of the dihydronicotinamide species and that the product shows an absorption near 298 nm. These results indicate that a reduced form of the flavin nucleus catalyzes the hydration of dihydronicotinamides. 相似文献
65.
We present absorption and circular dichroism (CD) spectra for the synthetic polymers poly d(AAT):d(AAT) and poly r(AAU):r(AAU), in both native and heat-denatured forms. As a means of evaluating the first-neighbor hypothesis, the CD spectra are compared with approximations derived from spectra of other synthetic polymers containing the same first-neighbor sequences. This is the first instance where such a comparison has been possible using spectra of double-stranded RNA sequences, and the agreement between the measured and approximated spectra for poly r(AAU):r(AUU) is surprisingly good. We have also subjected the CD spectrum of poly d(AAT):d(AAT) to a previously published analytical procedure for obtaining estimates of first-neighbor frequencies. In this first independent test of the procedure, we find that the analysis does infer the existence of a majority (86%) of AA, TT, AT, and TA first neighbors but does not precisely indicate their relative proportions. 相似文献
66.
67.
Lloyd Fallers 《American anthropologist》1968,70(6):1198-1199
68.
Sedimentation studies on the interaction of proflavine with deoxyribonucleic acid 总被引:2,自引:1,他引:1 下载免费PDF全文
A method is described of using photography to measure the concentrations of a small ligand (proflavine) above and below the boundary of a macromolecule (DNA, both native and denatured) sedimenting in the ultracentrifuge. The measurements are used to determine the extent of the binding of proflavine to DNA, and the results compared with those obtained by a spectrophotometric method. The results obtained by the two methods agree within 10%, thus validating the spectrophotometric technique under these conditions. The variation of the sedimentation coefficient with the extent of binding of proflavine was also studied. The results are discussed in relation to previously observed changes in the viscosity of the solutions. 相似文献
69.
Measurement of the weight of individual virus particles from untreated and antibody-treated populations was made by quantitative electron microscopy. The weight of antibody bound depended on the concentration of antibody in solution. One population of viruses exposed to an antibody concentration which resulted in 95% inhibition of hemagglutination showed a mass increase of 55%, corresponding to an absolute increase of 9.0 x 10-17 g in the median value. Another population, whose hemagglutination inhibition assay was 64%, showed a 39% increase in mass corresponding to an absolute median increase of 7.3 x 10-17 g. The larger viruses in each population bound a greater absolute amount of antibody than did the smaller ones, but the latter bound relatively more antibody in proportion to their mass. No cross-reactivity was found between the antibody to influenza A/PR8 and the influenza strain B/LEE. Influenza A/PR8 controls exposed to nonspecific gamma-globulin displayed a significant weight loss, at least in part owing to loss from the core, as judged from the electron micrographs. 相似文献
70.
D Lloyd 《Experimental cell research》1967,45(1):120-132