全文获取类型
收费全文 | 961篇 |
免费 | 80篇 |
国内免费 | 35篇 |
出版年
2023年 | 9篇 |
2022年 | 6篇 |
2021年 | 29篇 |
2020年 | 26篇 |
2019年 | 28篇 |
2018年 | 37篇 |
2017年 | 29篇 |
2016年 | 35篇 |
2015年 | 23篇 |
2014年 | 45篇 |
2013年 | 53篇 |
2012年 | 35篇 |
2011年 | 53篇 |
2010年 | 47篇 |
2009年 | 63篇 |
2008年 | 57篇 |
2007年 | 71篇 |
2006年 | 48篇 |
2005年 | 30篇 |
2004年 | 24篇 |
2003年 | 23篇 |
2002年 | 24篇 |
2001年 | 24篇 |
2000年 | 13篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 15篇 |
1996年 | 8篇 |
1995年 | 12篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有1076条查询结果,搜索用时 31 毫秒
91.
Cotton S Fowler K Pomiankowski A 《Evolution; international journal of organic evolution》2004,58(5):1038-1046
We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection. 相似文献
92.
93.
BRUNO STUDER FRANCO WIDMER JÜRG ENKERLI ROLAND K
LLIKER 《Molecular ecology resources》2006,6(4):1108-1110
Twelve microsatellite markers were isolated from Lolium multiflorum. Allelic variability and cross‐species amplification were assessed on 16 individuals of each of the three grassland species L. multiflorum, Lolium perenne and Festuca pratensis. Cross‐species amplification success was 100% for L. perenne and 83% for F. pratensis. The number of alleles detected ranged from one to 14 with an average of 3.4. While three microsatellite loci were polymorphic in all three species, one marker produced species‐specific alleles in all three species. These microsatellite markers provide a valuable tool for population genetic studies within and among species of the Festuca–Lolium complex. 相似文献
94.
The purpose of the present investigation was to increase the solubility and dissolution rate of rofecoxib by the preparation
of its solid dispersion with polyvinyl pyrrolidone K30 (PVP K30) using solvent evaporation method. Drug-polymer interactions
were investigated using differential scanning calorimetry (DSC), x-ray diffraction (XRD), and Fourier transform infrared spectroscopy
(FTIR). For the preparation of rofecoxib mouth dissolve tablets, its 1∶9 solid dispersion with PVP K30 was used with various
disintegrants and sublimable materials. In an attempt to construct a statistical model for the prediction of disintegration
time and percentage friability, a 32 randomized full and reduced factorial design was used to optimize the influence of the amounts of superdisintegrant and subliming
agent. The obtained results showed that dispersion of the drug in the polymer considerably enhanced the dissolution rate.
The drug-to-carrier ratio was the controlling factor for dissolution improvement. FTIR spectra revealed no chemical incompatibility
between the drug and PVP K30. As indicated from XRD and DSC data, rofecoxib was in the amorphous form, which explains the
better dissolution rate of the drug from its solid dispersions. Concerning the optimization study, the multiple regression
analysis revealed that an optimum concentration of camphor and a higher percentage of crospovidone are required for obtaining
rapidly disintegrating tablets. In conclusion, this investigation demonstrated the potential of experimental design in understanding
the effect of the formulation variables on the quality of mouth dissolve tablets containing solid dispersion of a hydrophobic
drug. 相似文献
95.
This article investigates enhancement of the dissolution profile of valdecoxib using solid dispersion with PVP. The article
also describes the preparation of fast-dissolving tablets of valdecoxib by using a high amount of superdisintegrants. A phase
solubility method was used to evaluate the effect of various water-soluble polymers on aqueous solubility of valdecoxib. Polyvinyl
pyrrolidone (PVP K-30) was selected and solid dispersions were prepared by the method of kneading. Dissolution studies, using
the USP paddle method were performed for solid dispersions of valdecoxib. Infrared (IR) spectroscopy, differential scanning
calorimetry (DSC), and x-ray diffractometry (XRD) were performed to identify the physicochemical interaction between drug
and carrier, hence its effect on dissolution. Tablets were formulated containing solid dispersion products and compared with
commercial products. IR spectroscopy, XRD, and DSC showed no change in the crystal structure of valdecoxib. Dissolution of
valdecoxib improved significantly in solid dispersion products (<85% in 5 minutes). Tablets containing solid dispersion exhibited
better dissolution profile than commercial tablets. Thus, the solid dispersion technique can be successfully used for improvement
of dissolution of valdecoxib.
Published: August 18, 2006 相似文献
96.
Tapan Biswas Olga E. Zolova Felipe Lombó Jose A. Salas Sylvie Garneau-Tsodikova 《Journal of molecular biology》2010,397(2):495-2570
Thiocoraline is a thiodepsipeptide with potent antitumor activity. TioX, a protein with an unidentified function, is encoded by a gene of the thiocoraline biosynthetic gene cluster. The crystal structure of the full-length TioX protein at 2.15 Å resolution reveals that TioX protomer shares an ancient βαβββ fold motif with glyoxalase I and bleomycin resistance protein families, despite a very low sequence homology. Intriguingly, four TioX monomers form a unique 2-fold symmetric tetrameric assembly that is stabilized by four intermolecular disulfide bonds formed cyclically between Cys60 and Cys66 of adjacent monomers. The arrangement of two of the four monomers in the TioX tetramer is analogous to that in dimeric bleomycin resistance proteins. This analogy indicates that this novel higher-order structural scaffold of TioX may have evolved to bind thiocoraline. Our equilibrium titration studies demonstrate the binding of a thiocoraline chromophore analog, quinaldic acid, to TioX, thereby substantiating this model. Furthermore, a strain of Streptomyces albus containing an exogenous thiocoraline gene cluster devoid of functional tioX maintains thiocoraline production, albeit with a lower yield. Taken together, these observations rule out a direct enzymatic function of TioX and suggest that TioX is involved in thiocoraline resistance or secretion. 相似文献
97.
98.
Yu Hirano Makoto Higuchi Hirozo Oh-oka Zheng-Yu Wang 《Journal of molecular biology》2010,397(5):1175-1187
In green sulfur photosynthetic bacteria, the cytochrome cz (cyt cz) subunit in the reaction center complex mediates electron transfer mainly from menaquinol/cytochrome c oxidoreductase to the special pair (P840) of the reaction center. The cyt cz subunit consists of an N-terminal transmembrane domain and a C-terminal soluble domain that binds a single heme group. The periplasmic soluble domain has been proposed to be highly mobile and to fluctuate between oxidoreductase and P840 during photosynthetic electron transfer. We have determined the crystal structure of the oxidized form of the C-terminal functional domain of the cyt cz subunit (C-cyt cz) from thermophilic green sulfur bacterium Chlorobium tepidum at 1.3-Å resolution. The overall fold of C-cyt cz consists of four α-helices and is similar to that of class I cytochrome c proteins despite the low similarity in their amino acid sequences. The N-terminal structure of C-cyt cz supports the swinging mechanism previously proposed in relation with electron transfer, and the surface properties provide useful information on possible interaction sites with its electron transfer partners. Several characteristic features are observed for the heme environment: These include orientation of the axial ligands with respect to the heme plane, surface-exposed area of the heme, positions of water molecules, and hydrogen-bond network involving heme propionate groups. These structural features are essential for elucidating the mechanism for regulating the redox state of cyt cz. 相似文献
99.
Mohammed Ali A. Shah 《Experimental parasitology》2010,124(3):330-333
The study describes cross protection experiments with chimeric DNA vaccine pVAX1-cSZ2-IL-2 to determine its efficacy against four important Eimeria species. Seven-day-old chickens were randomly divided into nine groups; group 1 negative control, groups 2, 3, 4, 5 positive controls; and groups 6, 7, 8 and 9 experimental groups. On days 7 and 14, groups 1-5 were injected with TE buffer, and groups 6-9 with the vaccine. At 21 days of age, all chickens were inoculated with 5 × 104 sporulated oocysts except for the negative control. Groups 2 and 6 were inoculated with Eimeria tenella, groups 3 and 7 with Eimerianecatrix, groups 4 and 8 with Eimeria acervulina and groups 5 and 9 with Eimeria maxima. Seven days later, all chickens were weighed and slaughtered to obtain intestinal samples. Efficacy of immunization was evaluated on the basis of oocyst decrease ratio, lesion score, body-weight gain and anti-coccidial index. The results indicated that the recombinant plasmid can induce host immune responses by alleviating intestinal lesions, body weight loss and oocyst ratio and imparting good protection against E. tenella and E.acervulina, medium protection against E. necatrix but little effect against E. maxima. It is concluded that the conserved antigen can provide cross protection and should be explored further. 相似文献
100.
Saccharophagus degradans belongs to a recently discovered group of marine bacteria equipped with an arsenal of sugar cleaving enzymes coupled to carbohydrate-binding domains to degrade various insoluble complex polysaccharides. The modular Sde-1182 protein consists of a family 2 carbohydrate binding module linked to a X158 domain of unknown function. The 1.9 Å and 1.55 Å resolution crystal structures of the isolated X158 domain bound to the two related polyisoprenoid molecules, ubiquinone and octaprenyl pyrophosphate, unveil a β-barrel architecture reminiscent of the YceI-like superfamily that resembles the architecture of the lipocalin fold. This unprecedented association coupling oxidoreduction and carbohydrate recognition events may have implications for effective nutrient uptake in the marine environment. 相似文献