全文获取类型
收费全文 | 38762篇 |
免费 | 2993篇 |
国内免费 | 2873篇 |
出版年
2024年 | 65篇 |
2023年 | 440篇 |
2022年 | 1054篇 |
2021年 | 2149篇 |
2020年 | 1356篇 |
2019年 | 1715篇 |
2018年 | 1724篇 |
2017年 | 1176篇 |
2016年 | 1644篇 |
2015年 | 2399篇 |
2014年 | 2828篇 |
2013年 | 3074篇 |
2012年 | 3575篇 |
2011年 | 3164篇 |
2010年 | 1987篇 |
2009年 | 1614篇 |
2008年 | 1968篇 |
2007年 | 1718篇 |
2006年 | 1586篇 |
2005年 | 1285篇 |
2004年 | 1052篇 |
2003年 | 909篇 |
2002年 | 758篇 |
2001年 | 664篇 |
2000年 | 588篇 |
1999年 | 628篇 |
1998年 | 351篇 |
1997年 | 364篇 |
1996年 | 344篇 |
1995年 | 316篇 |
1994年 | 332篇 |
1993年 | 263篇 |
1992年 | 311篇 |
1991年 | 242篇 |
1990年 | 213篇 |
1989年 | 189篇 |
1988年 | 127篇 |
1987年 | 101篇 |
1986年 | 92篇 |
1985年 | 86篇 |
1984年 | 59篇 |
1983年 | 53篇 |
1982年 | 34篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1976年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
851.
Xiaolin Pei Hongyu Zhang Lijun Meng Gang Xu Lirong Yang Jianping Wu 《Process Biochemistry》2013,48(12):1921-1927
A nitrile hydratase (NHase) gene from Aurantimonas manganoxydans was cloned and expressed in Escherichia coli BL21 (DE3). A downstream gene adjacent to the β-subunit was necessary for the functional expression of the recombinant NHase. The structural gene order of the Co-type NHase was α-subunit beyond β-subunit, different from the order typically reported for Co-type NHase genes. The NHase exhibited adequate thermal stability, with a half-life of 1.5 h at 50 °C. The NHase efficiently hydrated 3-cyanopyridine to produce nicotinamide. In a 1-L reaction mixture, 3.6 mol of 3-cyanopyridine was completely converted to nicotinamide in four feedings, exhibiting a productivity of 187 g nicotinamide/g dry cell weight/h. An industrial auto-induction medium was applied to produce the recombinant NHase in 10-L fermenter. A glycerol-limited feeding method was performed, and a final activity of 2170 U/mL culture was achieved. These results suggested that the recombinant NHase was efficiently cloned and produced in E. coli. 相似文献
852.
Meijuan Xu Rongzhen Zhang Xiangyu Liu Jinsong Shi Zhenghong Xu Zhiming Rao 《Process Biochemistry》2013,48(8):1166-1173
β-Mannanase can randomly hydrolyze the (1→4)-β-d-mannosidic linkages in mannans, galactomannans and glucomannans, yielding manno-oligosaccharides. In this study, the β-mannanase (MAN) from Bacillus subtilis B10-02 was overexpressed successfully in B. subtilis 168 as a hexa-histidine tagged, secreted protein. The recombinant enzyme BsMAN6H was not stable under acidic conditions, which restricts its use in food and feed industry. We aimed to improve the acid stability of BsMAN6H by changing several surface-exposed amino acid residues to acidic or neutral ones. Among the mutations, the His54Asp resulted in a shift in the optimal pH from 6.5 to 5.5. Accordingly, the acid stability was improved by a factor of a negative potential on the structure surface around the mutated site. Furthermore, the H54D variant showed the enzyme activity up to 3207.82 U/mL in bioreactors using the cheap Kojac powder as substrate. As a result, a bacterial β-mannanase was produced efficiently with increased acid stability, improving its applicability in the animal feed industry. 相似文献
853.
Cloud computing is an emerging computing paradigm in which IT resources and capacities are provided as services over the Internet. Promising as it is, this paradigm also brings forth new challenges for security when users want to securely outsource the computation of cryptographic operations to the untrusted cloud servers. As we know, modular exponentiation is one of the basic operations among most of current cryptosystems. In this paper, we present the generic secure outsourcing schemes enabling users to securely outsource the computations of exponentiations to the untrusted cloud servers. With our techniques, a batch of exponentiations (e.g. t exponentiations) can be efficiently computed by the user with only O(n+t) multiplications, where n is the number of bits of the exponent. Compared with the state-of-the-art algorithm, the proposed schemes are superior in both efficiency and verifiability. Furthermore, there are not any complicated pre-computations on the user side. Finally, the schemes are proved to be secure under the Subset Sum Problem. 相似文献
854.
Cloud computing provides many kinds of application services for cloud users, but security problems have caused great impact on Software as a Service (SaaS). As a commercial model, SaaS is related among different participants who could be malicious or dishonest. This paper presents a Software Service Signature (S3) to deal with several security issues in SaaS and keep the interests and rights of all participants in safety. Our design is based on ID-based proxy signatures from pairings. The analysis shows that the proposed scheme can effectively strengthen the security through authentication in cloud computing. 相似文献
855.
Zheyong Huang Yunli Shen Hongmin Zhu Jianfeng Xu Yanan Song Xinying Hu Zhang Shuning Xiangdong Yang Aijun Sun Juying Qian Junbo Ge 《Experimental Animals》2013,62(3):197-203
Cell delivery via the retrograde coronary route boasts less vessel embolism, myocardial
injury, and arrhythmogenicity when compared with those via antegrade coronary
administration or myocardial injection. However, conventional insertion into the coronary
sinus and consequent bleeding complication prevent its application in small animals. To
overcome the complication of bleeding, we described a modified coronary retroinfusion
technique via the jugular vein route in rats with myocardial infarction (MI). A flexible
wire with a bent end was inserted into the left internal jugular vein and advanced slowly
along the left superior vena cava. Under direct vision, the wire was run into the left
cardiac vein by rotating the wire and changing the position of its tip. A fine tube was
then advanced along the wire to the left cardiac vein. This modified technique showed less
lethal hemorrhage than the conventional technique. Retroinfusion via transjugular catheter
enabled efficient fluid or cell dissemination to the majority areas of the free wall of
the left ventricle, covering the infarcted anterior wall. In conclusion, transjugular
cardiac vein catheterization may make retrocoronary infusion a more safe and practical
route for delivering cell, drug, and gene therapy into the infarcted myocardium of
rats. 相似文献
856.
Grassland productivity in response to climate change and land use is a global concern. In order to explore the effects of climate change and land use on net primary productivity (NPP), NPP partitioning [fBNPP, defined as the fraction of belowground NPP (BNPP) to NPP], and rain‐use efficiency (RUE) of NPP, we conducted a field experiment with warming (+3 °C), altered precipitation (double and half), and annual clipping in a mixed‐grass prairie in Oklahoma, USA since July, 2009. Across the years, warming significantly increased BNPP, fBNPP, and RUEBNPP by an average of 11.6%, 2.8%, and 6.6%, respectively. This indicates that BNPP was more sensitive to warming than aboveground NPP (ANPP) since warming did not change ANPP and RUEANPP much. Double precipitation stimulated ANPP, BNPP, and NPP but suppressed RUEANPP, RUEBNPP, and RUENPP while half precipitation decreased ANPP, BNPP, and NPP but increased RUEANPP, RUEBNPP, and RUENPP. Clipping interacted with altered precipitation in impacting RUEANPP, RUEBNPP, and RUENPP, suggesting land use could confound the effects of precipitation changes on ecosystem processes. Soil moisture was found to be a main factor in regulating variation in ANPP, BNPP, and NPP while soil temperature was the dominant factor influencing fBNPP. These findings suggest that BNPP is critical point to future research. Additionally, results from single‐factor manipulative experiments should be treated with caution due to the non‐additive interactive effects of warming with altered precipitation and land use (clipping). 相似文献
857.
Little is known about the relevance of genetic polymorphisms to arsenic-related bladder cancer. A preliminary case-control study was conducted to explore the association between genetic polymorphisms of GSTT1, p53 codon 72 and bladder cancer in southern Taiwan, a former high arsenic exposure area. Fifty-nine urinary transitional cell carcinoma (TCC) patients from a referral centre in south-western Taiwan and 81 community controls matched on residence were recruited from 1996 to 1999. A questionnaire was administered to obtain arsenic exposure and general health information. Genotypes of p53 codon 72 and GSTT1 were analysed by polymerase chain reaction-restriction fragment length polymerase. The combined variant genotypes (heterozygous or homozygous variant) of p53 codon 72 and GSTT1 null were observed in 29% of cases and in 44% of controls, respectively. In this preliminary study, bladder cancer risk was slightly elevated for subjects carrying the variant genotype of p53 codon 72 or in subjects carrying the GSTT1 null genotype. Variants in p53 codon 72 increased the risk of bladder cancer among smokers. However, the results were not statistically significant and larger confirmatory studies are needed to clarify the role of candidate gene polymorphisms and bladder cancer risk in arsenic exposed populations. 相似文献
858.
Xiang-Yang Ge Yan Xu Xiang Chen 《Journal of industrial microbiology & biotechnology》2013,40(3-4):345-352
This study describes a novel strategy to improve the glycolysis flux of Saccharomyces cerevisiae at high temperature. The TSL1 gene-encoding regulatory subunit of the trehalose synthase complex was overexpressed in S. cerevisiae Z-06, which increased levels of trehalose synthase activity in extracts, enhanced stress tolerance and glucose consuming rate of the yeast cells. As a consequence, the final ethanol concentration of 185.5 g/L was obtained at 38 °C for 36 h (with productivity up to 5.2 g/L/h) in 7-L fermentor, and the ethanol productivity was 92.7 % higher than that of the parent strain. The results presented here provide a novel way to enhance the carbon metabolic flux at high temperature, which will be available for the purposes of producing other primary metabolites of commercial interest using S. cerevisiae as a host. 相似文献
859.
Dayu Yu Fuchao Xu Jonathan Valiente Siyuan Wang Jixun Zhan 《Journal of industrial microbiology & biotechnology》2013,40(1):159-168
A putative indigoidine biosynthetic gene cluster was located in the genome of Streptomyces chromofuscus ATCC 49982. The silent 9.4-kb gene cluster consists of five open reading frames, named orf1, Sc-indC, Sc-indA, Sc-indB, and orf2, respectively. Sc-IndC was functionally characterized as an indigoidine synthase through heterologous expression of the enzyme in both Streptomyces coelicolor CH999 and Escherichia coli BAP1. The yield of indigoidine in E. coli BAP1 reached 2.78 g/l under the optimized conditions. The predicted protein product of Sc-indB is unusual and much larger than any other reported IndB-like protein. The N-terminal portion of this enzyme resembles IdgB and the C-terminal portion is a hypothetical protein. Sc-IndA and/or Sc-IndB were co-expressed with Sc-IndC in E. coli BAP1, which demonstrated the involvement of Sc-IndB, but not Sc-IndA, in the biosynthetic pathway of indigoidine. The yield of indigoidine was dramatically increased by 41.4 % (3.93 g/l) when Sc-IndB was co-expressed with Sc-IndC in E. coli BAP1. Indigoidine is more stable at low temperatures. 相似文献
860.
Chong Sha Xiao-Wei Yu Meng Zhang Yan Xu 《Journal of industrial microbiology & biotechnology》2013,40(11):1241-1249
The lipase r27RCL from Rhizopus chinensis CCTCC M201021 was heterologously expressed in Pichia pastoris GS115 by simultaneous co-expression with two secretion factors ERO1p and PDI involved in the endoplasmic reticulum (ER). Compared to the expression of the lipase alone (12,500 U/ml), co-expression with these two proteins resulted in the production of larger total quantities of enzymes. The largest increase was seen when the combined ERO1p/PDI system was co-expressed, resulting in approximately 30 % higher enzyme yields (16,200 U/ml) than in the absence of co-expressed secretion factors. The extracellular protein concentration of the recombinant strain Co XY RCL-5 reached 9.39 g/l in the 7-l fermentor. Simultaneously, the fermentation time was also shortened by about 8 h compared to that of the control. The substrate-specific consumption rate (Qs) and the product-specific production rate (Qp) were both investigated in this research. In conclusion, the space–time yield was improved by co-expression with ERO1p and PDI. This is a potential strategy for high level expression of other heterologous proteins in P. pastoris. 相似文献