全文获取类型
收费全文 | 649310篇 |
免费 | 75105篇 |
国内免费 | 352篇 |
专业分类
724767篇 |
出版年
2016年 | 7051篇 |
2015年 | 9739篇 |
2014年 | 11343篇 |
2013年 | 16707篇 |
2012年 | 18666篇 |
2011年 | 18827篇 |
2010年 | 12693篇 |
2009年 | 11901篇 |
2008年 | 17009篇 |
2007年 | 17434篇 |
2006年 | 16670篇 |
2005年 | 16083篇 |
2004年 | 15748篇 |
2003年 | 15573篇 |
2002年 | 15041篇 |
2001年 | 27304篇 |
2000年 | 27529篇 |
1999年 | 22424篇 |
1998年 | 8345篇 |
1997年 | 8700篇 |
1996年 | 8478篇 |
1995年 | 7767篇 |
1994年 | 7929篇 |
1993年 | 7770篇 |
1992年 | 19347篇 |
1991年 | 18567篇 |
1990年 | 18438篇 |
1989年 | 18306篇 |
1988年 | 16897篇 |
1987年 | 16349篇 |
1986年 | 15152篇 |
1985年 | 15324篇 |
1984年 | 12621篇 |
1983年 | 11211篇 |
1982年 | 8690篇 |
1981年 | 7983篇 |
1980年 | 7541篇 |
1979年 | 12551篇 |
1978年 | 9849篇 |
1977年 | 9083篇 |
1976年 | 8754篇 |
1975年 | 9450篇 |
1974年 | 10142篇 |
1973年 | 9960篇 |
1972年 | 9179篇 |
1971年 | 8184篇 |
1970年 | 7240篇 |
1969年 | 6989篇 |
1968年 | 6307篇 |
1967年 | 5549篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
23.
P C de Visser N M A J Kriek P A V van Hooft A Van Schepdael D V Filippov G A van der Marel H S Overkleeft J H van Boom D Noort 《The journal of peptide research》2003,61(6):298-306
As part of a program towards the development of novel antibiotics, a convenient method for solid-phase synthesis of the cyclic cationic peptide polymyxin B1 and analogues thereof is described. The methodology, based on cleavage-by-cyclization using Kenner's safety-catch linker, yields crude products with purities ranging from 37-67%. Antibacterial assays revealed that analogues 23-26, in which the (S)-6-methyloctanoic acid moiety is replaced with shorter acyl chains, exhibit distinct antimicrobial activity. The results suggest that the length of the acyl chain is rather critical for antimicrobial activity. On the other hand, substitution of the hydrophobic ring-segment D-Phe-6/Leu-7 in polymyxin B1 with dipeptide mimics (i.e. analogues 27-33) resulted in almost complete loss of antimicrobial activity. 相似文献
24.
25.
Elevated levels of intracellular calcium are a major cause of myocardial dysfunction. To find possible mediators of the deregulated calcium we searched for EF-hand calcium-binding proteins of the S100 family. By PCR technology we identified three members of the S100 protein family (S100 alpha, CACY, and CAPL) in the human heart. We cloned the corresponding cDNAs and examined their expression levels in various human tissues by Northern blot analysis. All three proteins are expressed at high levels in the human heart. Whereas CACY and CAPL mRNAs are expressed ubiquitously, S100 alpha mRNA is restricted to heart, skeletal muscle, and brain. Interestingly, the expression pattern of S100 alpha, CACY, and CAPL in human tissues differs significantly from that in rodent tissues. 相似文献
26.
27.
28.
Transcriptional control, translation and function of the products of the five open reading frames of the Escherichia coli nir operon 总被引:3,自引:0,他引:3
Nerina R. Harborne Lesley Griffiths Stephen J. W. Busby Jeffrey A. Cole 《Molecular microbiology》1992,6(19):2805-2813
Five open reading frames designated nirB, nirD, nirE, nirC and cysG have been identified from the DNA sequence of the Escherichia coli nir operon. Complementation experiments established that the NirB, NirD and CysG polypeptides are essential and sufficient for NADH-dependent nitrite reductase activity (EC 1.6.6.4). A series of plasmids has been constructed in which each of the open reading frames has been fused in-phase with the beta-galactosidase gene, lacZ. Rates of beta-galactosidase synthesis during growth in different media revealed that nirB, -D, -E and -C are transcribed from the FNR-dependent promoter, p-nirB, located just upstream of the nirB gene: expression is co-ordinately repressed by oxygen and induced during anaerobic growth. Although the nirB, -D and -C open reading frames are translated into protein, no translation of nirE mRNA was detected. The cysG gene product is expressed from both p-nirB and a second, FNR-independent promoter, p-cysG, located within the nirC gene. No NADH-dependent nitrite reductase activity was detected in extracts from bacteria lacking either NirB or NirD, but a mixture of the two was as active as an extract from wild-type bacteria. Reconstitution of enzyme activity in vitro required stoichiometric quantities of NirB and NirD and was rapid and independent of the temperature during mixing. NirD remained associated with NirB during the initial stages of purification of the active enzyme, suggesting that NirD is a second structural subunit of the enzyme. 相似文献
29.
Results are presented on the analysis of three-dimensional motion of compound cilia or cirri in voltage-clamped specimens of the protozoan Stylonychia mytilus. Time series of three-dimensional data were obtained by using the anaxial illumination method for simultaneous recording of stereoscopic video images. Data processing involved the following steps: determination of a reference coordinate system based solely on features present in each stereo-pair; tracing of cirral axes in digitized images, conversion to parameter curves by means of least-squares polynomial approximation, conversion of pairs of two-dimensional data to a series of three-dimensional data; correction for distortion due to projective shortening and conversion to a series of polynomial triplets, and analysis of the periodical components of the motion pattern in the frequency domain. Reconstructed beating cycles show typical differences between hyperpolarization-induced ciliary activity and depolarization-induced ciliary activity. Reconstructions of the motion of the basal segment of a cirrus are in agreement with existing data. Analysis of the curvature and torsion of a cirral axis during beating does not reveal any simple pattern of propagated activity within the axoneme. The return stroke may be subdivided into two phases. First, a curvature peak develops proximally. Secondly, a region with increased torsion arises more distally and spreads out in proximal direction. Both curvature and torsion return to minimal values by the beginning of the power stroke. 相似文献
30.