全文获取类型
收费全文 | 2252篇 |
免费 | 128篇 |
国内免费 | 258篇 |
专业分类
2638篇 |
出版年
2024年 | 4篇 |
2023年 | 21篇 |
2022年 | 45篇 |
2021年 | 51篇 |
2020年 | 60篇 |
2019年 | 80篇 |
2018年 | 69篇 |
2017年 | 75篇 |
2016年 | 68篇 |
2015年 | 65篇 |
2014年 | 106篇 |
2013年 | 232篇 |
2012年 | 85篇 |
2011年 | 116篇 |
2010年 | 90篇 |
2009年 | 166篇 |
2008年 | 138篇 |
2007年 | 154篇 |
2006年 | 131篇 |
2005年 | 121篇 |
2004年 | 78篇 |
2003年 | 75篇 |
2002年 | 62篇 |
2001年 | 53篇 |
2000年 | 48篇 |
1999年 | 36篇 |
1998年 | 38篇 |
1997年 | 36篇 |
1996年 | 28篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 23篇 |
1992年 | 35篇 |
1991年 | 30篇 |
1990年 | 30篇 |
1989年 | 6篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 7篇 |
1979年 | 14篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1974年 | 3篇 |
排序方式: 共有2638条查询结果,搜索用时 8 毫秒
991.
灰树花活性多糖构效关系研究进展 总被引:2,自引:0,他引:2
灰树花是一种珍贵的食药用菌,具有降血糖、抗肿瘤、免疫调节和抗病毒等多种生物活性。灰树花多糖是其主要的活性成分,多糖的生物活性与多糖的结构密切相关。本文综述了从20世纪80年代起国内外已报道的灰树花活性多糖的结构表征。部分研究认为多糖的降血糖活性可能与β-1,6-葡聚糖主链化学结构相关,而灰树花多糖结构为β-1,6主链或β-1,3主链葡聚糖时具有较好的抗肿瘤活性。然而多糖结构异常复杂,精细结构的解析困难,导致目前灰树花多糖结构表征一般止步于单糖组成、分子量、糖苷键类型、分支结构和粗略分子链构象,但二维核磁(two dimensional nuclear magnetic resonance,2D-NMR)和高分辨质谱联用等技术的发展将有助于解开灰树花多糖构效关系,并为灰树花活性多糖的开发利用提供理论依据。 相似文献
992.
【目的】高山冰川是一类独特的生态系统,本研究探索从明永冰川地区分离和培养低温菌噬菌体,并对其特征进行研究。【方法】利用已分离的低温菌为宿主,采用"双层平板法"从明永冰川融水中分离纯化低温菌噬菌体;对噬菌体及其宿主进行电镜形态观察,并进行噬菌体基因组限制性酶切片段长度多态性分析、衣壳蛋白组成分析及噬菌体生理特征研究。【结果】从明永冰川融水中分离获得一株裂解性低温噬菌体,命名为MYSP03(Mingyong Flavobacterium Siphoviridae Bacteriophage),其宿主菌MYB03鉴定为Flavobacterium菌株。噬菌体MYSP03为长尾型,无囊膜,头部具典型的正多面体立体对称结构,直径约72 nm;尾管长约240 nm,直径约10 nm;4℃时具侵染活性,在4℃-20℃范围内均可产生边缘清晰、透明的噬菌斑,最适感染温度约10℃,pH耐受范围较广,最适感染pH约9.4,对氯仿不敏感,基因组为双链DNA,大小约66 kb。 相似文献
993.
Samar Ramzi 《Archives Of Phytopathology And Plant Protection》2013,46(17):2131-2138
Apodiphus amygdali or stink bug of fruit trees is one of the polyphagous species from pentatomid bugs that attack many of fruit trees and ornamental trees. In the current study, activities of α- and β-glucosidases were measured in the midgut of A. amygdali adults. It was found the higher activity of β-glucosidase than α-glucosidase in addition to different enzymatic properties of the enzymes. Optimal pHs for enzymatic activities were found to be 5 and 7 for α- and β-glucosidases, respectively. Values regarding optimal temperatures were obtained at 30?°C for both α- and β-glucosidases. Among ions used on α-glucosidase activity, K+ and Ca2+ significantly increased enzymatic activity, Na+ had no effect, and Cu2+, Fe2+ and Mg2+ had the significant negative effects on the enzyme activity. Ca2+ and Fe2+ increased β-glucosidase activity in the midgut of A. amygdali, Na+ had no effect, and other ions significantly decreased the enzyme activity. Ethylene glycol-bis (β-aminoethylether) N,N,N?,N-tetraacetic acid (EGTA), citric acid, ethylenediamide tetraacetic acid (EDTA) and sodium dodecylsulfate (SDS) significantly decreased α-glucosidase activity but EGTA, triethylenetetramine hexaacetic acid (TTHA), EDTA and SDS decreased β-glucosidase activity in the midgut of A. amygdali. Characterisation of digestive enzymes, especially the effect of inhibitors on enzyme activity, could be useful for better understanding of enzyme roles in nutritional physiology of insects in addition to reach safe and useful controls of insect pests. 相似文献
994.
《Biocatalysis and Biotransformation》2013,31(2):163-169
Pure sucrose is inexpensive and readily available, making this disaccharide a highly desirable starting material for new polymers. In order to achieve a clean polymerization, the disaccharide must be regioselectively monofunctionalized in good yield. This paper describes the practical, enzyme-mediated synthesis of sucrose-1 ′-methacrylate 2 from sucrose and vinyl methacrylate using subtilisin Carlsberg (Sigma, Protease VIII), a readily available bacterial serine protease. A key aspect of this process, ascertaining the positional selectivity of acylation, was unambiguously accomplished using 1H-detected (1H, 13C) one-bond shift correlation (HMQC) spectroscopy and1H-detected (1H, 13C) multiple bond correlation (HMBC) spectroscopy. 相似文献
995.
《Biocatalysis and Biotransformation》2013,31(4):113-118
AbstractCandida antarctica lipase catalyzes a number of elementary reactions like alcoholysis, ammoniolysis and aminolysis in poly(ethylene glycol) (PEG) media. Reaction rates were comparable to or better than those observed in conventional organic reaction media and ionic liquids. It is envisaged that PEGs could have added benefits for performing biotransformations with highly polar substrates, which are sparingly soluble in common organic solvents. 相似文献
996.
Lipase could catalyze the ammonolysis of trimethylsilylmethyl acetate in organic solvents and Novozym 435 was the best biocatalyst for the reaction. The influences of some factors on the reaction were investigated. Cyclohexane, n-hexane and heptane were found to be suitable reaction media and ammonium carbamate was the best ammonium source. The optimal initial water activity, temperature and pH value were 0.55–0.75, 35°C and 6.5 respectively, under which a substrate conversion of 97.6% could be achieved after reaction for 140 h. 相似文献
997.
Heinz Nika Edward Nieves David H. Hawke Ruth Hogue Angeletti 《Journal of biomolecular techniques》2013,24(1):17-31
A sample preparation method for protein C-terminal peptide isolation has been developed. In this strategy, protein carboxylate glycinamidation was preceded by carboxyamidomethylation and optional α- and ϵ-amine acetylation in a one-pot reaction, followed by tryptic digestion of the modified protein. The digest was adsorbed on ZipTipC18 pipette tips for sequential peptide α- and ϵ-amine acetylation and 1-ethyl-(3-dimethylaminopropyl) carbodiimide-mediated carboxylate condensation with ethylenediamine. Amino group-functionalized peptides were scavenged on N-hydroxysuccinimide-activated agarose, leaving the C-terminal peptide in the flow-through fraction. The use of reversed-phase supports as a venue for peptide derivatization enabled facile optimization of the individual reaction steps for throughput and completeness of reaction. Reagents were exchanged directly on the support, eliminating sample transfer between the reaction steps. By this sequence of solid-phase reactions, the C-terminal peptide could be uniquely recognized in mass spectra of unfractionated digests of moderate complexity. The use of the sample preparation method was demonstrated with low-level amounts of a model protein. The C-terminal peptides were selectively retrieved from the affinity support and proved highly suitable for structural characterization by collisionally induced dissociation. The sample preparation method provides for robustness and simplicity of operation using standard equipment readily available in most biological laboratories and is expected to be readily expanded to gel-separated proteins. 相似文献
998.
999.
《Bioscience, biotechnology, and biochemistry》2013,77(6):1103-1108
Phosphopentomutase catalyzes the transfer of an intramolecular phosphate on ribose or deoxyribose, and is involved in the salvage pathway of nucleoside synthesis. We identified a sequence 5′-upstream of the genes for the nucleoside phosphorylases of Bacillus stearothermophilus as the phosphopentomutase (ppm) gene. The novel gene corresponded to an open reading frame of 1,179 nucleotides that is translated into a putative 393-amino acid protein with a molecular weight of 43,735. The gene product, partially purified from ppm-overexpressing Escherichia coli cells, was judged to be a monomer of a 44-kDa polypeptide. The phosphopentomutase was found to catalyze the phosphotransfer on not only ribose or deoxyribose but also arabinose or dideoxyribose. 相似文献
1000.