全文获取类型
收费全文 | 526篇 |
免费 | 67篇 |
国内免费 | 67篇 |
专业分类
化学 | 443篇 |
晶体学 | 8篇 |
力学 | 32篇 |
综合类 | 4篇 |
数学 | 45篇 |
物理学 | 128篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 24篇 |
2021年 | 31篇 |
2020年 | 33篇 |
2019年 | 27篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 34篇 |
2015年 | 35篇 |
2014年 | 46篇 |
2013年 | 58篇 |
2012年 | 61篇 |
2011年 | 54篇 |
2010年 | 31篇 |
2009年 | 29篇 |
2008年 | 26篇 |
2007年 | 21篇 |
2006年 | 22篇 |
2005年 | 10篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 10篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 1篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有660条查询结果,搜索用时 31 毫秒
71.
采用高频感应炉燃烧–红外吸收法测定银杏叶、银杏果肉、银杏壳和银杏仁中的硫含量。样品以艾士卡试剂为熔融剂,在800℃马弗炉内熔融1 h,冷却后测定硫含量。硫的质量分数在0.40%~4.00%范围内与吸收峰面积呈良好的线性关系,线性相关系数为0.990 4,检出限为0.000 9%。测定结果的相对标准偏差为4.04%(n=11),平均加标回收率为101.03%。将红外吸收法与电感耦合等离子体原子发射光谱法及硫酸钡重量法进行比对试验,3种方法测定结果相一致。利用该方法测定了不同区域银杏叶中硫的含量,结果表明,风景区和居民区银杏叶干燥基全硫的含量较低,重工业区硫含量较高。对同一地区的银杏果肉、银杏壳、银杏仁中干燥基全硫的含量进行比较,结果表明银杏果肉硫含量最高,银杏壳次之,银杏仁最低。该方法灵敏度高,重现性好,可用于银杏及银杏叶中硫含量的准确测定。 相似文献
72.
芳香烃受体(Aryl hydrocarbon receptor,AhR)属于配体依赖性的转录因子蛋白。本文通过对AhR配体结合区域(Ligand binding domain,LBD)的结构功能及物种特异性分析,发现在其结合腔口有一些关键残基可能起到"门控"作用,进一步将野生型(WT)和3个突变模型(Phe289Ala、Tyr316Ala、Ile319Ala)进行分子动力学模拟,从蛋白稳定性、蛋白结构变化、蛋白结合腔变化及蛋白和配体结合能力4个方面分析3个残基的门控作用。研究发现,Phe289、Tyr316、Ile319氨基酸残基通过形成疏水作用为AhR LBD起到"门控"作用;而将这些氨基酸分别突变后,其蛋白稳定性降低,整体运动性增加,配体亲和力减弱,其中Tyr316、Ile319对腔内体积影响较大,Phe289使腔内环境稳定性降低。本研究可为基于芳香烃受体的药物设计提供相关理论指导。 相似文献
73.
阿莫西林是一种最常用的β-内酰胺抗生素,作为基本医疗系统中最重要药物之一被世界卫生组织基本药物标准清单收录。其杂质L,即:(2S,5R,6R)-6-((2S,5R,6R)-6-((R)-2-氨基-2-(4-羟基苯基)乙酰氨基)-3,3-二甲基-7-氧代-4-硫杂-1-氮杂双环[3.2.0]庚烷-2-甲酰胺)-3,3-二甲基-7-氧代-4-硫杂-1-氮杂双环[3.2.0]庚烷-2-甲酸(6-APA amoxicillin amide),由于含量少难以获取成为影响产品质量的因素之一,因此,我们以6-氨基青霉素烷酸和阿莫西林三水合物为原料,分别通过3步和1步转化为相应的中间体化合物3和6,并使用酰胺缩合试剂六氟磷酸苯并三唑-1-基-氧基三吡咯烷基磷(PyBOP)以76%的收率实现了化合物3和6的脱水缩合,得到的关键中间体化合物7在Pd/C催化条件下一步脱除苄基及两个苄氧羰基保护基,首次通过化学方法合成了β-内酰胺化合物阿莫西林杂质L,反应使用简单易得的原料,温和的反应条件,无需复杂的分离纯化处理,对以阿莫西林为代表的β-内酰胺类抗生素的过敏研究以及新型β-内酰胺抗生素的开发和作用机制研究等有着参考和借鉴意义。 相似文献
74.
Protein–protein interactions, particularly weak and transient ones, are often mediated by peptide recognition domains. Characterizing
the interaction interface of domain–peptide complexes and analyzing binding specificity for modular domains are critical for
deciphering protein–protein interaction networks. In this article, we report the successful use of an integrated computational
protocol to dissect the energetic profile and structural basis of peptide binding to third PDZ domain (PDZ3) from the PSD-95
protein. This protocol employs rigorous quantum mechanics/molecular mechanics (QM/MM), semi-empirical Poisson–Boltzmann/surface
area (PB/SA), and empirical conformational free energy analysis (CFEA) to quantitatively describe and decompose systematic
energy changes arising from, respectively, noncovalent interaction, desolvation effect, and conformational entropy loss associated
with the formation of 30 affinity-known PDZ3–peptide complexes. We show that the QM/MM-, PB/SA-, and CFEA-derived energy components
can work together fairly well in reproducing experimentally measured affinity after a linearly weighting treatment, albeit
they are not compatible with each other directly. We also demonstrate that: (1) noncovalent interaction and desolvation effect
donate, respectively, stability and specificity to complex architecture, while entropy loss contributes modestly to binding;
(2) P0 and P−2 of peptide ligand are the most important positions for determining both the stability and specificity of the PDZ3–peptide
complex, P−1 and P−3 can confer substantial stability (but not specificity) for the complex, and N-terminal P−4 and P−5 have only a very limited effect on binding. 相似文献
75.
超高效液相色谱-串联质谱法测定人尿液中全氟有机化合物 总被引:1,自引:0,他引:1
采用超高效液相色谱-串联质谱(UPLC-MS/MS)联用技术,建立了对人尿液中12种全氟有机化合物(PFCs)的分析方法。首先在尿液样品中加入相应的同位素内标,以2%(体积分数)甲酸甲醇溶液超声萃取、离心后,将提取液用弱阴离子交换固相萃取柱净化,采用UPLC-MS/MS测定,内标法定量。12种目标化合物在0.05~50 μg/L质量浓度范围内线性良好,相关系数(r)均大于0.992,检出限在0.44~3.47 ng/L之间。在20、100、500 ng/L添加水平下,平均回收率范围为80.3%~116.2%,相对标准偏差(n=6)在5.5%~13.8%之间。该方法灵敏度高、重现性好、回收率高、操作简单,适合人尿液中PFCs的测定。 相似文献
76.
Ng F Bae B Miyatake K Watanabe M 《Chemical communications (Cambridge, England)》2011,47(31):8895-8897
Novel ionomers based on polybenzimidazole block sulfonated poly(arylene ether sulfone) show excellent thermal properties. The ionic aggregation of sulfonic acid groups leads to well-developed phase separated morphology and thus high proton conductivity at wide humidity range, up to 65 mS cm(-1) at 90% relative humidity. 相似文献
77.
In this paper, the authors consider the problem of which
(generalized) moment-angle manifolds admit Ricci positive metrics.
For a simple polytope $P$, the authors can cut off one vertex $v$ of
$P$ to get another simple polytope $P_{v}$, and prove that if the
generalized moment-angle manifold corresponding to $P$ admits a
Ricci positive metric, the generalized moment-angle manifold
corresponding to $P_{v}$ also admits a Ricci positive metric. For a
special class of polytope called Fano polytopes, the authors prove
that the moment-angle manifolds corresponding to Fano polytopes
admit Ricci positive metrics. Finally some conjectures on this
problem are given. 相似文献
78.
Peng Zhou Hu Mei Feifei Tian Jiaona Wang Shirong Wu Zhiliang Li 《Frontiers of Chemistry in China》2007,2(1):55-63
Based on two-dimensional topological characters, a novel method called molecular electronegativity-interaction vector (MEIV)
is proposed to parameterize molecular structures. Applying MEIV into quantitative structure-spectrometry relationship studies
on ion mobility spectrometry collision cross-sections of 113 singly protonated peptides, three models were strictly obtained,
with correlative coefficient r and leave-one-out cross-validation q of 0.983, 0.979, 0.981, 0.979 and 0.980, 0.978, respectively. Thus, the MEIV is confirmed to be potent to structural characterizations
and property predictions for organic and biologic molecules.
Translated from Chinese Journal of Analytical Chemistry, 2006, 34(6) (in Chinese) 相似文献
79.
Nong Yue He Ya Fei Guo Song Li Jian-Xin Tang 《中国化学快报》2007,18(1):111-114
A novel maskless technique,self-driving micro-fluid porous type printing(SMPTP),was reported to in situ synthesize oligonucleotide arrays on glass slide,which has the merits of low cost,high quality and simple craft.In SMPTP for fabricating gene- chips,porous fiber tubes with a number of nanometric or micron channels functioned as“active letters”and were assembled in designed patterns,which are identical to the distribution of monomers in each layer of the array,and four patterns were needed for each layer.By means of capillarity,the synthesis solution was automatically taken into porous tubes assembled in a printing plate and reached the surface.An oligonucleotide array of 160 features with four different 15-mer probes was in situ synthesized using this technique.The four specific oligonucleotide probes,including the matched and the mismatched by the fluorescent target sequence,gave obviously different hybridization fluorescent signals. 相似文献
80.