全文获取类型
收费全文 | 1170篇 |
免费 | 22篇 |
国内免费 | 144篇 |
专业分类
化学 | 1263篇 |
晶体学 | 31篇 |
综合类 | 2篇 |
数学 | 7篇 |
物理学 | 33篇 |
出版年
2023年 | 7篇 |
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 20篇 |
2019年 | 17篇 |
2018年 | 12篇 |
2017年 | 21篇 |
2016年 | 28篇 |
2015年 | 33篇 |
2014年 | 44篇 |
2013年 | 92篇 |
2012年 | 141篇 |
2011年 | 70篇 |
2010年 | 70篇 |
2009年 | 79篇 |
2008年 | 83篇 |
2007年 | 81篇 |
2006年 | 57篇 |
2005年 | 65篇 |
2004年 | 61篇 |
2003年 | 46篇 |
2002年 | 36篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 15篇 |
1996年 | 18篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1336条查询结果,搜索用时 16 毫秒
1.
《Arabian Journal of Chemistry》2022,15(12):104368
Protein hydrolysates have the potential to be natural and safer sources of bioactive peptides. In this study, two proteases were used to hydrolyze Chinese sturgeon (Acipenser sinensis) protein, and the hydrolysates were then purified to yield antioxidant peptides. The degree of hydrolysis of 23.56 % and 18.14 % was obtained using papain and alcalase 2.4L, respectivly, and hydrolysates had 96.80 % and 87.24 % total amino acid content, respectivly. The papain hydrolysate (PH) and alcalase 2.4L hydrolysate (AH) showed good antioxidant activity against DPPH? (IC50 of 3.64 and 3.15 mg/mL) and ABTS?+ (IC50 of 1.92 and 1.58 mg/mL), respectively. The low-molecular-weight (<1000 Da) fraction of both hydrolysates demonstrated the highest antiradical activity (IC50 of 2.59 and 2.31 mg/mL, DPPH) and (IC50 of 1.54 and 1.36 mg/mL, ABTS), respectively. Nine peptides were separated from both hydrolysates using reverse phase high performance liquid chromatography (RP-HPLC). The IC50 for ABTS?+ scavenging activity of peptide P5 with valine, glycine and asparagine (MW of 282.13 Da) from PH, and peptide P3 with histidine, glycine and alanine (MW of 302.74 Da) from AH was 0.89 and 0.72 mg/mL, respectively. The fractions and purified peptides obtained from Chinese sturgeon hydrolysates could be utilized as natural antioxidant substitutes in pharmaceuticals and food products. 相似文献
2.
《Arabian Journal of Chemistry》2022,15(4):103707
Alsophila spinulosa is a tree-like fern, and many evidences suggested that plant polyphenols had the potential therapeutic for Alzheimer s disease (AD). Herein, polyphenols (ASP) was isolated from A. spinulosa leaves and its major constituent were isoorientin and vitexin. ASP displayed excellent antioxidant activity and obvious anti-lipid peroxidation capacity in vitro. ASP improved the survival rate of C. elegans under high temperature by enhancing the antioxidant enzymes activities and decreasing the lipid peroxidation level. Moreover, ASP alleviated β-amyloid (Aβ) induced paralysis and reduced Aβ deposition, decreased reactive oxygen species (ROS) accumulation and improved the level of skn-1 mRNA. In addition, ASP decreased the levels of pdk-1 and akt-1 mRNA in P13K/AKT signaling pathway. In conclusion, ASP may be a potential ingredient for the alleviation of AD. 相似文献
3.
《Acta Crystallographica. Section C, Structural Chemistry》2018,74(3):256-262
Using polynuclear metal clusters as nodes, many high‐symmetry high‐connectivity nets, like 8‐connnected bcu and 12‐connected fcu , have been attained in metal–organic frameworks (MOFs). However, construction of low‐symmetry high‐connected MOFs with a novel topology still remains a big challenge. For example, a uninodal 8‐connected lsz network, observed in inorganic ZrSiO4, has not been topologically identified in MOFs. Using 2,2′‐difluorobiphenyl‐4,4′‐dicarboxylic acid (H2L) as a new linker and 1,2,4‐triazole (Htrz) as a coligand, a novel three‐dimensional CdII–MOF, namely poly[tetrakis(μ4‐2,2′‐difluorobiphenyl‐4,4′‐dicarboxylato‐κ5O1,O1′:O1′:O4:O4′)tetrakis(N,N‐dimethylformamide‐κO)tetrakis(μ3‐1,2,4‐triazolato‐κ3N1:N2:N4)hexacadmium(II)], [Cd6(C14H6F2O4)4(C2H2N3)4(C3H7NO)4]n, (I), has been prepared. Single‐crystal structure analysis indicates that six different CdII ions co‐exist in (I) and each CdII ion displays a distorted [CdO4N2] octahedral geometry with four equatorial O atoms and two axial N atoms. Three CdII ions are connected by four carboxylate groups and four trz− ligands to form a linear trinuclear [Cd3(COO)4(trz)4] cluster, as do the other three CdII ions. Two Cd3 clusters are linked by trz− ligands in a μ1,2,4‐bridging mode to produce a two‐dimensional CdII–triazolate layer with (6,3) topology in the ab plane. These two‐dimensional layers are further pillared by the L2− ligands along the c axis to generate a complicated three‐dimensional framework. Topologically, regarding the Cd3 cluster as an 8‐connected node, the whole architecture of (I) is a uninodal 8‐connected lsz framework with the Schläfli symbol (422·66). Complex (I) was further characterized by elemental analysis, IR spectroscopy, powder X‐ray diffraction, thermogravimetric analysis and a photoluminescence study. MOF (I) has a high thermal and water stability. 相似文献
4.
Dayang Yu Guoliang Mao Huaixun Cai Shuhua Wang Jiandang Liu Pengju Pan Yongzhong Bao 《Journal of polymer science. Part A, Polymer chemistry》2021,59(9):754-763
2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene (PDD-TFE) copolymer is a good candidate to prepare gas separation membranes with excellent permeability due to its free volume characteristics. However, the influence of PDD-TFE copolymer structure on its free volume characteristics is less studied. In this paper, PDD-TFE copolymers with different compositions and molecular weights were synthesized, and their free volume characteristics were analyzed by positron annihilation lifetime spectroscopy and a molecular dynamics simulation. It indicated that the molar fraction of PDD in copolymers had a significant effect on free volume characteristics, while the molecular weight of copolymers exerted a slight influence on free volume when the molecular weight exceeded a critical region (intrinsic viscosity [η] > 68 ml g−1). PDD-TFE copolymers with greater PDD molar fractions (i.e., 72% and 84%) showed bimodal distributions in positron lifetime and free volume size distributions, while PDD-TFE copolymers with lower PDD molar fractions (i.e., 27% and 35%) exhibited a single peak. The long-lifetime parameter τ3 was assigned to micro-cavities formed by [-(TFE)y-PDD-] segments and τ4 was attributed to micro-cavities formed by [-(PDD)x-TFE-] segments. The cis and trans transitions of PDD led to a local multilayer spiral structure with a 2.6–4.3 Å layer spacing, which would also increase the free volume of copolymers. 相似文献
5.
The molecule rac-1,1'-binaphthalene-2,2'-diol (rac-1,1'-bi-2-naphthol, rac-BINOL) shows a propensity for supramolecular, charge-assisted O–H ··· O– hydrogen-bonded strand formation when crystallized with its deprotonated form BINOLAT2– and Cu2+ in conc. ammonia. The naphthyl-paneled cavities in the {(rac-BINOLAT2–)(rac-BINOL)2} strands host the [Cu(NH3)5]2+-guest cation through second-sphere N–H ··· O hydrogen bonding in the structure of [Cu(NH3)5]2+(rac-BINOLAT2–)(rac-BINOL)2. Decreasing the copper(II) and ammonia concentrations in the crystallization leads to {(rac-BINOLAT2–)(rac-BINOL)} strands, in which rac-BINOLAT2– coordinates to two copper(II) atoms in the structure of [Cu(NH3)2(μ-rac-BINOLAT2–-κ2O,O':κO)]2(rac-BINOL)2. In the {Cu2+(NH3)2} moiety two BINOLAT-oxide atoms act as bridging ligands. Both copper structures could be obtained by using the racemic rac-BINOL or the enantiomeric R- or S-BINOL, through an in-situ racemization of the latter. 相似文献
6.
以2,2′-二硫代二苯甲酸、2,2′-联吡啶、咪唑、硝酸锌和硝酸锰为原料,使用水热方法合成了配合物[Zn(EBLA)(2,2′-bipy)(H2O)](EBLA=2,2′-二苯甲酸硫醚)(2,2′-bipy=2,2′-联吡啶)(1),自然挥发法制备了配合物[Mn(EBSA)(im)2(H2O)]n(EBSA=2,2′-二硫代二苯甲酸)(im=咪唑)(2)。利用元素分析、红外和热重分析对其进行了表征。利用X-射线单晶衍射对结构进行了测定,并研究了配合物1的荧光性质。配合物1中,2,2′-二苯甲酸硫醚是通过水热方法由2,2′-二硫代二苯甲酸发生原位反应制备,Zn2+离子是五配位的四角锥双核结构;配合物2中,Mn2+离子是六配位的变形八面体一维链状结构。配合物中存在氢键和π-π堆积等弱的分子间作用力。 相似文献
7.
利用硅溶胶的成膜性、纳米二氧化钛-氧化锌大的比表面积及导电胶的粘结性,制备了纳米二氧化钛-氧化锌/硅溶胶/导电胶复合材料,基于此复合材料将联吡啶钌固定到金电极表面,制备了磷酸可待因电化学发光(ECL)传感器.在优化的实验条件(800 V负高压、扫描速度100 mV/s,磷酸盐缓冲体系(pH 6.5))下,可待因浓度在1.0×10-7~1.0×10-4 mol/L范围内与电化学发光强度呈良好的线性关系(r2=0.9973),检出限为2.56×10-8 mol/L (S/N=3).传感器表现出良好的重现性与稳定性,连续平行测定1.28×10-5 mol/L可待因溶液10次,发光强度的相对标准偏差(RSD)为2.7%;室温下保存10天后,发光强度为初始值的92%以上.测定可待因药物实际样品的加标回收率在99.3% ~ 102.5%之间. 相似文献
8.
Vadim A. Dubskikh Anna A. Lysova Denis G. Samsonenko Alexander N. Lavrov Konstantin A. Kovalenko Danil N. Dybtsev Vladimir P. Fedin 《Molecules (Basel, Switzerland)》2021,26(5)
Three new 3D metal-organic porous frameworks based on Co(II) and 2,2′-bithiophen-5,5′-dicarboxylate (btdc2−) [Co3(btdc)3(bpy)2]·4DMF, 1; [Co3(btdc)3(pz)(dmf)2]·4DMF·1.5H2O, 2; [Co3(btdc)3(dmf)4]∙2DMF∙2H2O, 3 (bpy = 2,2′-bipyridyl, pz = pyrazine, dmf = N,N-dimethylformamide) were synthesized and structurally characterized. All compounds share the same trinuclear carboxylate building units {Co3(RCOO)6}, connected either by btdc2– ligands (1, 3) or by both btdc2– and pz bridging ligands (2). The permanent porosity of 1 was confirmed by N2, O2, CO, CO2, CH4 adsorption measurements at various temperatures (77 K, 273 K, 298 K), resulted in BET surface area 667 m2⋅g−1 and promising gas separation performance with selectivity factors up to 35.7 for CO2/N2, 45.4 for CO2/O2, 20.8 for CO2/CO, and 4.8 for CO2/CH4. The molar magnetic susceptibilities χp(T) were measured for 1 and 2 in the temperature range 1.77–330 K at magnetic fields up to 10 kOe. The room-temperature values of the effective magnetic moments for compounds 1 and 2 are μeff (300 K) ≈ 4.93 μB. The obtained results confirm the mainly paramagnetic nature of both compounds with some antiferromagnetic interactions at low-temperatures T < 20 K in 2 between the Co(II) cations separated by short pz linkers. Similar conclusions were also derived from the field-depending magnetization data of 1 and 2. 相似文献
9.
《Arabian Journal of Chemistry》2021,14(11):103421
Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC50: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS+ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain. 相似文献
10.
《Mendeleev Communications》2022,32(6):726-728
Reaction between 5-cyano-3,6-di(het)aryl-1,2,4-triazines and arylhydrazines with the following aza-Diels–Alder autoclave reaction affords hardly available 2-amino-3,6-di(het)aryl-pyridines in up to 67% yield after two steps and in 75% yield for the one-pot way. The compounds obtained can be promising for medicinal chemistry. 相似文献