全文获取类型
收费全文 | 3317篇 |
免费 | 162篇 |
国内免费 | 163篇 |
专业分类
3642篇 |
出版年
2023年 | 40篇 |
2022年 | 50篇 |
2021年 | 48篇 |
2020年 | 65篇 |
2019年 | 74篇 |
2018年 | 92篇 |
2017年 | 63篇 |
2016年 | 61篇 |
2015年 | 73篇 |
2014年 | 119篇 |
2013年 | 254篇 |
2012年 | 117篇 |
2011年 | 172篇 |
2010年 | 94篇 |
2009年 | 144篇 |
2008年 | 149篇 |
2007年 | 150篇 |
2006年 | 149篇 |
2005年 | 146篇 |
2004年 | 111篇 |
2003年 | 120篇 |
2002年 | 114篇 |
2001年 | 79篇 |
2000年 | 82篇 |
1999年 | 66篇 |
1998年 | 81篇 |
1997年 | 65篇 |
1996年 | 55篇 |
1995年 | 63篇 |
1994年 | 60篇 |
1993年 | 64篇 |
1992年 | 56篇 |
1991年 | 62篇 |
1990年 | 44篇 |
1989年 | 30篇 |
1988年 | 34篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 54篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 38篇 |
1981年 | 41篇 |
1980年 | 31篇 |
1979年 | 26篇 |
1978年 | 15篇 |
1977年 | 19篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 11篇 |
排序方式: 共有3642条查询结果,搜索用时 15 毫秒
991.
Toshifumi Miyazawa Nobuhiro Yabuuchi Ryoji Yanagihara Takashi Yamada 《Biotechnology letters》1999,21(9):725-728
-Chymotrypsin catalyzed the highly enantioselective amidation of chiral amines such as 1-(1-naphthyl)ethylamine using N-benzyloxycarbonyl-(S)-phenylalanine carbamoylmethyl ester as acyl donor (E = 25–660) in acetonitrile with low water content. 相似文献
992.
Takahashi N Tsukamoto Y Shiosaka S Kishi T Hakoshima T Arata Y Yamaguchi Y Kato K Shimada I 《Glycoconjugate journal》1999,16(8):405-414
N-glycans of neuropsin (serine protease in the murine hippocampus) expressed in Trichoplusia ni cells were released from the glycopeptides by digestion with glycoamidase A (from sweet almond), and the reducing ends of the oligosaccharides were reductively aminated with 2-aminopyridine. The derivatized N-glycans were separated and structurally identified by a two dimensional high-performance liquid chromatography (HPLC) mapping technique on two kinds of HPLC columns. Fourteen different major N-glycan structures were identified, of which 6 were high-mannose type (9.1%), and the remaining 8 were paucimannosidic type. The presence of insect specific N-glycan structures containing both 1,3- and 1,6- di-fucosylated innermost N-acetylglucosamine residue (23.3%), as below, was also confirmed by 600 MHz 1H-NMR spectroscopy. 相似文献
993.
The main strategy developed to shift the equilibrium state of a hydrolase-catalyzed hydrolysis/synthesis reaction consists in reducing water activity by addition of organic solvents in the reaction medium. We have used several mixtures of water and 1,4-butanediol, ranging from pure water to pure 1,4-butanediol, to study the hydrolysis/synthesis reaction of the N-Cbz-L-tryptophanyl-glycineamide dipeptide, catalyzed by alpha-chymotrypsin. In the presence of 1,4-butanediol, alpha-chymotrypsin also catalyzed the esterification reaction between this diol and N-Cbz-L-tryptophan; this ester hydrolysis/synthesis reaction has thus also been examined. The dipeptide and ester equilibrium concentrations increase when the water content of the reaction medium is decreased. Using our experimental data, we have determined the equilibrium constants of the hydrolysis/synthesis equilibria involving the nonionized forms of the protected amino acids, the estimated values of which are Ksp = 8 10(5) for the dipeptide and Kse = 78 for the ester respectively. They are true thermodynamic equilibrium constants, each related to a single, well-defined reaction equilibrium and with water activity being taken into account. If an organic solvent is added to the reaction medium these equilibria can be shifted towards synthesis by decreasing the water activity but also by modifying the ionization/neutralization equilibrium constant of the ionizable groups. These two effects depend both on the water content and on the nature of the organic solvent used, and, in particular, on its dielectric constant. Because of the importance of this parameter in our study, we discuss using it as an indicator to select an appropriate organic solvent to perform an enzyme-catalyzed synthesis. 相似文献
994.
The addition of dimethyl phosphonate to six different hexofuranos-5-uloses in the presence of DBU, followed by esterification with methoxalyl chloride and then radical reduction, afforded 5-deoxy-5-dimethoxyphosphinyl-D- and L-hexofuranoses. The stereoselectivity of the deoxygenation and possible transition-state models are discussed. 相似文献
995.
Carvalho I Scheuerl SL Ravindranathan Kartha KP Field RA 《Carbohydrate research》2003,338(10):1039-1043
Mercuric bromide-promoted glycosylation of Fmoc-Ser-OBn and Fmoc-Thr-OBn with 2-acetamido-2-deoxy-3,4,6-tri-O-acetyl-alpha-D-glucopyranosyl chloride in refluxing 1,2-dichloroethane gave the corresponding beta-glycosides in good yields (64 and 62%, respectively). Direct coupling of the commercially available Fmoc-Ser-OH and Fmoc-Thr-OH carboxylic acids under similar conditions gave the corresponding beta-glycosides, possessing free carboxyl groups, in moderate yields (50 and 40%, respectively). 相似文献
996.
Lie Ken Jie MS Lau MM Lam CN Alam MS Metzger JO Biermann U 《Chemistry and physics of lipids》2003,125(2):93-101
Methyl santalbate (methyl trans-11-octadecen-9-ynoate) from Sandal wood seed oil, Santalbum alum) was epoxidized to methyl trans-11,12-epoxy-octadec-9-ynoate (1). Treatment of compound 1 with tetrabutylammonium dihydrogentrifluoride, and boron trifluoride etherate gave the corresponding anti- (2a) (57%) and syn- (2b) (35%) fluorohydrin derivatives, respectively. These reactions were regio- and stereoselective in nature. The structures of the anti- and syn- isomers were confirmed by NMR spectroscopy. Ring opening of the epoxy system of compound 1 with lithium chloride gave the anti-chlorohydrin derivative (3) (89%). Oxidation of either compound 2a or 2b gave the same fluoro-keto acetylenic fatty ester (4) (75%), and compound 3 on chromic acid oxidation yielded the corresponding chloro-keto acetylene (5) (73%). Isomerization of compounds 4 and 5 with potassium carbonate in dichloromethane furnished the requisite fluoro-allenic (6) (63%, methyl 11-fluoro-12-oxo-9,10-octadecadienoate) and chloro-allenic (7) (80%, methyl 11-chloro-12-oxo-9,10-octadecadienoate) C(18) fatty esters. All products were confirmed by a combination of spectrometric and spectroscopic techniques. 相似文献
997.
Cholesterol has been implicated in the pathogenesis of Alzheimer's disease, both through intracellular effects, and through an extracellular effect due to its physical interaction with plaque associated amyloid. Epidemiology studies have implicated high cholesterol as a risk factor for AD, and have shown that the use of cholesterol reducing agents (statins) can be protective against the disease. We, and others have shown that cholesterol levels modulate the processing of the amyloid precursor protein (APP) both in vivo and in vitro, affecting the accumulation of Abeta (A) peptides which may directly impact the risk of AD. This review describes the biology of sterols, and identifies how cholesterol may exacerbate the pathogenesis of AD. Data from in vivo and in vitro studies will then be presented to describe how treatments aimed at modulating lipid levels may be efficacious in treating AD. 相似文献
998.
999.
Aqueous methanol extracts of Citrus junos, C. unshiu and C. hassaku fruit peel inhibited the growth of the roots and hypocotyls of lettuce (Lactuca sativa L.) seedlings. Significant reductions in the root and hypocotyl growth were observed as the extract concentration increased in all bioassays. The inhibitory activity of C. junos extract on the growth of lettuce roots and hypocotyls was about 13- and 24-fold greater than that of C. unshiu and C. hassaku extracts, respectively. The concentration of abscisic acid--D-glucopyranosyl ester (ABA-GE) in fruit peel of C. junos, C. unshiu and C. hassaku was determined since ABA-GE had been found to be a possible cause of the growth inhibitory effect of C. junos. The concentration was 132, 10.6 and 5.0 µg g–1 dry weight in C. junos, C. unshiu and C. hassaku fruit peel, respectively. Thus, there was a good correlation between ABA-GE concentrations in C. junos, C. unshiu and C. hassaku fruit peel and the inhibitory activities of their aqueous extracts, which suggests that ABA-GE may be involved in the growth inhibitory effect of C. junos, C. unshiu and C. hassaku fruit peel. 相似文献
1000.
Enantiomeric discrimination of pyrethroic acid esters on polysaccharide derived chiral stationary phases 总被引:1,自引:0,他引:1
Enantiomeric separation of pyrethroic acid methyl and ethyl esters was examined on cellulose-based chiral stationary phases (CSPs): chiralcel OD (cellulose tris(3,5-dimethylphenyl carbamate)) and chiralcel OF (cellulose tris(4-chlorophenyl carbamate)). The good resolution of pyrethroic acid esters was achieved on chiralcel OD and OF. Separation factors ranged from 1.19-5.12 for Chiralcel OD and 1.00-1.59 for chiralcel OF. Hexane/2-propanol (100:0.15, v/v %) was used as the eluent. The resolution capability of CSPs was greater chiralcel OD than chiralcel OF in the case of the pyrethroic acid esters. The flow rate was 0.8 ml/min and detection was set at 230 nm. The results of the chromatographic data and molecular mechanics suggest that steric effect was a major factor in the enantioseparation. Furthermore, the hydrogen bond between analytes and CSP played an important role in the chiral recognition. 相似文献