全文获取类型
收费全文 | 4598篇 |
免费 | 278篇 |
出版年
2022年 | 15篇 |
2021年 | 38篇 |
2020年 | 20篇 |
2019年 | 34篇 |
2018年 | 47篇 |
2017年 | 44篇 |
2016年 | 80篇 |
2015年 | 133篇 |
2014年 | 136篇 |
2013年 | 312篇 |
2012年 | 289篇 |
2011年 | 284篇 |
2010年 | 171篇 |
2009年 | 169篇 |
2008年 | 267篇 |
2007年 | 248篇 |
2006年 | 281篇 |
2005年 | 250篇 |
2004年 | 254篇 |
2003年 | 246篇 |
2002年 | 237篇 |
2001年 | 97篇 |
2000年 | 92篇 |
1999年 | 82篇 |
1998年 | 60篇 |
1997年 | 66篇 |
1996年 | 63篇 |
1995年 | 61篇 |
1994年 | 47篇 |
1993年 | 51篇 |
1992年 | 64篇 |
1991年 | 62篇 |
1990年 | 59篇 |
1989年 | 49篇 |
1988年 | 51篇 |
1987年 | 31篇 |
1986年 | 25篇 |
1985年 | 42篇 |
1984年 | 32篇 |
1983年 | 22篇 |
1982年 | 29篇 |
1981年 | 39篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 37篇 |
1977年 | 19篇 |
1976年 | 22篇 |
1975年 | 14篇 |
1968年 | 7篇 |
1966年 | 10篇 |
排序方式: 共有4876条查询结果,搜索用时 218 毫秒
91.
Hydrophobic protein (H protein) was isolated from membrane fractions of Bacillus subtilis and constituted into artificial membrane vesicles with lipid of B. substilis. Glutamate was accumulated into the vesicle when a Na+ gradient across the membrane was imposed. The maximum effect of Na+ on the transport was achieved at a concentration of about 40 mM, while the apparent Km for Na+ was approximately 8 mM. On the other hand, Km for glutamate in the presence of 50 mM Na+ was about 8 μM. Increasing the concentration of Na+ resulted in a decrease in Km for glutamate, maximum velocity was not affected. The transport was sensitive to monensin (Na+ ionophore).Glutamate was also accumulated when pH gradient (interior alkaline) across the membrane was imposed or a membrane potential was induced with K+-diffusion potential. The pH gradient-driven glutamate transport was sensitive to carbonylcyanide m-chlorophenylhydrazone and the apparent Km for glutamate was approximately 25 μM.These results indicate that two kinds of glutamate transport system were present in H protein: one is Na+ dependent and the other is H+ dependent. 相似文献
92.
The 14N nuclear relaxation times T1 and T2 in egg yolk phosphatidylcholine have been observed in single bilayer vesicles dispersed in the media of different viscosities, 1H2O and 2H2O. The lateral diffusion coefficient of lipid molecule D has been calculated according to the method reported earlier: D = 2.2 × 10?8cm2s?1 in 1H2O and 2.1 × 10?8cm2s?1 in 2H2O at 20°C. They are in excellent agreement. This result gives a strong basis of usefulness of 14N NMR method in the evaluation of D without introducing any system perturbation. 相似文献
93.
The conformational transitions of the alternating maleic acid copolymers with styrene (MA-St)n, and alpha-methyl styrene (MA-MSt)(n) in aqueous solutions were studied by means of various methods. The following results were obtained: 1) The conformational transitions of (MA-St)n from the compact to extended coil form are observed in various salt solutions, as in aqueous NaCl, and the compact form is stabilized by Rb+ and Cs+, but destabilized by Li+. The coions, Br-, I-, ClO4- and SCN- affect scarcely the stability of the compact form. 2) The temperature coefficient of viscosity d In [eta]/dT of (MA-St)n in 0.09 M NaCl was positive for the compact form, but negative for the coil form, and it reflects the transition. 3) The difference between specific heats for the compact and coil forms of (MA-St)n in 0.03 M NaCl is determined to be about 15% of the corresponding heat of transfer of benzene to aqueous medium. 4) A remarkable dilution of the bound monomeric acridine orange to the compact form (MA-St)n is observed and the dimerization free energy of the dye in the compact form is about -2.1 kcal mole at 25 degrees C. 5) Potentiometric, dilatometric and viscometric titrations of (MA-MSt)n in aqueous NaCl at 25 degrees C show a similar conformational transition to that of (MA-St)n. Also, the difference in the molar extinction coefficient at 261 nm indicates the transition. The compact form of (MA-MSt)n is more unstable than that of (MA-St)n. From the results, the compact conformations and the transition mechanism of both the polyacids were discussed in comparison with the results for the maleic acid copolymers with n-alkyl vinyl ethers. 相似文献
94.
(AGCTG) (AGCTG) (AGCTG) (GGGTG) as the primordial sequence of intergenic spacers: the role in immunoglobulin class switch 总被引:5,自引:0,他引:5
S Ohno 《Differentiation; research in biological diversity》1981,18(2):65-74
The means employed for immunoglobulin heavy chain class switch appears to be no different from that by which meiotic intergenic crossing-overs at accomplished. As with other intergenic spacers, the 5' noncoding sequence of each Ig CH (immunoglobulin heavy chain constant region) gene apparently undergoes unconstrained sequence changes due to randomly sustained base substitutions, deletions, and duplications. Yet, there remains sufficient regional sequence homology between the Ig Cmu 5' noncoding sequence and those of its somatic recombination partners, e.g., Ig C gamma 1, Ig C gamma 2b, Ig C alpha, because each of these 5' noncoding sequences is made of multiple copies in various stages of degeneracy of one primordial 20 base pair-long sequence: (AGCTG) (AGCTG) (AGCTG) (GGGTG). 相似文献
95.
Stereochemistry of the biomimetic reduction of α-keto esters with NAD(P)H-model compounds has been investigated. The model compound with the R-configuration reduces the α-keto esters to the (R)-α-hydroxy esters, whereas (S)-α-hydroxy esters are afforded by the reduction with the S-configurational model compounds. It has been concluded that pro-R and -S hydrogens of the model compounds with R- and S-configuration, respectively, contribute predominantly to the reduction. 相似文献
96.
97.
The structure for trichoclin, (E)-8-(3-methyl-4-hydroxy-2-butenyloxy)-psoralen, a new furocoumarin isolated from Trichocline incana, has been established. Phellopterin and isopimpinellin were also obtained. The new side chain of trichoclin was confirmed by synthesis. 相似文献
98.
The potential use of antibodies that selectively recognize either X-bearing or Y-bearing sperm is self-evident. Thus our attention was directed to the fact that under optimal conditions, H-Y antibody lyses 50% of mouse spermatozoa. Accordingly, we asked whether expression of H-Y antigen is haploid in spermatozoa from XY male mice heterozygous for the autosomal dominantSxr gene, for if H-Y expression were haploid, H-Y antibody would be expected to kill 75% of spermatozoa derived from these XY,Sxr/- males. However, maximal lysis remained at the 50% level, which indicates that haploid expression of H-Y antigen and the potential immunoselection of Y-(or X-) bearing spermatozoa are unlikely. 相似文献
99.
Effect of the removal of the COOH-terminal region of bacteriorhodopsin on its light-induced H+ changes. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Removal of the COOH-terminal region of bacteriorhodopsin by digestion with trypsin or papain reduces the yield of light-induced H+ release by 50-70%. The rate of H+ release is not affected significantly, but the half time of H+ uptake increases almost twofold. However, there is no effect on the photocycle of bacteriorhodopsin as judged by the yield and decay kinetics of the M412 photointermediate. The H+:M ratio in enzyme-digested membranes is approximately 0.4-0.8, whereas untreated membranes have a H+:M ratio of approximately 2. Purple membrane sheets stored in distilled water at 4 degrees C for prolonged periods also have a low H+:M ratio, probably due to protease activity associated with bacterial contamination. Electrophoresis on sodium dodecylsulfate-polyacrylamide gels showed that both the enzyme-treated and the stored purple membrane samples have a higher electrophoretic mobility compared to the fresh preparation. The reduction in molecular weight can be accounted for by the loss of several residues from the COOH-terminal portion of the bacteriorhodopsin. We propose that the COOH-terminal region is partially responsible for the high yield of H+ release by the purple membrane. 相似文献
100.