全文获取类型
收费全文 | 392篇 |
免费 | 77篇 |
国内免费 | 2篇 |
出版年
2023年 | 19篇 |
2022年 | 13篇 |
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 12篇 |
2018年 | 16篇 |
2017年 | 18篇 |
2016年 | 6篇 |
2015年 | 25篇 |
2014年 | 24篇 |
2013年 | 48篇 |
2012年 | 11篇 |
2011年 | 19篇 |
2010年 | 14篇 |
2009年 | 19篇 |
2008年 | 16篇 |
2007年 | 19篇 |
2006年 | 20篇 |
2005年 | 21篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 22篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 5篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有471条查询结果,搜索用时 421 毫秒
431.
Shinichi Yoshida Akinobu Chatani Yoichi Honda Takashi Watanabe Masaaki Kuwahara 《Journal of Molecular Catalysis .B, Enzymatic》2000,9(4-6):173-182
The oxidation of various phenolics and aromatic amines by manganese-dependent peroxidase (MnP) of Bjerkandera adusta was examined in aqueous organic media. MnP retained its activities in several 70% (v/v) aqueous solutions of water-miscible organic solvents including ethylene glycol, diethylene glycol, acetone and acetonitrile. The absorption spectra of MnP in these aqueous organic media were similar to that observed in the reaction without solvent addition, indicating that the heme of MnP was little affected by the addition of these water-miscible organic solvents. MnP was also found to oxidize Mn(II) to Mn(III) in these 70% (v/v) aqueous organic media. The oxidation of Mn(II) by MnP was correlated with the Dimroth–Reichardt parameter, ET(30), of the solvents. Furthermore, MnP catalyzed the oxidation of anisidines, aminophenols, phenylenediamines and phenolics in aqueous 70% (v/v) acetone, acetonitrile and diethylene glycol media. Aromatic amines that have high hydrophobicity were shown to be suitable for the reaction of MnP in aqueous water-miscible organic media. 相似文献
432.
Ribonucleotide reductases and their occurrence in microorganisms: A link to the RNA/DNA transition 总被引:1,自引:0,他引:1
Jens Harder 《FEMS microbiology reviews》1993,12(4):273-292
Abstract: The evolution of a deoxyribonucleotide synthesizing ribonucleotide reductase might have initiated the transition from the ancient RNA world into the prevailing DNA world. At least five classes of ribonucleotide reductases have evolved. The ancient enzyme has not been identified. A reconstruction of the first ribonucleotide reductase requires knowledge of contemporary enzymes and of microbial evolution. Experimental work on the former focuses on few organisms, whereas the latter is now well understood on the basis of ribosomal RNA sequences. Deoxyribonucleotide formation has not been investigated in many evolutionary important microorganisms. This review covers our knowledge on deoxyribonucleotide synthesis in microorganisms and the distribution of ribonucleotide reductases in nature. Ecological constraints on enzyme evolution and knowledge deficiencies emerge from complete coverage of the phylogenetic groups. 相似文献
433.
Authenticity and quality coherence are the major elements in ensuring the consistency of the expected beneficial outcomes from the use of traditional or herbal remedies. Metabolomics offers the possibility of addressing these issues. Principal Component Analysis (PCA) was applied to select the best solvent system for sample extraction. Partial Least Square (PLS) regression analysis was found useful in evaluating the relationship between Nigella sativa seeds from four different origins on the basis of their metabolite profiles. In this study, different bioactivities were displayed by different samples with the Qasemi and Syrian samples exhibited high α-glucosidase inhibitory activity, which was correlated to the high fatty acid contents based on the PLS model. The Ethiopian sample exhibited high DPPH radical scavenging and nitric oxide (NO) inhibition activities, which may be related to the presence of high levels of thymoquinone and thymol. The method was also successfully used to classify “new test” samples into their proper groups. 相似文献
434.
Christine C. Winterbourn 《Biochimica et Biophysica Acta (BBA)/General Subjects》1985,840(2):204-210
The reactivities of myeloperoxidase-H2O2-Cl? and sodium hypochlorite with amino acids, uric acid, NADH, ascorbic acid, ADP, albumin, haemoglobin, α1-antitrypsin and some hydroxyl radical scavengers have been compared. The ability of each compound to inhibit chlorination of monochlorodimedon by both oxidants was measured. Relative reaction rates varied over a range of 105, but the reactivities of the two oxidants with each compound were very similar, from which it is concluded that the reactions of hypochlorite accurately reflect those of the myeloperoxidase system. Thiol compounds (cysteine and GSH) and methionine were more than 100-times more reactive than other amino acids, which had comparable reactivity to NADH and uric acid. Benzoate, dimethylsulphoxide and formate were very much less reactive. The significance of these reactions of myeloperoxidase in microbial killing and inflammation is discussed. 相似文献
435.
Abdulkarim Dakah Salim Zaid Mohamad Suleiman Sami Abbas Michael Wink 《Saudi Journal of Biological Sciences》2014,21(4):317-323
Ziziphora tenuior L. (Lamiaceae) is an aromatic herb used for its medicinal values against fungi, bacteria. Micropropagation can be used for large-scale multiplication of essential oil producing plants thus avoiding an overexploitation of natural resources. This work aims to develop a reliable protocol for the in vitro propagation of Z. tenuior, and to compare the antioxidant activity between in vitro propagated and wild plants.The explants were sterilized and cultured on MS medium containing different concentrations of growth regulators naphthalene acetic acid (NAA) or indole-3-butyric acid (IBA) with 0.5 mg/L of kinetin (Kin) callus formation was 70.2% after 45 days of incubation in dark on medium supplemented with 1.5 mg/L of NAA. After one month of callus culture on medium supplemented with 2 mg/L BA the shoot number was 5.12 and for the multiplication stage. The shoot number was 4.21 and length was 6.17 cm on medium supplemented with 1 mg/L Kin + 0.1 mg/L NAA.DPPH• reagent was used to test the antioxidant activity. The aqueous and methanol extracts of in vitro plants which were treated with 1.5 and 1 mg/L of kin plus 0.1 mg/L of NAA showed a strong DPPH• scavenging activity where IC50 was 0.307 and 0.369 mg/ml, respectively, while the IC50 of aqueous and methanol extracts of wild plants was 0.516 and 9.229 mg/ml, respectively. Our results suggested that plant growth regulators and in vitro culture conditions increased the antioxidant activity. 相似文献
436.
《Endocrine practice》2021,27(9):925-933
ObjectiveTo define optimal intact parathyroid hormone (iPTH) cut-off threshold predictive of hypocalcemia after total thyroidectomy for safe and effective postoperative management.MethodsThis prospective single center study was done in 2 phases. In phase I, predictors of symptomatic hypocalcemia were analyzed and the receiver operating characteristic curve was used to define the optimal iPTH cut-off threshold predictive of hypocalcemia. Phase II studied giving prompt prophylactic supplemental calcium and vitamin D to all patients who had iPTH levels below the calculated threshold, while phase I patients were given prompt selective supplementation if they had postoperative hypocalcemia or symptoms.ResultsUnivariate analysis of patients in phase I showed that postoperative iPTH was the only significant variable that can predict symptomatic hypocalcemia. Using receiver operating characteristic curve and Youden index, the confirmed optimal cut-off threshold predictive of hypocalcemia was iPTH 19.95 pg/mL, with area under the curve of 0.903, 100% sensitivity, negative predictive value, and highest Youden index, while iPTH 15 pg/mL and iPTH 10 pg/mL were less optimal. Symptomatic hypocalcemia occurred in 30% of the phase I cohort who received selective supplementation versus 3% of those in the phase II cohort who received prophylactic supplementation. Return to emergency department and need for intravenous calcium were also significantly better in phase II.ConclusioniPTH cut-off for post-thyroidectomy hypocalcemia was 19.95 pg/mL. Low-risk patients were discharged with no supplementation while all high-risk patients received prompt calcium and vitamin D supplementation, which led to effective hypocalcemia management and safe 24-hour discharge. 相似文献
437.
Madhur M Goyal Anjan Basak 《International Journal of Biochemistry and Molecular Biology》2012,3(3):282-289
Catalase is well known antioxidant enzyme which catalyses the dissociation of hydrogen peroxide directly into H2O and O2. Mammalian catalase has been considered as ‘a venerable enzyme with new mysteries’. Some aspects of its mechanism of action are mystifying and many of new findings are still unexplained. To fill up the gap we propose the ‘Hydroxyl Radical Generation Theory (HRGT)’ with possible mechanism. According to HRGT, mammalian catalase apart from its known catalytic reaction generates hydroxyl radicals (HRs). The HR generation mainly depends on concentration of specific substrate, hydrogen peroxide. The present theory is supported by previous experimental findings and has great deal of observational evidences. The proposed mechanism of generation of HRs answer several unexplained features of mammalian catalase, however, should be tested further. 相似文献
438.
Abstract In two recent researches on antioxidant mechanisms of flavonoids and isoflavonoids, quantum chemical method was employed to calculate the proton dissociation energies, the bond dissociation energies and the ionization potentials for the phenols and derived radicals to help determine the radical-scavenging mechanisms. As the solvent effect was left out of consideration, the conclusion drawn from the calculation incurred some controversies. In the current study, we re-calculated the parameters for genistein and its anion by employing a B3LYP/6–311+G(d,p)//6–31G(d,p) method with solvent effect. Accordingly, a more reasonable explanation on the experimentally observed behavior of genistein as a radical scavenger was obtained. Therefore, solvent effect should be considered in the investigation of radical-scavenging mechanisms of antioxidants in polar solvents. 相似文献
439.
The avian magnetic compass was analyzed in bird species of three different orders - Passeriforms, Columbiforms and Galliforms - and in three different behavioral contexts, namely migratory orientation, homing and directional conditioning. The respective findings indicate similar functional properties: it is an inclination compass that works only within a functional window around the ambient magnetic field intensity, it tends to be lateralized in favor of the right eye, and it is wavelength-dependent, requiring light from the short-wavelength range of the spectrum. The underlying physical mechanisms have been identified as radical pair processes, spin-chemical reactions in specialized photopigments. The iron-based receptors in the upper beak do not seem to be involved. The existence of the same type of magnetic compass in only very distantly related bird species suggests that it may have been present already in the common ancestors of all modern birds, where it evolved as an all-purpose compass mechanism for orientation within the home range. 相似文献
440.
Several derivatives of formulae [M(N4)(TCNQ)2] and [M(N4)(TCNQ)2](TCNQ) (M = Ni, Cu; N4 = 1,4,7,10-tetraazacyclododecane ([12] aneN4), 1,4,8,11-tetraazacyclotetradecane ([14] aneN4), 1,4,8,12-tetraazacyclopentadecane ([15] aneN4), 1,4,7,10-tetraazadecane (trien), N,N,N-tris(2-aminoethyl)amine (tren), 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,14-diene (trans-dieneN4)) have been obtained by metathesis reaction of the corresponding perchlorate or nitrate derivatives and LiTCNQ or (Et3NH)(TCNQ)2. The compounds [M(aneN4)(TCNQ)2] have a six coordinated metal atom surrounded by the four macrocyclic nitrogens and two nitrogens from σ coordinated TCNQ−. The overlap with a neighbouring 7,7,8,8-tetracyanoquinodimethane (TCNQ) forms the diamagnetic dianion [TCNQ]22−, and the whole structure can be seen as chains of metallomacrocyclic cations and TCNQ dianions alternating in the solid. The crystal structure of [Cu([15] aneN4)(TCNQ)2] confirms this fact. With open chain tetraamines the derivatives [Cu(trien)(TCNQ)2] and [Cu(tren)(TCNQ)2] are proposed to have the copper in a pentacoordinated environment, with only one coordinated TCNQ. In [M(trans-dieneN4)](TCNQ)2 both TCNQ− are uncoordinated and dimerized, as the crystal structure of the nickel derivative confirms. The derivatives with three TCNQ [M(aneN4)(TCNQ)2](TCNQ) are proposed to have a structure derived from that of the analogous [M(aneN4)(TCNQ)2], based on the metallomacrocycle-[TCNQ]2 chains connected through the extra TCNQ which remains uncoordinated and overlaps with the coordinated anions. This fact lowers the antiferromagnetic coupling inside the dimers and a small contribution for a thermally activated triplet state is observed in the magnetic susceptibility of these compounds. 相似文献