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排序方式: 共有153条查询结果,搜索用时 15 毫秒
1.
Molecular dynamics simulations with a combined quantum mechanical and molecular mechanical (QM/MM) potential have been carried out to investigate the squalene-to-hopene carbocation cyclization mechanism in squalene-hopene cyclase (SHC). The present study is based on free energy simulations by constructing the free energy surface for the cyclization steps along the reaction pathway. The picture that emerges for the carbocation cyclization cascade is a delicate balance of thermodynamic and kinetic control that ultimately favors the formation of the final hopanoids carbon skeleton. A key finding is that the five- to six-membered ring expansion process is not a viable reaction pathway for either C- or D-ring formation in the cyclization reaction. The only significant intermediate is the A/B-bicyclic cyclohexyl cation (III), from which two asynchronous concerted reaction pathways lead to, respectively, the 6,6,6,5-tetracyclic carbon skeleton and the 6,6,6,6,5-pentacyclic hopanoids. Experimentally, these two products are observed to have 1% and 99% yields, respectively, in the wild-type enzyme. We conclude that the product distribution in the wild-type enzyme is dictated by kinetic control of these two reaction pathways. 相似文献
2.
A. Anton Smith K. Kannan R. Manavalan N. Rajendiran 《Journal of inclusion phenomena and macrocyclic chemistry》2007,58(1-2):161-167
Absorption and fluorescence spectra of bicalutamide (BCA) in different solvents and aqueous β-cyclodextrin (β-CD) solutions
are reported. The solid inclusion complex of BCA with β-CD is prepared and investigated by FT-IR, 1H NMR, differential scanning colorimetry (DSC) and scanning electron microscopy (SEM). The Stokes shift of BCA is correlated
with various solvent polarity scales like E
T(30), BK and f(D,n). β-CD studies show that (i) the absorption and emission maxima of BCA are shifted to red, (ii) the absorbance is slightly
decreased whereas emission intensity is increased largely with an increasing β-CD concentration, (iii) BCA forms 1:1 inclusion
complex with β-CD and (iv) intramolecular charge transfer (ICT) emission is present in the excited state. 相似文献
3.
Jayshree Ramkumar 《Talanta》2007,71(3):1054-1060
The single ion transport of transition metal ions like Cu2+, Co2+, Ni2+ and Zn2+ were carried out through the H+ and alkali metal ion forms of Nafion membrane. These studies showed that the ion exchange selectivity coefficient of the permeating ion had an effect on its transport process. It was found that the diffusion coefficient values (D) were directly proportional to the selectivity coefficient (K). This shows that the initial stage of permeation is governed by ion exchange process (effect of K on D). 相似文献
4.
N. Rajendiran M. Swaminathan 《Journal of photochemistry and photobiology. A, Chemistry》1995,90(2-3):109-116
A comparative study of absorption and fluorescence maxima of 4,4′-diaminodiphenyl sulphone (4DADPS), 3,3′-diaminodiphenyl sulphone (3DADPS) and 2-aminodiphenyl sulphone (2ADPS) in different solvents reveals that (i) solvatochromic shifts are found to be mainly due to interaction of solvents with amino group, (ii) in any one solvent the net solvatochromic shifts of two amino groups are less than that of one amino group, (iii) fluorescence shift from cyclohexane to water is a maximum for 4DADPS and a minimum for 2ADPS and (iv) 4DADPS and 3DADPS possess more twisted intramolecular charge transfer character than 2ADPS. The excited-state acidity constants, determined by fluorimetric titration and Förster cycle methods, have been reported and discussed. 相似文献
5.
Rajendiran TM Kirk ML Setyawati IA Caudle MT Kampf JW Pecoraro VL 《Chemical communications (Cambridge, England)》2003,(7):824-825
Binuclear manganese complexes Mn2(III/IV)(dtsalpn)2DCBI, 1, Mn2(III/III)(dtsalpn)2HDCBI, 2, containing the ligand dicarboxyimidazole (DCBI) have been prepared in order to address the issue of imidazole bridged and ferromagnetically coupled Mn sites in high oxidation states of the OEC in Photosystem II (PS II). Temperature dependent magnetic susceptibility studies of 1 indicates that the interaction between the two Mn(III)/Mn(IV) ions is ferromagnetic (J = +1.4 cm(-1)). Variable temperature EPR spectra of 1 shows that a g = 2 multiline is as an excited state signal corresponding to S = 1/2. 相似文献
6.
Kalkote Uttam Ramrao Choudhary Anil Ramkumar Natu Arvind Anant Ayyangar Nagaraja Ramanuja 《合成通讯》2013,43(10-11):1129-1135
1-Alkylaminoanthraquinones ( 2 a-f) and 1,4-bisalkylaminoanthraquinones ( 4 a-c) were prepared from aminoanthraquinones ( 1,3 ) by alkylation with alkyl sulphate/alkyl halide in presence of powdered sodium hydroxide, potassium carbonate and phase transfer catalyst. 相似文献
7.
8.
Synthesis of conformationally restricted dispiro- and bis-dispiro-1,3-dioxolanes via three-component reaction of diazoamides, ketoamides/diketones, and aromatic/heteroaromatic aldehydes in the presence of rhodium(II) acetate dimer catalyst at room temperature involving carbonyl ylides is demonstrated with diastereoselectivity. Synthesis of macrocyclic dispiro-1,3-dioxolanes via intramolecular carbonyl ylide is also delineated in high yield. The conformationally restricted symmetrical as well as unsymmetrical dispiro-1,3-dioxolanes were obtained under mild conditions in a highly diastereo- and regioselective manner. 相似文献
9.
10.
G. Venkatesh J. Thulasidhasan N. Rajendiran 《Journal of inclusion phenomena and macrocyclic chemistry》2014,78(1-4):225-237
The inclusion complexation behavior of orciprenaline (ORC) and terbutaline (TER) with α-CD, β-CD, HP-α-CD and HP-β-CD are examined by absorption, fluorescence, life time and molecular modeling methods. ORC and TER forms 1:1 (CD/drug) inclusion complexes in lower CD concentrations and 1:2 (CD/drug) inclusion complexes with higher CD concentrations. The inclusion of both drugs with HP-CDs was stronger than that of native CDs. Both drugs exhibit dual emission (excimer) in the CD solution, whereas in water single emission is seen. The hydrogen bonding and van der Waals interaction between the drugs and the CD plays an important role in the inclusion complexes. Computational results show the side chain of the drugs encapsulated in the CD cavity. The molecular modeling results by PM3 were in good agreement with the experimental results. 相似文献