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Chemical Problems (chemical + problem)
Selected AbstractsCobalamin-Dependent and Cobalamin-Independent Methionine Synthases: Are There Two Solutions to the Same Chemical Problem?HELVETICA CHIMICA ACTA, Issue 12 2003Rowena Two enzymes in Escherichia coli, cobalamin-independent methionine synthase (MetE) and cobalamin-dependent methionine synthase (MetH), catalyze the conversion of homocysteine (Hcy) to methionine using N(5)-methyltetrahydrofolate (CH3 -H4folate) as the Me donor. Despite the absence of sequence homology, these enzymes employ very similar catalytic strategies. In each case, the pKa for the SH group of Hcy is lowered by coordination to Zn2+, which increases the concentration of the reactive thiolate at neutral pH. In each case, activation of CH3 -H4folate appears to involve protonation at N(5). CH3 -H4folate remains unprotonated in binary E,CH3 -H4folate complexes, and protonation occurs only in the ternary E,CH3 -H4folate,Hcy complex in MetE, or in the ternary E,CH3 -H4folate,cob(I)alamin complex in MetH. Surprisingly, the similarities are proposed to extend to the structures of these two unrelated enzymes. The structure of a homologue of the Hcy-binding region of MetH, betainehomocysteine methyltransferase, has been determined. A search of the three-dimensional-structure data base by means of the structure-comparison program DALI indicates similarity of the BHMT structure with that of uroporphyrin decarboxylase (UroD), a homologue of the MT2-A and MT2-M proteins from Archaea, which catalyze Me transfers from methylcorrinoids to coenzyme M and share the Zn-binding scaffold of MetE. Here, we present a model for the Zn binding site of MetE, obtained by grafting the Zn ligands of MT2-A onto the structure of UroD. [source] Living Yeast Cells as a Controllable Biosynthesizer for Fluorescent Quantum DotsADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Ran Cui Abstract There are currently some problems in the field of chemical synthesis, such as environmental impact, energy loss, and safety, that need to be tackled urgently. An interdisciplinary approach, based on different backgrounds, may succeed in solving these problems. Organisms can be chosen as potential platforms for materials fabrication, since biosystems are natural and highly efficient. Here, an example of how to solve some of these chemical problems through biology, namely, through a novel biological strategy of coupling intracellular irrelated biochemical reactions for controllable synthesis of multicolor CdSe quantum dots (QDs) using living yeast cells as a biosynthesizer, is demonstrated. The unique fluorescence properties of CdSe QDs can be utilized to directly and visually judge the biosynthesis phase to fully demonstrate this strategy. By such a method, CdSe QDs, emitting at a variety of single fluorescence wavelengths, can be intracellularly, controllably synthesized at just 30°C instead of at 300°C with combustible, explosive, and toxic organic reagents. This green biosynthetic route is a novel strategy of coupling, with biochemical reactions taking place irrelatedly, both in time and space. It involves a remarkable decrease in reaction temperature, from around 300 °C to 30 °C and excellent color controllability of CdSe photoluminescence. It is well known that to control the size of nanocrystals is a mojor challenge in the biosynthesis of high-quality nanomaterials. The present work demonstrates clearly that biological systems can be creatively utilized to realize controllable unnatural biosynthesis that normally does not exist, offering new insights for sustainable chemistry. [source] Myth and Reality in the Attitude toward Valence-Bond (VB) Theory: Are Its ,Failures' Real?HELVETICA CHIMICA ACTA, Issue 4 2003Sason Shaik According to common wisdom propagated in textbooks and papers, valence-bond (VB) theory fails and makes predictions in contradiction with experiment. Four iconic ,failures' are: a) the wrong prediction of the ground state of the O2 molecule, b) the failure to predict the properties of cyclobutadiene (CBD) viz. those of benzene, c) the failure to predict the aromaticity/anti-aromaticity of molecular ions like C5H and C5H, C3H and C3H, C7H and C7H, etc; and d) the failure to predict that, e.g., CH4 has two different ionization potentials. This paper analyzes the origins of these ,failures' and shows that two of them (stated in a and d) are myths of unclear origins, while the other two originate in misuse of an oversimplified version of VB theory, i.e., simple resonance theory that merely enumerate resonance structures. It is demonstrated that, in each case, a properly used VB theory at a simple and portable level leads to correct predictions, as successful as those made by use of molecular-orbital (MO) theory. This notion of VB ,failure', which is traced back to the VB-MO rivalry, in the early days of quantum chemistry, should now be considered obsolete, unwarranted, and counterproductive. A modern chemist should know that there are two ways of describing electronic structure, which are not two contrasting theories, but rather two representations or two guises of the same reality. Their capabilities and insights into chemical problems are complementary, and the exclusion of any one of them undermines the intellectual heritage of chemistry. [source] Semilocalized approach to investigation of chemical reactivityINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2003V. GineityteArticle first published online: 21 JUL 200 Abstract Application of the power series for the one-electron density matrix Gineityte, V., J Mol Struct Theochem 1995, 343, 183 to the case of two interacting molecules is shown to yield a semilocalized approach to investigate chemical reactivity, which is characterized by the following distinctive features: (1) Electron density (ED) redistributions embracing orbitals of the reaction centers of both molecules and of their neighboring fragments are studied instead of the total intermolecular interaction energy; (2) the ED redistributions are expressed directly in the basis of fragmental orbitals (FOs) without passing to the basis of delocalized molecular orbitals (MOs) of initial molecules; (3) terms describing the ED redistributions due to an intermolecular contact arise as additive corrections to the purely monomolecular terms and thereby may be analyzed independently; (4) local ED redistributions only between orbitals of the reaction centers of both molecules are described by lower-order ter s of the power series, whereas those embracing both the reaction centers and their neighborhoods are represented by higher-order terms. As opposed to the standard perturbative methods based on invoking the delocalized (canonical) MOs of isolated molecules, the results of the approach suggested are in-line with the well-known intuition-based concepts of the classic chemistry concerning reactivity, namely, with the assumption about different roles of the reaction center and of its neighborhood in a chemical process, with the expectation about extinction of the indirect influence of a certain fragment (substituent) when its distance from the reaction center grows, etc. Such a parallelism yields quantum chemical analogs for the classic concepts and thereby gives an additional insight into their nature. The scope of validity of these concepts also is discussed. Applicability of the approach suggested to specific chemical problems is illustrated by a brief consideration of the SN2 and AdE2 reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 302,316, 2003 [source] | ||||||||||