Preview: Chem. Eur. J. 24/2011

Written by Chemistry - A European Journal on May 27, 2011 – 5:00 am -


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Spotlights on our sister journals: Chem. Eur. J. 23/2011

Written by Chemistry - A European Journal on May 27, 2011 – 5:00 am -


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Corrigendum: Copper-Catalyzed CP Coupling through DecarboxylationP Coupling through Decarboxylation

Written by Jie Hu on May 27, 2011 – 5:00 am -


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Graphical Abstract: Chem. Eur. J. 23/2011

Written by Chemistry - A European Journal on May 27, 2011 – 5:00 am -


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Inside Cover: Chemical and Thermal Stability of Isotypic Metal–Organic Frameworks: Effect of Metal Ions (Chem. Eur. J. 23/2011)

Written by In Joong Kang on May 27, 2011 – 5:00 am -

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The stability of MOFs… … (metal–organic frameworks) has been compared to understand relative chemical and thermal stabilities of isotypic porous MOFs (metal–benzenedicarboxylates; M-BDCs). The chemical stability of MOFs depends on the inertness of central metal ions. On the other hand, the bond strength of common metal oxides determines the thermal stability of MOFs. For more details, see the Full Paper by S. H. Jhung et al. on page 6437 ff.


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Cover Picture: Hemoglobin as a Nitrite Anhydrase: Modeling Methemoglobin-Mediated N2O3 Formation (Chem. Eur. J. 23/2011)

Written by Kathrin H. Hopmann on May 27, 2011 – 5:00 am -

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The cover depicts the role of nitrite in hypoxic vasodilation. A key conundrum involves how nitrite-derived NO, generated within the red blood cells, reaches the endothelium while eluding recapture by deoxyhemoglobin. Quantum chemical studies lend credence to a recent suggestion that N2O3 (generated via TS1 or TS2) may be the actual NO-transporting vehicle. For more details see the Full Paper by A. Ghosh et al. on page 6348 ff.


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Hemoglobin as a Nitrite Anhydrase: Modeling Methemoglobin-Mediated N2O3 Formation

Written by Kathrin H. Hopmann on May 27, 2011 – 5:00 am -

Abstract

Nitrite has recently been recognized as a storage form of NO in blood and as playing a key role in hypoxic vasodilation. The nitrite ion is readily reduced to NO by hemoglobin in red blood cells, which, as it happens, also presents a conundrum. Given NO’s enormous affinity for ferrous heme, a key question concerns how it escapes capture by hemoglobin as it diffuses out of the red cells and to the endothelium, where vasodilation takes place. Dinitrogen trioxide (N2O3) has been proposed as a vehicle that transports NO to the endothelium, where it dissociates to NO and NO2. Although N2O3 formation might be readily explained by the reaction Hb-Fe3++NO2+NO⇌Hb-Fe2++N2O3, the exact manner in which methemoglobin (Hb-Fe3+), nitrite and NO interact with one another is unclear. Both an “Hb-Fe3+-NO2+NO” pathway and an “Hb-Fe3+-NO+NO2” pathway have been proposed. Neither pathway has been established experimentally. Nor has there been any attempt until now to theoretically model N2O3 formation, the so-called nitrite anhydrase reaction. Both pathways have been examined here in a detailed density functional theory (DFT, B3LYP/TZP) study and both have been found to be feasible based on energetics criteria. Modeling the “Hb-Fe3+-NO2+NO” pathway proved complex. Not only are multiple linkage-isomeric (N- and O-coordinated) structures conceivable for methemoglobin–nitrite, multiple isomeric forms are also possible for N2O3 (the lowest-energy state has an NN-bonded nitronitrosyl structure, O2NNO). We considered multiple spin states of methemoglobin–nitrite as well as ferromagnetic and antiferromagnetic coupling of the Fe3+ and NO spins. Together, the isomerism and spin variables result in a diabolically complex combinatorial space of reaction pathways. Fortunately, transition states could be successfully calculated for the vast majority of these reaction channels, both MS=0 and MS=1. For a six-coordinate Fe3+-O-nitrito starting geometry, which is plausible for methemoglobin–nitrite, we found that N2O3 formation entails barriers of about 17–20 kcal mol−1, which is reasonable for a physiologically relevant reaction. For the “Hb-Fe3+-NO+NO2” pathway, which was also found to be energetically reasonable, our calculations indicate a two-step mechanism. The first step involves transfer of an electron from NO2 to the Fe3+–heme–NO center ({FeNO}6) , resulting in formation of nitrogen dioxide and an Fe2+–heme–NO center ({FeNO}7). Subsequent formation of N2O3 entails a barrier of only 8.1 kcal mol−1. From an energetics point of view, the nitrite anhydrase reaction thus is a reasonable proposition. Although it is tempting to interpret our results as favoring the “{FeNO}6+NO2” pathway over the “Fe3+-nitrite+NO” pathway, both pathways should be considered energetically reasonable for a biological reaction and it seems inadvisable to favor a unique reaction channel based solely on quantum chemical modeling.

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Nitrite as signaling molecule: An unresolved aspect of nitric oxide physiology involves processes that prevent capture of NO by heme species. A possible scenario involves transformation of NO into an intermediate that is less likely to be captured, allowing it to diffuse to relevant target sites. A key candidate for such an intermediate is the N2O3 molecule. Evidence is presented in favor of hemoglobin's role as a nitrite anhydrase and of N2O3's viability as a NO vehicle (see figure).


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Drug-Delivery Strategies by Using Template-Synthesized Nanotubes

Written by Jillian L. Perry on May 27, 2011 – 5:00 am -

Abstract

Encapsulating drugs within hollow nanotubes offers several advantages, including protection from degradation, the possibility of targeting desired locations, and drug release only under specific conditions. Template synthesis utilizes porous membranes prepared from alumina, polycarbonate, or other materials that can be dissolved under specific conditions. The method allows for great control over the lengths and diameters of nanotubes; moreover, tubes can be constructed from a wide variety of tube materials including proteins, DNA, silica, carbon, and chitosan. A number of capping strategies have been developed to seal payloads within nanotubes. Combining these advances with the ability to target and internalize nanotubes into living cells will allow these assemblies to move into the next phase of development, in vivo experiments.

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Special order delivery! Template synthesis strategies provide excellent control over both the internal and external dimensions of nanotubes. Because these methods also allow internal and external surfaces to display disparate functional groups, template-synthesized nanotubes are very attractive starting points for drug-delivery vehicles (see graphic). Several key advances have recently been made that bring this goal closer to reality and are discussed in this Concept article.


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Contiguous Metal-Mediated Base Pairs Comprising Two AgI Ions

Written by Dominik A. Megger on May 27, 2011 – 5:00 am -

Abstract

The incorporation of transition-metal ions into nucleic acids by using metal-mediated base pairs has proved to be a promising strategy for the site-specific functionalization of these biomolecules. We report herein the formation of Ag+-mediated Hoogsteen-type base pairs comprising 1,3-dideaza-2′-deoxyadenosine and thymidine. By defunctionalizing the Watson–Crick edge of adenine, the formation of regular base pairs is prohibited. The additional substitution of the N3 nitrogen atom of adenine by a methine moiety increases the basicity of the exocyclic amino group. Hence, 1,3-dideazaadenine and thymine are able to incorporate two Ag+ ions into their Hoogsteen-type base pair (as compared with one Ag+ ion in base pairs with 1-deazaadenine and thymine). We show by using a combination of experimental techniques (UV and circular dichroism (CD) spectroscopies, dynamic light scattering, and mass spectrometry) that this type of base pair is compatible with different sequence contexts and can be used contiguously in DNA double helices. The most stable duplexes were observed when using a sequence containing alternating purine and pyrimidine nucleosides. Dispersion-corrected density functional theory calculations have been performed to provide insight into the structure, formation and stabilization of the twofold metalated base pair. They revealed that the metal ions within a base pair are separated by an Ag⋅⋅⋅Ag distance of about 2.88 Å. The Ag–Ag interaction contributes some 16 kcal mol−1 to the overall stability of the doubly metal-mediated base pair, with the dominant contribution to the Ag–Ag bonding resulting from a donor–acceptor interaction between silver 4d-type and 4s orbitals. These Hoogsteen-type base pairs enable a higher functionalization of nucleic acids with metal ions than previously reported metal-mediated base pairs, thereby increasing the potential of DNA-based nanotechnology.

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Loading up DNA: A novel silver(I)-mediated base pair comprising 1,3-dideazaadenine and thymine can accommodate two metal ions, thereby doubling the amount of transition-metal ions that can be site-specifically attached to DNA. Nucleic acids containing contiguous base pairs of this type are still capable of reversible self-assembly (see scheme).


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Chemical and Thermal Stability of Isotypic Metal–Organic Frameworks: Effect of Metal Ions

Written by In Joong Kang on May 27, 2011 – 5:00 am -

Abstract

Chemical and thermal stabilities of isotypic metal–organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal–oxygen bond in common oxides like Al2O3, Cr2O3, and V2O5. In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.

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Stability of MOFs: The chemical and thermal stabilities of isotypic MOFs (metal–organic frameworks) were evaluated in a wide range of conditions to understand the effect of central metal ions on the stabilities. The chemical stabilities in acids, bases and water increase with increasing inertness of the central metal ions. On the other hand, the thermal stability depends on the strength of the metal–oxygen bond of common oxides.


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