The chemistry of graphene oxide is discussed in this critical review Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references)

/pdf/the-chemistry-of-graphene-oxide-507zcedhhu.pdf

The chemistry of graphene oxide

Constructing magnetic molecular solids by employing three-atom ligands as bridges

https://hal.archives-ouvertes.fr/hal-00565442/document

High-temperature structural evolution of caesium and rubidium triiodoplumbates

This review highlights the use and great potential of liquid metals as exotic and powerful solvents (i.e. fluxes) for the synthesis of intermetallic phases. The results presented demonstrate that considerable advances in the discovery of novel and complex phases are achievable utilizing molten metals as solvents. A wide cross-section of examples of flux-grown intermetallic phases and related solids are discussed and a brief history of the origins of flux chemistry is given. The most commonly used metal fluxes are surveyed and where possible, the underlying principal reasons that make the flux reaction work are discussed.

/pdf/the-metal-flux-a-preparative-tool-for-the-exploration-of-4gmt0kl7ri.pdf

The metal flux: a preparative tool for the exploration of intermetallic compounds.

The high specificity of incorporation of nucleotides into DNA by polymerase enzymes is crucial for maintaining fidelity of information transfer in cellular replication. The initial insertion event is the first point at which mutations of the genome are avoided.1,2 Mismatched pairing at this step occurs on the level of only ∼1 in 103–105 insertions, indicating a selectivity of at least 4 kcal/mol.1 It is clear that polymerases enhance the selectivity of nucleotide choice at the active site relative to the much lower pairing differences observed at the duplex terminus in the absence of enzyme.2

While many of the kinetic details of replication have been studied in recent years,3 the precise physical origins of this selectivity enhancement are poorly understood. Mechanisms involving both kinetic and binding selectivity between correct and incorrect nucleotides have been proposed.3 Base–base hydrogen bonding, base stacking, base pair geometry, and interactions between the enzyme, DNA, and nucleotides have all been invoked as potentially important interactions; however, the relative importance of these different effects remains unclear. Many DNA nucleotide analogs with altered or reduced H-bonding potential have been examined as substrates for polymerases;4 most of those analogs are quite poor substrates, and result in less discriminate incorporation fidelity than do the natural nucleotides. This general finding has been used as evidence that the number and strength of hydrogen bonds in a given pair determine efficiency and fidelity of DNA synthesis. Indeed, most if not all current models for replication fidelity hold that the specificity of hydrogen bonds formed in the new base pair is a central contributor to the observed selectivity.

Here we present evidence, however, that a DNA polymerase can exert high fidelity even when a base pair completely lacks conventional hydrogen bonds. The difluorotoluene nucleoside 1 has recently been constructed as a nonpolar shape mimic for natural thymidine (2).5,6 Its “base” moiety cannot measurably form paired complexes with adenine derivatives even in chloroform, a solvent in which H-bonded complexes are much more stable than they are in water.7 When placed within a DNA strand paired opposite adenine, moreover, it actually destabilizes the helix by ∼4–5 kcal relative to thymine at the same position. In addition, 1 shows no inherent pairing selectivity among the four natural bases, also consistent with its nonpolar, non-hydrogen-bonding nature (ref 7 and work in progress). We felt therefore that 1 would serve as a good test for the importance of thymidine’s hydrogen bonding groups on fidelity, because 1 lacks the strongly localized charges but retains nearly the exact steric shape of the natural molecule. If, as current models suggest, such polar interactions are important for achieving high fidelity, then 1 would be expected to be very inefficient and highly nonselective as a template for replication.

We were surprised to find, however, that nucleoside 1 (also called F) serves as a very good template for DNA synthesis. Initial single-nucleotide insertion experiments with the Klenow fragment of DNA Polymerase I (exo-) and a 23nt primer (Figure 1) indicated that at 20 µM dNTP concentrations, adenine, and only adenine, is inserted well opposite F (not shown). We then carried out more extensive studies using multiple nucleotide synthesis with a template sequence containing F, T, or an abasic nucleoside (ϕ) at the sixth position downstream of an 18nt primer (Figure 1). An equimolar mixture of dATP, dCTP, dTTP, and dGTP at 20 µM each was used, and short and long time points were examined. The results show that replacement of thymine in the template by 1 has little or no measurable effect on DNA synthesis; in both cases the primer is efficiently extended to the end of the template, with no pause either before or after the site in question even at the shortest (2 min) time point. By comparison, substitution in the template by an abasic nucleoside causes a strong, long-lived stop immediately after a nucleotide (deoxyadenosine) is inserted at that position.



Figure 1

(A) Sequences of DNA template/primer duplexes in this study. Compound 1 is designated F (from difluorotoluene). (B) Products of “running-start” DNA synthesis on synthetic duplex templates containing analog 1 or thymidine at the sixth position ...









To examine quantitatively the ability of nonpolar analog 1 to serve as a template for DNA synthesis, we carried out kinetic measurements of single-nucleotide insertions with the same enzyme. The 23nt template sequence is shown in Figure 1A. Steady-state methods8 were used to determine the efficiency (Vmax/Km) of insertion of natural nucleotides opposite T or opposite 1 in the same sequence (Table 1). Examination of relative efficiencies for each of the four bases allows a quantitative measure of fidelity (selectivity of nucleotide insertion) at the position in question.8,9



Table 1

Steady-State Kinetic Parameters for Insertion of Single Nucleotides into a Template–Primer Duplex by the KF (exo-) Polymerasea



The results show, remarkably, that analog 1 promotes efficiency and fidelity of nucleotide insertion nearly as high as that observed for natural thymine (Table 1). The apparent efficiency of insertion of deoxyadenosine opposite 1 is only ∼4-fold lower than that opposite T. In addition, the selectivity for insertion of A rather than C, T, or G is surprisingly similar (Figure 2): With T in the template, A is preferred by 3.0–4.5 log units over the other three bases, and with 1 in the template, A is preferred by 2.9–4.2 log units. These data indicate that the energetic barrier leading to the newly synthesized A–F base pair is within 0.9 kcal of that for the A–T pair. Moreover, A is inserted with ∼3.6–6.0 kcal of selectivity (relative to C, T, G) opposite F in the template. By comparison, the results for the abasic nucleoside in the template show that A is incorporated opposite an abasic site nearly two orders of magnitude less efficiently than across from 1 (Table 1), and the abasic site causes a strong stall in further strand elongation, while 1 does not (Figure 1). In addition, our measured fidelity of A insertion opposite F is much higher than reported values opposite an abasic site.10 Thus, our findings with analog F are quite distinct from the known “A rule”, in which adenine is preferentially incorporated opposite abasic sites.10,11



Figure 2

Histogram of fidelity of nucleotide insertion opposite T and isostere F (compound 1) with the 23nt primer template. Data are taken from Table 1.



These observations suggest that conventional hydrogen bonds may not be necessary for high efficiency and fidelity in DNA synthesis, and that shape complementarity may play a more important role in fidelity than previously believed.11 We cannot rule out a very weak contribution from an unconventional C–F···H–N hydrogen bond in the F–A base pair being synthesized;12 however, these effects must be quite small since our previous studies have shown clearly that difluorotoluene is a highly hydrophobic species which (i) does not pair with adenine even in chloroform, (ii) is strongly destabilizing when paired with adenine in DNA, and (iii) shows no inherent pairing selectivity in the absence of the enzyme.7

Previous polymerase experiments with hydrogen bonding-impaired analogs of natural DNA bases have nearly always resulted in poor nucleotide insertion or elongation.4 Those earlier analogs have considerable steric alterations (relative to the natural bases) resulting from alkylation or removal of H-bonding groups. We hypothesize that the reason that analog 1 is so successful in DNA replication is that it retains the steric shape and conformation of the natural nucleoside. It will be useful in continuing studies to compare analog 1 with closely related structures to evaluate the importance of the shape mimicry. Future studies will also address the kinetic behavior using pre-steady-state methods, and will explore the generality of F–A base pair recognition by different polymerases and in different sequence contexts. It will also be of interest to investigate whether the nucleotide triphosphate derivative of 1 is viable in DNA synthesis.

Difluorotoluene, a Nonpolar Isostere for Thymine, Codes Specifically and Efficiently for Adenine in DNA Replication.

PbMo5O8 and Tl0.8Sn0.6Mo7O11, Novel Clusters of Molybdenum and Thallium

Orange-red transparent single crystals of LiEu2(NCN)I3 and LiEu4(NCN)3I3 were synthesized from fluxes of europium iodide, sodium cyanide, sodium azide, and lithium iodide at elevated temperatures and structurally characterized by X-ray diffraction. While LiEu2(NCN)I3 crystallizes in the cubic system (Fd3m, a = 15.1427(17) A, V = 3472.2(7) A3, Z = 16, R1 = 0.0322), LiEu4(NCN)3I3 adopts the hexagonal system (P63/mmc, a = 10.6575(11) A, c = 6.8232(10) A, V = 671.16(14) A3, Z = 2, R1 = 0.0246). Both extended structures are composed of complex frameworks built from europium tetrahedra coordinated by carbodiimide (symmetrical NCN2-) units on the one side and condensed iodine octahedra around lithium cations on the other. Within LiEu2(NCN)I3, vertex-sharing of the LiI6 subunits together with isolated europium tetrahedra results in two three-dimensional networks interpenetrating each other. Within LiEu4(NCN)3I3, face-sharing of the europium tetrahedra results in bitetrahedral units which further connect via two ...

Formation of complex three- and one-dimensional interpenetrating networks within carbodiimide chemistry: NCN2(-)-coordinated rare-earth-metal tetrahedra and condensed alkali-metal iodide octahedra in two novel lithium europium carbodiimide iodides, LiEu2(NCN)I3 and LiEu4(NCN)3I3.

The Crystal Structure of Mn2O7

Synthese und Kristallstruktur von PbMo5O8; ein reduziertes Oxomolybdat mit Mo10O28‐Oktaederdoppeln

In3Mo11O17 besitzt ein Molybdatgerust Mo22O348−, das zur allgemeinen Reihe Mo4n+2O6n+4x− (mit n = 5) gehort. Die in sternformigen Kristallaggregaten wachsende Phase kristallisiert im orthorhombischen System (a = 988,0(2) pm, b = 951,2(2) pm, c = 3 176,7(4) pm). Es liegen oligomere Cluster aus funf trans-kantenverknupften Mo6-Oktaedern vor, die nach dem Verknupfungsprinzip Mo22OOO uber allen freien Kanten von O-Atomen umgeben sind. Im verbleibenden Hohlraum liegt ein In68+−Polykation, das vollstandig dem entsprechenden Polykation aus In11Mo40O62 gleicht. Auch fur die Mo—O− und Mo—Mo-Abstande innerhalb des Clusters werden gleiche Werte wie in In11Mo40O62 erreicht, doch gibt es in In3Mo11O17 mit 306 pm kurzere inter-Cluster-Abstande. In der Struktur treten Fehlordnungen auf, die mit dem Vorliegen von Konstitutionsisomeren interpretiert werden.



Wahrend der elektrische Widerstand von PbMo5O8 dem eines stark gestorten Metalls ahnelt (mit lokalem Minimum bei ca. 120 K), ist Tl0,8Sn0,6Mo7O11 aufgrund der Temperaturcharakteristik ein Halbleiter. Oxomolybdate mit langeren Oligomeren wie In11Mo40O62 und In3Mo11O17 zeigen metallische Leitfahigkeit. Dieser Gang korrespondiert mit der Grose der Cluster und ihrer elektronischen Kopplung untereinander, die in der Lange der inter-Cluster-Abstande zum Ausdruck kommt.



Mit wachsender Clustergrose steigen sowohl das effektive magnetische Moment von 0,97 μB (PbMo5O8) auf 1,40 mB (In11Mo40O62) pro Cluster (Ausnahme: In3Mo11O17) als auch der Anteil des temperaturunabhangigen paramagnetischen Beitrags von 890 auf 2 191 × 10−6 pro Cluster stetig an. Ein kondensierter Oktaeder hat daher einen Beitrag von 440 × 10−6 wie bei entsprechenden M6X12-Halogenidclustern. In11Mo40O62 zeigt einen auffalligen Wechsel im Temperaturverhalten der magnetischen Suszeptibilitat.



On the Crystal Structure of In3Mo11O17 and the Physical Properties of Oligomeric Oxomolybdates



In3Mo11O17 is characterized by its molybdate framework Mo22O348− belonging to the general series Mo4n+2O6n+4x− (with n = 5). The phase grows in star-like aggregates and crystallizes within the orthorhombic system (a = 988.0(2) pm, b = 951.2(2) pm, c = 3 176.7(4) pm). There are oligomeric clusters built from five trans edge-sharing Mo6 octahedra, surrounded by O atoms over all empty edges according to the Aufbau principle Mo22OOO. In the remaining structural channel one finds an In68+ polycation which is geometrically equivalent with the one of In11Mo40O62. The Mo—O and Mo—Mo distances within the cluster are the same like in In11Mo40O62, too, but there are shorter inter cluster distances (306 pm) in In3Mo11O17. Disorder in the structure may be understood in terms of the presence of constitutional isomers.



While the electrical resistivity of PbMo5O8 resembles the one of a strongly disturbed metal (with a local minimum around 120 K), the temperature characteristic of Tl0.8Sn0.6Mo7O11 is typical for a semiconductor. Oxomolybdates with even longer oligomers like In11Mo40O62 and In3Mo11O17 show metallic conductivity. This course corresponds with the sizes of the clusters and their electronic intercoupling which can be estimated from the specific lengths of the inter cluster distances.



The effective magnetic moment grows with increasing cluster length from 0.97 μB (PbMo5O8) to 1.40 μB (In11Mo40O62) per cluster (exception: In3Mo11O12), and so does the contribution of the temperature-independent paramagnetism (from 890 to 2191 × 10−6 per cluster). Thus, a single condensed octahedron carries roughly 440 × 10−6 as a temperature-independent paramagnetism, similar to the M6X17 halide clusters. In11Mo40O62 shows an interesting change in the temperature dependence of its magnetic suszeptibility.

Richard Dronskowski

Papers

PbMo5O8 and Tl0.8Sn0.6Mo7O11, Novel Clusters of Molybdenum and Thallium

Formation of complex three- and one-dimensional interpenetrating networks within carbodiimide chemistry: NCN2(-)-coordinated rare-earth-metal tetrahedra and condensed alkali-metal iodide octahedra in two novel lithium europium carbodiimide iodides, LiEu2(NCN)I3 and LiEu4(NCN)3I3.

The Crystal Structure of Mn2O7

Synthese und Kristallstruktur von PbMo5O8; ein reduziertes Oxomolybdat mit Mo10O28‐Oktaederdoppeln

Über die Kristallstruktur von In3Mo11O17 sowie über physikalische Eigenschaften oligomerer Oxomolybdate