Synthese und Struktur eines zweikernigen Gadolinium(III)-Komplexes: Magnetische Austauschwechselwirkungen in alkoxyverbrückten Komplexen der Lanthanoiden

Synthesis and Structure of a Binuclear Gadolinium(III) Complex: Magnetic Exchange Interactions in Alkoxy Bridged Lanthanide Complexes The Schiff Base ligand N-salicylidene-2-(bis-(2-hydroxyethyl)amino)ethylamine (H₃sabhea) reacts with Gd(NO₃)₃ · 6 H₂O in methanol solution to yield the alkoxy bridged binuclear gadolinium(III) complex [{Gd(Hsabhea)(NO₃)}₂] · 2MeOH ( 1 ). 1 crystallizes in the monoclinic space group P2₁/c with a = 1014.8(2), b = 2059.2(4), c = 867.5(2) pm, β = 106.72(2)°, and Z = 2. The two gadolinium atoms are bridged by two alkoxide oxygen atoms with angles of 107.60(11)° at the oxygen bridgeheads and a Gd? Gd separation of 376.43(7) pm. A variable-temperature magnetic susceptibility study (2 to 280 K) of 1 revealed an antiferromagnetic coupling between the Gd(III) ions with J = ?0.198 cm^?1 (g = 1.975).     

Micropacked capillary nucleic acid analysis

Nucleic acid compositional analysis is discussed, followed by a brief review of the current state of the technology applied in which the benefits and limitations of current methodology are enumerated. An alternative method is presented, namely the use of a micropacked capillary ion exchange column to separate nucleoside monophosphates via an anion exchange mechanism. The separated nucleotides are then quantitated by means of post-column photodiode-array detection. The use of a photodiode array also enables the verification of peak identity and purity by acquisition of UV spectra at any point in the separation. The technique has applications in the compositional analysis of both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).     

Reaction of 3/2-formylindoles with TOSMIC: formation of indolyloxazoles and stable indolyl primary enamines

3-Formylindole and its 1-substituted and 1,5-disubstituted derivatives react with TOSMIC in presence of potassium carbonate in methanol under reflux to furnish 5-(3′-indolyl)oxazoles, new stable E-2-(3′-indolyl)-2-tosylethenamines and two diastereomers of N-[2-(3′-indolyl)-1,2-dimethoxy]ethylformamides. In contrast, 2-formylskatole furnishes N-(1-tosyl-2-skatolyl)ethenylformamide.     

The coordination chemistry of (py)2BF2 and related difluoroboron cations

The substitution and decomposition reactions of the difluorobis(pyridine)boron(III) cation, and of several substituted pyridine and other difluoroboron cations (D)₂BF₂⁺, are explored. Donor molecule displacement is greatly accelerated by the presence of electron-withdrawing meta and para pyridine substituents in (substituted pyridine)₂BF₂⁺ cations, and by electron-donating substituents on pyridine when the substituted pyridine is the attacking species. Increased steric hindrance of D in D₂BF₂⁺, and of the attacking base D′, slows or prevents displacement of D by D′. (py)₂BF₂⁺·PF₆⁻ undergoes slow decomposition in acetone, giving initially py·BF₃ and finally py·H⁺·BF₄⁻ as the major fluoroboron species. A second decomposition pathway, giving rise to BF₃OH⁻ and several additional fluorine-containing products, is favoured by the presence of strong bases such as amidines and by the presence of NH and OH bonds. Increased susceptibility to ligand displacement facilitates the synthesis of new fluoroboron cations, but also increases the susceptibility to decomposition.     

Studies on the Extension of Sequence‐independence and the Enhancement of DNA Triplex Formation

A more elaborate sequence‐independent triple‐helix formation viability study was carried out and extended from a recombination‐like triple‐helical DNA motif of a previous study (J. Mol. Recognition 14, 122–139 (2001)). The intended triple‐helix was formed by mixing one part of a DNA hairpin duplex and one part of a single (or third) strand identical to one of the duplex strands and complementary to the other strand. In contrast to the common purine and pyrimidine motifs in triple‐stranded DNA, the strands of the recombination‐like motif are not monotonously built from pyrimidine only, or purine only, in the sequence. The stability of the recombination‐like motif triplexes with varying sequences was monitored by UV thermal melting curves. The results showed that the order of the stability of the R‐form DNA base triads (J. Mol. Biol., 239, 181–200 (1994)) is G*(G ○ C) > C*(C ○ G) > A*(A ○ T) >T*(T ○ A) (the Watson‐Crick base pair is denoted in the parentheses) in 200 mM NaCl, at pH 7. In an attempt to increase the stability of the triplex in the recombination‐like motif, we replaced cytidine by 5‐methylcytidine (^mC) of the third strand. There is a general trend that ^mC modification stabilizes the complex (<2 °C per ^mC). The complex is furthermore stabilized by Mg²⁺ ion. The Tm increases from 7 to 2 °C from less stable to highly stable triplex by 20 mM Mg²⁺ ion in solution.     

The effect of sample mass and heating rate on DSC results when studying the fractional composition and oxidative stability of lube base oils

The method of dynamic microdistillation instrumented through DSC has been further elaborated and validated using distillate and residual base oils as model systems. The two major experimental factors of the method—sample mass and heating rate were varied to determine the optimal (standard) experimental conditions for better fraction resolution and thus more reliable quality assessment of petroleum products (lube oils). If these are increased, the fraction resolution ability of the method is reduced—the lighter fractions evaporate at temperatures close to those of the heavier ones, and generally all the fractions evaporate/oxidize at higher temperatures. Two major types of reactions in the heated sample were identified—the one occurring on the metal surface of the crucible leading to a polymer (lacquer) film formation and the other the oxidative cracking in the bulk of the sample leading to the formation of gaseous products. The extent of the lacquer film formation on the crucible metal surface can be reduced by increasing the heating rate and/or the sample mass, while their reduction results in better fraction resolution.     

Oligonucleotides Containing 7‐Deaza‐2′‐deoxyxanthosine: Synthesis,Base Protection,and Base‐Pair Stability

Oligonucleotides incorporating 7‐deaza‐2′‐deoxyxanthosine ( 3 ) and 2′‐deoxyxanthosine ( 1 ) were prepared by solid‐phase synthesis using the phosphoramidites 6 – 9 and 16 which were protected with allyl, diphenylcarbamoyl, or 2‐(4‐nitrophenyl)ethyl groups. Among the various groups, only the 2‐(4‐nitrophenyl)ethyl group was applicable to 7‐deazaxanthine protection being removed with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) by β‐elimination, while the deprotection of the allyl residue with Pd⁰ catalyst or the diphenylcarbamoyl group with ammonia failed. Contrarily, the allyl group was found to be an excellent protecting group for 2′‐deoxyxanthosine ( 1 ). The base pairing of nucleoside 3 with the four canonical DNA constituents as well as with 3‐bromo‐1‐(2‐deoxy‐β‐D ‐erythro‐pentofuranosyl)‐1H‐pyrazolo[3,4‐d]pyrimidine‐4,6‐diamine ( 4 ) within the 12‐mer duplexes was studied, showing that 7‐deaza‐2′‐deoxyxanthosine ( 3 ) has the same universal base‐pairing properties as 2′‐deoxyxanthosine ( 1 ). Contrary to the latter, it is extremely stable at the N‐glycosylic bond, while compound 1 is easily hydrolyzed under slightly acidic conditions. Due to the pK_a values 5.7 ( 1 ) and 6.7 ( 3 ), both compounds form monoanions under neutral conditions (95% for 1 ; 65% for 3 ). Although both compounds form monoanions at pH 7.0, pH‐dependent T_m measurements showed that the base‐pair stability of 7‐deaza‐2′‐deoxyxanthosine ( 3 ) with dT is pH‐independent. This indicates that the 2‐oxo group is not involved in base‐pair formation.     

Sample-to-sample fluctuations in the conductivity of a disordered medium

We investigate the sample-to-sample fluctuations in the conductivity of a random resistor network—equivalently, in the diffusivity of a disordered medium with symmetric hopping rates. We argue that whenever the effective conductivity ^* is strictly positive, then the fluctuations are normal, i.e., proportional to (volume)^–1/2. If the local conductivities are allowed to be zero, then ^* vanishes when approaching the percolation thresholdp _c. Close top _c the fluctuations are anomalous. From the renormalization group on hierarchical lattices we find that atp _c fluctuations and mean scale in the same fashion, i.e., there is no independent scaling exponent for the fluctuations.