Thermodynamic Properties and Melting Behavior of Bi–Sn–Zn Alloys

Summary. The partial and integral enthalpies of mixing of liquid Bi–Sn–Zn alloys were determined at 500°C by a drop calorimetric technique using a Calvet-type microcalorimeter. The ternary interaction parameters in the Bi–Sn–Zn system were fitted using the Redlich-Kister-Muggianu model for substitutional solutions, and isoenthalpy curves of the integral molar enthalpy of mixing at 500°C were constructed. Furthermore, a DSC technique was used to determine the liquidus temperatures in three sections (3, 5, and 7 at.% Zn) as well as the invariant reaction temperature of the ternary eutectic L ⇄ (Bi) + (Sn) + (Zn). The ternary eutectic reaction was found at 135°C.     

Hard-soft acid-base interactions of silylenes and germylenes

A detailed investigation of the electrophilic and nucleophilic character of singlet silylenes and germylenes, divalent compounds of silicon and germanium, respectively, substituted by first- and second-row elements is presented. In a first part, the Lewis acid properties of these compounds were studied through their complexation reaction with the Lewis bases NH3, PH3, and AsH3. The results indicate that this complexation is most favorable with the hardest base NH3, classifying these compounds as hard Lewis acids. This is confirmed by the linear correlation between the interaction energies and the value of the electrostatic potential, used as an approximation to the local hardness, near the empty p orbital of these compounds, indicating a charge-controlled interaction in the complex. Also the electrophilicity index, proposed by Parr et al., computed both at the global and the local level, correlates linearly with the complexation energies of the compounds with NH3. The Lewis base character of these silylenes has been investigated, through their interaction with the acids BH3 and AlH3. Also in this case, the electrostatic potential can be used to probe the reactivity of the compounds. It will finally be demonstrated that an increasing stability of the silylenes and germylenes is accompanied by an increase in their nucleophilicity and a decrease of the electrophilicity.     

Combining the best in triplex recognition: synthesis and nucleic acid binding of a BQQ-neomycin conjugate

Synthesis of a BQQ-neomycin conjugate is reported. The conjugate combines two ligands, one known to intercalate triplexes (BQQ) and another known to bind in the triplex groove (neomycin). The conjugate stabilizes T.A.T, as well as mixed base DNA triplex, better than neomycin, BQQ, or a combination of both. The conjugate selectively stabilizes the triplex (in the presence of physiological salt concentrations), with as little as 4 muM of the ligand leading to a DeltaTm of >60 degrees C. Competition dialysis studies show a clear preference for the drug binding to triplex DNA/RNA over the duplex/single strand structures. Modeling studies suggest a structure of neomycin bound to the larger W-H (Watson-Hoogsteen) groove with BQQ intercalated between the triplex bases.     

A 13C- and 195Pt-NMR. Study of the Complexes trans-PtCl2 (amine) (CH2CH2)

¹³C- and ¹⁹⁵Pt-NMR. spectra for the complexes trans-PtCl₂(amine)(CH₂?CH₂) have been measured. For amine = (S)-N-methyl-α-methylbenzylamine the two diastereomers present may be distinguished from the values ³J(Pt,C). The ¹⁹⁵Pt-chemical shift is shown to be sensitive to subtle differences stemming from intramolecular non-bonded interactions.     

Catalyzed oxidation of alcohols and aldehydes with iodosylbenzene

RuCl₂(PPh₃)₃ catalyzes oxidation of alcohols to carbonyi compounds by iodosylbenzene and that of aldehydes to carboxylic acids. Catalyzed oxidation of primary alcohols with phenyliodosodiacetate affords aldehydes.     

CORRELATION BETWEEN THE BINDING OF FORMATE AND DECREASED RATES OF CHARGE TRANSFER THROUGH THE PHOTOSYSTEM II QUINONES

Abstract— The binding parameters of bicarbonate to the thylakoid membrane at different formate concentrations have been established [Stemler and Murphy (1983) Photochem. Phorobiol. 38, 701–707]. Based on these parameters, predictions could be made concerning the effects of bicarbonate and formate on photosynthetic electron flow. In this work these effects of various concentrations of bicarbonate and formate are measured and compared to predictions from the binding study. Electron flow is measured between Q_A and Q_B (the primary and secondary quinone acceptors) and Q_B and the plastoquinone pool. Also, these same concentration effects are determined for silicomolybdate supported oxygen evolution. It is found that the results of the bicarbonate binding study are in good agreement with the concentration dependence determined for the quinone reactions, as well as the silicomolybdate reaction. The bicarbonate concentrations required for half-maximal effects are approximately 100 μM, 300 μM and 1.3 mM in the presence of 0, 20 mM and 100 mM formate, respectively. It is concluded that a hierarchy of possible electron flow rates exist. The slowest rates occur when formate is bound. A substantially higher rate occurs when neither formate nor bicarbonate (< 2 μM) are present, but only chloride is present. The highest rates of electron flow occur when bicarbonate is bound. The Q_A- Q_B→ Qa Qb^? Qa^? Qb^2– PQ → Q_a Q_b- PQ^2–, and the silicomolybdate reactions all have the same concentration dependence on formate and bicarbonate. From this it is concluded that a single binding site for formate and bicarbonate affect all of these reactions. The possibility that multiple sites exist with approximately equal affinities for bicarbonate cannot be excluded.     

A nonenzymic model for the coenzyme B12-dependent isomerization of methylmalonyl-SCoA to suiccinyl-SCoA

Dimethyl bromomethylmalonate (IV) reacts with vitamin B_12s in aqueous solution yielding a relatively unstable carbon-cobalt bonded adduct V, which shows visible spectra in good accord with expectation. The adduct V was allowed to decompose in water, in the dark, at room temperature and at physiological pH. Three products: succinic acid (VI), methylmalonic acid (VIII) and malonic acid (VII) were formed in 3, 18, and 13% yields respectively. Isolation of the succinic acid rearrangement product provides support for the intermediacy of the carbon-cobalt bond in the coenzyme B₁₂ dependent enzymic carbon-skeleton rearrangement of methylmalonyl-SCoA to succinyl-SCoA.     

The structure of malabricol

A single crystal X-ray analysis has confirmed the constitution and established the stereochemistry of the triterpene, malabaricol.     

Anisotropic hydrogel thickness gradient films derivatized to yield three-dimensional composite materials

We report the preparation of in-plane density gradients of amino-terminated molecules and gold particles through derivatization of laterally varying thickness gradients of poly(acrylic acid) (PAA) or poly(acrylamide) (PAAm) films. PAA and PAAm gradients were formed by Zn(II)-catalyzed electropolymerization of acrylic acid (AA) or acrylamide (AAm) in the presence of an in-plane electrochemical potential gradient applied to Au or indium-tin-oxide (ITO) working electrodes. PAA thickness gradients were converted into density gradients of fluorocarbons or biocompatible groups by derivatizing with NH(2)CH(2)(CF(2))(6)CF(3) or an Arg-Gly-Asp (RGD)-containing peptide, respectively. X-ray photoelectron spectroscopy (XPS) and XPS imaging were used to characterize the modified PAA gradients. Transition regions as narrow as 104 mum were achieved for fluorocarbon gradients. PAAm gradients were treated with gold particles to form a density gradient of gold particles. Surface plasmon resonance imaging and scanning electron microscopy (SEM) as well as UV-visible absorption measurements were used to characterize the gold particle density gradients. It is likely that the gold particles were attached both on the surface and inside the PAAm film.     

Sigma-aromaticity and sigma-antiaromaticity in saturated inorganic rings

The magnetic properties of a series of inorganic saturated rings, (SiH2)n, (GeH2)n, (NH)n, (PH)n, (AsH)n, On, Sn, and Sen (n = 3-6), exhibit zigzag behavior with ring size resembling that of aromatic and antiaromatic Hückel pi-systems and (CH2)n rings. Computed GIAO-SCF nucleus-independent chemical shifts (NICS) and localized (LMO) NICS analysis indicate that the sigma-ring electrons are chiefly responsible for this zigzag behavior. This evidence for sigma-aromaticity is further supported by theoretical strain energy (TSE). The Hückel 4n + 2/4n aromaticity/antiaromaticity rule for pi-electron systems applies well to the smaller saturated rings.