Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)- semiquinone binuclear species

The monoanion of pyrazincarboxylic acid (PcA^–), the dianion of quinizarine (Qz⁼) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose Mn^II:Qz⁼:PcA^– stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at –0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a Mn^II-Mn^IIImixed-valence species. The latter is, in turn, reduced at –1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits themagnetic characteristic of manganese(III) and thesemiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.     

Bi-Lipschitz parameterization of surfaces

We consider surfaces Z homeomorphic to the plane with complete, possibly singular Riemannian metrics. If we have _ZK⁺<2– for the positive and _ZK^–<C for the negative part of the integral curvature, then Z is L-bi-Lipschitz equivalent to ² with L depending only on >0 and C>0. This result implies a conjecture by J. Fu.Supported by NSF grant DMS-0200566.     

Palladium-catalyzed cross-coupling reactions and electrocyclizations—efficient combinations for new cascade reactions

Palladium-catalyzed cross-coupling reactions and electrocyclic transformations as well as cycloadditions can efficiently be combined in one-pot sequences to build up complex molecules from simple precursors. 1,3-Dicyclopropyl-1,2-propadiene (1) could be coupled to various aryl halides, and the 1,3,5-hexatrienes resulting after rearrangement were trapped with different dienophiles. Further exploration of the highly reactive building block bicyclopropylidene (8) demonstrates the combinatorial potential of the two reaction modes in terms of two different three-component reactions, and even a novel four-component reaction was readily developed. Additional increase of complexity was gained by combining an intramolecular Heck reaction with consecutive trapping by 8, which can be carried out with or without 6π-electrocyclization. Finally a Stille-Heck-coupling sequence of substituted 2-bromocycloalkenyl triflates with functionally substituted alkenylstannanes and acrylic esters led to highly substituted 1,3,5-hexatrienes, which smoothly underwent 6π-electrocyclization at elevated temperatures to yield bi- and tricyclic skeletons. With this methodology at hand, a new versatile access to steroid-analogues with a diene moiety in the B ring and a functionality at C-7 was developed.     

Synthesis of 3-aminolactams as X-Gly constrained pseudodipeptides and conformational study of a Trp-Gly surrogate

3-Amino-delta-valerolactams trans-11a-c were synthesized through conjugate addition and Curtius rearrangement and converted into Fmoc-[Trp-Gly], Fmoc-[Ile-Gly], and Fmoc-[Phe-Gly] pseudodipeptides. Conformational analyses of tripeptide analogues Ac-[Trp-Gly]-Leu-NH(2) 17a and 17b by NMR experiments and molecular modeling calculations showed that diastereomer 17a adopted a gamma-turn/distorted type II beta-turn structure, whereas diastereomer 17b adopted mainly a gamma-turn structure.     

Packing optimization for automated generation of complex system's initial configurations for molecular dynamics and docking

Molecular Dynamics is a powerful methodology for the comprehension at molecular level of many chemical and biochemical systems. The theories and techniques developed for structural and thermodynamic analyses are well established, and many software packages are available. However, designing starting configurations for dynamics can be cumbersome. Easily generated regular lattices can be used when simple liquids or mixtures are studied. However, for complex mixtures, polymer solutions or solid adsorbed liquids (for example) this approach is inefficient, and it turns out to be very hard to obtain an adequate coordinate file. In this article, the problem of obtaining an adequate initial configuration is treated as a "packing" problem and solved by an optimization procedure. The initial configuration is chosen in such a way that the minimum distance between atoms of different molecules is greater than a fixed tolerance. The optimization uses a well-known algorithm for box-constrained minimization. Applications are given for biomolecule solvation, many-component mixtures, and interfaces. This approach can reduce the work of designing starting configurations from days or weeks to few minutes or hours, in an automated fashion. Packing optimization is also shown to be a powerful methodology for space search in docking of small ligands to proteins. This is demonstrated by docking of the thyroid hormone to its nuclear receptor.     

Variability in goethite surface site density: evidence from proton and carbonate sorption

Goethite is a representative iron oxide in natural environments due to its abundance and thermodynamic stability and may be responsible for many surface-mediated processes, including ion retention and mobility in aqueous settings. A large variability in morphologies and specific surface areas of goethite crystals exists but little work has been done to compare surface reactivity between them. The present work offers experimental evidence for the existence of an inverse relationship between sorption capacity for protons and carbonate ions and specific surface area of goethite for three synthetic goethite preparations spanning surface area differences by a factor of 2. An explanation for this was found by assuming a variable reactive site density between preparations in direct relationship to their sorption capacity based on congruency of carbonate sorption computed on a per-site basis. Previous evidence of maximum sorption capacities supports this explanation, and site density ratios between the goethites studied here were obtained. Triple layer surface complexation modeling was successful in describing adsorption data for all goethite preparations using equal stoichiometries. A new formulation of standard state for activities of surface species based on a 1.0 mole fraction of sites on the solid allowed transformation of the conventional molar concentration-based affinity constants to values based on site occupancy. In this fashion, by applying the appropriate site density ratios, a single set of affinity constant values was found that described accurately the adsorption data for all preparations.     

Real Interpolation and Two Variants of Gehring's Lemma

Let be a fixed open cube in Rⁿ. For r[1, ) and [0, ) we define where Q is a cube in Rⁿ (with sides parallel to the coordinateaxes) and _Q stands for the characteristic function of the cubeQ. A well-known result of Gehring [5] states that if (1.1) for some p(1, ) and c(0, ), then there exist q(p, ) and C=C(p,q, n, c)(0, ) such that for all cubes Q, where |Q| denotes the n-dimensional Lebesguemeasure of Q. In particular, a function fL¹() satisfying (1.1)belongs to L^q(). In [9] it was shown that Gehring's result is a particular caseof a more general principle from the real method of interpolation.Roughly speaking, this principle states that if a certain reversedinequality between K-functionals holds at one point of an interpolationscale, then it holds at other nearby points of this scale. Usingan extension of Holmstedt's reiteration formulae of [4] andresults of [8] on weighted inequalities for monotone functions,we prove here two variants of this principle involving extrapolationspaces of an ordered pair of (quasi-) Banach spaces. As an applicationwe prove the following Gehring-type lemmas.     

Dynamics, Thermodynamics, and Time Asymmetry

There are two schools, or lines, of thoughtwhich attempt to unify the apparently divergent laws ofdynamics and thermodynamics and to explain the observedtime asymmetry of the universe and most of its subsystems in spite of the fact that thesesystems are driven by time-symmetric evolutionequations. They will be called the coarse-graining andthe extended dynamics schools (even if these names onlypartially describe their philosophy). The coarse-grainingschool obtains time asymmetry via a projection of thestate space onto a space of relevantstates. The corresponding projection of the primitivereversible evolution laws yields effective irreversibleevolution laws for the relevant states. Extendeddynamics always uses the same primitive reversibleevolution laws. But these laws (in adequate extensionsof the usual spaces where they are formulated)have a set of solutions S that can be decomposed intotwo subsets S₊ and S_- oftime-asymmetric solutions. Time asymmetry is establishedby choosing one of these two sets as the arena in which to formulate thetheory. This paper explains in the simplestself-contained and unbiased way the main characteristicsof both schools and points out the advantages anddisadvantages of each, in such a way as to make explicit thedebate between the schools. Some cosmological featuresof the theory are also considered, mainly the problem ofthe low-entropy initial state of the universe.     

Acid-catalysed,nucleophile-catalysed and thermal decomposition of 2-amino-3-(2′,2′-dimethylaziridino)-1,4-naphthoquinone

The course of the thermal, acid-catalysed and iodide-catalysed decomposition of 2-amino-3-(2′,2′-dimethylaziridino)-1,4-naphthoquinone (III) was investigated. Thermal and iodide-catalysed decompositions gave mainly 2,3-diamino-1,4-naphthoquinone (VI) and 2-amino-3-(2′-methylallylamino)-1,4-naphthoquinone (V) together with low amounts of 2,2-dimethyl-1,2,3,4,5,10-hexahydrobenzo[g]quinoxaline (IV) and 2-isopropyl-1H-naphthoimid-azole-4,9-dione (VII). The acid catalysed isomerization of the aziridinonaphthoquinone III with halohydric acids or with acetic acid readily gave the opening of the aziridine ring; the corresponding salts of 2-amino-3-(2′-haloisobutylamino)-1,4-naphthoquinones (VIIIa-c) and 2-amino-3-(2′-acetoxyisobutylamino)-1,4-naphthoqunone (X) were formed by cleavage of the carbon-nitrogen bond at the substituted carbon atom. Hypotheses on the mechanism of these reactions are given.