Assessment of the Equilibrium between Solid and Liquid Cobalt-Zinc Solutions

Experimental and literature informations on the phase equilibria f.c.c. solid/liquid Co Zn solutions, and about the thermodynamic properties of these alloys have been combined for the evaluation of two sets of polynomial coefficients serving to calculate thermodynamic or phase boundary values.     

The Activity of Substrates in the Catalyzed Nucleation of Undercooled Melts and Aqueous Aerosols

The aim of the present contribution is to analyze and to correlate the experimental evidence on the catalyzed nucleation in the crystallization of undercooled melts. This is done by using a new, generalized theoretical approach accounting for both the thermodynamics of adhesion (i.e. the interface bonding) and for correcting structural factors. Beside the usually considered lattice discrepancy substrate/overgrowing crystal in the present contribution as an additional structural corrective is introduced the polarizability of the building units of both interface constituents. It is shown that such a combined approach gives for the first time a way to correlate quantitatively experiment and theory in the case both of heterogeneous meteorolgical important nucleation catalysis and in the crystallization of glassforming organic and inorganic polymer melts. This approach leads to the possibility of predicting further promising nucleation catalysts and especially appropriate substrates in the induced crystallization of ice from undercooled water. An additional advantage of the theoretical concept derived here is that it allows also the determination of nucleation activity, Φ, of amorphous substrates or of crystallization cores with distorted structure – i.e. in cases, where the lattice discrepancy approach is impossible from the very beginning. Two main sources of experimental data are used: crystallization of aqueous aerosols induced by various crystallization cores and the heterogeneous crystallization of undercooled glassforming melts. Thus a new solution of a classical problem is attempted which may be of use in both meteorology and in the technical applications of induced nucleation by using foreign substrates.     

Algorithms for optimal area triangulations of a convex polygon

Given a convex polygon with n vertices in the plane, we are interested in triangulations of its interior, i.e., maximal sets of non-intersecting diagonals that subdivide the interior of the polygon into triangles. The MaxMin area triangulation is the triangulation of the polygon that maximizes the area of the smallest triangle in the triangulation. Similarly, the MinMax area triangulation is the triangulation that minimizes the area of the largest area triangle in the triangulation. We present algorithms that construct MaxMin and MinMax area triangulations of a convex polygon in O(n²logn) time and O(n²) space. The algorithms use dynamic programming and a number of geometric properties that are established within the paper.     

Hydroxyl radical and hydroxide ion in liquid water: a comparative electron density functional theory study

Ab initio density functional theory molecular dynamics simulations of the solvated states of the hydroxyl radical and hydroxide ion are performed using the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional (Becke, A. D. Phys. Rev. A 1988, 38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785). The structures of the solvation shells of the two species are examined. It is found that the OH radical forms a relatively well-defined solvation complex with four neighboring water molecules. Three of these molecules are hydrogen bonded to the OH, while the fourth is hemibonded via a three-electron two-centered bond between the oxygen atoms of the OH and water. The activity and the diffusion mechanism of the OH radical in water is discussed in comparison with the OH- ion. Although the results are partially influenced by the tendency of the BLYP density functional to overestimate hemibonded structure, the present simulations suggest that the widely accepted picture of rapid diffusion of OH radical in water through hydrogen exchange reaction may need to be reconsidered.     

Acid catalyzed intramolecular attack of β‐phenylthioureido group on amide function. Parallel formation of thiodihydrouracil and 4‐iminothiodihydrouracil. Different pathways in the Edman degradation reaction in the formation of six‐ versus five‐membered cyclic intermediates

Contrary to the cleavage of α‐phenylthioureido peptides 1 proceeding through intermediate 2‐anilinothiazolinone 2 , the b‐analog cis‐2‐(3‐phenylthioureido)cyclopentane‐carboxamide 5 forms transiently 4‐imino‐2‐thioxopyrimidine 6 . Monitoring amide cyclization and hydrolysis of iminopyrimidine 6 in acid by UV showed that an equilibrium between 5 and 6 was reached followed by slower conversion of both compounds into 2‐oxo‐4‐thioxopyrimidine 7 . Both processes were characterized by isosbestic points, the first due to parallel conversion of 5 into 6 and 7 (or 6 into 5 and 7 ) at a constant ratio while the second identical for both reactants – to conversion of equilibrated 5 and 6 into 7 . The special isosbestic points allowed the determination of the individual constants of Scheme 2. Further confirmation was obtained from NMR product analysis and following the cyclization of amide 5 in DMSO:DCl. Product 2‐oxo‐4‐ thioxopyrimidine 7 hydrolyzed reversibly to thioureido acid 8 . The cyclization rate of 8 allowed the participation of 6‐oxothiazine 10 formed by sulfur attack to be excluded. The absence of sulfur attack in the six‐membered case is explained by the longer C? S bond bringing about greater bond angle strain at the tetrahedral ring atoms due to the geometrical characteristics of five‐ and six‐membered rings with planar segments. The cyclizations of amide 5 to iminopyrimidine 6 and to thiodihydrouracil 7 are first order in [H⁺], while the reactions of protonated imine 6 H⁺ are zero order to amide and ?1 to thiodihydrouracil. The reaction orders can be reconciled by assuming a rate determining proton transfer from the tetrahedral intermediate in amide cyclization. Copyright © 2007 John Wiley & Sons, Ltd.     

Local cohomology modules with infinite dimensional socles

In this paper we prove the following generalization of a result of Hartshorne: Let be a commutative Noetherian local ring of dimension at least two, , and . Let be a homogeneous element of such that the coefficients of form a system of parameters for . Then the socle of is infinite dimensional.

     

Ribozymomimetic Chemical Synthesis of Peptide Bond Using Phosphorous Acid/Oxirane Mediators

Abstract

A model system is created that mimics natural ribosome as well as the way of ribozyme action. It uses oxirane analogue as condensation reagent and H₃PO₃ as activator for the carboxyl group and as protecting reagent for the NH₂ function. In order to demonstrate the ability of the H₃PO₃/oxirane system to mimic natural peptide bond synthesis on the ribosome as well as to mimic the ribozyme synthesis and hydrolysis of an inter biomonomeric bond, four model compounds containing an amide bond were synthesized using phosphorous acid/oxirane mediators.

GRAPHICAL ABSTRACT