Conditions for Permutability of Congruences in Implication Algebras

In this paper we give conditions on an implication algebra A so that two congruences θ ₁, θ ₂ on A permute, i.e. θ ₁ ∘ θ ₂ = θ ₂ ∘ θ ₁. We also provide simpler conditions for permutability in finite implication algebras. Finally we present some applications of these characterizations. The support of Universidad Nacional del Sur and CONICET is gratefully acknowledged.     

UV radiation as a potential driving force for zooplankton community structure in Patagonian lakes

This article explores the potential role of UV radiation (UVR) as an influence on zooplankton communities. In the first section we provide a general overview of UVR effects on freshwater zooplankton, with an emphasis on Argentine and Chilean environments. In the second section we present the results of a survey involving 53 temperate lakes across a gradient of UVR exposure to determine patterns of species richness and specific diversity. These community characteristics decreased at high potential UVR exposure (i.e. high mean water column irradiance or low lake optical density). A threshold value of mean water column irradiance of approximately 10% of the surface level seems to limit both richness and diversity to minimum values. On the basis of the collected evidence it is not possible to definitely conclude that UVR rather than another covarying factor is responsible for the decrease in specific diversity observed at the lowest end of lake optical depth. However, lakes with values above the previous threshold are likely to exhibit highly depauperate zooplankton communities regardless of the mechanism. As a cautionary note we suggest that changes in the optical characteristics (i.e. changes due to atmospheric conditions, precipitation patterns or vertical displacement of the tree line) may result in sudden shifts in zooplankton community structure.     

Genetically engineered synthesis and structural characterization of cobalt-precorrin 5A and -5B, two new intermediates on the anaerobic pathway to vitamin B12: definition of the roles of the CbiF and CbiG enzymes

Two new cobalt corrinoid intermediates, cobalt-precorrin 5A and cobalt-precorrin 5B, have been synthesized with the aid of overexpressed enzymes of the vitamin B(12) pathway of Salmonella entericaserovar typhimurium. These compounds were made in several regioselectively (13)C-labeled forms, and their structures have been established by multidimensional NMR spectroscopy. The addition of CbiF to the enzymes known to synthesize cobalt-precorrin 4 resulted in the formation of cobalt-precorrin 5A, and the inclusion of CbiG with CbiF produced cobalt-precorrin 5B, which has allowed us to define the role of these enzymes in the anaerobic biosynthetic pathway. CbiF is the C-11 methylase, and CbiG, an enzyme which shows homology with CobE of the aerobic pathway, is the gene product responsible for the opening of the ring A delta-lactone and extrusion of the "C(2)" unit. The discovery of these long-sought intermediates paves the way for defining the final stages of the anaerobic pathway. It is of considerable evolutionary interest that nature uses two distinct pathways to vitamin B(12), both conserved over several billion years and featuring completely different mechanisms for ring-contraction of the porphyrinoid to the corrinoid ring system. Thus the aerobic pathway utilizes molecular oxygen to trigger the events at C-20 leading to contraction and expulsion of the "C(2)" unit as acetic acid from a metal-free intermediate, whereas the anaerobic route features internal delivery of oxygen from a carboxylic acid terminus to C-20 followed by extrusion of the "C(2)" unit as acetaldehyde, using cobalt complexes as substrates.     

Highly convergent three component benzyne coupling: the total synthesis of ent-clavilactone B

The first total synthesis of (+)-clavilactone B, a potent antifungal agent and novel tyrosine kinase inhibitor, is described. The absolute configuration of clavilactones has been unambiguously established by using Sharpless asymmetric epoxidation to generate the enantiomerically pure substrate. The strategy highlights the use of a powerful and convergent three-component benzyne coupling with a methylallyl Grignard and a chiral epoxy-aldehyde to generate two C-C bonds and install the carbon skeleton of clavilactone. Oxidative lactonization, ten-membered ring construction by ring closing metathesis, and oxidation gave clavilactone B.     

Geometry of phase plane and radial solutions for nonlinear elliptic equations with extremal operators

We study the existence of positive radially symmetric solutions to a class of nonlinear elliptic problems involving extremal operators and nonlinearity of exponential or polynomial type. According to the values of a parameter, we describe situations where the equation has one, finitely many, infinitely many or no solutions, by using the geometry structure of phase plane.     

High-energy solutions for a phase transition problem

In this article we study the existence and profile of high-energy solutions for the phase transition model

     

Hydrogenation of citral on Ir/SiO2 catalysts. Effect of the addition of Nb2O5 on surface and catalytic properties

Hydrogenation of citral over Ir supported on SiO₂, Nb₂O₅ and Ir/Nb₂O₅-SiO₂ catalysts reduced at 473 K (LT) and 773 K (HT) was studied. High selectivity to unsaturated alcohol was found, the main reaction products being geraniol and nerol. The LT catalysts show higher conversion levels, the one with a higher Nb content displays highest conversion.