Inhibition and acceleration of deuterium exchange in amide-functionalized monolayer-protected gold clusters

Hydrogen/deuterium exchange rates in amide-functionalized monolayer protected gold clusters (MPCs) are controlled by the radial nature of the surface, with inhibition and catalysis observed at different chain lengths.     

Finding Optimal Routings in Hamming Graphs

A routing R in a graph consists of a simple path p_uvfromu to v for each ordered pair of distinct vertices (u, v). We will call R optimal if all the paths p_uvare shortest paths and if edges of the graph occur equally often in the paths of R. In 1994, Solé gave a sufficient condition involving the automorphism group for a graph to have an optimal routing in this sense. Graphs which satisfy Solé’s condition are called orbital regular graphs. It is often difficult to determine whether or not a given graph is orbital regular. In this paper, we give a necessary and sufficient condition for a Hamming graph to be orbital regular with respect to a certain natural subgroup of automorphisms.     

On Primitive Representations of Finite Alternating and Symmetric Groups with a 2-Transitive Subconstituent

In this paper we analyse primitive permutation representations of finite alternating and symmetric groups which have a 2-transitive subconstituent. We show that either the representation belongs to an explicit list of known examples, or the point stabiliser is a known almost-simple 2-transitive group and acts primitively in the natural representation of the associated alternating or symmetric group.     

On orbital partitions and exceptionality of primitive permutation groups

Let and be transitive permutation groups on a set such that is a normal subgroup of . The overgroup induces a natural action on the set of non-trivial orbitals of on . In the study of Galois groups of exceptional covers of curves, one is led to characterizing the triples where fixes no elements of ; such triples are called exceptional. In the study of homogeneous factorizations of complete graphs, one is led to characterizing quadruples where is a partition of such that is transitive on ; such a quadruple is called a TOD (transitive orbital decomposition). It follows easily that the triple in a TOD is exceptional; conversely if an exceptional triple is such that is cyclic of prime-power order, then there exists a partition of such that is a TOD. This paper characterizes TODs such that is primitive and is cyclic of prime-power order. An application is given to the classification of self-complementary vertex-transitive graphs.

     

Dioxygenase-catalysed oxidation of monosubstituted thiophenes: sulfoxidation versus dihydrodiol formation

Toluene dioxygenase (TDO)-catalysed sulfoxidation, using Pseudomonas putida UV4, was observed for the thiophene substrates 1A-1N. The unstable thiophene oxide metabolites, 6A-6G, 6K-6N, spontaneously dimerised yielding the corresponding racemic disulfoxide cycloadducts 7A-7G, 7K-7N. Dimeric or crossed [4 + 2] cycloaddition products, derived from the thiophene oxide intermediates 6A and 6D or 6B and 6D, were found when mixtures of thiophene substrates 1A and 1D or 1B and 1D were biotransformed. The thiophene sulfoxide metabolite 6B was also trapped as cycloadducts 17 or 18 using stable dienophiles. Preferential dioxygenase-catalysed oxidation of the substituent on the thiophene ring, including exocyclic sulfoxidation (1H-1J) and cis-dihydroxylation of a phenyl substituent (1G and 1N), was also observed. An enzyme-catalysed deoxygenation of a sulfoxide in P. putida UV4 was noticed when racemic disulfoxide cyclo-adducts 7A, 7B and 7K were converted to the corresponding enantioenriched monosulfoxides 8A, 8B and 8K via a kinetic resolution process. The parent thiophene 1A and the 3-substituted thiophenes 1K-1N were also found to undergo ring dihydroxylation yielding the cis/trans-dihydrodiol metabolites 9A and 9K-9N. Evidence is provided for a dehydrogenase-catalysed desaturation of a heterocyclic dihydrodiol (9Kcis/9Ktrans) to yield the corresponding 2,3-dihydroxythiophene (24) as its preferred thiolactone tautomer (23). A simple model to allow prediction of the structure of metabolites, formed from TDO-catalysed bacterial oxidation of thiophene substrates 1, is presented.     

Ethynyl and Propynylpyrene Inhibitors of Cytochrome P450

Abstract  

The single-crystal X-ray structures and in vivo activities of three aryl acetylenic inhibitors of cytochromes P450 1A1, 1A2, 2A6, and 2B1 have been determined and are reported herein. These are 1-ethynylpyrene, 1-propynylpyrene, and 4-propynylpyrene. To investigate electronic influences on the mechanism of enzyme inhibition, the experimental electron density distribution of 1-ethynylpyrene has been determined using low-temperature X-ray diffraction measurements, and the resulting net atomic charges compared with various theoretical calculations. A total of 82,390 reflections were measured with Mo Kα radiation to a (sinθ/λ)_max = 0.985 Å⁻¹. Averaging symmetry equivalent reflections yielded 8,889 unique reflections. A least squares refinement procedure was used in which multipole parameters were added to describe the distortions of the atomic electron distributions from spherical symmetry. A map of the model electron density distribution of 1-ethynylpyrene was obtained. Net atomic charges calculated from refined monopole population parameters yielded charges that showed that the terminal acetylenic carbon atom (C18) is more negative than the internal carbon (C17). Net atomic charges calculated by ab initio, density functional theory, and semi-empirical methods are consistent with this trend suggesting that the terminal acetylenic carbon atom is more likely to be the site of oxidation. This is consistent with the inhibition mechanism pathway that results in the formation of a reactive ketene intermediate. This is also consistent with assay results that determined that 1-ethynylpyrene acts as a mechanism-based inhibitor of P450s 1A1 and 1A2 and as a reversible inhibitor of P450 2B1. Crystallographic data: 1-ethynylpyrene, C₁₈H₁₀, P2₁/c, a = 14.571(2) Å, b = 3.9094(5) Å, c = 20.242(3) Å, β = 105.042(2)°, V = 1,113.5(2) Å³; 1-propynylpyrene, C₁₉H₁₂, P2₁/n, a = 8.970(2) Å, b = 10.136(1) Å, c = 14.080(3) Å, β = 99.77(2)°, V = 1,261.5(4) Å³; 4-propynylpyrene, C₁₉H₁₂, Pbca, a = 9.904(1) Å, b = 13.174(2) Å, c = 19.401(1) Å, V = 2,531.4(5) Å³.     

Rapid annulation of tropolone units via a pyrylium ylide 1,3-dipolar cycloaddition reaction

The tropolone subunit of the naturally occurring alkaloid rubrolone aglycon is synthesized via a short reaction sequence starting with a 1,3-dipolar cycloaddition of a pyrylium ylide and indenone, followed by enone oxidation, oxygen-bridge elimination and finally hydroxy group oxidation.     

Logarithmic asymptotics for a single-server processing distinguishable sources

We consider a single-server first-in-first-out queue fed by a finite number of distinct sources of jobs. For a large class of short-range dependent and light-tailed distributed job processes, using functional large deviation techniques we prove a large deviation principle and logarithmic asymptotics for the joint waiting time and queue lengths distribution. We identify the paths that are most likely to lead to the rare events of large waiting times and long queue lengths. A number of examples are presented to illustrate salient features of the results.