Operators that are points of spectral continuity

In this paper a characterization is obtained of those bounded operators on a Hilbert space at which the spectrum is continuous, where the spectrum is considered as a function whose domain is the set of all operators with the norm topology and whose range is the set of compact subsets of the plane with the Hausdorff metric. Similar characterizations of the points of continuity of the Weyl spectrum, the spectral radius, and the essential spectral radius are also obtained.The first author was supported by National Science Foundation Grant MCS 77-28396.     

Minimal-energy clusters of hard spheres

What is the tightest packing ofN equal nonoverlapping spheres, in the sense of having minimal energy, i.e., smallest second moment about the centroid? The putatively optimal arrangements are described forN≤32. A number of new and interesting polyhedra arise.     

Electrocatalytic effect of the oxide film at Pt anodes on Cl• recombination kinetics in chlorine evolution

In aqueous medium, Cl₂ evolution on noble metal anodes occurs on a surface oxide film, the properties of which, rather than those of the metal, determine the kinetics of the reaction. The kinetics of Cl₂ evolution on an oxidized Pt electrode in aqueous KCl are compared with those for Cl₂ evolution on a demonstrably unoxidized Pt surface in anhydrous trifluoroacetic acid (TFA) as solvent. Hence the effects of development of an oxide film on Pt in aqueous medium can be determined in relation to the kinetic behaviour of Cl₂ evolution on the unoxidized Pt metal surface. In both TFA and water, and their mixtures, it is shown that Cl₂ evolution kinetics are recombination-controlled and non-diffusion-controlled limiting currents, i_lim, are observed. The i_lim values in aqueous medium, i.e. for the oxide-covered surface, are some 45 times larger than on the free Pt surface in TFA; since the mechanism is shown to be the same in the two solvents, and their mixtures, this observation indicates a substantial electrocatalytic enhancement of the recombination rate for Cl^? on the oxide in comparison with the Pt metal surface. This is presumably due to weaker binding of Cl^? on oxidized Pt than on the “bare” Pt surface. Effects due to specific adsorption of Cl^? ion are also important and give rise to dependence of the recombination catalysis on Cl^? ion concentration. A method is proposed for analyzing the recombination kinetics by means of linear plots which give both the recombination rate constant and the quasi-equilibrium constant for the prior Cl^? ion discharge step as a function of water content.     

Ammonium formate-based one-pot reductive Heck reactions for the construction of cyclic sulfonamides

A modified method is reported for the conversion of unsaturated sulfonamides into their cyclic saturated counterparts. This method utilises a single palladium catalyst for an intramolecular Heck reaction and subsequent transfer hydrogenation, which is achieved in one-pot following the addition of ammonium formate. Accordingly, a range of fourteen structural variations are reported and under optimal conditions the adducts were generated in typically good to excellent yields. Notably, discrimination of differentially substituted dienes can be accomplished in the case of compounds 28 and 29 and the process was only observed to fail with the more sterically hindered precursor 32.     

Unexpected Acetylation of Endogenous Aliphatic Amines by Arylamine N-Acetyltransferase NAT2

N-Acetyltransferases play critical roles in the deactivation and clearance of xenobiotics, including clinical drugs. NAT2 has been classified as an arylamine N-acetyltransferase that mainly converts aromatic amines, hydroxylamines, and hydrazines. Herein, we demonstrate that the human arylamine N-acetyltransferase NAT2 also acetylates aliphatic endogenous amines. Metabolomic analysis and chemical synthesis revealed increased intracellular concentrations of mono- and diacetylated spermidine in human cell lines expressing the rapid compared to the slow acetylator NAT2 phenotype. The regioselective N⁸-acetylation of monoacetylated spermidine by NAT2 answers the long-standing question of the source of diacetylspermidine. We also identified selective acetylation of structurally diverse alkylamine-containing drugs by NAT2, which may contribute to variations in patient responses. The results demonstrate a previously unknown functionality and potential regulatory role for NAT2, and we suggest that this enzyme should be considered for re-classification.