2,4‐Dioxa‐7‐aza‐, 2,4‐Dioxa‐8‐aza‐, and 2,4‐Dioxa‐9‐aza‐3‐phosphadecalins as Rigid Acetylcholine Mimetics: Syntheses and Characterization

Phosphorylation of suitable piperidine precursors yielded a series of novel decalin‐type O,N,P‐heterocycles. The title compounds, P(3)‐axially and P(3)‐equatorially X‐substituted, cis‐ and trans‐configurated 2,4‐dioxa‐7‐aza‐, 2,4‐dioxa‐8‐aza‐, and 2,4‐dioxa‐9‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides (X=Cl, F, 4‐nitrophenoxy, and 2,4‐dinitrophenoxy), are configuratively fixed and conformationally constrained P‐analogues of acetylcholine and as such represent acetylcholine (7‐aza and 9‐aza isomers) or γ‐homo‐acetylcholine mimetics (8‐aza isomers). Being irreversible inhibitors of acetylcholinesterase (AChE), the compounds are considered to be suitable probes for the investigation of the stereochemical course of the inhibition reaction by ³¹P‐NMR spectroscopy. Moreover, the design of these mimetics will enable studies of molecular interactions with AChE, in particular, the recognition conformation of acetylcholine.     

On cycle permutation graphs

Permutation graphs were first introduced by Chartrand and Harary in 1967 [5]. The purpose of this paper is to study some properties of cycle permutation graphs. A determination of some of their crossing numbers, in keeping with the topological nature of the Chartrand and Harary paper is followed by the determination of all permutations yielding certain isomorphic permutation graphs, extending the algebraic results for planar graphs obtained by those authors.     

Survey of results on the maximum genus of a graph

Some of the early questions concerning the maximum genus of a graph have now been answered. In this paper we survey the progress made on such problems and present some recent results, outlining proofs for some of the major theorems.     

Generalized petersen graphs which are cycle permutation graphs

A cycle permutation graph is obtained by taking two n-cycles each labeled 1, 2,…, n, along with the edges obtained by joining i in the first copy to α(i) in the second, where α ∈ S_n. A characterization of the intersection between cycle permutation graphs and the generalized Petersen graphs as defined by Watkins (J. Combin. Theory6 (1969), 152–164), is given.     

Vapor deposition of intact polyethylene glycol thin films

Thin films of polyethylene glycol (PEG) of average molecular weight, 1400 amu, were deposited by both matrix-assisted pulsed laser evaporation (MAPLE) and pulsed laser deposition (PLD). The deposition was carried out in vacuum (∼10^-6 Torr) with an ArF (λ=193 nm) laser at a fluence between 150 and 300 mJ/cm². Films were deposited on NaCl plates, Si(111) wafers, and glass slides. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy (FTIR), electrospray ionization (ESI) mass spectrometry, and matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometry. The results show that the MAPLE films nearly identically resemble the starting material, whereas the PLD films do not. These results are discussed within the context of biomedical applications such as drug delivery coatings and in vivo applications where there is a need for transfer of polymeric coatings of PEG without significant chemical modification. Received: 2 March 2001 / Accepted: 5 March 2001 / Published online: 23 May 2001     

A circular-arc characterization of certain rectilinear drawings

In this paper we give a construction that produces exactly those graphs having maximum rectilinear crossing number equal to the subthrackle bound. We then prove a theorem characterizing these graphs in terms of proper circular-arc graphs. © 1996 John Wiley & Sons, Inc.     

Stereochemistry of the Inhibition of δ-Chymotrypsin with Optically Active Bicyclic Organophosphates: 31P-NMR studies

The inhibition of δ-chymotrypsin with optically active, axially and equatorially substituted trans-3-(2,4-dini-trophenoxy)-2,4-dioxa-3λ⁵-phospbubicyclo[4.4.0]decan-3-ones ( = hexahydro-4H-1,3,2-benzodioxaphosphorin 3-oxides) was investigated. Their inhibitory power was determined by kinetic measurements, and the stereochemical course of the reaction of stoichiometric amounts of the enzyme and inhibitor was monitored with ³¹P-NMR spectroscopy at pH 7.8. The irreversible inhibitors show significant enantioselectivity (the (S_p)-enantiomer reacting faster) and yield diastereoisomeric, covalently phosphorylated derivatives of δ-chymotrypsin. ³¹P-NMR Spectroscopic studies of the inhibition by the axially substituted inhibitor revealed for the racemic (±) -2a first a resonance at –4.4 ppm and later, while inhibition proceeded, a second one at –4.5 ppm. The reaction with optically active (+) -2a showed only one signal at –4.4 ppm and its enantiomer (–) -2a only one signal at –4.5 ppm. Using the equatorially substituted racemic epimer (±) -2b , we observed the main resonance at –5.3 ppm and two minor ones at –4.4 and –4.5 ppm. The optically active compound (+) -2b showed two peaks at –4.5 and –5.3 ppm, whereas its antipode (–) -2b revealed two signals at –4.4 and –5.3 ppm. Comparing the ³¹P chemical shifts of the corresponding covalent phosphoserine derivatives 4a (-5.7 ppm, axial) and 4b (-4.5 ppm, equatorial) shows the inhibition with the axial compounds 2a to proceed via neat inversion of the configuration at the P-atom, whereas the equatorial epimers 2b with a higher conformational flexibility seem to follow a different stereochemical pathway which results in both inversion and retention.     

Cyclic voltammetric analysis of the electron transfer of Shewanella oneidensis MR-1 and nanofilament and cytochrome knock-out mutants

Shewanella is frequently used as a model microorganism for microbial bioelectrochemical systems. In this study, we used cyclic voltammetry (CV) to investigate extracellular electron transfer mechanisms from S. oneidensis MR-1 (WT) and five deletion mutants: membrane bound cytochrome (?mtrC/ΔomcA), transmembrane pili (ΔpilM-Q, ΔmshH-Q, and ΔpilM-Q/ΔmshH-Q) and flagella (?flg). We demonstrate that the formal potentials of mediated and direct electron transfer sites of the derived biofilms can be gained from CVs of the respective biofilms recorded at bioelectrocatlytic (i.e. turnover) and lactate depleted (i.e. non-turnover) conditions. As the biofilms possess only a limited bioelectrocatalytic activity, an advanced data processing procedure, using the open-source software SOAS, was applied. The obtained results indicate that S. oneidensis mutants used in this study are able to bypass hindered direct electron transfer by alternative redox proteins as well as self-mediated pathways.     

An additivity theorem for maximum genus of a graph

Let G be a finite, connected graph with no loops or multiple edges. If G is the union of two blocks, then a necessary and sufficient condition is given for the maximum genus of G to be the sum of the maximum genera of its blocks. If in addition the blocks of G are upper embeddable, then a necessary and sufficient condition is given for the upper embeddability of G.