Anion induced and inhibited circumrotation of a [2]catenane

The first example of a catenane capable of performing circumrotation via an anion switching methodology is described; of particular interest is a conformational locking mechanism which results from chloride coordination in the catenane binding cavity.     

Unusual odd-electron fragments from even-electron protonated prodiginine precursors using positive-ion electrospray tandem mass spectrometry

Reports of anticancer and immunosuppressive properties have spurred recent interest in the bacterially produced prodiginines. We use electrospray tandem mass spectrometry (ES-MS/MS) to investigate prodigiosin, undecylprodiginine, and streptorubin B (butyl-meta-cycloheptylprodiginine) and to explore their fragmentation pathways to explain the unusual methyl radical loss and consecutive fragment ions that dominate low-energy collision-induced dissociation (CID) mass spectra. The competition between the formation of even-electron ions and radical ions is examined in detail. Theoretical calculations are used to optimize the structures and calculate the energies of both reactants and products using the Gaussian 03 program. Results indicate that protonation occurs on the nitrogen atom that initially held no hydrogen, thus allowing formation of a pseudo-seven-membered ring that constitutes the most stable ground state [M + H](+) structure. From this precursor, experimental data show that methyl radical loss has the lowest apparent threshold but, alternatively, even-electron fragment ions can be formed by loss of a methanol molecule. Computational modeling indicates that methyl radical loss is the more endothermic process in this competition, but the lower apparent threshold associated with methyl radical loss points to a lower kinetic barrier. Additionally, this characteristic and unusual loss of methyl radical (in combination with weaker methanol loss) from each prodiginine is useful for performing constant neutral loss scans to quickly and efficiently identify all prodiginines in a complex biological mixture without any clean-up or purification. The feasibility of this approach has been proven through the identification of a new, low-abundance prodigiosin analog arising from Hahella chejuensis.     

Conjugacy languages in groups

We study the regularity of several languages derived from conjugacy classes in a finitely generated group G for a variety of examples including word hyperbolic, virtually abelian, Artin, and Garside groups. We also determine the rationality of the growth series of the shortlex conjugacy language in virtually cyclic groups, proving one direction of a conjecture of Rivin.     

Voltammetric sizing of particles: chronoamperometry of impact events in acoustically agitated particulate suspensions

This communication presents initial results into the use of chronoamperometry of acoustically agitated particulate suspensions at a dual electrode as a means of approximately sizing the suspended particles.     

Steiner triple systems with disjoint or intersecting subsystems

The existence of incomplete Steiner triple systems of order υ having holes of orders w and u meeting in z elements is examined, with emphasis on the disjoint (z = 0) and intersecting (z = 1) cases. When and , the elementary necessary conditions are shown to be sufficient for all values of z. Then for and υ “near” the minimum of , the conditions are again shown to be sufficient. Consequences for larger orders are also discussed, in particular the proof that when one hole is at least three times as large as the other, the conditions are again sufficient. © 2000 John Wiley & Sons, Inc. J Combin Designs 8: 58–77, 2000     

Maximal sets of triangle-factors on v = 6m + 3 vertices

We show that the necessary condition m + 1 ≤ k ≤ 3m + 1 that there exist a maximal set of k triangle-factors on 6m + 3 ≥ 15 vertices is also sufficient, except possibly when k = m + 1, or when 6m + 3ϵ{45, 57, 69, 81, 93, 237, 261, 309, 333, 381}. © 1998 John Wiley & Sons, Inc. J Combin Designs 6: 309–323, 1998     

Frequency‐Response Evidence for Parallel Diffusion Processes in the Bimodal Micropore System of an Activated Carbon

The rates of mass transport of gases such as propane, carbon dioxide, and ammonia have been studied in an activated carbon sorbent. The frequency‐response (FR) method was used to measure these rates. Small‐angle X‐ray scattering (SAXS) and nitrogen‐adsorption isotherm at 77 K showed that the activated carbon contains small micropores with diameters around 0.64 and 0.92 nm, respectively. The FR‐rate spectra distinguished two parallel diffusion processes with different time constants, suggesting that the smaller and the larger micropores were not interconnected.     

The Effect of Conducting Boundaries on the Onset of Convection in a Porous Layer Which is Heated from Below by Internal Heating

The onset of convection in a porous layer heated from below is considered, and we determine how the presence of two solid but heat-conducting bounding plates of finite thickness alters the manner in which convection ensues. Heat is generated by the lower plate (with an insulating lower boundary), but the upper one is passive with a fixed upper boundary temperature. It is shown that this composite layer may mimic in turn one of the three different types of classical single-layer onset problems which are well-known in the literature. The type which is selected (or indeed whether it corresponds to a transitional case) depends quite critically on the precise values of the relative thickness of the solid layers and their conductivity ratio. It is also shown that care needs to be taken over declaring that the solid plates are thin: extreme values of the conductivity ratio can yield a stability criterion which appears to be different from that suggested by the imposed boundary conditions.