Pulsed EPR studies of polycrystalline cesium hexamethyl hexacyclen sodide

The electron spin echo envelope modulation (ESEEM) technique of pulsed EPR spectroscopy has been used to measure weak¹³³Cs hyperfine couplings to trapped electrons in polycrystalline cesium hexamethyl hexacyclen sodide. Two magnetically distinct groups of weakly coupled¹³³Cs nuclei were found — one with an isotropic electron-nuclear hyperfine (Fermi contact) coupling of 0.34 MHz and the other with a contact coupling less than 0.1 MHz. Analysis of these data, by computer simulation, shows that the number of Cs nuclei that give rise to these interactions and their distance from the paramagnetic center is consistent with the hypothesis that the trapped electrons occupy vacant anion sites. The results indicate that the spread of unpaired electron spin density, along the axis formed by the “contact ion pair,” may be greater than that in the plane perpendicular to this axis.     

Joint pricing and ordering policy for an advance booking system with partial order cancellations

In this paper, an inventory model for deteriorating items with price-dependent demand is developed. The cycle is divided into two periods, where an advance sales period is followed by a spot sales period. In practice, customers with reservations may cancel their orders before receiving them. During the advance sales period, the rate of reservations which will not be cancelled is dependent on the length of the waiting time for the receiving order. During the spot sales period, all customers receive their orders at the time of the purchase. We prove the existence of the realistic relationship that the advance sales price is smaller than the spot sales price. We also develop some useful properties and provide an iterative procedure for solving the maximization problem. Numerical examples are given to demonstrate the effectiveness of the proposed approach and we conclude the paper with suggestions for possible future research.     

An EOQ model for deteriorating items with time varying demand and partial backlogging

In the classical economic order quantity model, it is often assumed that the shortages are either completely backlogged or completely lost. However, in some inventory systems, it is more reasonable to assume that the backlogging rate is dependent on the length of the waiting time for the next replenishment. The longer the waiting time is, the smaller the backlogging rate would be. In this paper, we focus on the effect of the backlogging rate on the economic order quantity decision. Numerical examples are presented to illustrate the model.     

Reexamination of lead(II) coordination preferences in sulfur-rich sites: implications for a critical mechanism of lead poisoning

Recent studies suggest that the developmental toxicity associated with childhood lead poisoning may be attributable to interactions of Pb(II) with proteins containing thiol-rich structural zinc-binding sites. Here, we report detailed structural studies of Pb(II) in such sites, providing critical insights into the mechanism by which lead alters the activity of these proteins. X-ray absorption spectroscopy of Pb(II) bound to structural zinc-binding peptides reveals that Pb(II) binds in a three-coordinate Pb(II)-S(3) mode, while Zn(II) is known to bind in a four-coordinate mode in these proteins. This Pb(II)-S(3) coordination in peptides is consistent with a trigonal pyramidal Pb(II)-S(3) model compound previously reported by Bridgewater and Parkin, but it differs from many other reports in the small molecule literature which have suggested Pb(II)-S(4) as a preferred coordination mode for lead. Reexamination of the published structures of these "Pb(II)-S(4)" compounds reveals that, in almost all cases, the coordination number of Pb is actually 5, 6, or 8. The results reported herein combined with this new review of published structures suggest that lead prefers to avoid four-coordination in sulfur-rich sites, binding instead as trigonal pyramidal Pb(II)-S(3) or as Pb(II)-S(5-8). In the case of structural zinc-binding protein sites, the observation that lead binds in a three-coordinate mode, and in a geometry that is fundamentally different from the natural coordination of zinc in these sites, explains why lead disrupts the structure of these peptides and thus provides the first detailed molecular understanding of the developmental toxicity of lead.