Effects of the hyperfine interactions on the decay of the collective nuclear excited states in α-hematite

In this paper the time dependence of the coherent decay of nuclear excited state in an α-⁵⁷Fe₂O₃ single crystal is presented. The experiment was carried out in diffraction geometry. A highly monochromatized and collimated beam of synchrotron radiation was used for the excitation of nuclear levels. Quantum beat spectra taken below and above the (7, 7, 7) pure nucleation reflection of hemative show a characteristic pattern corresponding to the magnetic an quadrupole hyperfine interactions. This work was performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC02-76CH00016.     

BORONIC ACIDS AS MECHANISTIC PROBES FOR THE BACTERIAL LUCIFERASE REACTION

Bacterial luciferase uses long chain aldehydes as substrates. Alkylboronic acid analogs of these substrates with chain lengths of C₇ and C₉ have been synthesized, characterized, and used as mechanistic probes for the light emitting reaction. They behave as inhibitors in the in vitro luminescence reaction. Contrary to an earlier report (Macheroux and Ghisla, 1985, Nachr. Chem. Tech. Lab. 33,785–790) they are not substrates for bacterial luciferase, in that they do not lead to light emission and are not oxidized by the flavin-4a-hydroperoxide to the products boric acid and the corresponding alcohol, as would be expected from a Baeyer-Villiger reaction. However, the particular conformation of a putative boronic acid hydroperoxide at the active center might be such that it would preclude a Baeyer-Villiger fragmentation. Thus, while the results do not support the postulate that luciferase proceeds via a Baeyer-Villiger mechanism, they also do not exclude it. A further observation was that the endogenous light emission (no added aldehyde) decays more rapidly than does the luciferase bound flavin-4a-hydroperoxide. This suggests that the endogenous light is not caused by the decomposition of the flavin-4a-hydroperoxide.     

BOOK REVIEW

STABILITY OF BIOLOGICAL COMMUNITIES, Yu M. Svirezhev D.O. Logofet, MIR Publishers, Moscow, 1983, 319 pp.     

Minimum Cost Maintenance in Heavy Haul Rail Track

This paper describes two complementary studies concerning the maintenance and replacement of heavy haul railway track. Minimum cost policies are derived using decision networks in one case and a group replacement model in the other. The results indicate relatively large cost savings on current practice.     

Characterization of depleted uranium oxides fabricated using different processing methods

Identifying both physical and chemical characteristics of Special Nuclear Material (SNM) production processes is the corner stone of nuclear forensics. Typically, processing markers are based on measuring an interdicted sample’s bulk chemical properties, such as the elemental or isotopic composition, or focusing on the chemical and physical morphology of only a few particles. Therefore, it is imperative that known SNM processes be fully characterized from bulk to trace level for each particle size range. This report outlines a series of particle size measurements and fractionation techniques that can be applied to a bulk SNM powders, categorizing both chemical and physical properties in discrete particle size fractions. This will be demonstrated by characterizing the process signatures of a series of different depleted uranium oxides prepared at increasing firing temperatures (350–1100 °C). Results will demonstrate how each oxides’ material density, particle size distribution, and morphology varies.     

Simultaneously bound guests and chiral recognition: a chiral self-assembled supramolecular host encapsulates hydrophobic guests

Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple species is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural products. Upon encapsulation of chiral molecules into the racemic host, diastereomeric host-guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.     

Calculation of the vibrational properties of chlorophyll a in solution

Chlorophyll a (Chl-a) is at the heart of solar energy capture and conversion in plants. Because of this, Chl-a has been the subject of innumerable studies. Recently, we have been able to use quantum mechanical methods to calculate the vibrational properties of neutral and oxidized Chl-a in the gas phase [Wang, R.; Parameswaran, S.; Hastings, G. Vib. Spectrosc. 2007, 44, 357-368]. The calculated vibrational properties do not agree with experiment, however. One factor ignored in our calculations was how solvents could impact the vibrational properties. Here we calculate the vibrational properties of Chl-a and Chl-a+ in several solvents that span a wide range of dielectric constant. The calculated and experimental (Chl-a+-Chl-a) infrared difference spectra now show a remarkable similarity. However, the composition of the calculated vibrational modes are very different from that suggested from experiment. We therefore use our calculated data to make new suggestions as to the origin of the bands in experimental (Chl-a+-Chl-a) FTIR difference spectra. We indicate why bands in experimental spectra may have been misassigned. We also point to other experimental data that support our new band assignments. Assignment of bands in (Chl-a+-Chl-a) FTIR difference spectra were first made nearly 20 years ago. These assignments have formed the basis for evaluating all "cation minus neutral" FTIR difference spectra obtained for all photosynthetic systems since then. All of these experimental FTIR difference spectra should be re-examined in light of our new assignments.     

Analytically measurable selection of epsilon optimal transition kernals

The problem of existence of ah ε-optimal transition kernel for a canonical continuous time stochastic process with a general cost variable is considered. An analytically measurable, ε-optimal kernel exists if the state space is a compact Banach space, the cost variable is lower-semi-analytic, and the graph of the admissibility function is an analytic set. The result is applied to a problem in which the controller is to optimally select transition probabilities for a non-Markovian step process based on statistical estimates of holding time distributions.