SOME SPECTRAL CHARACTERISTICS OF THE LIGHT EMITTING SYSTEM OF THE POLYNOID WORMS

Abstract— The bioluminescence emission spectrum of the light emitted from the elytra of certain poly-noid worms is similar to the fluorescence of the scales and to the fluorescence of flavins. In extracts the fluorescence is confined to the particulate fraction. Stimulation of bioluminescence from such particles was observed upon the addition of Fe²⁺ ions.     

Cryptic functions of enzymes in chemical catalysis

Using a number of examples, this article demonstrates how the functional groups responsible for the catalytic activity of an enzyme must be studied within the context of the enzyme-substrate complex. Very often a substrate will actively cooperate with the enzyme to bring about its own transformation. The so-called cryptic functions of enzymes are considered in the case of seryl proteases which, according to the type of substrate or structural modification introduced in the enzyme, may exhibit esterase, amidase, protease, racemase or dehydratase activity. The cryptic functions may possess a physiological significance which reflects the evolutionary history of the protein. Alternatively they may offer a simple way of exploiting the enzymes as catalysts capable of taking part in the chemical reactions of biotechnological interest but little physiological importance.This work is dedicated to Prof.E. Wünsch (München) at the occasion of his 60^th birthday.     

Single-ion magnetism in the extended solid-state: insights from X-ray absorption and emission spectroscopy

Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L_2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine Fe^I dopant ions to be linearly coordinated, occupying a D_6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L_2,3-edge XAS from which the energy reduction of 3d_z² due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L₃-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm⁻¹), that corresponds with Orbach relaxation via the first excited, M_J = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.

Taking advantage of synchrotron light source methods, we present the geometric and electronic structure of iron doped in lithium nitride.     

Flux-line lattice distortion in PrOs4Sb12

We report that the flux-line lattice in the cubic superconductor Pr(Os4Sb12 is strongly distorted from an ideal hexagonal lattice at very low temperatures in a small applied field. We attribute this to the presence of gap nodes in the superconducting state on at least some Fermi-surface sheets.     

Analysis of β-agonists by HPLC/ESI-MS(n) in horse doping control

A sensitive method using LC/ESI-MS(n) has been developed on a quadrupole linear ion trap mass analyser for the detection of nine β(2) agonists (cimaterol, clenbuterol, fenoterol, formoterol, mabuterol, terbutaline, ractopamine, salbutamol and salmeterol) in horse urine. The method consists of solid-phase extraction on CSDAU cartridges before analysis by LC/ESI-MS(n) . The efficiency of extraction combined with the sensitivity and the selectivity of MS(n) allowed the detection of these compounds at pg/mL levels. Administration studies of fenoterol and formoterol are reported and show their possible detection after inhalation. The method is applicable for screening and confirmatory analysis.     

Spin structure of surface-supported single-molecule magnets from isomorphous replacement and X-ray magnetic circular dichroism

Surface-supported arrays of Fe(4)-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe(4)(L)(2)(dpm)(6)] (1) on Au, an isomorphous compound [Fe(3)Cr(L)(2)(dpm)(6)] was synthesized and structurally and magnetically characterized (H(3)L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr(3+) ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L(2,3) edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L(2,3) spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe(3+) ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.     

Molecular organization of DHP membrane fragments

The phase transition of dihexadecyl phosphate (DHP) bilayered disks has been studied using EPR spectroscopy. In the acid form of DHP, a phase transition temperature exists, that we have monitored through the spin-spin interaction between the nitroxide molecules at high concentration (8%) in DHP bilayers. This spin-spin interaction is due to the gathering of solutes in a fluid defect of the membrane: the border. The fluorescence quenching of two probes by the nitroxide stearic acids in DHP bilayers has been studied by stationary and time-resolved fluorescence measurements. The quenching process is mainly static. Both magnetic and fluorescent probes are localized in the periphery of the bilayered disks. An erratum to this article is available at .     

Probing the beta-1,3:1,4 glucanase, CtLic26A, with a thio-oligosaccharide and enzyme variants

The substrate binding regions of a beta-1,3:1,4 glucanase are revealed through structural analysis with a thio-oligosaccharide and kinetics of enzyme variants.