Corrected asymmetry plots

Presenting experimental data as asymmetry plotsA _i (t), rather than as single histogramsN _±i (t), removes the muon lifetime which allows the time dependence of the ensemble muon polarization to be viewed directly (particularly useful for longitudinal and zero field experiments). The combining of two histograms effectively increases the statistical accuracy by a factor of 2. However, differences in efficiency within a pair of counters, differences in solid angle subtended by a pair of counters, counter pairs which are not exactly 180 degrees apart and errors in the random background estimate can distort the calculation of the asymmetry. This paper describes the expected distortions and explains how to correct for them.     

Full vector treatment of the polarization evolution in an arbitrary field distribution

This paper presents a vector representation of the ensemble muon polarizationP(t) for static field distributions which emphasizes the different behavior resulting from correlated and non-correlated field distributions. For example, with most local field distributions, such as random dipolar fields, the net muon polarization in the limit of infinite timeP(t) is parallel to the initial muon spin direction when the using transverse field (TF) or longitudinal field (LF) geometries. In general, however, it need not be; this will be demonstrated using the field distribution of an anisotropic superconductor.     

Vanadium oxides on aluminum oxide supports. 1. Surface termination and reducibility of vanadia films on alpha-Al2O3(0001)

Using density functional theory and statistical thermodynamics, we obtained the phase diagram of thin VnOm films of varying thickness (approximately 2-6 A, 1-6 vanadium layers) supported on alpha-Al2O3(0001). Depending on the temperature, oxygen pressure, and vanadium concentration, films with different thickness and termination may form. In ultrahigh vacuum (UHV), at room temperature and for low vanadium concentrations, an ultrathin (1 x 1) O=V-terminated film is most stable. As more vanadium is supplied, the thickest possible films form. Their structures and terminations correspond to previous findings for the (0001) surface of bulk V2O3 [Kresse et al., Surf. Sci. 2004, 555, 118]. The presence of surface vanadyl (O=V) groups is a prevalent feature. They are stable up to at least 800 K in UHV. Vanadyl oxygen atoms induce a V(2p) core-level shift of about 2 eV on the surface V atoms. The reducibility of the supported films is characterized by the energy of oxygen defect formation. For the stable structures, the results vary between 4.11 and 3.59 eV per 1/2O2. In contrast, oxygen removal from the V2O5(001) surface is much easier (1.93 eV). This provides a possible explanation for the lower catalytic activity of vanadium oxides supported on alumina compared to that of crystalline vanadia particles.     

Tetrathiafulvalene (TTF)‐Bridged Resorcin[4]arene Cavitands: Towards New Electrochemical Molecular Switches

We report the synthesis of novel resorcin[4]arene‐based cavitands featuring two extended bridges consisting of quinoxaline‐fused TTF (tetrathiafulvalene) moieties. In the neutral form, these cavitands were expected to adopt the vase form, whereas, upon oxidation, the open kite geometry should be preferred due to Coulombic repulsion between the two TTF radical cations (Scheme 2). The key step in the preparation of these novel molecular switches was the P(OEt)₃‐mediated coupling between a macrocyclic bis(1,3‐dithiol‐2‐thione) and 2 equiv. of a suitable 1,3‐dithiol‐2‐one. Following the successful application of this strategy to the preparation of mono‐TTF‐cavitand 3 (Scheme 3), the synthesis of the bis‐TTF derivatives 2 (Scheme 4) and 19 (Scheme 5) was pursued; however, the target compounds could not be isolated due to their insolubility. Upon decorating both the octol bowl and the TTF cavity rims with long alkyl chains, the soluble bis‐TTF cavitand 23 was finally obtained, besides a minor amount of the novel cage compound 25a featuring a highly distorted TTF bridge (Scheme 6). In contrast to 25a , the deep cavitand 23 undergoes reversible vase → kite switching upon lowering the temperature from 293 to 193 K (Fig. 1). Electrochemical studies by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) provided preliminary evidence for successful vase → kite switching of 23 induced by the oxidation of the TTF cavity walls.     

Inclusion by a fluorenyl host with volatile guests: structures, thermal stability and kinetics

The structures of the inclusion compounds formed by the host H = 9,9'-(biphenyl-4,4'-diyl)bis(fluoren-9-ol) with N,N-dimethylacetamide (H.2DMA and H.4DMA), 1,4-dioxane (H.3DIOX), methyl ethyl ketone (H.2MEK), as well as that of the apohost have been elucidated. The compounds were characterised by thermal analysis and solid state NMR, and the kinetics of desorption of H.4DMA, H.2DMA and H.3DIOX have been examined.     

Increasing the kinase specificity of k252a by protein surface recognition

[reaction: see text] Here we describe a miniature protein (1) that presents the cAMP-dependent protein kinase (PKA) recognition epitope found within the heat-stable Protein Kinase Inhibitor protein (PKI) and a miniature protein conjugate (1-K252a) in which 1 is joined covalently to the high-affinity but nonselective kinase inhibitor K252a. Miniature protein 1 recognizes PKA with an affinity that rivals that of PKI and, in the context of 1-K252a, leads to a dramatic increase in kinase specificity.     

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.     

New isomerization operations for fullerene graphs

The problem of topological isomerization operations for fullerenes is considered. Two basic classes of isomerization transformations admissible for fullerene graphs are proposed: rotation and mirror reflection of a chosen part of fullerene graph. Pyracylene and generalized Stone-Wales rearrangements are only the specific cases of extended isomerization operations for the fullerene graphs.     

Utilization of alternate substrates by the first three modules of the epothilone synthetase assembly line

The epothilones, a family of macrolactone natural products produced by the myxobacterial species Sorangium cellulosum, are of current clinical interest as antitumor agents. Inspection of the structure of the epothilones suggests a hybrid polyketide/nonribosomal peptide biosynthetic origin, and the recent sequencing of the epothilone biosynthetic gene cluster has validated this proposal. Here we have examined unnatural substrates with the first two enzymes of the biosynthetic pathway, EpoA and EpoB, to investigate the enzymatic construction of alternate heterocyclic structures and the subsequent elongation of these products by the third enzyme of the pathway, EpoC. The epothilone biosynthetic machinery can utilize serine to install an oxazole in place of a thiazole in the epothilone structure and will tolerate functionalized donor groups from the EpoA-ACP domain to produce epothilone fragments modified at the C21 position. These studies with the early enzymes of the epothilone biosynthesis cluster suggest that combinatorial biosynthesis may be a viable means for producing a variety of epothilone analogues that incorporate diversity into the heterocycle starter unit.