Controlling Through-Space and Through-Bond Exchange Pathways in Bis-Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin-like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene-bridged biscobaltocenes with competing through-space and through-bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through-space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non-intuitive behavior. These results are of interest for molecular spintronics, as naphthalene-linked cobaltocenes can form wires on surfaces for potential spin-only information transfer.     

Oxidative phenol coupling reactions catalyzed by OxyB: a cytochrome P450 from the vancomycin producing organism. implications for vancomycin biosynthesis

OxyB is a cytochrome P450 enzyme that catalyzes the first phenol coupling reaction during the biosynthesis of vancomycin-like glycopeptide antibiotics. The phenol coupling reaction occurs on a linear peptide intermediate linked as a C-terminal thioester to a peptide carrier protein (PCP) domain within the multidomain glycopeptide nonribosomal peptide synthetase (NRPS). Using model peptides with the sequence (R)(NMe)Leu-(R)Tyr-(S)Asn-(R)Hpg-(R)Hpg-(S)Tyr-S-PCP and (R)(NMe)Leu-(R)Tyr-(S)Asn-(R)Hpg-(R)Hpg-(S)Tyr-(S)Dpg-S-PCP (where Hpg = 4-hydroxyphenylglycine, and Dpg = 3,5-dihydroxyphenylglycine), or containing (R)Leu instead of (R)(NMe)Leu, attached to recombinant PCPs derived from modules-6 and -7 in the vancomycin NRPS, we show that cross-linking of Hpg4 and Tyr6 by OxyB can occur in both hexapeptide- and heptapeptide-PCP conjugates. Thus, whereas OxyB may act preferentially on a hexapeptide still linked to the PCP-6 in NRPS subunit-2, it is possible that a linear heptapeptide intermediate linked to PCP-7 in NRPS subunit-3 may also be transformed into monocyclic product. For turnover, OxyB requires electrons, which in vitro can be supplied by spinach ferredoxin and E. coli flavodoxin reductase. Turnover is also dependent upon the presence of molecular oxygen. The model substrate (R)(NMe)Leu-(R)Tyr-(S)Asn-(R)Hpg-(R)Hpg-(S)Tyr-S-PCP is transformed into cross-linked product by OxyB with a kcat of 0.1 s-1 and Km in the range 4-13 muM. Equilibrium binding of this substrate to OxyB, monitored by UV-vis, is accompanied by a typical low-to-high spin state change in the heme, characterized with a Kd of 17 +/- 5 muM.     

Kneelike structure in the spectrum of the heavy component of cosmic rays observed with KASCADE-Grande

We report the observation of a steepening in the cosmic ray energy spectrum of heavy primary particles at about 8×10(16) eV. This structure is also seen in the all-particle energy spectrum, but is less significant. Whereas the "knee" of the cosmic ray spectrum at 3-5×10(15) eV was assigned to light primary masses by the KASCADE experiment, the new structure found by the KASCADE-Grande experiment is caused by heavy primaries. The result is obtained by independent measurements of the charged particle and muon components of the secondary particles of extensive air showers in the primary energy range of 10(16) to 10(18) eV. The data are analyzed on a single-event basis taking into account also the correlation of the two observables.     

Molecular-Recognition-Directed Self-Assembly of Supramolecular Polymers

Abstract

The first part of this paper discusses the molecular design of selected examples of structural units containing taper-shaped exo-receptors and various crown ether, oligooxyethylenic, and H-bonding-based endo-receptors, which self-assemble into cylindrical channel-like architectures via principles resembling those of tobacco mosaic virus. The ability of these structural units to self-assemble via a delicate combination of exo-and endo-recognition processes will be presented. A comparison between various supramolecular (generated via H-bonding, ionic, and electrostatic interactions) and molecular “polymer backbones” will be made. The present limitations concerning the ability to engineer the structural parameters of these supramolecular channel-like architectures and some possible novel material functions derived from them will be briefly mentioned. The second part of this paper discusses our research on the molecular design of a novel class of macrocyclics which self-assemble via intramolecular recognition processes into supramolecular “rodlike” collapsed macrocyclics which display thermotropic liquid crystalline mesophases. It is demonstrated that these macrocyclics have a higher ability to form liquid crystalline phases than the corresponding linear compounds which have identical or even higher degrees of polymerization. Therefore, they represent the ideal molecular architectures which generate mesophases.     

Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm^–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap.