Conformations of GABA analogues. I: Crystal and molecular structure of guanidinoacetic acid

The crystal and molecular structure of guanidinoacetic acid has been determined by single-crystal X-ray diffraction techniques. The crystals are monoclinic, space groupP2₁/c, witha = 4.942(3),b = 6.010(3),c = 21.911(14) Å, = 128.60(4) °, andZ = 4. The structure was solved by direct methods, and refined by least-squares procedures toR = 0.070 for 552 observed diffraction maxima. The molecules are zwitterions linked by an infinite three-dimensional network of hydrogen bonds via the primary amine nitrogen and carboxylate oxygen atoms. The secondary amine nitrogen atom is not involved in hydrogen bonding.     

Identification of special structure constraints in linear programs

In large LPs it may be that a major fraction of the constraints has a particularly simple form. Frequently this simple form may be exploited by either decomposition or implicit representation methods. In these cases, the effective size and computational difficulty are more closely related to the number of remaining nonspecial constraints than to the number of special constraints. A unified computational procedure is presented for mechanically identifying near maximal sets of special structure constraints for most of the kinds of special structures which have been identified thus far as being exploitable.     

The symmetry of two-photon excited states as determined by time-resolved fluorescence depolarization experiments

A new experimental and theoretical approach is presented for the quantitative determination and assignment of the two-photon absorption tensor of fluorophores dissolved in liquid solutions. Two linearly independent time-resolved fluorescence anisotropies and the two-photon polarization ratio were determined from experiments based on using the time-correlated single photon counting technique. The data were analyzed in a global manner under the assumption of prevailing diffusive molecular reorientations and when accounting for the influence of rapid unresolved reorientations. The method has been applied in fluorescence studies of perylene, two-photon excited at 800 nm. The analysis suggests that the two-photon transition is mediated via vibronic coupling including at least two vibrations of different symmetry, and also that the first singlet excited electronic state acts as a dominating intermediate state.     

New iso-propoxides, tert-butoxides and neo-pentoxides of niobium(V): synthesis, structure, characterization and stabilization by trifluoroheteroarylalkenolates and pyridine ligands

The synthesis and characterization of nine new heteroleptic alkoxides of niobium is described. Metathesis reactions of Nb(2)Cl(10) with (t)BuCH(2)OH and pyridine (py) or 4-dimethylaminopyridine (DMAP) affords monomeric octahedral complexes Nb(OCH(2)(t)Bu)(5)py (1) and Nb(OCH(2)(t)Bu)(5)DMAP (2), respectively, in high yields (>60%). The same reaction with (t)BuOH resulted in a chloro functionalized alkoxide Nb(O(t)Bu)(4)pyCl (3) and could not be pushed to complete removal of remaining Cl(-) ligand. The introduction of a chelating bidental ligand 3,3,3-trifluoro-1-(pyridine-2-yl)propen-2-ol (2-PyCHCOHCF(3)) (4') in the dimeric framework of Nb(2)(O(i)Pr)(10) (4') produced a heteroleptic, monomeric niobium complex Nb(O(i)Pr)(4)(2-PyCHCOCF(3)) (4) with significantly enhanced stability and volatility. As a comparison to (4), five different heteroaryl systems (5-9) with the same side chain have been synthesized and examined in order to understand the influence upon physio-chemical properties. All the new compounds (1-9) have been characterized by microanalysis, variable temperature multinuclear NMR spectroscopy, mass spectrometry, thermal analysis and single crystal X-ray diffraction studies ((3), (4) and (9)). The molecular structure of (3) revealed mononuclear species with Nb atoms present in the distorted octahedral environment of four (t)BuO, one chloride and one pyridine ligand. Compounds (4) and (9) consisting of four (i)PrO and a trifluoroheteroarylenolate exhibited a stronger distortion in the molecular geometry due to the rigidity of chelating β-alkenolate moiety.     

The point and line space of a compact projective plane are homeomorphic

The local Hopf bundles at the points and lines of a compact projective plane with manifold lines are all weakly equivalent. In particular, the point space and the line space of such a projective plane are homeomorphic. This is a consequence of the following topological result. Theorem. Let $\xi,\xi'$ be orientable topological $\mathbb{R}^n$ -bundles over an n-dimensional CW-complex. If $\xi$ and $\xi'$ are fibre homotopy equivalent and stably equivalent, then $\xi$ and $\xi'$ are equivalent. Received: 10 December 1998 / Accepted: 18 May 1999     

Simplest Monodentate Imidazole Stabilization of the oxy‐Tyrosinase Cu2O2 Core: Phenolate Hydroxylation through a CuIII Intermediate

Tyrosinases are ubiquitous binuclear copper enzymes that oxygenate to Cu^II₂O₂ cores bonded by three histidine Nτ‐imidazoles per Cu center. Synthetic monodentate imidazole‐bonded Cu^II₂O₂ species self‐assemble in a near quantitative manner at ?125 °C, but Nπ‐ligation has been required. Herein, we disclose the syntheses and reactivity of three Nτ‐imidazole bonded Cu^II₂O₂ species at solution temperatures of ?145 °C, which was achieved using a eutectic mixture of THF and 2‐MeTHF. The addition of anionic phenolates affords a Cu^III₂O₂ species, where the bonded phenolates hydroxylate to catecholates in high yields. Similar Cu^III₂O₂ intermediates are not observed using Nπ‐bonded Cu^II₂O₂ species, hinting that Nτ‐imidazole ligation, conserved in all characterized Ty, has functional advantage beyond active‐site flexibility. Substrate accessibility to the oxygenated Cu₂O₂ core and stabilization of a high oxidation state of the copper centers are suggested from these minimalistic models.     

Viscoelastic properties of poly(ε‐caprolactone) – hydroxyapatite micro‐ and nano‐composites

Various composites have been proposed in the literature for the fabrication of bioscaffolds for bone tissue engineering. These materials include poly(ε‐caprolactone) (PCL) with hydroxyapatite (HA). Since the biomaterial acts as the medium that transfers mechanical signals from the body to the cells, the fundamental properties of the biomaterials should be characterized. Furthermore, in order to control the processing of these materials into scaffolds, the characterization of the fundamental properties is also necessary. In this study, the physical, thermal, mechanical, and viscoelastic properties of the PCL‐HA micro‐ and nano‐composites were characterized. Although the addition of filler particles increased the compressive modulus by up to 450%, the thermal and viscoelastic properties were unaffected. Furthermore, although the presence of water plasticized the polymer, the viscoelastic behavior was only minimally affected. Testing the composites under various conditions showed that the addition of HA can strengthen PCL without changing its viscoelastic response. The results found in this study can be used to further understand and approximate the time‐dependent behavior of scaffolds for bone tissue engineering. Copyright © 2012 John Wiley & Sons, Ltd.