SSTR1- and SSTR3-selective somatostatin analogues

We prepared the two enantiomers of 3‐(3′‐quinolyl)‐alanine (Qla, 1 ) in multigram scale by asymmetric hydrogenation. These amino acids, protected as Fmoc derivatives, were then used in the solid‐phase synthesis of two new somatostatin 14 (SRIF‐14) analogues 8 a and 8 b , tetradecapeptides in which the tryptophan residue (Trp8) is replaced by one of the two enantiomers of 3‐(3′‐quinolyl)‐alanine (Qla8) and therefore lack the N? H bond in residue 8. The selectivity of these new analogues for the somatostatin receptors, SSTR1–5, was measured. Substitution with L ‐Qla8 yielded peptide 8 a , which was highly selective for SSTR1 and SSTR3, with an affinity similar to that of SRIF‐14. Substitution by D ‐Qla gave the relatively selective analogue 8 b , which showed high affinity for SSTR3 and significant affinity for SSTR1, SSTR2 and SSTR5. The biological results demonstrate that bulky and electronically poor aromatic amino acids at position 8 are compatible with strong activity with SSTR1 and SSTR3. Remarkably, these high affinity levels were achieved with peptides in which the conformational mobility was increased with respect to that of SRIF‐14. This observation suggests that conformational rigidity is not required, and might be detrimental to the interaction with receptors SSTR1 and SSTR3. The absence of an indole N proton in Qla8 might also contribute to the increased flexibility observed in these analogues.     

Double-Cubane [8Fe9S] Clusters: A Novel Nitrogenase-Related Cofactor in Biology

Three different types of electron-transferring metallo-ATPases are able to couple ATP hydrolysis to the reduction of low-potential metal sites, thereby energizing an electron. Besides the Fe-protein known from nitrogenase and homologous enzymes, two other kinds of ATPase with different scaffolds and cofactors are used to achieve a unidirectional, energetic, uphill electron transfer to either reduce inactive Co-corrinoid-containing proteins (RACE-type activators) or a second iron-sulfur cluster-containing enzyme of a unique radical enzymes family (archerases). We have found a new cofactor in the latter enzyme family, that is, a double-cubane cluster with two [4Fe4S] subclusters bridged by a sulfido ligand. An enzyme containing this cofactor catalyzes the ATP-dependent reduction of small molecules, including acetylene. Thus, enzymes containing the double-cubane cofactor are analogous in function and share some structural features with nitrogenases.     

Effect of clay dispersion on the rheological properties and flammability of polyurethane‐clay nanocomposite elastomers

Polyurethane‐clay nanocomposite elastomers were synthesized using polyol‐clay blends with different levels of dispersion, which affected the final elastomer microstructure. A PU‐clay microcomposite elastomer containing partially dispersed clay showed poorer mechanical and similar fire properties to the unmodified polyurethane. More complete dispersion of the clay into the polyol led to an exfoliated structure in the final elastomer. This showed a higher modulus and kept a viscoelastic behavior to higher temperature than the pristine PU. The enhancement of mechanical and thermal properties in the nanocomposite elastomer can be attributed to the degree of clay exfoliation, and this also prevented dripping during the UL 94 fire test. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nanoindentation-based study of the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites

Bulk poly-supercrystalline ceramic-organic nanocomposites were produced and characterized with a nanoindentation-based study. These nanocomposites were processed using two different routines, to compare their properties with and without crosslinking the organic ligands interfacing the ceramic nanoparticles. Together with the expected material strengthening induced by crosslinking, a distinct response emerges when using Berkovich and cube-corner indenters. The supercrystalline materials are prone to compaction, cracking and chipping phenomena that become more severe when a sharper tip is employed, implying that a Berkovich indenter is more suitable for the evaluation of elastic modulus and hardness. The cube-corner tip, on the other hand, is employed for the investigation of fracture toughness, comparing two methods and multiple models available from the literature. The fracture toughness outcomes suggest that cracks evolve with a quarter-penny shaped profile at the indent’s corners, and that extrinsic toughening mechanisms, such as plastic-like deformation and crack deflection, play a significant role.     

Polymeric pseudocrown ethers. II. Complexation with acids in aqueous and organic solvents

A macroreticular polystyrene incorporating polyoxyethylene (14) units (Pseudocrown-14) was shown to complex strongly with both protic and Lewis acids. The distribution coefficients depend on the dielectric constant of the solvent. In non polar organic solvents such as benzene or chloroform, the capacity of the polymer for acid reaches the theoretical value of 13.60 mmole hydrogen ions per gram polymer. Column tests have shown that the polymers can be used to reduce acidity in solutions to 10^?4M or less, and that the polymers can be fully regenered using polar solvents such as methanol or water. The comparison of HBr complexation with pseudocrown-14 and linear or macrocyclic ethers, shows that pseudocrown-14 behaves similarly to a macrocyclic crown ether.