Claudia Zucchi

Volatile components of Grana Parmigiano-Reggiano type hard cheese

Abstract GC–MS analysis of volatile components of Grana Parmigiano-Reggiano type Italian hard cheese was performed by solid phase micro extraction (SPME) and Purge & Trap (PT) methods. Half of the 24 samples analysed were produced in flat land (

Molecular Elements of Ion Permeation and Selectivity within Calcium Channels

Voltage-dependent calcium channels are located in the plasma membrane and form a highly selective conduit by which Ca2+ ions enter all excitable cells and some nonexcitable cells. Extensive characterization studies have revealed the existence of one low (T) and five high-voltage-activated calcium channel types (L, N, P, Q, and R). The high voltage-activated calcium channels have been found to exist as heteromultimers, consisting of an alpha1, beta, alpha2/delta, and gamma subunit. Molecular cloning has revealed the existence of 10 channel transcripts, and expression of these cloned calcium channel genes has shown that basic voltage-activated calcium channel function is strictly carried by the corresponding alpha1 subunits. In turn, the auxiliary subunits serve to modulate calcium channel function by altering the voltage dependence of channel gating, kinetics, and current amplitude, thereby creating a likelihood for calcium channels with multiple properties. Although for calcium channels to be effective, Ca2+ ions must enter selectively through the pore of the alpha1-subunit, bypassing competition with other extracellular ions. The structural determinants of this highly selective Ca2+ filter reside within the four glutamic acid residues located at homologous positions within each of the four pore-forming segments. Together, these residues form a single or multiple Ca2+ affinity site(s) that entrap calcium ions, which are then electrostatically repulsed through the intracellular opening of the pore. This mechanism of high-selectivity calcium filtration, the spatial arrangement of pore glutamic acid residues, and the coordination chemistry of calcium binding are discussed in this review.

Isotope Chirality and Asymmetric Autocatalysis: A Possible Entry to Biological Chirality

Natural-abundance isotopic substitution in isotopically prochiral groups of otherwise achiral molecules can provide stochastically formed enantiomeric excesses which exceed the sensitivity threshold of sensitive asymmetric autocatalytic (Soai-type) reactions. This kind of induction of chirality should be taken into consideration in in vitro model experiments and offer a new kind of entry into primary prebiotic or early biotic enantioselection in the earliest stages of molecular evolution.

Mono- or polynuclear surface species: Models and tendencies with cobalt and rhodium

Abstract Models of surface reactions of cobalt and rhodium carbonyls with oxide or thiolate type supports were prepared and characterized. For Co carbonyls it was found that simple noncarbonyl ligands that can be formed in reductive CO reactions may promote clusterification under conditions where the support alone causes declusterification. Model complexes of the interaction of rhodium with silica and aromatic thiolate supports were structurally characterized by X-ray diffraction. Model compounds indicate that organic thiolate supports promote clusterification with cobalt and declusterification with rhodium.

Asymmetric induction by amino acid ligands in chromium(II)-assisted reduction of ketones

Abstract Asymmetric reduction of acetophenone by Cr(II)/amino acid/water/DMF system is reported. Chemical yields up to 94%, enantiomeric excesses up to 74% were observed. The relevance to the Nozaki–Hiyama–Kishi reaction is discussed.

Enantioselective reduction of prochiral ketones by chromium(II) complexes with amino acid ligands as the source of chirality

Abstract Prochiral ketones were reduced to enantiomerically enriched secondary alcohols by Cr(II)[L-amino acid] complexes in good yields and moderate enantioselectivity.

Preparation and molecular structures of benzyl- and phenylacetylcobalt carbonyls

The benzyl-type cobalt carbonyl complexes ( para - t BuC 6 H 4 CH 2 )Co(CO) 3 PPh 3 ( 4 ) and [ para -ClC 6 H 4 CH 2 C(O)]Co(CO) 3 PPh 3 ( 5 ) were prepared and characterized by analyses, spectra and X-ray single-crystal diffraction. The overall structures both of the alkylcobalt-type 4 and the acylcobalt-type 5 display trigonal bipyramidal geometry, with the two noncarbonyl ligands in the two axial positions. The relevance of the stereochemistry of complexes 4 and 5 to the supposed mechanism of the CO insertion/deinsertion on cobalt is discussed.

Bimetallic cyclooligosiloxanolate complexes of copper and nickel

The bimetallic cyclosiloxanolate cluster complexes Na[PhSiO 2 ) 6 Cu 4 Ni 2 ( μ 6 -Cl)(PhSiO 2 ) 6 ] ( 1 ) and Na[(PhSiO 2 ) 6 Cu 3 Ni 3 ( μ 6-Cl)(PhSiO 2 ) 6 ] ( 2 ) were prepared by Na + and Ni 2+ ion exchange from in situ generated Na 2 {[(PhSiO 2 ) 6 ] 2 Na 4 Ni 4 (OH) 2 }. Complexes 1 and 2 were characterized by analytical, spectroscopic and electrochemical methods as well as complex 2 by single-crystal X-ray diffraction. The X-ray structure shows a sandwich-type array comprising two superimposed cyclosiloxanolate rings and an M 6 Cl unit in between. For the first time the regioselectivity of the metal ion exchange could be deduced from the X-ray structural parameters.

Alkylcobalt carbonyls: XI. Ligand-assisted carbonylation—decarbonylation reactions of alkylcobalt carbonyls

Abstract An [(alkoxycarbonyl)methyl]cobalt tetracarbonyl ( 1 ) derivative (alkyl = Et, 1a ) was treated with tertiary phosphorus ligands. A monosubstituted acyl derivative EtOC(O)CH 2 C(O)Co(CO) 3 L ( 2 ), L = PPh 3 ( 2f ) was isolated, and other new complexes of type 2 were detected spectroscopically (L = PEt 3 , 2a ; P( i Pr) 3 , 2c ; P( t Bu) 3 , 2d ; PMePh 2 , 2e ; P(NEt 2 ) 3 , 2g ; P(OMe) 3 , 2h ; P(OEt) 3 , 2i ; P(OSiMe 3 ) 3 , 2j ; P(S i Pr) 3 , 2k ). The corresponding alkyl derivatives, EtOC(O)CH 2 Co(CO) 3 L( 3 ), were obtained by thermal decarbonylation. Disubstituted acyl-, EtOC(O)CH 2 C(O)Co(CO) 2 L 2 ( 4 ), and alkyl-, EtOC(O)CH 2 Co-(CO) 2 L 2 ( 5 ) derivatives were obtained by direct substitution at ca. 30°C and ca. 70°C, respectively. The geometries of the new complexes were deduced from spectroscopic data. The monosubstituted derivatives 2 and 3 all show a trigonal bipyramidal array with axial organyl an PR 3 ligands. All the disubstituted complexes display a trigonal bipyramidal geometry with axial organyl groups, whereas an axial-equatorial distribution of the phosphines was assigned in the case of compounds 4a , b and 5a , b , e , h , i , and an equatorial-equatorial distribution in the case of complexes 4j and 5g , j . For the alkyl complexes 5 a solvation-like interaction between the ester group and the cobalt atom (autosolvation) was indicated by spectroscopic data.

Astrobiology and Biological Chirality

The emerging discipline of astrobiology could gain valuable support from research dealing with the problems of biological chirality. The most profitable fields of common interest are: (a) living organisms under extraterrestrial conditions, (b) extraterrestrial signatures of life and (c) origin(s) of biological chirality. These areas of complementary and overlapping fields are analysed on the basis of selected references.