Silvano Geremia

Polyoxometalate Embedding of a Tetraruthenium(IV)-oxo-core by Template-Directed Metalation of (γ-SiW10O36) 8- : A Totally Inorganic Oxygen-Evolving Catalyst

Solid state and solution evidence confirms the embedding of an adamantane-like, Ru4O6 fragment by the divacant, gamma-decatungstosilicate ligand. The resulting complex catalyzes water oxidation to oxygen with TON up to 500 and TOF > 450 h-1.

Vitamin B12: unique metalorganic compounds and the most complex vitamins.

The chemistry and biochemistry of the vitamin B12 compounds (cobalamins, XCbl) are described, with particular emphasis on their structural aspects and their relationships with properties and function. A brief history of B12, reveals how much the effort of chemists, biochemists and crystallographers have contributed in the past to understand the basic properties of this very complex vitamin. The properties of the two cobalamins, the two important B12 cofactors Ado- and MeCbl are described, with particular emphasis on how the Co-C bond cleavage is involved in the enzymatic mechanisms. The main structural features of cobalamins are described, with particular reference to the axial fragment. The structure/property relationships in cobalamins are summarized. The recent studies on base-off/base-on equilibrium are emphasized for their relevance to the mode of binding of the cofactor to the protein scaffold. The absorption, transport and cellular uptake of cobalamins and the structure of the B12 transport proteins, IF and TC, in mammals are reviewed. The B12 transport in bacteria and the structure of the so far determined proteins are briefly described. The currently accepted mechanisms for the catalytic cycles of the AdoCbl and MeCbl enzymes are reported. The structure and function of B12 enzymes, particularly the important mammalian enzymes methyltransferase (MetH) and methyl-malonyl-coenzymeA mutase (MMCM), are described and briefly discussed. Since fast proliferating cells require higher amount of vitamin B12 than that required by normal cells, the study of B12 conjugates as targeting agents has recently gained importance. Bioconjugates have been studied as potential agents for delivering radioisotopes and NMR probes or as various cytotoxic agents towards cancer cells in humans and the most recent studies are described. Specifically, functionalized bioconjugates are used as “Trojan horses” to carry into the cell the appropriate antitumour or diagnostic label. Possible future developments of B12 work are summarized.

HCV-NS3 and IgG-Fc crossreactive IgM in patients with type II mixed cryoglobulinemia and B-cell clonal proliferations.

We demonstrate that in three cases of MC (two with immunocytoma), the IgM-RF+ component of their cryoprecipitated represents the circulating counterpart of the B-cell receptor (BCR) of the monoclonal overexpanded B-cell population. These IgMs were isolated and used to demonstrate a crossreactivity against both hepatitis C virus (HCV) NS3 antigen and the Fc portion of IgG. Epitopes were identified in a fraction of exemplary samples by using epitope excision approach (NS31250–1334 and IgG Fc345–355). The same phenomenon of crossreactivity has been shown to occur in vivo after immunization of a mouse with the NS31251−1270 peptide. To verify if the same reaction was also present in MC samples characterized by an oligo/polyclonal B-cell proliferation, IgM crossreactivity was tested in 14 additional samples. Five out of the 14 were reactive against HCV NS3 and 11 out of 14 were reactive against IgG-Fc peptide. The data support the role of HCV NS3 antigen in a subset of patients with MC, whereas the high frequency of the IgG-Fc epitope suggests that these B cells originate from precursors strongly selected for auto-IgG specificity. We suggest that engagement of specific BCRs by NS3 (or NS3-immunocomplex) antigen could explain the prevalence of IgM cryoglobulins in these patients

Enantioselective hydrogen transfer reactions from propan-2-ol to ketones catalyzed by pentacoordinate iridium(I) complexes with chiral Schiff bases

Abstract Some diastereoisomeric pentacoordinate complexes of the type [Ir(COD)-(NNR ★ )I] (COD = cis , cis -1,5-cyclooctadiene; NNR ★ = 2-pyridinal-1-phenylethylimine (PPEI) (I), 2-acetylpyridine-1-phenylethylimine (APPEI) (II)) have been synthesized. The complexes are active and selective catalysts for asymmetric hydrogen transfer from propan-2-ol to prochiral ketones. Optical yields of up to 84% have been obtained in the reduction of t-butyl phenyl ketone. The structure and absolute configuration of complexes I and II were determined by X-ray diffraction.

Nanoporous crystals of calixarene/porphyrin supramolecular complex functionalized by diffusion and coordination of metal ions

A highly nanoporous material has been obtained by self-assembly of calixarene and porphyrin building blocks. This supramolecular zeolite-like structure was successively functionalized by diffusion and coordination of metal ions to form a new bifunctionalized nanoporous material containing a porphyrinic pigment together with a metal center.

Structural study on ligand specificity of human vitamin B12 transporters.

Studies comparing the binding of genuine cobalamin (vitamin B12) to that of its natural or synthetic analogues have long established increasing ligand specificity in the order haptocorrin, transcobalamin and intrinsic factor, the high-affinity binding proteins involved in cobalamin transport in mammals. In the present study, ligand specificity was investigated from a structural point of view, for which comparative models of intrinsic factor and haptocorrin are produced based on the crystal structure of the homologous transcobalamin and validated by results of published binding assays. Many interactions between cobalamin and its binding site in the interface of the two domains are conserved among the transporters. A structural comparison suggests that the determinant of specificity regarding cobalamin ligands with modified nucleotide moiety resides in the beta-hairpin motif beta3-turn-beta4 of the smaller C-terminal domain. In haptocorrin, it provides hydrophobic contacts to the benzimidazole moiety through the apolar regions of Arg357, Trp359 and Tyr362. Together, these large side chains may compensate for the missing nucleotide upon cobinamide binding. Intrinsic factor possesses only the tryptophan residue and transcobalamin only the tyrosine residue, consistent with their low affinity for cobinamide. Relative affinity constants for other analogues are rationalized similarly by analysis of steric and electrostatic interactions with the three transporters. The structures also indicate that the C-terminal domain is the first site of cobalamin-binding since part of the beta-hairpin motif is trapped between the nucleotide moiety and the N-terminal domain in the final holo-proteins.

Rhodium(III) analogues of antitumour-active ruthenium(III) compounds: the crystal structure of [ImH][trans-RhCl4(Im)2] (Im= imidazole)

A series of neutral and anionic Rh(III)-chloride compounds bearing ammonia or imidazole (Im) ligands, and isostructural to Ru(III) complexes endowed with antineoplastic activity, were synthesized and characterized spectroscopically. The X-ray crystal structure of [ImH][ trans -RhCl 4 (Im) 2 ] was determined. Crystal data: monoclinic, space group C 2/ c , Z =4, a =13.133(3), b =7.977(1), c =16.683(4) A, β =113.84(2)°. The solution behaviour and some biological parameters of the new rhodium compounds, including cytotoxicity, in vitro interactions with DNA and in vivo antitumour activity against a tumour model, were investigated and compared with those of the corresponding ruthenium analogues.

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

Four novel π-conjugated chromophores with an azobenzene core (1–4) have been synthesized exploiting Pd-catalysed cross-coupling reactions between ethynyl-bearing azobenzene cores and suitably-designed peripheral groups. While in molecules 2 and 3 the azobenzene core is equipped, respectively, with ethynyl and 1,3-butadiyne spacers terminated with a substituted aniline, molecule 4 is an homologue of derivative 2 in which the terminal moieties are replaced by meso-substituted Zn-porphyrins. X-Ray crystallographic studies of substituted azobenzene 2 reveal a nearly planar arrangement of the four phenyl rings and the trans configuration of the NN central unit. The UV-Vis absorption spectrum of molecule 1 in cyclohexane (CHX) is very similar to that of unsubstituted azobenzenes; upon irradiation at the maximum of the intense π–π absorption feature (360 nm), 1 undergoes trans → cisphotoisomerization reaching a photostationary state. The process is fully reversible both photochemically and thermally (ca. 120 min in the dark). The UV-Vis electronic absorption features of 2–4 are dramatically different compared to those of 1, but the photochemical process can still be traced and exhibits full reversibility in CHX. Also in the case of compound 4, where the photoreactive azobenzene excited states might be quenched by the low-lying porphyrin electronic levels, the photoreaction does occur. Extensive STM investigations of self-assembled monolayers (SAMs) of 2 and 3 at the solid/liquid interface were performed by means of scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG). It is evidenced that only the trans isomer can be physisorbed on the surface whereas the cis form, either produced under illumination in situ or prepared by irradiation of the solution prior to deposition (ex-situ), is never observed on the surface. The smallest azobenzene 1 and the bisporphyrin system 4 did not physisorb onto the surface because of the very small size and the bulky 3,5-di(tert-butyl)phenyl groups hindering flat adsorption on HOPG, respectively.

Evidence of the interaction between steric and electronic influence in rhodoximes and cobaloximes. Synthesis of pyRh(DH)2I and X-ray structure of pyRh(DH)2Cl, pyCo(DH)2Cl and pyRh(DH)2I

Abstract The crystal structure of pyRh(DH)2Cl (I) and pyCo(DH)2Cl (II) are reported together with the synthesis and the crystal structure of pyRh(DH)2I (III).I: RhClO4N5C13H19; a=7.5090(8), b=8.588(1), c=14.479(2) A; α=96.45(8), β=87.07(8), γ=76.12(9)°; triclinic P 1 ; Z=2; R=0.022. II: CoClO4N5C13H19; a=14.738(2), b=8.606(1), c=13.369(2) A; β=96.30(1)°; monoclinic Cc; Z=4; R=0.022. III: IRhO4N5C13H19; a=8.875(1), b=14.741(2), c=14.163(3) A; β=97.47(8)°; monoclinic P21/c; Z=4; R=0.031. The Rh-py distance of 2.046(1) A in I, compared to that of 2.079(3) A in III, indicates that I is a slightly better trans-influencing ligand than Cl. The difference in the M-py distances when X=Cl and i-pr is 0.18 A for M=Rh and 0.14 A for M=Co. This result suggests that the interaction between steric and electronic factors is reduced in rhodoximes, owing to the larger size of RhIII.

Host–Guest‐Driven Copolymerization of Tetraphosphonate Cavitands

The outstanding complexing properties of tetraphosphonate cavitands towards N-methylpyridinium salts were exploited to realise a new class of linear and cyclic AABB supramolecular polymers through host-guest interactions. The effectiveness of the selected self-association processes was tested by (1)H NMR studies, whereas microcalorimetric analyses clarified the binding thermodynamics and revealed the possibility of tuning entropic contributions by acting on the flexibility of the guest linker. Although the formation of linear polymeric chains for a rigid system was demonstrated by X-ray analysis, the presence of a concentration-dependent ring-chain equilibrium was indicated by solution viscosity measurements in the case of a very flexible ditopic BB guest co-monomer.