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Isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) represent the two central intermediates in the biosynthesis of isoprenoids. The recently discovered deoxyxylulose 5-phosphate pathway generates a mixture of IPP and DMAPP in its final step by reductive dehydroxylation of 1-hydroxy-2-methyl-2-butenyl 4-diphosphate. This conversion is catalyzed by IspH protein comprising a central iron-sulfur cluster as electron transfer cofactor in the active site. The five crystal structures of IspH in complex with substrate, converted substrate, products and [PP.sub.i] reported in this article provide unique insights into the mechanism of this enzyme. While IspH protein crystallizes with substrate bound to a [4Fe-4S] cluster, crystals of IspH in complex with IPP, DMAPP or inorganic pyrophosphate feature [3Fe-4S] clusters. The IspH:substrate complex reveals a hairpin conformation of the ligand with the C(1) hydroxyl group coordinated to the unique site in a [4Fe-4S] cluster of aconitase type. The resulting alkoxide complex is coupled to a hydrogen-bonding network, which serves as proton reservoir via a Thr167 proton relay. Prolonged x-ray irradiation leads to cleavage of the C(1)-O bond (initiated by reducing photo electrons). The data suggest a reaction mechanism involving a combination of Lewis-acid activation and proton coupled electron transfer. The resulting allyl radical intermediate can acquire a second electron via the iron-sulfur cluster. The reaction may be terminated by the transfer of a proton from the [beta]-phosphate of the substrate to C(1) (affording D MAPP) or C(3) (affording IPP). iron-sulfur protein | LytB protein | nonmevalonate pathway | terpene biosynthesis | isoprenoid biosynthesis doi/10.1073/pnas.0913045107

The synthesis and the spectroscopic and structural characterization of lower-rim-silylated and rare-earth-metalated calix[4]arenes are described. Depending on the steric bulk of their silyl groups, disilazane reagents HN(SiMe3)(sub 2) and HN(SiMe2)(sub 2) selectively yield bis- and tetrakis-silylated calix[4]arene derivatives and yttrium silylamide Y[N(SiHMe2)(sub 2)](sub 3)(THF)(sub 2) forms aryloxide bond-linked and silylated moieties on the lower-rim calix[4]arene surface

Density functional calculations were undertaken on ansa-bridged rare earth disilylamide compounds.

An account of the extended silylamide route for the synthesis of C2-symmetric ansa-lanthanidocene complexes is reported. Spectroscopic and structural features are also described.