Charge-induced distortion and stabilization of surface transfer doped porphyrin films.

The interaction between zinc-tetraphenylporphyrin (ZnTPP) and fullerenes (C60 and C60F48) are studied using ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy (STM). Low temperature STM reveals highly ordered ZnTPP monolayers on Au(111). In contrast to C60, a submonolayer coverage of C60F48 results in long-range disorder of the underlying single ZnTPP layer and distortion of individual ZnTPP molecules. This is induced by substantial charge transfer at the organic-organic interface, revealed by the interface energetics from UPS. However, a second layer of ZnTPP prevents C60F48 guests from breaking the self-assembled porphyrin template. This finding is important for understanding the growth behaviour of "bottom-up" functional nanostructures involving strong donor-acceptor heterojunctions in molecular electronics.

Surface transfer doping of hydrogen-terminated diamond by C60F48: energy level scheme and doping efficiency.

Surface sensitive C1s core level photoelectron spectroscopy was used to examine the electronic properties of C(60)F(48) molecules on the C(100):H surface. An upward band bending of 0.74 eV in response to surface transfer doping by fluorofullerene molecules is measured. Two distinct molecular charge states of C(60)F(48) are identified and their relative concentration determined as a function of coverage. One corresponds to ionized molecules that participate in surface charge transfer and the other to neutral molecules that do not. The position of the lowest unoccupied molecular orbital of neutral C(60)F(48) which is the relevant acceptor level for transfer doping lies initially 0.6 eV below the valence band maximum and shifts upwards in the course of transfer doping by up to 0.43 eV due to a doping induced surface dipole. This upward shift in conjunction with the band bending determines the occupation of the acceptor level and limits the ultimately achievable hole concentration with C(60)F(48) as a surface acceptor to values close to 10(13) cm(-2) as reported in the literature.

Mimicking the active site of aldehyde dehydrogenases: stabilization of carbonyl hydrates through hydrogen bonds.

Aldehyde hydrates are important but highly unstable, transient intermediates in biological and synthetic oxidations to carboxylic acids. We here report N-oxides as the first class of chemical reagents capable of stabilizing such water adducts. This stabilizing effect (studied in solution and in the solid state) seems to be based on the formation of hydrogen bonds.

The total synthesis of C-glycosides with completely resolved seven-carbon backbone polyol stereochemistry: stereochemical correlations and access to L-configured and other rare carbohydrates.

The de novo synthesis of a full set of hydroxymethyl C-glycosides from only two precursors is described. The seven-carbon target molecules contain five stereocentres and bridge the stereochemical gap between natural D-configured and non-natural L-configured series of hexoses. Key steps include hydroxylation, differential protection, stereoselective reduction and desymmetrization of 8-oxabicyclo[3.2.1]oct-6-enes. C-Terminus differentiation and C-terminus excision of the seven-carbon polyol backbone lead to hexoses, including those of the L-series. A stereochemical and genetic classification of C-glycosides is presented.

Engineering of complex polyketide biosynthesis--insights from sequencing of the monensin biosynthetic gene cluster.

The biosynthesis of complex reduced polyketides is catalysed in actinomycetes by large multifunctional enzymes, the modular Type I polyketide synthases (PKSs). Most of our current knowledge of such systems stems from the study of a restricted number of macrolide-synthesising enzymes. The sequencing of the genes for the biosynthesis of monensin A, a typical polyether ionophore polyketide, provided the first genetic evidence for the mechanism of oxidative cyclisation through which polyethers such as monensin are formed from the uncyclised products of the PKS. Two intriguing genes associated with the monensin PKS cluster code for proteins, which show strong homology with enzymes that trigger double bond migrations in steroid biosynthesis by generation of an extended enolate of an unsaturated ketone residue. A similar mechanism operating at the stage of an enoyl ester intermediate during chain extension on a PKS could allow isomerisation of an E double bond to the Z isomer. This process, together with epoxidations and cyclisations, form the basis of a revised proposal for monensin formation. The monensin PKS has also provided fresh insight into general features of catalysis by modular PKSs, in particular into the mechanism of chain initiation.