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Density functional theory (DFT) calculations have supported a concerted mechanism for xanthine oxidase and aldehyde oxidase hydride displacement from the [sp.sup.2] carbon of 6-substituted 4-quinazolinones. The variations in the transition state structure have shown that C-O bond formation is nearly complete in the transition state and the transition state changes are anti-Hammond with the C-H and C-O bond lengths being more product-like for the faster reactions.

A typical model substrate, formamide, is used for reinvestigating several plausible reaction mechanisms of the oxidation process by xanthine oxidase. The results have shown that the oxidation reaction proceeds by the deprotonated active site through the one-step mechanism.

[His.sub.6]-tagged xanthine/[alpha]-ketoglutarate ([alpha] KG) dioxygenase (XanA) of Aspergillus nidulans was purified from the fungal mycelium and recombinant Escherichia coli cells. The properties of these two forms of protein were compared.

Density functional theory (DFT) calculations of the 'very rapid' Mo(V) intermediate of xanthine oxidase (XO) are presented. The results have suggested that the structure of the intermediate contains the substrate bonding to the metal in end-on coordination with a bond distance of 3.1-3.2 Angstrom.

The reaction between (2,4,5-trimethoxyphenyl)(2-hydroxyphenyl)methanone and ceric ammonium nitrate which resulted in the xanthone, 2,3-dimethoxy-9H-xanthen-9-one is reported. The reaction could also applied under the same conditions for (1,4-dimethoxynaphthalen-2-yl)(2-hydroxyphenyl)methanone to generate 12a-methoxy-5H-benzo[c]xanthenes 5,7(12aH)-dione.

The crystal structures of the urate complexes of the demolybdo-form of the D428A mutant of rat xanthine oxidoreductase and of the reduced bovine milk enzyme are determined in order to distinguish between the two models described for the binding mode of the substrate xanthine and its activation mechanism by xanthine oxidoreductase. The results have shown the catalytically relevant binding mode of the structure xanthine.

The synthesis, purification and x-ray structural characterization of two enzymes, NYSGXRC-9339a and NYSGXRC-9236b identified from DNA isolated from the Sargasso Sea as part of the Global Ocean Sampling Project are reported. The identified enzymes which efficiently catalyze the deamination of isoxanthopterin and pterin 6-carboxylate could also be used to discover 24 other genes which are predicted to deaminate isoxanthopterin.

Ultrafast time-resolved fluorescence spectroscopic experiments were used to probe picosecond dynamics of the theophylline-binding RNA aptamer. Picosecond dynamics measured for the modified aptamer that binds with higher affinity to 3-methylxanthine than theophylline revealed that the modified aptamer exhibit less heterogeneity in the aptamer-3-methylxanthine complex than those observed in the theophylline aptamer-theophylline complex.

Quantum mechanical/molecular mechanical (QM/MM) methods were used to explore the conversion of xanthine to uric acid in xanthine oxidase. An analysis of the computational results provides detailed insigth into the necessary mechanistic role of the active-site residues of xanthine oxidase.

The Julia-Stille sequence is a valuable convergent approach to construct the complete skeleton of highly functionalized acetylenic C37-norcarotenoid butenolides. The palladium-induced isomerization, which proved beneficial during the second Stille coupling in the case of peridinin, could be a limitation, as shown in the present approach to pyrrhoxanthin.

A facile access to 1,3,7-trihydroxyxanthone and its two different regioselective coupling reactions with prenal to obtain osajaxanthone and nigro-lineaxanthone F is demonstrated. The linear product osajaxanthone had a higher melting point than the corresponding angular nigrolineaxanthone F but the confirmatory structural discrimination of these two natural products on the basis of H and C NMR data was a difficult task but was confirmed by using X-ray crystallographic data.

The electron paramagnetic resonance (EPR) is a simple method and it directly checks the active sites of molybdenum enzymes like sulfite, xanthine oxidase and dimethyl sulfoxide reductase. Furthermore, the close association among the illustrated and computed EPR parameters for increased symmetry representation system gives more support to expand the theoretical computation of spectral parameters to enzyme sites of low symmetry.

Raman and UV-visible absorption spectroscopy is used to investigate the binding of the inhibitor 8 nitroxanthine to urate oxidase. The results suggest that binding to the urate oxidase active site does induce procatalytic changes in the electronic structure of the ligand.

Molybdenum K-edge X-ray absorption studies performed on the oxidized active site of xanthine oxidase at pH 6 and 10 is described. The results indicated that the active site possesses one terminal oxygen ligand, two thiolate ligands, one terminal sulfido ligand and one Mo-OH moiety.

The influence of different lipids on the solubility of the xanthophyll cycle pigments diadinoxanthin (Ddx) and violaxanthin (Vx) and on the efficiency of Ddx and Vx de-epoxidation by the enzymes Vx de-epoxidase (VDE) from wheat and Ddx de-epoxoidase (DDE) from the diatom Cyxlotella meneghiniana is examined. The results reveal that the lipid MGDG and PE are able to solubalize both xanthrophyll cycle pigments in an aqueous medium.

A theoretical study based on density functional theory calculations on the mechanism of the reductive half-reaction of xanthine oxidase is reported. The calculated results indicate that the anionic catalyst model acts as a nucleophile and is reactive for the oxidation of aldehyde substrates that are relevant for nucleophilic additions.

The arsenite coordination to the molybdenum through an oxygen bridge in the desulfo form of the enzyme is demonstrated. The coordination pattern of the arsenite indicates that the substrate reacts with the labile water ligand of Mo site to form a Mo-O-C bond.

The preparation of the initial structural analogues of oxidized active sites of the xanthine oxidoreductase (XO) enzyme family which uses tungsten to stabilize the [M.sup.VI] state is described.

The mechanism of xanthine oxidase carryout substrate hydroxylation can now be understood in the context of the crystal structures of several enzymes. Xanthine oxidase most likely operates via two-electron chemistry, with the formation of the initially appearing LMo(super IV)O(SH)(OR) intermediate proceeding in a single two-electron process involving nucleophilic attack on substrate followed by hydride transfer to the molybdenum center.