Background: Aromatic peroxygenases (APOs) are the "missing link" between heme peroxidases and P450-monooxygenases. Results: Based on two crystal structures the substrate conversion of APOs is elucidated. Conclusion:The specific design of the heme cavity and the distal heme access channel govern substrate specificity. Significance: APOs can be utilized in biotechnology and organic synthesis having significant advantages when compared with cytochrome P450 enzymes.
Background: DyP-type peroxidases catalyze biotechnologically important reactions. Results: Based on the crystal structure of a fungal DyP, the conformational flexibility of Asp-168 is elucidated. Tyr-337 is identified as a surface-exposed substrate interaction site. Conclusion: Asp-168 and Tyr-337 are key residues directly involved in AauDyPI-catalysis. Significance: Peroxidases are biocatalysts, much sought after and ubiquitous enzymes in nature.
The titanates derived from C(,C(,LY ',ol'-tetraaryl-l,3-dioxolane-4,5-dimethanols (TADDOLs, prepared from tartrate) act as catalysts for enantioselective additions of dialkylzinc compounds to aldehydes. For the standard reaction chosen for this investigation of the mechanism, the addition of diethylzinc to benzaldehyde, there is very little change of selectivity with different aryl substituents on the TADDOLate ligands (Tables 2 4 , examples). With 0.02 to 0.2 equiv. of the chiral titanates, selectivities above 90% are observed only in the presence of excess tetraisopropyl titanate! According to NMR measurements (Fig. 2), the chiral bicyclic titanate and the achiral titanate do not react to give new species under these conditions. From experiments with different stoichiometries of the components, and with different achiral or chiral OR groups on the Ti-atom of the seven-membered ring titanate, it is concluded i ) that a single chiral titanate is involved in the product-forming step, ii) that the bulky TADDOLate ligand renders the Ti-center catalytically more active than that of (i-PrO)4Ti, due to fast dynamics of ligand exchange on the sterically hindered Ti-center (Table 5, Fig.3), and iii) that the role of excess (i-PrO),Ti is to remove -by ligand exchange -the product alkoxides (R*O) from the catalytically active Ti-center (Scheme 4, Table 6 ) . Three new crystal structures of TADDOL derivatives (two clathrates with secondary amines, and a dimethyl ether) have been determined by X-ray diffraction (Figs. 5-7), and are compared with those previously reported. The distances between the C(aryl)*O oxygen atoms in the C2-and C1-symmetrical structures vary from 2.58 to 2.94 A, depending upon the conformation of their dioxolane rings and the presence or absence of an intramolecular H-bond (Fig. 8). A single-crystal X-ray structure of a spiro-titanate, with two TADDOLate ligands on the Ti-atom, is described (Fig. 9 ) ; it contains six different seven-membered titanate-ring conformations in the asymmetric unit (Fig. lo), which suggests a highly flexible solution structure. The structures of Ti TADDOLate complexes are compared with those of C,-symmetrical Ru, Rh, and Pd disphosphine chelates ( Table 7). A common topological model is presented for all nucleophilic additions to aldehydes involving Ti TADDOLates (Si attack with (R,R)-derivatives, relative topicity unlike ; Fig. 11 ). Possible structures of complexes containing bidentate substrates for Ti TADDOLate-mediated ene reactions and cycloadditions are proposed (Fig. 12). A simple six-membered ring chair-type arrangement of the atoms involved can be used to describe the result of TADDOLate-mediated nucleophilic additions to aldehydes and ketones, with Ti, Zr, Mg, or A1 bearing the chiral ligand (Scheme 6). A proposal is also made for the geometry of the intermediate responsible for enantioselective hydrogenation of N-(acety1amino)cinnamate catalyzed by Rh complexes containing C2-symmetrical dipbosphines (Fig. 13).
Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid. A σ-allyl complex was identified as the resting state of the catalytic transformation and characterized by X-ray crystallographic analysis. By means of ESI-MS investigations we were able to detect a reactive intermediate of the catalytic cycle.
Density functional calculations have been carried out on free-base porphyrin (1) and its seven possible isomers (2−8) with an N4-metal coordination core. A total of 27 structures resulting from geometrical isomerism ((E/Z)-configurations) and NH tautomerism were studied. Geometries were fully optimized with the nonlocal density functional approximation (BLYP) using the 3-21G and 6-31G** basis sets. The calculated geometries compare favorably with the available X-ray crystal structures. Porphycene (2) is predicted to be the most stable among the eight isomers and is about 2 kcal/mol more stable than porphyrin due to its exceptionally strong hydrogen bonding. Compounds 5−8 are much less stable than porphyrin due to severe ring strain in these compounds. When a -(CH) n - linker is in a (Z)-configuration, each compound is planar or nearly planar with significant π-delocalization; the corresponding (E)-configured structures are predicted to be somewhat distorted into bowl-like geometries in order to avoid severe steric interactions involving the inner hydrogens.
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