A cationic 24-MC-8 manganese cluster with ring metals possessing three oxidation states [Mn(II)4Mn(III)6Mn(IV)2(mu4-O)2(mu3-O)4(mu3-OH)4(mu3-OCH3)2(pko)12](OH)(ClO4)3

Reaction of Mn(ClO4)2 with di-pyridyl ketone oxime, (2-py)2C=NOH, gives the novel cluster [Mn(II)4Mn(III)6Mn(IV)2(mu4-O)2(mu3-O)4(mu3-OH)4(mu3-OCH3)2(pko)12](OH)(ClO4)3 1. It is the only example of a 24-MC-8, and the first metallacrown with ring metal ions in three different oxidation states. Magnetic measurements show antiferromagnetic behavior.     

Methyl 3‐(4‐methoxyphenyl)prop‐2‐enoate

The title mol­ecule, C₁₁H₁₂O₃, is almost planar, with an average deviation of the C and O atoms from the least‐squares plane of 0.146 (4) Å. The geometry about the C=C bond is trans. The phenyl ring and –COOCH₃ group are twisted with respect to the double bond by 9.3 (3) and 5.6 (5)°, respectively. The endocyclic angle at the junction of the propenoate group and the phenyl ring is decreased from 120° by 2.6 (2)°, whereas two neighbouring angles around the ring are increased by 2.3 (2) and 0.9 (2)°. This is probably associated with the charge‐transfer interaction of the phenyl ring and –COOCH₃ group through the C=C double bond. The mol­ecules are joined together through C—H?O hydrogen bonds between the methoxy and ester groups to form characteristic zigzag chains extended along the c axis.     

Orthorhombic polymorphs of two trans‐4‐aminoazoxybenzenes

The two isomeric compounds 4‐amino‐ONN‐azoxy­benzene [or 1‐(4‐amino­phenyl)‐2‐phenyl­diazene 2‐oxide], i.e. the α isomer, and 4‐amino‐NNO‐azoxy­benzene [or 2‐(4‐amino­phenyl)‐1‐phenyl­diazene 2‐oxide], i.e. the β isomer, both C₁₂H₁₁N₃O, crystallized from a polar solvent in orthorhombic space groups, and their crystal and molecular structures have been determined using X‐ray diffraction. There are no significant differences in the bond lengths and valence angles in the two isomers, in comparison with their monoclinic polymorphs. However, the conformations of the mol­ecules are different due to rotation along the Ar—N bonds. In the α isomer, the benzene rings are twisted by 31.5 (2) and 14.4 (2)° towards the plane of the azoxy group; the torsion angles along the Ar—N bond in the β isomer are 24.3 (3) and 23.5 (3)°. Quantum‐mechanical calculations indicate that planar conformations are energetically favourable for both isomers. The N—H?O hydrogen bonds observed in both networks may be responsible for the deformation of these flexible mol­ecules.     

Polyketide double bond biosynthesis. Mechanistic analysis of the dehydratase-containing module 2 of the picromycin/methymycin polyketide synthase

Picromycin/methymycin synthase (PICS) is a modular polyketide synthase (PKS) that is responsible for the biosynthesis of both 10-deoxymethynolide (1) and narbonolide (2), the parent 12- and 14-membered aglycone precursors of the macrolide antibiotics methymycin and picromycin, respectively. PICS module 2 is a dehydratase (DH)-containing module that catalyzes the formation of the unsaturated triketide intermediate using malonyl-CoA as the chain extension substrate. Recombinant PICS module 2+TE, with the PICS thioesterase domain appended to the C-terminus to allow release of polyketide products, was expressed in Escherichia coli. Purified PICS module 2+TE converted malonyl-CoA and 4, the N-acetylcysteamine thioester of (2S,3R)-2-methyl-3-hydroxypentanoic acid, to a 1:2 mixture of the triketide acid (4S,5R)-4-methyl-5-hydroxy-2-heptenoic acid (5) and (3S,4S,5R)-3,5-dihydroxy-4-methyl-n-heptanoic acid-delta-lactone (10) with a combined kcat of 0.6 min(-1). The triketide lactone 10 is formed by thioesterase-catalyzed cyclization of the corresponding d-3-hydroxyacyl-SACP intermediate, a reaction which competes with dehydration catalyzed by the dehydratase domain. PICS module 2+TE showed a strong preference for the syn-diketide-SNAC 4, with a 20-fold greater kcat/K(m) than the anti-(2S,3S)-diketide-SNAC 14, and a 40-fold advantage over the syn-(2R,3S)-diketide-SNAC 13. PICS module 2(DH(0))+TE, with an inactivated DH domain, produced exclusively 10, while three PICS module 2(KR(0))+TE mutants, with inactivated KR domains, produced exclusively or predominantly the unreduced triketide ketolactone, (4S,5R)-3-oxo-4-methyl-5-hydroxy-n-heptanoic acid-delta-lactone (7). These studies establish for the first time the structure and stereochemistry of the intermediates of a polyketide chain elongation cycle catalyzed by a DH-containing module, while confirming the importance of key active site residues in both KR and DH domains.     

Crystallization of the $${\text{Mn}}^{{\text{II}}}[12{\text{-MC}}_{{\text{Mn}}^{{\text{III}}}{\text{(N)shi}}}{\text{-}}4]^{2+}$$ Mn II [ 12 -MC Mn III (N)shi - 4 ] 2 + Structure with 1,2,4-Triazolate from Methanol

Journal of Chemical Crystallography - The metallacrown (MC) dimer complex {MnII(OAc)(1,2,4-trz) $$[12{\text{-MC}}_{{\text{Mn}}^{{\text{III}}}{\text{(N)shi}}}{\text{-}}4]$$...     

Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster

Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi‐isotopic broadband rotational spectra. This heterodimer is held together by a primary C? H???π hydrogen bond, assisted by multiple weak C? H???F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C? H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum.     

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor O? O distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three‐body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical O? O distances are in good agreement. The cooperativity effects revealed by the hydrogen bond O? O distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.