Models for the Active Center of Pterin-Containing Molybdenum Enzymes: Crystal structure of a molybdenum complex with sulfur and pterin ligands

The first crystal structure of a molybdenum complex 9 with a hydrogenated pterin and a sulfur ligand contributes to the discussion about the active center of molybdenum and tungsten enzymes containing a molybdopterin cofactor. Complex 9 was synthesized through a redox reaction of [Mo^VIO₂ (LN-S₂)] ( 8 ; LN-S₂ = pyridine-2, 6-bis(methanethiolato)) with 5, 6, 7, 8-tetrahydropterin ( 7 ). 2 HCl (H₄Ptr.2 HCl). The complex crystallizes, with a non-coordinating Cl-atom acting as a counterion, in the monoclinic space group C2/c (No. 15) with cell dimensions a = 22.900(5), b = 10.716(2), c = 17.551(4) Å, β = 120.36(3)°, and Z = 8. We interpret 9 as [Mo^IVO(LN-S₂)(H⁺-q-H₂Ptr)]Cl (q = quinonoid; H₂Ptr = dihydropterin), i.e., a Mo^IV monooxo center coordinated by a pyridine-2, 6-bis(methanethiolato) ligand and a protonated dihydropterin. The spectroscopic properties of this new complex are comparable to those of other crystalline molybdenum complexes of hydrogenated pterins without additional S-coordination. The slightly H₂O-soluble complex 9 reacts with the natural enzyme substrate DMSO very slowly, possibly due to the lack of easily dissociable ligands at the metal center.     

Synthese, 13C-NMR-Spektren und räumliche Struktur von ‘Push-Pull’-Diacetylenen

Synthesis, ¹³C-NMR Spectra, and X-Ray Investigation of ‘Push-Pull’ Diacetylenes Phenyl-substituted ‘push-pull’ diacetylenes 1f and 1g have been prepared by acetylation and benzoylation of the appropriate lithiodiynylamines 4 (Scheme 2). ¹³C-NMR spectra of diacetylenes 1a–g (Table 1) are discussed with respect to the expected polarisation of the diacetylene unit by ‘push’ and ‘pull’ substituents. X-Ray investigations of 1c , 1e , and 1f have been performed in view of the planned solid-state polymerisation of ‘push-pull’ diacetylenes. In the crystalline state, diacetylenes 1c and 1f are stacked, however, the stacking parameters do not allow a solid-state polymerisation.     

A 13C and 31P N.M.R. Investigation of restricted rotation in [Pt(η3-allyl){P(cyclohexyl)3}2]+ [PF6]- and related compounds

The dynamic behaviour of [Pt(/gh³-allyl){P(cyclohexyl)₃}₂]⁺[PF₆]^- has been reinvestigated, and the earlier interpretation of restricted rotation about the Pt—P endorsed. The activation parameters were obtained. [Pt(/gh³-allyl)(PPrⁱ₃)₂]⁺[PF₆]^- behaves similarly, while it has not prove possible to stop the rotation in [Pt(/gh³-allyl){P(CH₂Ph)₃}₂]⁺[PF₆]^-.     

Approaching supramolecular functionality

Functional molecules require a high degree of complexity which is difficult to achieve by covalent synthesis. This article discusses supramolecular approaches to the creation of larger architectures through noncovalent bonds, self-assembly, and template strategies. It highlights selected examples for the structural and conformational control of function and attempts to identify difficulties and challenges which may arise in future.     

Convergence behavior of the density-matrix renormalization group algorithm for optimized orbital orderings

The density-matrix renormalization group algorithm has emerged as a promising new method in ab initio quantum chemistry. However, many problems still need to be solved before this method can be applied routinely. At the start of such a calculation, the orbitals originating from a preceding quantum chemical calculation must be placed in a specific order on a one-dimensional lattice. This ordering affects the convergence of the density-matrix renormalization group iterations significantly. In this paper, we present two approaches to obtain optimized orderings of the orbitals. First, we use a genetic algorithm to optimize the ordering with respect to a low total electronic energy obtained at a predefined stage of the density-matrix renormalization group algorithm with a given number of total states kept. In addition to that, we derive orderings from the one- and two-electron integrals of our test system. This test molecule is the chromium dimer, which is known to possess a complicated electronic structure. For this molecule, we have carried out calculations for the various orbital orderings obtained. The convergence behavior of the density-matrix renormalization group iterations is discussed in detail.     

New aspects in the reaction of azomethines with cyclic CH-acidic compounds

Summary Treatment of substituted benzylidene anilines1 a – df with cyclic CH-acidic compounds2a–m in ethanol at room temperature yields in additon/elimination reactions the corresponding arylidene derivatives4 and the 2:1 adducts5. The addition products3, which are formed as intermediates, could not be isolated in any case. The donor/acceptor effect of the substituents on the benzylidene moiety influences to a significant extent the reactivity towards the azomethine carbon.

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Neue Aspekte der Reaktion von Azomethinen mit cyclischen CH-aciden Verbindungen

Zusammenfassung Bei der Umsetzung der substituierten Benzylidenaniline1 a – f mit den cyclischen CH-aciden Verbindungen2 a – m in Ethanol bei Raumtemperatur erhält man in Additions/Eliminierungsreaktionen die Arylidenderivate4 und die 2:1-Addukte5. Die als Intermediat gebildeten Additionsprodukte3 konnten in keinem Fall isoliert werden. Die Donor-bzw. Acceptorwirkung der Substituenten am Benzylidenrest beeinflußt maßgebend die Reaktivität am Azomethinkohlenstoff.

     

Synthesis and self-assembly of galactose-terminated alkanethiols and their ability to resist proteins

The synthesis of two galactose-terminated alkanethiols with the structural formula X-OC2H5NHCO(CH2)15SH (X = 2,3,4,6-tetra-O-methyl-beta-D-Gal or beta-D-Gal) is described. Single-component and mixed self-assembled monolayers (SAMs) of the methylated and nonmethylated compounds were prepared on gold and subsequently characterized with ellipsometry, contact angle goniometry, and infrared reflection-absorption spectroscopy. Studies of the irreversible protein adsorption onto the SAMs using ex-situ ellipsometry revealed very low levels of fibrinogen and lysozyme adsorption onto mixed SAMs displaying advancing water contact angles between 24 degrees and 45 degrees and below 45 degrees , respectively. A monomethylated compound (X = 6-O-methyl-beta-D-Gal) was also synthesized and assembled on gold. This particular compound was found to possess wettability properties corresponding to the low adsorption regime of the mixed SAMs, and the results from the same set of fibrinogen and lysozyme adsorption experiments showed very low levels of protein adsorption. Our findings suggest that the protein rejecting properties rely on a fine balance between the surface energy and/or hydrogen bond donating/accepting properties of the SAM surface.     

Diastereoface Selectivity During Pthalimidonitrene Additions to (E)-and (Z)-configurated α, β-unsaturated esters,induced by a chiral center in the γ-position

In-situ-generated phthalimidonitrene was added to five α, β-unsaturated esters containing a chiral secondary O-function at C(γ). The additions were fully suprafacial, inasmuch as the (E)-isomers 1 afforded only the trans-aziridines 2 and 3 (J(β, γ) = 4.8?5.1 Hz) and the (Z)-isomers 4 only the cis-aziridines 5 and 6 (8.2?8.5 Hz). The products 2 , 3 , 5 , and 6 where shown to possess the arabino-, xylo-, ribo-, and lyxo- configuration, respectively, by X-ray structure analysis of 2b , 2d , and 6a . The diastereoface selectivity of the nitrene additions, induced by the chiral substructure around C(γ), resulted in more 2 than 3 from 1 , but more 6 than 5 from 4 , which means that the preference of attack at the double bond switches from one side to the other depending on the C=C configuration. The preferences were higher at lower temperature. The aziridines 2a , 2d , and 3d exhibit ¹H-NMR-visible isomerism at the ring N-atom; the major (78?95 %)invertomer A is always the one with the phthalimido group in trans-position to the (larger) substructure around C(γ). The other aziridines only show ¹H-NMR signals of one invertomer, which – by steric reasoning - ought to be A ; this is confirmed by a ¹H-NMR argument for 3a , 5a , 6a , 5c , and 6c .     

Bilden sich aus Pentafulven und Cyclopentadien [6+4]-Cycloaddukte??

Does [6+4] Cycloaddition between Pentafulvene and Cyclopentadiene Take Place? Reaction of a 1:1 mixture of cyclopentadiene (CPD) and pentafulvene ( 1a ) at 20° gives a complex mixture. The low-molecular-weight part mainly consists of pure and mixed dimers (ca. 73 %) besides corresponding trimers (ca. 20%) and some corresponding oligomers according to GC/MS investigations (Fig. 1). The 3 predominant ‘mixed dimers’ between CPD and 1a have been separated, and structures 4 – 6 (Scheme 3) are assigned according to 400- and 600-MHz ¹H-NMR investigations. These results show that HOMO(CPD)-LUMO(fulvene) interactions are important in pentafulvene cycloadditions. Dimer 6 results from [6+4] cycloaddition followed by [1,5]-H shifts.