Carbon-13 spectral studies of some chalcones and flavanones

Carbon-13 signal assignments of 2′,4′-dihydroxychalcone, flemichapparin, pashanone, isodidymocarpin, pedicellin, pedicinin, methylpedicinin, didymocarpin-A and the corresponding quinoflavanone dehydrodidymocarpin-A are reported.     

Spectroscopic Study of Cerium-Doped Silica Gel Monoliths and Their Densified Derivatives

Cerium is exploited as a probe cation for elucidating the structure of an alkoxide-derived silica gel and its progressive evolution to a glass network as a function of heat-treatment up to 1000°C. At intermediate temperatures, the host structure exhibits inhomogeneity due to insufficient formation of siloxane bonds, which is reflected by at least two different sites and co-ordination spheres (termed “high” and “low” water ligation) for cerium. This is proved by the response of the gels heated up to 700°C to rehydration. Further formation of Si−O−Si network (900°C) leads to the destruction of the “high water” sites of cerium and progression towards a glassy structure. It is, however, only after heat-treatment at 1000°C that a dense silica glass network, not responding to rehydration, is finally obtained with cerium ions embedded in it.     

TiCl(4)-promoted Baylis-Hillman reactions of substituted 5-isoxazolecarboxaldehydes with cycloalkenones)

The Baylis-Hillman (BH) reaction of substituted 5-isoxazolecarboxaldehydes with cyclohexenone in the presence of TiCl(4) invariably lead to the formation of hemiacetals beside the BH adducts. A similar reaction in the presence of DABCO, DBU, or 3-HQN yielded minor quantities of phenols in addition to the usual BH adducts. Similar to 5-isoxazolecarboxaldehydes, the TiCl(4)-mediated BH reaction of cyclohexenone with various benzaldehydes also furnishes hemiacetals in considerable yields. The reaction mechanism involving the formation of alpha-chloromethyl enone as an intermediate has been proposed. The synthesis of hemiacetals 5 and 14 from compound 4 in the presence of cyclohexenone and cyclopentenone, respectively, under acidic conditions indicates that enolization and aromatization of the cyclohexene ring are the key steps in the reaction mechanism.     

A ruthenium nitrosyl that rapidly delivers NO to proteins in aqueous solution upon short exposure to UV light

Two Ru(III) complexes, [Ru(PaPy(3))(Cl)](BF(4)) (2) and [Ru(PaPy(3))(NO)](BF(4))(2) (3) (PaPy(3)H = N,N'-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide), have been synthesized and characterized by spectroscopy and X-ray diffraction. Nitrosyl complex 3 has been prepared by passage of purified NO gas to the hot methanolic solution of the chloro derivative 2. Complex 3 exhibits nu(NuOmicron) stretching frequency at 1899 cm(-)(1) indicating a [Ru-NO](6) configuration. Clean (1)H NMR spectra of 3 in D(2)O and CD(3)CN confirm the S = 0 ground state. When an aqueous solution of [Ru(PaPy(3))(NO)](BF(4))(2) is exposed to low intensity UV light, it rapidly loses NO and forms [Ru(PaPy(3))(H(2)O)](2+). This reaction can be conveniently used to transfer NO to proteins such as myoglobin (Mb) and cytochrome c oxidase. The NO transfer reaction is clean and occurs upon short exposure to light.     

First [Fe-NO](6) complex with an N(2)S(3)Fe-NO core as a model of NO-inactivated iron-containing nitrile hydratase. Are thiolates and thioethers equivalent donors in low-spin iron complexes?

The spectroscopic and structural properties of [(bmmp-TASN)FeNO]BPh(4) (1) (bmmp-TASN = 4,7-bis(2'-methyl-2'-mercaptopropyl)-1-thia-4,7-diazacyclononane) have been determined and are compared with the nitric oxide inactivated form of iron-containing nitrile hydratase, NHase(dark). [(bmmp-TASN)FeNO]BPh(4) is prepared from the addition of NO to (bmmp-TASN)FeCl followed by addition of sodium tetraphenylborate. [(bmmp-TASN)FeNO]BPh(4) crystallizes from acetonitrile-methanol solutions upon ether vapor diffusion as dark blue plates in the monoclinic space group P2(1)/c with a = 11.9521(14) A, b = 11.3238(13) A, c = 26.624(3) A, beta = 98.280(2), and Z = 4. The nu(NO) stretching frequency of 1856 cm(-)(1) and the M?ssbauer parameters, delta = 0.06 mm/s and DeltaE(q) = 1.75 mm/s, compare favorably with those of NHase(dark). The similarities of the iron-sulfur bond distances to the thiolate, 2.284(2) A and 2.291(2) A, versus thioether, 2.285(2) A, are attributed to the low-spin configuration of the iron. The relationship between this structural observation and the spectroscopic properties of the complex are discussed.     

Impact of cholesterol on voids in phospholipid membranes

Free volume pockets or voids are important to many biological processes in cell membranes. Free volume fluctuations are a prerequisite for diffusion of lipids and other macromolecules in lipid bilayers. Permeation of small solutes across a membrane, as well as diffusion of solutes in the membrane interior are further examples of phenomena where voids and their properties play a central role. Cholesterol has been suggested to change the structure and function of membranes by altering their free volume properties. We study the effect of cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC) bilayers by means of atomistic molecular dynamics simulations. We find that an increasing cholesterol concentration reduces the total amount of free volume in a bilayer. The effect of cholesterol on individual voids is most prominent in the region where the steroid ring structures of cholesterol molecules are located. Here a growing cholesterol content reduces the number of voids, completely removing voids of the size of a cholesterol molecule. The voids also become more elongated. The broad orientational distribution of voids observed in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a distribution where orientation along the bilayer normal is favored. Our results suggest that instead of being uniformly distributed to the whole bilayer, these effects are localized to the close vicinity of cholesterol molecules.     

An unusual dinuclear ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis,structure, and UV-vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states

Reaction of [Ru(acac)(2)(CH(3)CN)(2)] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H(2)L) results in formation of an unexpected dinuclear complex [(acac)(2)Ru(III)(L(1))Ru(III)(acac)(2)] (1) in which the bridging ligand [L(1)](2)(-) contains an (-)HN[bond]C[double bond]N[bond]N[double bond]C[bond]NH(-) unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H(2)L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (Delta E(1/2) = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (Delta E(1/2) = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1](-) has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V(ab) approximately 1100 cm(-1), characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1](+), no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.