New amphiphilic polymers: Poly(N-alkylacrylamide-CO-vinylpyridinium)

A series of new amphiphilic polymers containing various amounts of N-methyl-4-vinylpyridinium units as hydrophilic part and N-dodecylacrylamide or N-decylacrylamide units as hydrophobic part have been prepared by quaternizing the cognate copolymers. Their behaviour in aqueous solutions has been studied by viscometry, as well as by fluorescence spectroscopy and tensiometry. Results are discussed in detail and a conformational model of the macromolecules at the interface (water/methylcyclohexane) is proposed in relation to the formation of microdomains in the aqueous media.     

Precursors to [FeFe]-hydrogenase models: syntheses of Fe2(SR)2(CO)6 from CO-free iron sources

This report describes routes to iron dithiolato carbonyls that do not require preformed iron carbonyls. The reaction of FeCl 2, Zn, and Q 2S 2C n H 2 n (Q (+) = Na (+), Et 3NH (+)) under an atmosphere of CO affords Fe 2(S 2C n H 2 n )(CO) 6 ( n = 2, 3) in yields >70%. The method was employed to prepare Fe 2(S 2C 2H 4)( (13)CO) 6. Treatment of these carbonylated mixtures with tertiary phosphines, instead of Zn, gave the ferrous species Fe 3(S 2C 3H 6) 3(CO) 4(PR 3) 2, for R = Et, Bu, and Ph. Like the related complex Fe 3(SPh) 6(CO) 6, these compounds consist of a linear arrangement of three conjoined face-shared octahedral centers. Omitting the phosphine but with an excess of dithiolate, we obtained the related mixed-valence triiron species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (-). The highly reducing all-ferrous species [Fe 3(S 2C n H 2 n ) 4(CO) 4] (2-) is implicated as an intermediate in this transformation. Reactive forms of iron, prepared by the method of Rieke, also combined with dithiols under a CO atmosphere to give Fe 2(S 2C n H 2 n )(CO) 6 in modest yields under mild conditions. Studies on the order of addition indicate that ferrous thiolates are formed prior to the onset of carbonylation. Crystallographic characterization demonstrated that the complexes Fe 3(S 2C 3H 6) 3(CO) 4(PEt 3) 2 and PBnPh 3[Fe 3(S 2C 3H 6) 4(CO) 4] feature high-spin ferrous and low-spin ferric as the central metal, respectively.     

The lipidated membrane anchor of full length N-Ras protein shows an extensive dynamics as revealed by solid-state NMR spectroscopy

Many proteins involved in signal transduction are equipped with covalently attached lipid chains providing a hydrophobic anchor targeting these molecules to membranes. Despite the considerable biological significance of this membrane binding mechanism for 5-10% of all cellular proteins, to date very little is known about structural and dynamical features of lipidated membrane binding domains. Here we report the first comprehensive study of the molecular dynamics of the C-terminus of membrane-associated full-length lipidated Ras protein determined by solid-state NMR. Fully functional lipid-modified N-Ras protein was obtained by chemical-biological synthesis ligating the expressed water soluble N-terminus with a chemically synthesized (2)H or (13)C labeled lipidated heptapeptide. Dynamical parameters for the lipid chain modification at Cys 181 were determined from static (2)H NMR order parameter and relaxation measurements. Order parameters describing the amplitude of motion in the protein backbone and the side chain were determined from site-specific measurements of (1)H-(13)C dipolar couplings for all seven amino acids in the membrane anchor of Ras. Finally, the correlation times of motion were determined from temperature dependent relaxation time measurements and analyzed using a modified Lipari Szabo approach. Overall, the C-terminus of Ras shows a versatile dynamics with segmental fluctuations and axially symmetric overall motions on the membrane surface. In particular, the lipid chain modifications are highly flexible in the membrane.     

Selective halogenation at the pnictogen atom in Lewis-acid/base-stabilised phosphanylboranes and arsanylboranes

The halogenation of Lewis-acid/base-stabilised phosphanylboranes () and arsanylboranes () with CX(4) (X = Cl, Br) leads selectively to the substitution of both protons at the pentel atom and the new compounds [(CO)(5)W(X(2)PBH(2).NMe(3))] (: X = Cl, : X = Br) and [(CO)(5)W(X(2)AsBH(2).NMe(3))] (: X = Cl, : X = Br), respectively, are obtained. The new products were comprehensively characterised by spectroscopic methods and by X-ray crystallography. While compounds and show an antiperiplanar arrangement of the Lewis acid (W(CO)(5)) and the Lewis base (NMe(3)) in the solid state, a synclinal arrangement in and , respectively, was observed. Computational calculations of the optimised antiperiplanar and synclinal geometries of the compounds and in the gas phase slightly favour the antiperiplanar arrangement of the Lewis acid and the Lewis base for both compounds.     

A selective synthesis of hydroxyborate anions as novel anchors for zirconocene catalysts

A new family of hydroxytris(pentafluorophenyl)borate anions [B(C6F5)3OH](-) associated with organic and aprotic cations c+ (imidazolium, pyrrolidinium and phosphonium) has been prepared by a general one-pot synthesis that implies the chloride borate analogues [B(C6F5)3Cl](-)[c]+. The [c]+[B(C6F5)3OH](-) salts have been isolated and fully characterized. The borate anion [B(C6F5)3OH](-) has been shown to protonate the Zr-Me bond in the Cp2ZrMe2 complex forming CH4 and the first published example of anionic [Cp2Zr(Me)OB(C6F5)3](-) species. Standard spectroscopic methods demonstrate the covalent character of the Zr metal center and the anionic character of the boron atom. This protonolysis methodology using [B(C6F5)3OH](-) anion affords a new route for the incorporation of a covalently bonded anionic functionality on organometallic complexes. This provides a new way to immobilize transition metal complexes in ionic liquids.     

Ultrafast ring-closure reaction of photochromic indolylfulgimides studied with UV-pump-IR-probe spectroscopy

The ring-opening and ring-closure reactions of a photochromic indolylfulgimide are investigated with femtosecond vibrational spectroscopy. Spectral signatures due to excited-state decay and vibrational cooling are seen in the mid-IR region. For the ring-opening reaction triggered with visible pulses, a lifetime of the excited electronic state of 4 ps was obtained in polar solution. In a nonpolar solvent, this time constant is reduced to 2 ps. The ring-closure reaction induced with UV pulses displays an excited-state lifetime and thus a building of the photoproduct of roughly 0.5 ps. For all processes, the subsequent cooling occurs on a 15-ps time scale lasting up to approximately 50 ps. The time-resolved IR measurements do not support the existence of any long-living intermediate states.     

Synthesis of 1,3-diamino-7,8,9,10-tetrahydropyrido[3,2-f]-quinazolines. Inhibitors of Candida albicans dihydrofolate reductase as potential antifungal agents

A series of novel 1,3-diamino-7,8,9,10-tetrahydropyrido[3,2-f]quinazolines were synthesized starting from 6-amino-5-cyanoquinoline (4). These compounds inhibited Candida albicans dihydrofolate reductase with K_i values of ≤0.60 aM. One analogue exhibited moderate in vivo efficacy in a C. albicans-infected mouse model.     

Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.     

Validation of a simple, sensitive method for the determination of beta-estradiol in bovine urine using gas-chromatography negative-ion chemical ionization mass spectrometry

A new method is presented for the analysis of 17[small beta]-estradiol in bovine urine. After deconjugation, the sample is cleaned up using an OASIS[trade mark sign] HLB disposable cartridge and extracted into 1-chlorobutane. The hormone is derivatized using pentafluorobenzoyl chloride. The derivatized estradiol is quantitated using gas-chromatography negative-ion chemical ionization mass spectrometry. Calibrations, obtained using spiked blank urine, are linear in the range of 100-1000 pg mL(-1) with CC[small alpha] approximately 170 pg mL(-1) and CC[small beta] of 287 pg mL(-1). Recoveries are in the range of 80 to 130%. The method is rugged, rapid and sensitive when compared to other hormone methods.     

Treating molecules in arbitrary spin states using the parametric two-electron reduced-density-matrix method

Minimizing the electronic energy with respect to a parameterized two-electron reduced density matrix (2-RDM) is known as a parametric variational 2-RDM method. The parametric 2-RDM method with the M 2-RDM parametrization [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)] is extended to treat molecules in arbitrary spin states. Like its singlet counterpart, the M parametric 2-RDM method for arbitrary spin states is derived using approximate N-representability conditions, which allow it to capture more correlation energy than coupled cluster with single and double excitations at a lower computational cost. We present energies, optimized bond lengths, potential energy curves, and occupation numbers for a set of molecules in a variety of spin states using the M and K parametric 2-RDM methods as well as several wavefunction methods. We show that the M parametric 2-RDM method can describe bond breaking of open-shell molecules like triplet B(2) and singlet and triplet OH(+) even in the presence of strong correlation. Finally, the computed 2-RDMs are shown to be nearly N-representable at both equilibrium and non-equilibrium geometries.