Geometric and electronic structure of clusters

The interplay between electronic and geometric structure is investigated for covalently bonded phosphorus clusters. We use a modification of the molecular dynamics/ density functional (MD/DF) method of Car and Parrinello, describing the electronic structure by a simplified linear combination of atomic orbitals (LCAO) approach. The results show clearly the tendency of phosphorus to threefold coordination, and substantial variations in bond angles lead to a large variety of isomers.     

Extreme stability of an unsolvated alpha-helix

High-temperature ion mobility measurements have been performed for alpha-helical Ac-A15K+H+ and globular Ac-KA15+H+ peptides. The alpha-helical and globular conformations do not melt into random coils as the temperature is raised. Instead, both conformations survive to the point where the peptide signals vanishes due to fragmentation. This occurs at 600 K for the globular Ac-KA15+H+ peptide and at 725 K for the alpha-helical Ac-A15K+H+. For the helical Ac-A15K+H+ peptide it appears that fragmentation is triggered by disruption of the helical conformation.     

Infrared spectra and rotational isomerism of epifluorohydrin and other propene oxide derivatives

The infrared spectrum for the vapour, liquid and solid states of epifluorohydrin has been studied, and evidence has been obtained for the presence of rotational isomers. The dominant conformer in all phases corresponds to a gauche structure, which was the only form identified in the vapour by microwave spectroscopy [1]. Similar evidence for conformational equilibria has been found for the chloro, bromo and iodo derivatives, and the enthalpy differences between the rotational isomers of these compounds were measured respectively as 4580 ± 970, 3950 ± 400, and 2350 ± 170 J mol^?1 respectively. At low temperatures, epiiodohydrin was found to crystallize in one of two possible crystalline phases. These two phases correspond to the two rotational isomeric conformations of the molecule.     

Differential binding of Mg2+, Zn2+, and Cd2+ at two sites in a hammerhead ribozyme motif, determined by 15N NMR

A decamer duplex model of Domain II of the hammerhead ribozyme was synthesized with [8-13C-1,7,NH2-15N3]-guanosine at the known metal binding site, G10.1 and, for comparison, [2-13C-1,7,NH2-15N3]-guanosine at G16.2. The 15N NMR chemical shifts of the labeled N7s monitored during addition of Mg2+, Cd2+, and Zn2+ showed the same preference for binding at G10.1 over G16.2 for each metal. These results demonstrate that 15N labeling can be used to evaluate the binding of different metals, including Mg2+, to a given nitrogen, as well as to compare the binding potential of different sites.     

Platinum(II) and palladium(II) complexes of bisphosphine ligands bearing o-N,N-dimethylanilinyl substituents: a hint of catalytic olefin hydration

Platinum(II) and palladium(II) complexes of the potentially hexadentate P,N-donor ligand family Ar2P-X-PAr2 (X = (CH2)2 [dmape], cyclic-C5H8 [dmapcp]; Ar = o-N,N-dimethylanilinyl) are described. In CH2Cl2, the dmape complexes exist as equilibrium mixtures of MCl2(P,P'-dmape) and [MCl(P,P',N-dmape)]Cl isomers (M = Pd, Pt), governed by deltaH(o) = -19 +/- 4 kJ mol(-1) and deltaS(o) = -100 +/- 30 J mol(-1) K(-1) for M = Pt, and deltaH(o) = -11 +/- 7 kJ mol(-1) and deltaS(o) = -60 +/- 20 J mol(-1) K(-1) for M = Pd. The water-soluble dmapcp complexes exist solely in the [MCl(P,P',N-dmapcp)]Cl form, but the free and coordinated anilinyl rings in these complexes are in slow diastereoselective exchange. X-ray crystal structures for MCl2(P,P'-dmape) (M = Pd, Pt), and the [PdCl(P,P',N-dmape)]+ and [PtCl(P,P',N-dmapcp)]+ cations, are presented. Some of the complexes show marginal activity in water for the catalyzed hydration of maleic to malic acid, giving about 6-7% conversion in 24 h at 100 degrees C and substrate:catalyst loadings of 100:1. Attempts to synthesize a PdCl(P,P',N-dmapm)+ species led instead to isolation of [Pd(mu-Cl)(P,P'-dmapm)]2[PF6]2 (dmapm = Ar2PCH2Ar2).     

Reactions of a carbene stabilised indium trihydride complex, [InH3{CN(Mes)C2H2N(Mes)}] Mes = mesityl,with transition metal complexes

The reactivity of the carbene stabilised indium trihydride complex, [InH₃(IMes)] IMes = 1,3-dimesitylimidazol-2-ylidene, toward a variety of transition metal complexes has been investigated. The study has shown that the InH₃ complex can act as a carbene and/or hydride transfer reagent to transition metal centres but does not yield heterobimetallic materials. Two new complexes, [Cp₂Ti(-Cl)₂Zn(IMes)Cl] and [CpNi(H)(IMes)], have resulted from this work, both of which have been spectroscopically and structurally characterised.     

Macroscale high-performance liquid chromatographic separation and instrumental identification of components of diethylaminoethyl murine epidermal growth factor

Murine epidermal growth factor (m-EGF), a polypeptide produced as a chromatographically homogeneous peak on diethylaminoethyl (DEAE) cellulose by the method of Savage and co-workers, and characterised as a single compound, has been shown by ourselves and several other groups to be a mixture. The present contribution extends our previously reported work and discusses the separation of this material, termed DEAE-m-EGF, into its components by preparative ion-pair reversed-phase high-performance liquid chromatography (RP-HPLC) on C18 mu Bondapak in quantities up to 50 mg per run. Isocratic elution was used and the mobile phase was acetonitrile-water (26:74, v/v, 0.04 M in triethylamine acetate); pH was 5.6, temperature 40 degrees C, and detection was by ultraviolet absorption at 254 nm, and (for some runs) by differential refractometry. Seven significant peaks, four major, three minor, were detected. Of the major peaks, two designated alpha- and beta-EGF, constituted 70% of the total mass and were the most important to our work. Each of the eluted peaks was recovered by lyophilisation, and this product checked for homogeneity by ion-pair RP-HPLC on a C18 mu Bondapak analytical column, with ultraviolet detection as before. All recovered peaks were found to be homogeneous by this criterion. These chromatographically homogeneous compounds were investigated by modern physicochemical instrumentation to determine their structure. The molecular weight of each of the species was determined by fast atom bombardment mass spectrometry. High-field proton magnetic resonance at 270 MHz provided structural and conformational information. Polarimetry and ultraviolet absorption were also used to characterise the compound. alpha-EGF, for example, had a molecular weight of 6040 corresponding to the 53 amino acid residue peptide previously designated EGF; beta-EGF had a molecular weight of 5930. This molecular weight differential of 110 suggested the hypothesis that beta-EGF was a 52 residue peptide corresponding to alpha-EGF minus the terminal asparagine at position 1. Proton magnetic resonance difference spectroscopy (beta spectrum subtracted from alpha) provided powerful confirmatory evidence for this hypothesis. All materials recovered from RP-HPLC were tested in the sheep and found to retain their biological activity.