Complexes with metal-carbon bonds,part III(1). Preparation and properties of binuclear diarylplatinum(II) compounds

Summary Reaction ofcis-PtCl₂(R₂S)₂, where R=Et, n-Pr or i-Pr, with an excess of PhLi orp-tolylLi in ether yielded the dimeric [PtR₂(R₂S)]₂ complexes where R=Ph orp-tolyl. Usingtrans-PtCl₂(R₂S)₂ under the same conditions with PhLi, both [PtPh₂(Et₂S)]₂ and PtPh₂(Et₂S)₂ were isolated.The dimericcis-[PtR₂(Et₂S)]₂ complexes reacted with a range of neutral ligands to give PtR₂L₂ where L=Et₂S, Me₂S, PPh₃, t-BuNC, 0.5 (bipyridine) or pyridine. With CO, thecis-PtR₂(Et₂S)(CO) compounds were formed. Thecis-Pt(C₆F₅)₂(Et₂S)₂ complex reacted stepwise with PPh₃ and both Pt(C₆F₅)₂(Et₂S)(PPh₃) and Pt(C₆F₅)₂(PPh₃)₂ were isolated.     

Conformational analysis of furanoid epsilon-sugar amino acid containing cyclic peptides by NMR spectroscopy,molecular dynamics simulation,and X-ray crystallography: evidence for a novel turn structure

Sugar amino acids (SAAs) are useful building blocks for the design of peptidomimetics and peptide scaffolds. The three-dimensional structures of cyclic hybrid molecules containing the furanoid epsilon-SAA III and several amino acids were elucidated to study the preferred conformation of such an epsilon-SAA and its conformational influence on the backbone of cyclic peptides. NMR-based molecular dynamics simulations and empirical calculations of the cyclic tetramer 1, consisting of two copies of the SAA residue and two amino acids, revealed that it is conformationally restrained. The two SAA residues adopt different conformations. One of them forms an unusual turn, stabilized by an intraresidue nine-member hydrogen bond. The methylene functionalities of the other SAA residue are positioned in such a way that an intraresidue H bond is not possible. The X-ray crystal structure of 1 strongly resembles the solution conformation. Molecular dynamics calculations in combination with NMR analysis were also performed for compounds 2 and 3, which contain the RGD (Arg-Gly-Asp) consensus sequence and were previously shown to inhibit alpha(IIb)beta(3)-receptor-mediated platelet aggregation. The biologically most active compound 2 adopts a preferred conformation with the single SAA residue folded into the nine-member H bond-containing turn. Compound 3, containing an additional valine residue, as compared with compound 2, is conformational flexible. Our studies demonstrate that the furanoid epsilon-SAA III is able to introduce an unusual intraresidue hydrogen bond-stabilized beta-turn-like conformation in two of the three cyclic structures.     

Synthese und chemie von 4R-silacyclohexa-2,5-dienen und 4R-silacyclohexa-2,4-dienen

The 1(Z),4(Z)-1,5-dilithium-3R-3-methoxypenta-1,4-dienes, available by cleavage of 1,1-di-n-butyl-4R-4-methoxy-1-stannacyclohexane-2,5-dienes with n-BuLi in ether, react with dichlorosilanes (R′R″ SiCl₂; R′, R″ = H, alkyl, aryl, alkoxy) to give 1R′,1R″,4R-4-methoxy-1-silacyclohexa-2,5-dienes.Claevage of the ether group with BBr₃, BCl₃ or PCl₃ gives 6-chloro(bromo)-1-silacyclohexa-2,4-dienes or 1,5-dichloro-1-silacyclohexane-2,4-dienes, respectively.Ether cleavage of 4R-4-methoxy-1-silacyclohexa-2,5-dienes with sodium results in the formation of 4R-1-silacyclohexadienyl anions, which can be hydrolyzed to give 1-silacyclohexa-2,4-dienes which reach further with trichloromethylsilane to give 6-trimethylsilyl-1-silacyclohexa-2,4-dienes.Phase transfer-catalyzed conversion of 1-chlorosilacyclohexadienes into the fluoro derivatives as well as further substitution reactions of 1-chlorosilacyclohexadienes are described.The ¹H NMR, ¹³C NMR, IR and mass spectra of the silacyclohexadienes are discussed.     

ESI Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FT-ICR-MS): A Rapid High-Resolution Analytical Method for Combinatorial Compound Libraries

The direct determination of the elemental compositions of the components of compound collections from combinatorial chemistry is achieved by ESI-FT-ICR mass spectrometry. Coupling with HPLC opens up a new dimension in high-resolution, informative analysis. The improved resolution of ESI-FT-ICR mass spectrometry in comparison to quadrupole mass spectrometry in the measurement of a compound obtained by solid-phase synthesis is illustrated.     

Determination of polybutylene terephthalate polycondensation equilibrium constant using a batch reactor

The PBT polycondensation equilibrium constant at 255°C was determined using a batch reactor. Starting from a Polybutylene terephthalate (PBT) prepolymer having a degree of polymerisation of 12.7, equilibrium experiments were performed in the range of 1 to 50 mbar. The equilibrium degree of polymerisation (i) was determined indirectly using dilute solution viscometry of a solution of 1 weight % PBT in m - cresol. The degree of polymerisation of PBT obtained at equilibrium in the range of 1 to 50 mbar at 255°C as a function of the BDO partial pressure (mbar) could be expressed by: i = 111.47 − 86.18 exp(−1.14 equation/tex2gif-stack-1.gif). The equilibrium solubility of 1,4 butanediol (BDO) in the PBT melt was derived form the BDO partial pressure using the Flory - Huggins theory. The PBT polycondensation reaction equilibrium constant was related to the degree of polymerisation by the equation: in the range i = 26 − 100. The PBT polycondensation equilibrium constant at high degrees of polymerisation is in line with literature data and thermodynamics.     

INDUCTION and DISAPPEARANCE OF THYMINE DIMERS IN HUMAN SKIN EXPOSED TO UVB RADIATION: FLOW CYTOMETRIC MEASUREMENTS IN REPLICATING and NONREPLICATING EPIDERMAL CELLS

We have earlier reported on determining UV-induced DNA damage in murine epidermal cell suspensions by flow cytometric analysis of the fluorescence from a fluorescein isothiocyanate-labeled antibody (H3) directed against thymine dimers (T>2-M-phase cells can further be distinguished from cell doublets by pulse-shape discrimination. Thus, T>(i.e. G₀-G₁, S or G₂-M phases) can be determined after in vivo exposure of human skin to environmentally relevant UVB (280–315nm) doses. The method was applied to measure the decrease of T>0-G₁ phase) and replicating cells (S phase or G₂-M phase) from seven volunteers exposed to twice their minimal erythema dose. The reduction in the average T>0-G₁ cells and 70% (ranging between 37% and 100%) for the S + G₂-M cells. The difference was statistically highly significant. Determination of individual DNA repair capacities with this method can become a convenient diagnostic tool for patients with DNA repair disorders, or it may even be used to identify individuals with low repair proficiencies and increased risk of developing skin cancers.     

On the implementation of symmetric and antisymmetric periodic boundary conditions for incompressible flow

In this paper we consider symmetric and antisymmetric periodic boundary conditions for flows governed by the incompressible Navier-Stokes equations. Classical periodic boundary conditions are studied as well as symmetric and antisymmetric periodic boundary conditions in which there is a pressure difference between inlet and outlet. The implementation of this type of boundary conditions in a finite element code using the penalty function formulation is treated and also the implementation in a finite volume code based on pressure correction. The methods are demonstrated by computation of a flow through a staggered tube bundle.     

Photodissociation of carbonmonoxy-porphyrin complexes

Summary Partial photodissociation of two carbonmonoxy-porphyrin complexes, a bridged and a fenced one, has been achieved. The results from monitoring the recombination point to an important influence of the solvent matrix on the recombination rates. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.