Consequences of Strain for the Structure of Aliphatic Molecules

The chemist is accustomed to deriving structures and preferred conformations of organic compounds from rigid molecular models and standard values for bond lengths, bond angles, and torsional profiles. In the case of strained compounds, this rigid structural model has to be abandoned and replaced by a flexible one which takes individual conditions of strain into consideration. It is shown, on the basis of new experimental structure data, that the force field method is suitable and highly reliable for the calculation of structural parameters and preferred conformations of strained compounds. It is, therefore, capable of replacing the rigid molecular model. Furthermore, the systematic analysis of strain induced angle and bond deformation gives a new pivot for the development of a qualitative discussion of deformation in strained molecules and hence for improved conformational analysis. — In the course of this work we were able to isolate two rotamers of D,L -3,4-di(1-adamantyl)-2,2,5,5-tetramethylhexane; this is the first isolation of a rotamer pair of an aliphatic hydrocarbon.     

Mehrfachbindungen zwischen hauptgruppenelementen und Übergangsmetallen. 158. Photochemische und photophysikalische Eigenschaften des epoxidierungskatalysators CH3ReO(O2)2·H2O

The photolysis of CH₃ReO(O₂)₂·H₂O in methylene chloride yields, like the thermolysis, molecular oxygen in the triplet spin state. The quantum yield Q_Ph of photolysis shows a remarkable dependence the wavelength, increasing from 0.12 at 365 nm to 1.0 at 248 nm. One single excited state is responsible for this behaviour. The wavelength-dependent quantum yield profile corresponds in a first approximation to the ratio between the LMCT-band and the total absorption spectrum was made on a mathematical basis using symmetrical Gauss curves. This is the first time that a fluorescence and phosphorescence emission of an alkyl transition-metal complex of d⁰-configuration has been detected, thus allowing for the determination of both the S₁ and the T₁-energy levels. The quantum yield of the fluorescence (Q_F) is below 10⁻³; that of the phosphorescence is below 0.04.

Zusammenfassung

Die Photolyse von CH₃ReO(O₂)₂·H₂O in Methylenchlorid führt ebenso wie die Thermolyse dieses Komplexes zur Abspaltung von molekularem Sauerstoff im Triplettgrundzustand. Die Photolyse-Quantenausbeute Q_Ph zeigt eine bemerkenswerte Abhängigkeit von der eingerstrahlten Wellenlänge und steigt von 0.12 bei 365 nm bis zu 1.0 bei 248 nm an. Hierfür kann hauptsächlich ein einziger angeregter Zustand verantwortich gemacht werden. Der Kurvenverlauf der wellenlängenabhängigen Quantenausbeute entspricht in guter Näherung dem Verhältnis der LMCT-Bande zum Gesamtabsorptionsspektrum. Die Zerlegung des Absorptionsspektrums in die einzelnen Banden erfolgte durch rechnerische Kurvenanalyse in symmetrische Gauss-Kurven. Erstmals konnte die Fluoreszenz- und die Phosphoreszenz-emission eines alkylhaltigen d⁰-Übergangsmetall-Komplexes detektiert werden und somit sowohl die S₁-als auch die T₁-Energielage bestimmt werden. Die Quantenausbeute der Fluoreszenz Q_F ist kleiner als 10⁻⁴, die Phosphoreszenzquantenausbeute beträgt weniger als 0.04.     

Synthese homokonjugierter Polyene: 2,4-Dimethylenbicyclo[3.2.0]oct-6-en und 2,5-Dimethylenbicyclo[4.2.0]non-7-en

The synthesis of the two homoconjugated trienes has been achieved by cleavage of the strained -bonds of suitable cyclopropane and cyclobutane precursors. Ionization energies are discussed.

     

Construction of a full three-dimensional model of the transpeptidase domain of Streptococcus pneumoniae PBP2x starting from its Cα-atom coordinates

A new method is described for generating all-atom protein structures from C-atom information. The method, which combines both local structural trace alignments and comparative side chain modeling with ab initio side chain modeling, makes use of both the virtual-bond and the dipole-path methods. Provided that 3D structures of structurally and functionally related proteins exist, the method presented here is highly suitable for generating all-atom coordinates of partly solved, low-resolution crystal structures. Particularly the active site region can be modeled accurately with this procedure, which enables investigation of the binding modes of different classes of ligands with molecular dynamics simulations. The method is applied to the trace of Streptococcus pneumoniae, in order to construct an all-atom structure of the transpeptidase domain. Since after generation of full coordinates of the transpeptidase domain the structure had been solved to 2.4 Å resolution, new X-ray coordinates for the worst modeled loop (residues T370 to M386; 17 out of a total number of 351 residues constituting the transpeptidase domain) were incorporated, as kindly provided by Dr. Dideberg. The structure was relaxed with molecular dynamics simulations and simulated annealing methods. The RMS deviation between the 144 aligned C-atoms and the corresponding ones in the originally solved 3.5 Å resolution crystal structure was 0.98. The 351 C-atoms of the whole transpeptidase domain of the final model showed an RMS deviation of 1.58. The Ramachandran plot showed that 79.3% of the residues are in the most favored regions, with only 1.0% occurring in disallowed regions. The model presented here can be used to investigate the three-dimensional influences of mutations around the active site of PBP2x.     

Model studies on the utility of nucleophiles bound to insoluble supports for enzymatic peptide synthesis

Thermolysin, -chymotrypsin, and papain were used as biocatalysts for the coupling of several carboxyl components to silica-supported leucine amide as a nucleophile. A spacer length of 21 covalent bonds corresponding to 7 amino acid residues was required for successful coupling. This approach offers the possibility of using proteases for racemization-free segment condensations on insoluble polymeric supports.

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Modellstudien zur Anwendbarkeit von an unlösliche Träger gebundenen Nucleophilen für die enzymatische Peptidsynthese

Zusammenfassung Thermolysin, -Chymotrypsin und Papain wurden als Biokatalysatoren für die Kupplung verschiedener Carboxylkomponenten an kieselgel-gebundenes Leucinamid als Nucleophil eingesetzt. Für erfolgreiche Kupplungen war eine Spacerlänge von 21 kovalenten Bindungen entsprechend 7 Aminosäureresten erforderlich. Diese Synthesevariante eröffnet die Möglichkeit, racemisierungsfreie Segmentkondensationen an unlöslichen polymeren Trägern mit Hilfe von Proteasen durchzuführen.

     

Dihydrodiazepine aus valenzisomeren diazatricyclo[3.2.0.0.2,4]Heptanen1)

Cycloaddition of diazoalkanes to diazabicyclo[2.2.0]hexenes and subsequent extrusion of nitrogen afford diazatricyclo[3.2.0.0^2,4]heptanes that are easily valence isomerized to novel dihydrodiazepines.