Cucurbit[7]uril‐Dimethyllysine Recognition in a Model Protein

Here, we provide the first structural characterization of host–guest complexation between cucurbit[7]uril ( Q7 ) and dimethyllysine (KMe₂) in a model protein. Binding was dominated by complete encapsulation of the dimethylammonium functional group. While selectivity for the most sterically accessible dimethyllysine was observed both in solution and in the solid state, three different modes of Q7 ‐KMe₂ complexation were revealed by X‐ray crystallography. The crystal structures revealed also entrapped water molecules that solvated the ammonium group within the Q7 cavity. Remarkable Q7 ‐protein assemblies, including inter‐locked octahedral cages that comprise 24 protein trimers, occurred in the solid state. Cucurbituril clusters appear to be responsible for these assemblies, suggesting a strategy to generate controlled protein architectures.     

Ground state sigmatropic and electrocyclic rearrangements in some monoterpenes: The pyrolysis of α-pinene

Alloocimene 7, obtained via ocimene 6 by heating α-pinene 2, was further pyrolysed to a complex mixture of products derived primarily from antarafacial 1,7- and suprafacial 1,5-hydrogen migrations, and 6-eleetron disrotatory electrocyclizations.     

Supramolecular recognition forces: an examination of weak metal-metal interactions in host-guest formation

Molecular receptors consisting of two parallel-disposed terpy-M-Cl units (M = Pd2+, Pt2+) are used to form host-guest adducts with aromatic molecules and with neutral square-planar Pt2+ complexes. Host-guest formation is controlled by several factors including pi-pi interactions and, in some cases, weak Pt-Pt interactions between the host and the guest. This latter interaction was examined by comparing the host-guest stability of adducts formed by isoelectronic Pt2+ and Au3+ complexes with the Pt2+ receptor. Consistently, the former is more stable.     

Total synthesis of the ristocetin aglycon

The first total synthesis of the ristocetin aglycon is described employing a modular and highly convergent strategy. An effective 12-step (12% overall) synthesis of the ABCD ring system 3 from its amino acid subunits sequentially features an intramolecular aromatic nucleophilic substitution reaction for formation of the diaryl ether and closure of the 16-membered CD ring system (65%), a respectively diastereoselective (3:1, 86%) Suzuki coupling for installation of the AB biaryl linkage on which the atropisomer stereochemistry can be further thermally adjusted, and an effective macrolactamization (51%) for closure of the 12-membered AB ring system. A similarly effective 13-step (14% overall) synthesis of the 14-membered EFG ring system 4 was implemented employing a room-temperature intermolecular S(N)Ar reaction of an o-fluoronitroaromatic for formation of the FG diaryl ether (69%) and a key macrolactamization (92%) with formation of the amide linking residues 1 and 2. The two key fragments 3 and 4 were coupled, and the remaining 16-membered DE ring system was closed via diaryl ether formation to provide the ristocetin tetracyclic ring system (15 steps, 8% overall) enlisting an unusually facile (25 degrees C, 8 h, DMF, >/=95%) and diastereoselective (>/=15:1) aromatic nucleophilic substitution reaction that benefits from substrate preorganization.     

Transport of organic solutes through amorphous teflon AF films

Fluorous media have great potential for selective extraction (e.g., as applied to organic synthesis). Fluorous polymer films would have significant advantages in fluorous separations. Stable films of Teflon AF 2400 were cast from solution. Films appear defect-free (SEM; AFM). Rigid aromatic solutes are transported (from chloroform solution to chloroform receiving phase) in a size-dependent manner (log permeability is proportional to -0.0067 times critical volume). Benzene's permeability is about 2 orders of magnitude higher than in comparable gas-phase experiments. The films show selectivity for fluorinated solutes in comparison to the hydrogen-containing control. Transport rates are dependent on the solvent making up the source and receiving phases. The effect of solvent is, interestingly, not due to changes in partition ratio, but rather it is due to changes in the solute diffusion coefficient in the film. Solvents plasticize the films. A less volatile compound, -COOH-terminated poly(hexafluoropropylene oxide) (4), plasticizes the films (T(g) = -40 degrees C). Permeabilities are decreased in comparison to 4-free films apparently because of decreased diffusivity of solutes. The slope of dependence of log permeability on critical volume is not changed, however.     

A one pot multi-component CuAAC “click” approach to bidentate and tridentate pyridyl-1,2,3-triazole ligands: Synthesis,X-ray structures and copper(II) and silver(I) complexes

A one pot, multi-component CuAAC reaction has been developed for the generation of alkyl, benzyl or aryl substituted bi and tridentate pyridyl-1,2,3-triazole ligands from their corresponding halides, sodium azide and alkynes in excellent yields. The ligands have been fully characterized by elemental analysis, HR-ESMS, IR, ¹H and ¹³C NMR and in the ferrocenyl substituted cases the structures were confirmed by X-ray crystallography. Additionally, we have examined the coordination chemistry of these ligands and found that a variety of geometrically diverse Cu(II) and Ag(I) complexes, including interesting tri and tetrasilver complexes, can be formed.     

Structural study of a small molecule receptor bound to dimethyllysine in lysozyme

Lysine is a ubiquitous residue on protein surfaces. Post translational modifications of lysine, including methylation to the mono-, di- or trimethylated amine result in chemical and structural alterations that have major consequences for protein interactions and signalling pathways. Small molecules that bind to methylated lysines are potential tools to modify such pathways. To make progress in this direction, detailed structural data of ligands in complex with methylated lysine is required. Here, we report a crystal structure of p-sulfonatocalix[4]arene (sclx₄) bound to methylated lysozyme in which the lysine residues were chemically modified from Lys-NH₃ ⁺ to Lys-NH(Me₂)⁺. Of the six possible dimethyllysine sites, sclx₄ selected Lys116-Me₂ and the dimethylamino substituent was deeply buried in the calixarene cavity. This complex confirms the tendency for Lys-Me₂ residues to form cation–π interactions, which have been shown to be important in protein recognition of histone tails bearing methylated lysines. Supporting data from NMR spectroscopy and MD simulations confirm the selectivity for Lys116-Me₂ in solution. The structure presented here may serve as a stepping stone to the development of new biochemical reagents that target methylated lysines.     

The interaction of H(2)O(2) with ice surfaces between 203 and 233 K

The interaction of H(2)O(2) with ice surfaces at temperatures between 203 and 233 K was investigated using a low pressure, coated-wall flow tube equipped with a chemical ionisation/electron impact mass spectrometer. Equilibrium surface coverages of H(2)O(2) on ice were measured at various concentrations and temperatures to derive Langmuir-type adsorption isotherms. H(2)O(2) was found to be strongly partitioned to the ice surface at low temperatures, with a partition coefficient, K(linC), equal to 2.1 × 10(-5) exp(3800/T) cm. At 228 K, this expression results in values of K(linC) which are orders of magnitude larger than the single previous determination and suggests that H(2)O(2) may be significantly partitioned to the ice phase in cirrus clouds. The partition coefficient for H(2)O(2) was compared to several other trace gases which hydrogen-bond to ice surfaces and a good correlation with the free energy of condensation found. For this class of trace gas a simple parameterisation for calculating K(linC)(T) from thermodynamic properties was established.     

Mechanistic Insights into Catalytic Ethanol Steam Reforming Using Isotope‐Labeled Reactants

The low‐temperature ethanol steam reforming (ESR) reaction mechanism over a supported Rh/Pt catalyst has been investigated using isotope‐labeled EtOH and H₂O. Through strategic isotope labeling, all nonhydrogen atoms were distinct from one another, and allowed an unprecedented level of understanding of the dominant reaction pathways. All combinations of isotope‐ and non‐isotope‐labeled atoms were detected in the products, thus there are multiple pathways involved in H₂, CO, CO₂, CH₄, C₂H₄, and C₂H₆ product formation. Both the recombination of C species on the surface of the catalyst and preservation of the C?C bond within ethanol are responsible for C₂ product formation. Ethylene is not detected until conversion drops below 100 % at t=1.25 h. Also, quantitatively, 57 % of the observed ethylene is formed directly through ethanol dehydration. Finally there is clear evidence to show that oxygen in the SiO₂‐ZrO₂ support constitutes 10 % of the CO formed during the reaction.