Host-guest interactions in acid-porphyrin complexes

In this report we use the weak interactions of acid-porphyrin complexes to selectively bind competing acids to the faces of a rigid cyclic porphyrin dimer, and characterise the resulting interactions by NMR spectroscopy and nano-electrospray ionisation spectrometry.     

A mechanistic study of the 2-thienylmethyl + HO2 radical recombination reaction

Radical recombination reactions are important in the combustion of fuel oils. Shale oil contains alkylated heteroaromatic species, the simplest example of which is the 2-thienylmethyl radical. The ab initio potential energy surface for the reaction of the 2-thienylmethyl radical with the HO(2) radical has been examined. Seventeen product channels corresponding to either addition/elimination or direct hydrogen abstraction have been characterized for the first time. Direct hydrogen abstract from HO(2) proceeds via a weakly bound van der Waals complex, which leads to 2-methylthiophene, 2-methylene-2,3-dihydrothiophene, or 2-methylene-2,5-dihydrothiophene depending upon the 2-thienylmethyl radical reaction site. The addition pathway for the two radical reactants is barrierless with the formation of three adducts, as distinguished by HO(2) reaction at three different sites on the 2-thienylmethyl radical. The addition is exothermic by 37-55 kcal mol(-1) relative to the entrance channel, and these excess energies are available to promote further decomposition or rearrangement of the adducts, leading to nascent products such as H, OH, H(2)O, and CH(2)O. The reaction surfaces are characterized by relatively low barriers (most lower than 10 kcal mol(-1)). Upon the basis of a careful analysis of the overall barrier heights and reaction exothermicities, the formations of O(2), OH, and H(2)O are likely to be important pathways in the radical recombination reactions of 2-thienylmethyl + HO(2).     

A sequence-dependent exonuclease activity from <Emphasis Type="Italic">Tetrahymena thermophila</Emphasis>

Background  

Telomere function requires a highly conserved G rich 3'- overhang. This structure is formed by 5'-resection of the C-rich telomere strand. However, while many nucleases have been suggested to play a role in processing, it is not yet clear which nucleases carry out this 5'-resection.     

Ion mobility mass spectrometry of two tetrameric membrane protein complexes reveals compact structures and differences in stability and packing

Here we examined the gas-phase structures of two tetrameric membrane protein complexes by ion mobility mass spectrometry. The collision cross sections measured for the ion channel are in accord with a compact configuration of subunits, suggesting that the native-like structure can be preserved under the harsh activation conditions required to release it from the detergent micelle into the gas phase. We also found that the quaternary structure of the transporter, which has fewer transmembrane subunits than the ion channel, is less stable once stripped of detergents and bulk water. These results highlight the potential of ion mobility mass spectrometry for characterizing the overall topologies of membrane protein complexes and the structural changes associated with nucleotide, lipid, and drug binding.     

Structures of urea/thiourea 1,3-disubstituted thia[4]calixarenes and corresponding monofunctional receptors and their anion recognition properties

Two thiacalix[4]arenes in 1,3-alternate conformation functionalized by two (CH₂)₂NH(C=X)NHC₆H₄-NO₂-p groups (X = S,O) as well as two related monofunctional receptors MeO(CH₂)₂NH(C=X)NHC₆H₄-NO₂-p were prepared and characterized by X-ray crystal structures. The thioureido and ureido derivatives have E,Z and E,E conformations respectively both in monofunctional receptors and thiacalixarenes. The thiacalixarene attached thiourea groups are well separated from each other, but respective urea groups are much closer to each other and have mutual parallel orientation making the bisurea derivative a better preorganized receptor as compared to bisthiourea. Binding of Cl^?, F^?, H₂PO₄ ^? and AcO^? anions in chloroform and DMSO was studied by spectrophotometric and NMR titrations. In chloroform both bisurea and bisthiourea thiacalix[4]arenes bind anions 3–5 times stronger than corresponding monofunctional compounds in spite of better preorganization of the urea derivative. In DMSO simultaneous deprotonation of ureido NH groups of receptors and hydrogen bonding reactions are observed. Deprotonation by H₂PO₄ ^? is accompanied by a strong association between liberated H₃PO₄ and H₂PO₄ ^? (log K = 3.9). For hydrogen bonding associations the binding constants of H₂PO₄ ^? and AcO^? with bisurea thiacalixarene are up to two orders of magnitude larger than those with corresponding monofunctional receptor, but with bisthiourea thiacalixarene the effect is less than two-fold. Thus in this solvent in contrast to chloroform the preorganization is an important factor.     

Proving and Probing the Presence of the Elusive C−H⋅⋅⋅O Hydrogen Bond in Liquid Solutions at Room Temperature

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C?H group and an electron‐rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C?H group and an amide carbonyl oxygen atom in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6–20 kJ mol^?1. Ab‐initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen‐bond making and breaking.     

Synthesis and heteronuclear inclusion properties of a novel thiacalix[4]arene-based hard-soft receptor with 1,3-alternate conformation

A novel thiacalix[4]arene ditopic receptor with 1,3-alternate conformation and possessing two complexation sites for hard and soft cations, 5,11,17,23-tetra-tert-butyl-25,27-bis[(N,N-diethylaminocarbonyl)methoxy]-26,28-bis[(pyridylmethyl)oxy]-2,8,14,20-tetrathiacalix[4]arene is prepared. Regioselective synthesis of distal-bis[(N,N-diethylaminocarbonyl)methoxy]thiacalix[4]arene is accomplished by a protection-deprotection method using benzyl groups as a protecting group. The deprotection of benzyl group was succeeded in the presence of solid superacid (Nafion-H) under refluxing benzene. Its complexation behavior is examined by ¹H-NMR titration experiments. The formation of 1:2 homo- and heteronuclear complexes demonstrates that the preorganization, subtle conformational changes and affinity have a pronounced effect on the complexation of the receptor.     

Structure of a rearrangement product: 1-methyl-3-(5-amino-1-benzylimidazol-4-yl)-1,2,4-triazole,C13H14N6

The title compound was prepared during the course of an attempt to synthesize 8-amino-3-benzyl-6-methylimidazo[4,5-e][1,2,4]triazepine. The crystals are monoclinic, space groupP2₁/c,a=6.2375(5),b=9.1070(8),c=22.182(2) Å,=91.797(7)°,Z=4. The structure was solved by direct methods and refined by least squares toR=0.063 on all 2142 measured reflections. The imidazol-4-yl-triazole system is planar, conjugated, and aromatic. One hydrogen atom of the exocyclic amino group is involved in a bifurcated hydrogen bond, of which one branch is an intramolecular attraction to a triazole ring nitrogen atom, while the other branch is intermolecular. The crystal is held together by two N-HN hydrogen bonds between molecules related by translation along thea axis and by- stacking interactions of the imidazol-4-yl-triazole ring systems and the phenyl rings across centers of inversion.Visiting scientist, The Institute for Cancer Research.     

Observation of the nuclear magnetic octupole moment of 133Cs

We describe our high-resolution measurements of the 133Cs 6p (2)P(3/2) state hyperfine structure. An optically narrowed diode laser excites perpendicularly a highly collimated atomic beam. The spectra are calibrated with a stable reference diode laser using a rf locking scheme allowing us to determine the splittings with an accuracy of < or =2 kHz, an order of magnitude better than previous results. The magnetic dipole a, electric quadrupole b, and magnetic octupole c hyperfine coupling constants are determined. The values we obtained are a=50.288 27(23) MHz, b=-0.4934(17) MHz, and c=0.56(7) kHz. This work represents the first observation of the magnetic octupole moment of the cesium nucleus. We carry out atomic-structure calculations and determine the nuclear electric quadrupole moment Q= -3.55(4) mb and nuclear magnetic octupole moment Omega=0.82(10) b x mu(N).