Discriminative response of surface-confined metalloporphyrin molecules to carbon and nitrogen monoxide

The binding of small gas molecules to metalloporphyrins is of both fundamental scientific and technological interest. It plays a key role in the transport of respiratory gases, catalytic processes in biological systems, and artificial nanostructures for sensing. Here, we present a detailed molecular-level investigation regarding the interaction of nitrogen monoxide (NO) and carbon monoxide (CO) with metallo-tetraphenylporphyrin (M-TPP, M = Co, Fe) arrays, anchored on a noble metal Ag(111) surface, providing M-TPP species with a distinct saddle-shape conformation. Scanning tunneling microscopy and spectroscopy experiments reveal that the impact of CO and NO is strikingly different on both species. In the case of CO, the M-TPP core can be dressed by either one or two carbon monoxide ligands, whereby the porphyrin geometric and electronic structure remains nearly unaffected. In contrast, following NO exposure exclusively a mononitrosyl species evolves. The NO axial ligation induces a relaxation of the adsorption-induced molecular deformation and markedly modifies the electronic structure of the porphyrin.     

Influence of concentration and temperature on the dynamics of water in the hydrophobic hydration shell of tetramethylurea

We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water, using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable of resolving different water species, their relative fractions, and their corresponding reorientation dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller.     

The glass transition and crystallization of ball milled cellulose

Samples of ball milled cellulose were prepared by ball milling pulps from eucalyptus and softwood (spruce/pine). Water sorption isotherms were obtained by both dynamic vapor sorption and equilibration over saturated salt solutions, in the water content range of 5–42% db (db = dry basis; water as a % age of total solids). Dynamic mechanical analysis using a pocket technique showed a water content dependent thermal transition occurring at the same temperature for the two pulp samples, which was interpreted as a glass transition. Fitting the data to a Couchman–Karasz relationship predicted a value for T _g of the dry cellulose of approximately 478 K, which was similar to values previously reported for other dry polysaccharides. No clear glass transition could be observed calorimetrically, although an endotherm at approximately 333 K was measured, which in polymers is normally attributed to enthalpic relaxation, however the lack of dependence of this endotherm on water content suggests that the melting of some weak associations, such as residual hydrogen bonds, could be a more credible explanation. An exotherm was also observed on heating, which was dependent on water content and which was attributed to partial crystallization of the cellulose. This was confirmed by Wide angle X-ray diffraction and cross polarization magic angle spinning ¹³C NMR (CPMAS NMR). The recrystallisation was predominantly to form I of cellulose. This was thought to be caused by a small amount of residual form I (probably less than 5%) acting as a template for the crystallizing material. Differential scanning calorimetry reheat curves showed the appearance of freezable water for water contents higher than 20%, as a result of a transfer of water to the amorphous phase following crystallization. The increase in cellulose rigidity following crystallization was also confirmed by CPMAS NMR relaxation. Low resolution proton NMR T ₂ relaxation suggested the presence of proton water/cellulose exchange, which was active at water contents of 20% and above.     

Mechanical manipulation of molecular lattice parameters in smectic elastomers

Smectic liquid crystalline elastomers (SLCE) represent unique materials that combine a 1-D molecular lattice arrangement and orientational order with rubber-elasticity mediated by a polymer network. Such materials may exhibit large thermo-mechanical, opto-mechanical and electro-mechanical effects, due to the coupling of macroscopic sample geometry and microscopic structural features. It is shown that the molecular layer dimensions in the smectic phases can be influenced reversibly by macroscopic strain of the material. We present a microscopic model on the basis of experimental results obtained by mechanical dilatation measurements, optical interferometry, X-ray scattering, (13)C NMR, FTIR and polarizing microscopy data. The model gives an explanation of the controversial results obtained in different types of smectic elastomers.     

On the trapping of SOx on CaO-Al2O3-based novel high capacity sorbents

Calcium-aluminum mixed oxide based materials doped with Na and Mn were explored as sulfur trapping materials. The materials showed a three times higher total storage capacity and a higher time on stream with complete SO2 removal compared to a second generation SOx trapping material which was mesoporous with calcium mainly present in oxidic form. Combining in situ XANES at the S K-edge and IR spectroscopy the key properties of the storage materials and the affiliated storage processes were identified. CaO-Al2O3 acts as the primary support and storage component, while Na+ cations adjust the base strength and enhances the storage capacity. Manganese cations provide the appropriate oxidation capacity in absence and presence of up to 10% water. The transport into the bulk phase, which is markedly influenced by a layer of sorbed water, is the rate-limiting step in presence of Mn cations. In the absence of manganese cations the oxidation step appears controlling the rate. The overall reaction network, identified by in situ IR spectroscopy and the 2D Correlation Analysis, is similar on all materials.     

p53--a natural cancer killer: structural insights and therapeutic concepts

Every single day, the DNA of each cell in the human body is mutated thousands of times, even in absence of oncogenes or extreme radiation. Many of these mutations could lead to cancer and, finally, death. To fight this, multicellular organisms have evolved an efficient control system with the tumor-suppressor protein p53 as the central element. An intact p53 network ensures that DNA damage is detected early on. The importance of p53 for preventing cancer is highlighted by the fact that p53 is inactivated in more than 50 % of all human tumors. Thus, for good reason, p53 is one of the most intensively studied proteins. Despite the great effort that has been made to characterize this protein, the complex function and the structural properties of p53 are still only partially known. This review highlights basic concepts and recent progress in understanding the structure and regulation of p53, focusing on emerging new mechanistic and therapeutic concepts.     

Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C1-symmetric diphosphines in rhodium-catalysed hydrogenations

The new unsymmetrical, optically active ligands 1,2-C(2)H(4)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(a)) and 1,3-C(3)H(6)(PPh(2))(2'R,5'R-2',5'-dimethylphospholanyl) (L(b)) form complexes of the type [Rh(L)(cyclooctadiene)][BF(4)] where L = L(a) (1a) or L(b) (1b), [PtCl(2)(L)] where L = L(a) (2a) or L(b) (2b) and [PdCl(2)(L)] where L = L(a) (3a) or Lb (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt delta-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine L(a) gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L(b). The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh-diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.