Simple 10-dimensional supergravity in superspace

Differential geometry is used to formulate supergravity in a 10-dimensional superspace. From the knowledge of the supersymmetric set of fields in x-space we derive the constraints on the supertorsion and on a super 3-form field strength. We then solve the equations which follow from the Bianchi identities. The solution obtained, which is on the mass shell, is shown to be completely described in terms of a scalar superfield.     

Electron beam damage of chemisorbed surface species: A tunneling spectroscopy study

Electron beam damage is a problem for low-energy electron diffraction, Auger electron spectroscopy, and electron microscopy. We have used tunneling spectroscopy to study the damage caused by 30 keV incident electrons on chemisorbed monolayer films of hexanoic acid, 2,4-hexadienoic acid, and benzoic acid. Our results on monolayer films are compared to existing work on bulk samples. Damage cross sections are similar to bulk values; molecules with more delocalized electrons are more resistant to damage. In contrast to bulk results, however, we find little if any conjugation or cross linking after irradiation.     

The structure and absolute configuration of verticillol,a macrocyclic diterpene alcohol from the wood of sciadopitys verticillata sieb. et zucc. (taxodiaceae)

The structure of verticillol diepoxide 4 has been established by direct single crystal X-ray analysis. The structure of verticillol 5 follows from the chemical correlation to the diepoxide 4 as well as from NMR-LIS studies on verticillol which also provide evidence for the conformation of this alcohol. The absolute configuration of verticillol 5 has been assigned on the basis of CD data for the verticillol norketodiepoxide 6a.     

An effective calculation procedure for two-phase equilibria in multireaction systems

In this work, we present a method for the calculation of two-phase equilibria in multireaction systems. The procedure uses a transformed composition variable in the orthogonal derivatives of the Gibbs function and the tangent plane equation to form a system of non-linear equations. We solve this system with a Newton–Raphson method and our initialization procedure uses results from the reactive stability analysis and the reactive equal area rule. With this initialization strategy, convergence occurs with only a few iterations in the numerical method. Several examples with multiple chemical reactions demonstrate the performance of our approach.     

Stacking interactions and the twist of DNA

The importance of stacking interactions for the Twist and stability of DNA is investigated using the fully ab initio van der Waals density functional (vdW-DF). Our results highlight the role that binary interactions between adjacent sets of base pairs play in defining the sequence-dependent Twists observed in high-resolution experiments. Furthermore, they demonstrate that additional stability gained by the presence of thymine is due to methyl interactions with neighboring bases, thus adding to our understanding of the mechanisms that contribute to the relative stability of DNA and RNA. Our mapping of the energy required to twist each of the 10 unique base pair steps should provide valuable information for future studies of nucleic acid stability and dynamics. The method introduced will enable the nonempirical theoretical study of significantly larger pieces of DNA or DNA/amino acid complexes than previously possible.     

Reconstitution of rhodopsin into polymerizable planar supported lipid bilayers: influence of dienoyl monomer structure on photoactivation

G-protein-coupled receptors (GPCRs) play key roles in cellular signal transduction and many are pharmacologically important targets for drug discovery. GPCRs can be reconstituted in planar supported lipid bilayers (PSLBs) with retention of activity, which has led to development of GPCR-based biosensors and biochips. However, PSLBs composed of natural lipids lack the high stability desired for many technological applications. One strategy is to use synthetic lipid monomers that can be polymerized to form robust bilayers. A key question is how lipid polymerization affects GPCR structure and activity. Here we have investigated the photochemical activity of bovine rhodopsin (Rho), a model GPCR, reconstituted into PSLBs composed of lipids having one or two polymerizable dienoyl moieties located in different regions of the acyl chains. Plasmon waveguide resonance spectroscopy was used to compare the degree of Rho photoactivation in fluid and poly(lipid) PSLBs. The position of the dienoyl moiety was found to have a significant effect: polymerization near the glycerol backbone significantly attenuates Rho activity whereas polymerization near the acyl chain termini does not. Differences in cross-link density near the acyl chain termini also do not affect Rho activity. In unpolymerized PSLBs, an equimolar mixture of phosphatidylethanolamine and phosphatidylcholine (PC) lipids enhances activity relative to pure PC; however after polymerization, the enhancement is eliminated which is attributed to stabilization of the membrane lamellar phase. These results should provide guidance for the design of robust lipid bilayers functionalized with transmembrane proteins for use in membrane-based biochips and biosensors.     

Photocatalytic reactions at the graphite/ice interface

We have studied the photodissociation of water molecules at the ice/graphite interface in the presence of submonolayer amounts of potassium. The ice films were grown at cryogenic temperatures and ultrahigh vacuum conditions. They are transparent for 240-900 nm photons, but the strong light absorption in the uppermost layers of the graphite substrate generates energetic charge carriers that may drive photoreactions at the interface. Similar schemes have been demonstrated and investigated before in the monolayer regime, without an ice layer (D. Chakarov, M. Gleeson and B. Kasemo, J. Chem. Phys., 2001, 115, 9477). Here, using ice films of tens of monolayer thickness and with different morphologies, we have investigated the confinement effects due to the ice layer, and the ice permeability for reaction products.     

Further studies on silatropic carbonyl ene cyclisations: beta-crotyl(diphenyl)silyloxy aldehyde substrates; synthesis of 2-deoxy-2-C-phenylhexoses

Silatropic carbonyl ene cyclisations of beta-(allylsilyloxy)- and beta-(crotylsilyloxy)butyraldehydes are shown to proceed with high stereoselectivity but at a much reduced rate in comparison to the cyclisation of analogous alpha-substrates. In the second section, olefin cross-metathesis is explored as a route to substituted alpha-(allylsilyloxy)aldehydes and the method applied to the synthesis of diastereomeric 2-deoxy- and 2-deoxy-2-C-phenyl hexose derivatives from butanediacetal-protected D-glyceraldehyde.     

Complex DNA binding kinetics resolved by combined circular dichroism and luminescence analysis

We recently reported that ruthenium complexes, with general structure [mu-bidppz(bipy)4Ru2](4+) (B) or [mu-bidppz(phen)4Ru2](4+) (P) (bidppz=11,11'-bi(dipyrido[3,2- a:2',3'-c]phenazinyl)), show extreme kinetic selectivity for long AT tracts over mixed-sequence calf thymus DNA (ct-DNA), a selectivity that also varies markedly with the size (between B and P) and sense of chirality of the complex. Earlier studies, exploiting the great increase in luminescence intensity when the compound intercalates, have yielded complex kinetics indicating the presence of both first- and second-order processes. Even with a homogeneous DNA sequence, such as poly(dAdT)2, the luminescence kinetics generally requires more than a single exponential for a satisfactory fit. We here reveal that at least part of the complexity is a result of the extreme sensitivity of the effective quantum yield of the complexes, so that the luminescence trajectories also reflect subtle variations in the environment and binding geometry that the complex is sampling on its path to its final binding site. By monitoring the rearrangement process using circular dichroism (CD), we show that threading of both enantiomers of B and P into poly(dAdT)2 is effectively a monoexponential process, as expected if the compounds are not affecting each other during the intercalation process. Thus, the complex luminescence trajectories may be explained by slow relaxations in the binding geometry (DNA conformation) and associated changes in the environment of the entering complexes. To further disentangle the intriguing features of the threading intercalation kinetics, and how they may depend on the flexibility and size of the ruthenium complexes, we have also designed and studied two new ruthenium complexes, [mu-dtpf(phen)4Ru2](4+) (F) (dtpf=4,5,9,12,16,17,21,25-octaaza-23 H-ditriphenyleno[2,3-b:2,3-h]fluorene), in which the bridging ligand is made totally rigid, and [mu-bidppz([12]aneS4) 2Ru2](4+) (S), which has less bulky, nonaromatic ancillary ligands. The threading of F into poly(dAdT)2, also found to be a monoexponential process, is about 3 times slower than for P, indicating that the flexibility of the bridging ligand is an important factor for the intercalation rate. Surprisingly, in contrast to all other compounds, S requires two exponentials to fit its binding kinetics as monitored by CD. Also surprisingly, in view of the smaller steric bulk, even the fastest phase is roughly 2 times slower for S than for B and P. Thus, not only the size of the ancillary ligand but also other properties that can influence the energy landscape of the threading path are rate-determining factors. With mixed-sequence ct-DNA, threading of B and that of P are both multiphasic processes when monitored with CD as well as with luminescence. The rate constants for threading into ct-DNA show much larger variations between complexes than for poly(dAdT)2, confirming earlier results based on luminescence data.