Comparing contact and dipolar interactions in a Bose-Einstein condensate

We have measured the relative strength epsilon dd of the magnetic dipole-dipole interaction compared with the contact interaction in a dipolar chromium Bose-Einstein condensate. We analyze the asymptotic velocities of expansion of the condensate with different orientations of the atomic magnetic moments. By comparing the experimental results with numerical solutions of the hydrodynamic equations for dipolar condensates, we obtain epsilon dd = 0.159+/-0.034. We use this result to determine the s-wave scattering length a = (5.08+/-1.06 x 10(-9)) m = (96+/-20) a0 of 52Cr. This is fully consistent with our previous measurements on the basis of Feshbach resonances and therefore confirms the validity of the theoretical approach used to describe the dipolar Bose-Einstein condensate.     

Efficiency of initiating cell adhesion in hydrodynamic flow

We theoretically investigate the efficiency of initial binding between a receptor-coated sphere and a ligand-coated wall in linear shear flow. The mean first passage time for binding decreases monotonically with increasing shear rate. Above a saturation threshold of the order of a few 100 receptor patches, the binding efficiency is enhanced only weakly by increasing their number and size, but strongly by increasing their height. This explains why white blood cells in the blood flow adhere through receptor patches localized to the tips of microvilli, and why malaria-infected red blood cells form elevated receptor patches (knobs).     

Statistical mechanics of combinatorial auctions

Combinatorial auctions are formulated as frustrated lattice gases on sparse random graphs, allowing the determination of the optimal revenue by methods of statistical physics. Transitions between computationally easy and hard regimes are found and interpreted in terms of the geometric structure of the space of solutions. We introduce an iterative algorithm to solve intermediate and large instances, and discuss competing states of optimal revenue and maximal number of satisfied bidders. The algorithm can be generalized to the hard phase and to more sophisticated auction protocols.     

Site‐Resolved Observation of Vibrational Energy Transfer Using a Genetically Encoded Ultrafast Heater

Allosteric information transfer in proteins has been linked to distinct vibrational energy transfer (VET) pathways in a number of theoretical studies. Experimental evidence for such pathways, however, is sparse because site‐selective injection of vibrational energy into a protein, that is, localized heating, is required for their investigation. Here, we solved this problem by the site‐specific incorporation of the non‐canonical amino acid β‐(1‐azulenyl)‐l ‐alanine (AzAla) through genetic code expansion. As an exception to Kasha's rule, AzAla undergoes ultrafast internal conversion and heating after S₁ excitation while upon S₂ excitation, it serves as a fluorescent label. We equipped PDZ3, a protein interaction domain of postsynaptic density protein 95, with this ultrafast heater at two distinct positions. We indeed observed VET from the incorporation sites in the protein to a bound peptide ligand on the picosecond timescale by ultrafast IR spectroscopy. This approach based on genetically encoded AzAla paves the way for detailed studies of VET and its role in a wide range of proteins.     

Intermolecular,Branch‐Selective,and Redox‐Neutral Cp*IrIII‐Catalyzed Allylic C−H Amidation

Herein, we report the redox‐neutral, intermolecular, and highly branch‐selective amidation of allylic C?H bonds enabled by Cp*Ir^III catalysis. A variety of readily available carboxylic acids were converted into the corresponding dioxazolones and efficiently coupled with terminal and internal olefins in high yields and selectivities. Mechanistic investigations support the formation of a nucleophilic Ir^III–allyl intermediate rather than the direct insertion of an Ir–nitrenoid species into the allylic C?H bond.     

Probing Membrane Protein Insertion into Lipid Bilayers by Solid-State NMR

Determination of the environment surrounding a protein is often key to understanding its function and can also be used to infer the structural properties of the protein. By using proton-detected solid-state NMR, we show that reduced spin diffusion within the protein under conditions of fast magic-angle spinning, high magnetic field, and sample deuteration allows the efficient measurement of site-specific exposure to mobile water and lipids. We demonstrate this site specificity on two membrane proteins, the human voltage dependent anion channel, and the alkane transporter AlkL from Pseudomonas putida. Transfer from lipids is observed selectively in the membrane spanning region, and an average lipid-protein transfer rate of 6 s⁻¹ was determined for residues protected from exchange. Transfer within the protein, as tracked in the ¹⁵N-¹H 2D plane, was estimated from initial rates and found to be in a similar range of about 8 to 15 s⁻¹ for several resolved residues, explaining the site specificity.     

Site‐Selective C−H Oxygenation via Aryl Sulfonium Salts

Herein, we report a two‐step process forming arene C?O bonds in excellent site‐selectivity at a late‐stage. The C?O bond formation is achieved by selective introduction of a thianthrenium group, which is then converted into C?O bonds using photoredox chemistry. Electron‐rich, ‐poor and ‐neutral arenes as well as complex drug‐like small molecules are successfully transformed into both phenols and various ethers. The sequence differs conceptually from all previous arene oxygenation reactions in that oxygen functionality can be incorporated into complex small molecules at a late stage site‐selectively, which has not been shown via aryl halides.     

Effect of β3‐Amino Acids on the Performance of the Peptidic Catalyst H‐dPro‐Pro‐Glu‐NH2

The effect of β³‐amino acids on the conformation and catalytic performance of the peptidic catalyst H‐d Pro‐Pro‐Glu‐NH₂ was investigated. Analogues of the peptidic catalyst bearing instead of the α‐amino acids the respective β³‐amino acids were prepared and their reactivity and stereoselectivity was investigated in conjugate addition reactions of aldehydes to nitroolefins. Additional computational studies provided insights into the preferred conformations of the peptidic catalysts. The results show that conformational flexibility at the N‐terminus has a severe effect on the stereoselectivity but is tolerated at the C‐terminus.     

Transformation of Imine Cages into Hydrocarbon Cages

In contrast to organic cages which are formed by exploiting dynamic covalent chemistry, such as boronic ester cages, imine cages, or disulfide cages, those with a fully carbonaceous backbone are rarer. With the exception of alkyne metathesis based approaches, the vast majority of hydrocarbon cages need to be synthesized by kinetically controlled bond formation. This strategy implies a multiple step synthesis and no correction mechanism in the final macrocyclization step, both of which are responsible for low overall yields. Whereas for smaller cages the intrinsic drawbacks are not always obvious, larger cages are seldom synthesized in yields beyond a few tenths of a percent. Presented herein is a three‐step method to convert imine cages into hydrocarbon cages. The method has been successfully applied to even larger structures such as derivatives of C₇₂H₇₂ , an unknown cage suggested by Fritz Vögtle more than 20 years ago.     

Turning DNA Binding Motifs into a Material for Flow Cells

Nanoscale assemblies of DNA strands are readily designed and can be generated in a wide range of shapes and sizes. Turning them into solids that bind biomolecules reversibly, so that they can act as active material in flow cells, is a challenge. Among the biomolecular ligands, cofactors are of particular interest because they are often the most expensive reagents of biochemical transformations, for which controlled release and recycling are desirable. We have recently described DNA triplex motifs that bind adenine-containing cofactors, such as NAD, FAD and ATP, reversibly with low micromolar affinity. We sought ways to convert the soluble DNA motifs into a macroporous solid for cofactor binding. While assemblies of linear and branched DNA motifs produced hydrogels with undesirable properties, long DNA triplexes treated with protamine gave materials suitable for flow cells. Using exchangeable cells in a flow system, thermally controlled loading and discharge were demonstrated. Employing a flow cell loaded with ATP, bioluminescence was induced through thermal release of the cofactor. The results show that materials generated from functional DNA structures can be successfully employed in macroscopic devices.