Hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers

Molecular dynamics computer simulations are used to study hydrogen-bond structure and dynamics at the interface between water and carboxylic acid-functionalized self-assembled monolayers (CAFSAMs). Water-water, water-CAFSAM, and internal CAFSAM hydrogen bonds are examined. Roughly half of all adjacent carboxylic acid-terminated hydrocarbon chains are hydrogen-bonded to one another. This is consistent with experimental results reflecting two pKa values for CAFSAMs. Hydrogen-bond dynamics are expressed in terms of hydrogen-bond population autocorrelation functions and are found to be nonexponential. The water-water hydrogen-bond dynamics are slower at the interface than in the bulk, which is similar to what was found at the interface between water and weakly polar liquids such as nitrobenzene. The water-CAFSAM hydrogen bonds are found to be long-lived, on the order of tens of picoseconds. Internal CAFSAM chain-chain hydrogen bonds show almost no relaxation on the simulation time scale.     

Convergent diastereoselective preparation of adjacent quaternary stereocenters in an acyclic system

The combined carbometalation-zinc homologation-allylation reaction of the resulting stereodefined 3,3-disubstituted allylmetal species with ketones allow the preparation of allylic vicinal diol derivatives in good yields with excellent diastereomeric ratios from commercially available alkynes. Two adjacent quaternary centers are formed with the concomitant formation of three new carbon-carbon bonds in a single-pot operation in an acyclic system. The bulky substituent of the ketone occupies a pseudo-axial position in the Zimmerman-Traxler transition state.     

Controlled assembly of hybrid bulk-heterojunction solar cells by sequential deposition

This work presents a technique to create ordered and easily characterized hybrid nanocrystal-polymer composites by sequential deposition of tetrapod-shaped cadmium telluride (CdTe) nanocrystals and poly(3-hexlythiophene). With controlled fabrication and composite morphology, these devices offer several advantages over traditional co-deposited hybrid cells and provide a model system for detailed investigation into the operation of bulk-heterojunction cells.     

Computing in fault tolerant broadcast networks and noisy decision trees

We consider a fault tolerant broadcast network of n processors each holding one bit of information. The goal is to compute a given Boolean function on the n bits. In each step, a processor may broadcast one bit of information. Each listening processor receives the bit that was broadcast with error probability bounded by a fixed constant ?. The errors in different steps, as well as for different receiving processors in the same step, are mutually independent. The protocols that are considered in this model are oblivious protocols: At each step, the processors that broadcast are fixed in advanced and independent of the input and the outcome of previous steps. We present here the first linear complexity protocols for several classes of Boolean functions. This answer an open question of Yao (Invited talk in the 5th ISTCS Conf., 1997), considering this fault tolerant model that was introduced by El Gamal (Open problems presented at the 1984 workshop on Specific Problems in Communication and Computation sponsored by Bell Communication Research) and studied also by Gallager 10 . © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2009     

Fast and reliable reconstruction of phylogenetic trees with indistinguishable edges

Phylogenetic reconstruction methods attempt to reconstruct a tree describing the evolution of a given set of species using sequences of characters (e.g. DNA) extracted from these species as input. A central goal in this area is to design algorithms which guarantee reliable reconstruction of the tree from short input sequences, assuming common stochastic models of evolution. The fast converging reconstruction algorithms introduced in the last decade dramatically reduced the sequence length required to guarantee accurate reconstruction of the entire tree. However, if the tree in question contains even few edges which cannot be reliably reconstructed from the input sequences, then known fast converging algorithms may fail to reliably reconstruct all or most of the other edges. This calls for an adaptive approach suggested in this paper, called adaptive fast convergence, in which the set of edges which can be reliably reconstructed gradually increases with the amount of information (length of input sequences) available to the algorithm. This paper presents an adaptive fast converging algorithm which returns a partially resolved topology containing no false edges: edges that cannot be reliably reconstructed are contracted into high degree vertices. We also present an upper bound on the weights of those contracted edges, which is determined by the length of input sequences and the depth of the tree. As such, the reconstruction guarantee provided by our algorithm for individual edges is significantly stronger than any previously published edge reconstruction guarantee. This fact, together with the optimal complexity of our algorithm (linear space and quadratic‐time), makes it appealing for practical use. © 2011 Wiley Periodicals, Inc. Random Struct. Alg., 40, 350–384, 2011     

Tetrazole Diversification of Amino Acids and Peptides via Silver-Catalyzed Intermolecular Cycloaddition with Aryldiazonium Salts

Selective structural modification of amino acids and peptides is a central strategy in organic chemistry, chemical biology but also in pharmacology and material science. In this context, the formation of tetrazole rings, known to possess significant therapeutic properties, would expand the chemical space of unnatural amino acids but has received less attention. In this study, we demonstrated that the classic unimolecular Wolff rearrangement of α-amino acid-derived diazoketones could be replaced by a faster intermolecular cycloaddition reaction with aryldiazonium salts under identical practical conditions. This strategy provides an efficient synthetic platform that could transform proteinogenic α-amino acids into a plethora of unprecedented tetrazole-decorated amino acid derivatives with preservation of the stereocenters. Density functional theory studies shed some light on the reaction mechanism and provided information regarding the origins of the chemo- and regioselectivity. Furthermore, this diazo-cycloaddition protocol was applied to construct tetrazole-modified peptidomimetics and drug-like amino acid derivatives.     

Selective Terminal Functionalization of Linear Alkanes**

A two-step sequential strategy involving a biocatalytic dehydrogenation/remote hydrofunctionalization, as a unified and versatile approach to selectively convert linear alkanes into a large array of valuable functionalized aliphatic derivatives is reported. The dehydrogenation is carried out by a mutant strain of a bacteria Rhodococcus and the produced alkenes are subsequently engaged in a remote functionalization through a metal-catalyzed hydrometalation/migration sequence that subsequently react with a large variety of electrophiles. The judicious implementation of this combined biocatalytic and organometallic approach enabled us to develop a high-yielding protocol to site-selectively functionalize unreactive primary C−H bonds.