Investigation into the ROMP copolymerization of peptide‐ and PEG‐functionalized norbornene derivatives

The ring‐opening metathesis polymerizations (ROMP), using RuCl₂ (PCy₃)₂CHPh, of a series of peptide‐functionalized norbornene derivatives have been investigated. Incorporation of a PEG‐monomer was found to prevent premature precipitation of polymer strands during the course of polymerization reactions and yield water compatible polymers in high conversions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3178–3190, 2007     

Sugar–phosphite–oxazoline and phosphite–phosphoroamidite ligand libraries for Cu-catalyzed asymmetric 1,4-addition reactions

Sugar–phosphite–oxazoline L1–L5a–g and phosphite–phosphoroamidite L6a–c ligand libraries were tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of cyclic and acyclic enones. Systematically varying the electronic and steric properties of the oxazoline and biaryl phosphite substituents and the functional groups attached to the basic sugar-backbone had a strong effect on the catalytic performance. In general, good activities and enantioselectivities were obtained. The enantioselectivity (up to 80%) was optimized with catalyst precursors containing the phosphite–oxazoline ligands L1c and L1f that contain encumbered biaryl phosphite moieties and a phenyl oxazoline group.     

Lipase-mediated resolution of 3-hydroxy-4-trityloxybutanenitrile: synthesis of 2-amino alcohols,oxazolidinones and GABOB

Lipase-mediated kinetic resolution of 3-hydroxy-4-trityloxybutanenitrile gave the (S)-alcohol and (R)-acetate in good yields and high enantioselectivities. The resolution using Pseudomonas cepacia lipase (Burkholderia cepacia) immobilized on modified ceramic particles (PS-C) in diisopropyl ether gave the best results. The use of base additives in this transesterification drastically reduces the reaction time without effecting the yields or enantioselectivities. Resolved 3-hydroxy-4-trityloxybutanenitrile has been utilized for the synthesis of enantiomerically pure 5-tosyloxymethyl-1,3-oxazolidine-2-one, which is an important intermediate for the preparation of β-adrenergic blocking agents and oxazolidinone based antimicrobial agents. Enantiomerically pure (R)-3-hydroxy-4-trityloxybutanenitrile and (S)-5-tosyloxymethyl-1,3-oxazolidine-2-one have been utilized in the enantioconvergent synthesis of (R)-GABOB.     

A note on Fisher’s correlation coefficient

This note uses a geometric approach to discuss connections between different definitions of the correlation coefficient.     

Electrospray deposition in vacuum

We have used the established technique of electrospray in developing a portable vacuum electrospray system which can deposit, in vacuo, dissolved molecules onto a sample which may then be analysed by UHV techniques. As an initial test of the system we have analysed silicon samples with an electrosprayed layer of poly(ethylene) oxide (PEO) using atomic force microscopy (AFM). The polymer forms different structures depending on the voltage applied to the emitter, and solution composition. The system is part of our ongoing effort to deposit other materials such as nanoparticles, and large dye molecules for developing molecular dye sensitised solar cells.     

Coherence and stochastic resonance in threshold crossing detectors with delayed feedback

We consider the dynamical behavior of threshold systems driven by external periodic and stochastic signals and internal delayed feedback. Specifically, the effect of positive delayed feedback on the sensitivity of a threshold crossing detector (TCD) to periodic forcing embedded in noise is investigated. The system has an intrinsic ability to oscillate in the presence of positive feedback. We first show conditions under which such reverberatory behavior is enhanced by noise, which is a form of coherence resonance (CR) for this system. Further, for input signals that are subthreshold in the absence of feedback, the open-loop stochastic resonance (SR) characteristic can be sharply enhanced by positive delayed feedback. This enhancement is shown to depend on the stimulus period, and is maximal when this period is matched to an integer multiple of the delay. Reverberatory oscillations, which are particularly prominent after the offset of periodic forcing, are shown to be eliminated by a summing network of such TCDs with local delayed feedback. Theoretical analysis of the crossing rate dynamics qualitatively accounts for the existence of CR and the resonant behavior of the SR effect as a function of delay and forcing frequency.     

Superbranching processes and projections of random Cantor sets

We study sequences (X ₀, X ₁, ...) of random variables, taking values in the positive integers, which grow faster than branching processes in the sense that , for m, n0, where the X _n (m, i) are distributed as X _n and have certain properties of independence. We prove that, under appropriate conditions, X _n ^1/n almost surely and in L ¹, where =sup E(X _n )^1/n . Our principal application of this result is to study the Lebesgue measure and (Hausdorff) dimension of certain projections of sets in a class of random Cantor sets, being those obtained by repeated random subdivisions of the M-adic subcubes of [0, 1] ^d . We establish a necessary and sufficient condition for the Lebesgue measure of a projection of such a random set to be non-zero, and determine the box dimension of this projection.Work done partly whilst visiting Cornell University with the aid of a Fulbright travel grant     

High-resolution spectroscopy of induced chiral dimers: a study of the dimers of ethanol by Fourier transform microwave spectroscopy

We present the first recording of the high-resolution spectrum of an induced chiral dimer. Three conformers of the induced chiral dimers of ethanol have been observed using a pulsed molecular-beam Fourier transform microwave spectrometer. The rotational constants of the normal isotopomers of the three species have been determined to be (a) A=5113.826(5), B=1329.7214(4), and C=1257.5151(3) MHz, (b) A=5086.459(5), B=1316.6508(4), and C=1243.6329(4) MHz, and (c) A=4851.608(5), B=1369.7558(6), and C=1243.4184(4) MHz. The observed species have been assigned to calculated structures via Kraitchman double substitution analyses and ab initio calculations. The Kraitchman analyses and the fitted centrifugal distortion parameters suggest that the deuterium bond is significantly stronger than the hydrogen bond in the dimers of ethanol.     

Freezing and folding behavior in simple off-lattice heteropolymers

We have performed parallel tempering Monte Carlo simulations using a simple continuum heteropolymer model for proteins. All 10 heteropolymer sequences which we have studied have shown first-order transitions at low temperature to ordered states dominated by single chain conformations. These results are in contrast with the theoretical predictions of the random energy model for heteropolymers, from which we would expect continuous transitions to glassy behavior at low temperatures.