Polymerization studies on allylic compounds. III. α(substituted methyl)styrenes

The free-radical polymerization of a series of α-(substituted methyl)styrenes was investigated. These compounds were found to be inactive in homopolymerizations but copolymerized with methyl methacrylate and styrene with a retarding effect. Copolymerization characteristics were followed by rate and viscosity measurements.     

A generalization of the method of stimulated emission pumping

Drawing on the results of an analysis of the nature of the pulse that generates complete transfer of population from one to another level in a system with a discrete spectrum, a generalization of the method of stimulated emission pumping is proposed. It is shown that a small subset of the Fourier components of the optimal pulse will, if their relative amplitudes are the same as in the optimal pulse, generate almost as efficient a population transfer, thereby generating the opportunity to prepare a system in a selected state with a selected population.     

Chimeric (alpha/beta + alpha)-peptide ligands for the BH3-recognition cleft of Bcl-XL: critical role of the molecular scaffold in protein surface recognition

Molecules that bind to specific surface sites on proteins are of great interest from both fundamental and practical perspectives. We are exploring a ligand development strategy that is based on oligomers with discrete folding propensities ("foldamers"); we target a specific cleft on the cancer-associated protein Bcl-xL because this system is well characterized structurally. In vivo, this cleft binds to alpha-helical segments (BH3 domains) of other proteins. We evaluated several types of helical foldamer, built entirely from beta-amino acid residues or from mixtures of alpha- and beta-amino acid residues, and ultimately identified foldamers in the latter class that bind very tightly to Bcl-xL. Our results suggest that combining different types of foldamer backbones will be an effective and general strategy for creating high-affinity and specific ligands for protein surface sites.     

DNA photoreactivating enzyme from human tissues

Photoreactivating enzyme activity has been quantitated in human fetal skin, kidney, lung, liver, brain and intestine, and in neonatal human foreskin. In all the tissues examined there were at least two activities: one nominally greater than 10,000 Da, and one nominally less than 10,000 Da. Both can photolyze pyrimidine dimers in DNA using only light of wavelengths greater than 320 nm, thus excluding tryptophan-mediated dimer splitting as an important mechanism for these activities. The activities are inactivated by digestion with trypsin or pronase, and decreased partially or totally by heating to 65 degrees C. The activities from all six tissues, as well as that from neonatal foreskin, act catalytically in dimer photolysis. The properties of macromolecular size, heat lability, protease sensitivity and catalytic pyrimidine dimer photolysis by a non-tryptophan-mediated mechanism correspond to those of a true photoreactivating enzyme.     

Longitudinal nuclear relaxation measurements in hcp H2

Measurements of T₁ in the hep phase of H₂, over the temperature range 2°–12°K and the ortho concentration range between 0.5 and 0.97 are presented. At temperatures below 10°K, the thermally activated self-diffusion is negligible and the mechanism for nuclear relaxation is that attributed by Moryia and Motizuki and by Harris to intramolecular dipolar interaction, modulated by intennolecular electric quadrupole-quadrupole (EQQ) interaction. The gaussian approximation for the correlation function was used by these authors to predict T₁. From the comparison between experiment and theory, we determine the EQQ parameter Γ/k_B to be 0.67°K. Above 10°K the effect of diffusion influences T₁, and the experimental results for an 88 per cent ortho H₂ sample up to the melting point suggest that the relaxation mechanisms resulting from EQQ interaction and diffusion are not independent of one another.     

Projective Modules over the Non-Commutative Sphere

The positive cone of the K₀-group of the non-commutative sphereB is explicitly determined by means of the four basic unboundedtrace functionals discovered by Bratteli, Elliott, Evans andKishimoto. The C*-algebra B is the crossed product A x Z₂ ofthe irrational rotation algebra A by the flip automorphism defined on the canonical unitary generators U, V by (U) = U*,(V) = V*, where VU = e²ⁱ UV and is an irrational real number.This result combined with Rieffel's cancellation techniquesis used to show that cancellation holds for all finitely generatedprojective modules over B. Subsequently, these modules are determinedup to isomorphism as finite direct sums of basic modules. Italso follows that two projections p and q in a matrix algebraover B are unitarily equivalent if, and only if, their vectortraces are equal: [p] = [q]. These results will have the following ramifications. They areused (elsewhere) to show that the flip automorphism on A isan inductive limit automorphism with respect to the basic buildingblock construction of Elliott and Evans for the irrational rotationalgebra. This will, in turn, yield a two-tower proof of thefact that B is approximately finite dimensional, first provedby Bratteli and Kishimoto.     

Planar graphs,Hamilton cycles and extreme independence number

If a graph has too many edges, it must be Hamiltonian. We show how to construct graphs near the threshold: they have as many edges as possible without sacrificing planarity but are not Hamiltonian. We base our construction on the concepts of independent sets and 1-toughness.The work of the author was partially supported by CNPq-Brasil.     

R andRτ ratios at the five-loop level of perturbative QCD

We study perturbative QCD at the five-loop level. In particular we considerR = _tot(e ⁺ e ^– hadrons)/(e ⁺ e ^{– + –}) andR = ( v+hadrons)/( ev). We use our method to estimate the five-loop coefficients. As a result, we obtain _s (M _z ) = 0.1186(11) and _s (34 GeV) = 0.1396(16), which are accurate at the 1% level. We also findR = 3.8350(18), which is consistent withR and is accurate to 0.05%.