13C NMR coupling constants in delocalized organo-alkali metal compounds. Models of polymerization systems

2,5-Diphenyl-2,5-dipotassiohexane, 2-lithio-4,4-dimethyl-2-phenylpentane, and 1-lithio-2,5,5-trimethylhexene-2 have been prepared, all labelled with¹³C in the position adjacent to the alkali metal atom. The¹³C NMR spectra of these compounds have been measured and the¹³CC coupling constants found for the charged atom. The first two compounds have coupling constants consistent with an sp² hybridized C_α, with relatively little effect of the charge on the coupling constant. The third compound, when dissolved in either THF or benzene, gave much smaller coupling constants, which are more difficult to interpret.     

A sequence and structural study of transmembrane helices

A comparison is made between the distribution of residue preferences, three dimensional nearest neighbour contacts, preferred rotamers, helix-helix crossover angles and peptide bond angles in three sets of proteins: a non-redundant set of accurately determined globular protein structures, a set of four-helix bundle structures and a set of membrane protein structures. Residue preferences for the latter two sets may reflect overall helix stabilising propensities but may also highlight differences arising out of the contrasting nature of the solvent environments in these two cases. The results bear out the expectation that there may be differences between residue type preferences in membrane proteins and in water soluble globular proteins. For example, the -branched residue types valine and isoleucine are considerably more frequently encountered in membrane helices. Likewise, glycine and proline, residue types normally associated with `helix-breaking' propensity are found to be relatively more common in membrane helices. Three dimensional nearest neighbour contacts along the helix, preferred rotamers, and peptide bond angles are very similar in the three sets of proteins as far as can be ascertained within the limits of the relatively low resolution of the membrane proteins dataset. Crossing angles for helices in the membrane protein set resemble the four helix bundle set more than the general non-redundant set, but in contrast to both sets they have smaller crossing angles consistent with the dual requirements for the helices to form a compact structure while having to span the membrane. In addition to the pairwise packing of helices we investigate their global packing and consider the question of helix supercoiling in helix bundle proteins.     

Absolute propagation constants in vinyl polymerization

The determination of the individual rate constants in a reaction involving more than a single step is part of the basic knowledge required to understand the process itself. The history of the chain mechanism of vinyl polymerization is presented briefly. The techniques needed to measure the chain propagation step are discussed for the three basic mechanisms: free-radical, cationic, and anionic polymerization. Illustrative examples of the rate constants obtained are given, with stress placed on the monomers styrene and methyl methacrylate, which have the advantage of being able to be polymerized by all three or two of the mechanisms, respectively. This allows a comparison of propagation constants between mechanisms. Some factors influencing the magnitudes of the constants are mentioned, and some problems involved in specific cases are discussed. © 1999 Government of Canada. Exclusive worldwide publication rights in the article have been transferred to John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4467–4477, 1999     

Microstructures of polybutadienes prepared by anionic polymerization in polar solvents. Ion-pair and solvent effects

The microstructure of polybutadiene produced by anionic initiation in diethyl ether and tetrahydrofuran with counterions Li⁺, Na⁺ and K⁺ was determined by ¹H- and ¹³C-NMR. Ionization suppressing salts were added in tetrahydrofuran to ensure that only the ion-pair reaction was studied. Results are compared with older published data. In general, the 1,4 content of the polymer increases with increasing counterion size but varies somewhat with solvent with a given counterion. The cis component of the 1,4 structures changes with temperature and counterion. It is suggested that this change reflects the proportion of cis and trans centers that carry the reaction.