Fast Raman spectroscopy of cytochrome c using intracavity resonance Raman amplification

The resonance Raman spectrum of cytochrome c (5 × 10^?4 M) was obtained using the method of resonance Raman amplification (RRA) in a dye laser resonator in 30 ns. The second harmonic of a Q-switched Nd glass laser was used as an excitation source and the spectra were recorded using a photographic plate.     

Preparation and polymerization of methylenecyclobutene and 1-methyl-3-methylenecyclobutene

Methylenecyclobutene (MCB) and 1-methyl-3-methylenecyclobutene (MMCB) were synthesized, characterized, and polymerized by anionic and cationic initiators. Structural analyses of the polymers were carried out by infrared and NMR spectros-copy. The cationic polymerization of MCB appeared to proceed entirely by a 1,5-propagation mechanism to form low molecular weight polymers in low yields. Anionic polymerization of this monomer, on the other hand, proceeded primarily through a 1,2-propagation path, again forming only low molecular weight polymeric products in low yield. In contrast to MCB, the methyl-substituted monomer, MMCB, polymerized readily with cationic initiators to produce unusually high molecular weight polymers in high conversions. On the basis of both infrared and NMR spectroscopic analyses, it was concluded that the polymers also contained essentially only 1,5-addition repeating units. Anionic initiators such as n-BuLi were unable to induce polymerization of this monomer, but polymerization by Ziegler-Natta catalysts proceeded readily to yield polymers virtually identical in structure and molecular weight to those obtained with cationic initiators.     

A comparison of the absolute reactivity of vinyl monomers. I. Kinetic studies on radical polymerization of vinyl monomers initiated by a Mn3+–diglycolic acid redox system

The kinetics of polymerization of acrylamide (AM), acrylic acid (AA), and acrylonitrile (AN) initiated by the redox system Mn³⁺–diglycolic acid (DGA) was studied. All three systems followed the same mechanism; namely, initiation by an organic free radical arising from the oxidation of diglycolic acid and termination by the interaction of polymer radicals with Mn³⁺ ion. The rate coefficients k_i/k₀ and k_p/k_t were related to monomer and polymer radical reactivity, respectively. An inverse relation between monomer and polymer radical reactivity was observed. Monomers with higher Q values gave higher k_i/k₀ values but lower k_p/k_t values. The e values of the monomers were important in determining the reactivities of monomers with nearly the same Q values.     

Extinction probability,regularity and asymptotic growth of Markovian populations

"The distribution of the maximum and the extinction probability for a Markovian population is derived. Asymptotic growth is described, using the sequence of sojourn times. A regularity criterion for the processes under consideration exists under certain assumptions. For a class of processes with specific population-dependent transition rates the asymptotic behaviour is given explicitly."     

Crystallization-induced reactions of copolymers. III. Ester interchange reorganization of poly(cis/trans-1,4-cyclohexylenedimethylene terephthalate)

Random copolymers of cis- and trans-1,4-cyclohexylenedimethylene terephthalate were permitted to undergo ester-interchange reorganization at temperatures just below the melting point. As predicted from the principles of crystallization-induced reactions of semicrystalline copolymers proposed in the first two papers of this series, the copolymers were observed to undergo changes in physical properties which are associated with the conversion of a random to a block copolymer. The driving force for this antiequilibrium ordering process is believed to be the irreversible expansion of the crystalline regions following replacement of cis by trans glycol units. Solubility, crystallinity, and crystallization properties were monitored to determine the effects of copolymer composition, temperature, catalyst, and molecular weight on the reorganization rate. This type of process is also believed to be responsible for the direct preparation of block copolymers by a solid-state polycondensation reaction used in this study.     

Preparation of a polypentenamer having pendant ionic groups and its hydrogenated derivatives

Polypentenamers having pendant carboxylate groups including esters, acids, and salts and their hydrogenated derivatives have been prepared and characterized, and some of their properties investigated. Reaction conditions were established to control the amount of pendant thioglycolate groups incorporated into the polypentenamer by the free-radical addition reactions of thioglycolic acid derivatives and subsequently to completely hydrogenate the polymers so formed, which contained thioglycolate contents ranging from less than 1 mole-% to about 16 mole-%, without backbone degradation and crosslinking reactions. The glass transition temperatures and the melting points of these polymers were observed to depend upon the pendant group content, as expected, but there appears to be little difference within these properties between polymers in which the pendant groups were in the salt form as compared to those of the corresponding carboxylic acid form. The thermal stabilities of the polymers having the pendant groups were somewhat lower than those of the parent polymers, apparently because of side reactions involving the thioglycolate groups. The calcium salt derivative showed a particularly low thermal stability compared to that of polymers with monovalent salts.     

Folding probabilities: a novel approach to folding transitions and the two-dimensional Ising-model

The theoretical concept of folding probability, p(fold), has proven to be a useful means to characterize the kinetics of protein folding. Here, we illustrate the practical importance of p(fold) and demonstrate how it can be determined theoretically. We derive a general analytical expression for p(fold) and show how it can be estimated from simulations for systems where the transition rates between the relevant microstates are not known. By analyzing the Ising model we are able to determine the scaling behavior of the numerical error in the p(fold) estimate as function of the number of analyzed Monte Carlo runs. We apply our method to a simple, newly developed protein folding model for the formation of alpha helices. It is demonstrated that our technique highly parallelizes the calculation of p(fold) and that it is orders of magnitude more efficient than conventional approaches.     

Combinatorics of the subshift associated with Grigorchuk’s group

We study combinatorial properties of the subshift induced by the substitution that describes Lysenok’s presentation of Grigorchuk’s group of intermediate growth by generators and relators. This subshift has recently appeared in two different contexts: on the one hand, it allowed embedding Grigorchuk’s group in a topological full group, and on the other hand, it was useful in the spectral theory of Laplacians on the associated Schreier graphs.     

Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers

Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecular motions such as trans-gauche isomerization and ring flips can be detected. The chain order parameter of the nematic melt is S(ZZ) = 0.85. In addition, the chains adopt a highly extended conformation, evidenced by a trans population of n(t) = 0.8 throughout the entire spacer. This microorder is retained, when the polymer is cooled below the melting point and glass transition, respectively.