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The influence of pH on the activation of yeast enolase by Mn2+ was measured by steady-state kinetics. The pH influence on the binding of Mn2+ to apoenolase and the enolase-substrate complex was measured by EPR spectroscopy. At pH values above 6.6, activation by Mn2+ is fit by Michaelis-Menten kinetics, but at higher concentrations of Mn2+, inhibition is observed. Under conditions analogous to the kinetic studies, the enzyme binds two Mn2+ per dimer with a Kd in the micromolar range. In the presence of the substrate 2-phosphoglycerate, three thermodynamically distinct cation binding sites per monomer are detected and the binding constants are determined by a fit to the data. As the pH decreases, the reaction velocity decreases and the cation inhibition becomes minimal. Under these conditions, only two Mn2+ binding sites per monomer are observed; the third site must be the inhibitory site. The velocity and kinetic constants are minimally affected by buffer except at pH 5.8 with PIPES. Under these conditions, the velocity is only about 40% that observed with other buffers and only a single binding site for Mn2+ per monomer is detected in the presence or absence of substrate. A direct role in the catalytic mechanism by the second cation is called to question. The binding constant for Mn2+ at site I is independent of pH over the range from 7.5 to 5.2, and the binding at site II increases only slightly over this same pH range. These results indicate that the cation sites at positions I and II contain ligands that are pH independent over this range.(ABSTRACT TRUNCATED AT 250 WORDS)

Comonomer effect on the mechanical and morphological behavior of the calcite (stearic acid coated calcium carbonate)-filled polypropylene (PP), poly(propylene-random-ethylene) copolymer (CoPP), and poly(propylene-co-ethylene-co-1-butene) terpolymer (TerPP) composites were investigated by using dumbbell bar and film specimens. The tensile properties of the calcite-filled PP, CoPP, and TerPP composites exhibited lower values than those of the pure polymers (calcite-unfilled polymers), whereas the complex viscosity of the calcite-filled polymers exhibited slightly higher values than that of the pure polymers. Mechanical properties studied by using various strain rates and draw ratios rationalized in terms of comonomer units and contents in various PP systems. Morphological behavior of the specimens stretched at various strain rates and draw ratios was investigated by using SEM microphotographs and the mechanism of the formation of air holes was proposed. The air hole initiated from crack propagation and followed by dewetting between the calcite surface and the polymer interface in the weakened region. The crack propagated along the transverse direction; then the air hole developed parallel to the machine direction with fibril structure of the resin in PP and CoPP systems. However, TerPP composite exhibited no cracks in the beginning of the elongation, but the air hole was initiated due to dewetting; then its enlargement was exhibited by broken fibril structure of the resin. In the final stage of stretching, the air hole was dominated by merging of the neighboring air holes. Thus, different comonomer units, which are the small content of ethylene and 1-butene in CoPP and TepPP, are responsible for these systems behaving in a different manner on the mechanical and morphological properties.

Stable, spherical, polystyrene particles were synthesized in ab initio dispersion polymerization by using the poly(methacrylic acid)[PMAA] macro-RAFT agent. The presence of the PMAA macro-RAFT agent on the polystyrene (PS) particles was confirmed by NMR and FTIR spectroscopy. The PS particle size was influenced by the concentration of the RAFT agent and monomer due to the initial nucleation. When the concentration of the PMAA macro-RAFT agent was increased from 2 to 10 wt% relative to the monomer, the average particle size decreased from 2.31 to 1.36 μm, the conversion decreased from 93.3 to 88.9%, the weight-average molecular weight increased from 46,300 to 150,200 g mol -1 and the PDI decreased from 2.79 to 1.94, respectively. In particular, the incorporation of 10 wt% of PMAA macro-RAFT agent produced monodisperse PS spheres of 1.36 μm with a coefficient of variation (CV) of 6.44%. Thus, the PMAA macro-RAFT agent worked as a reactive steric stabilizer providing monodisperse, micron-sized, PS particles.