Influence of size and polarity of residue 31 in porcine pancreatic phospholipase A2 on catalytic properties

Residue 31 of porcine pancreatic phospholipase A₂ (PLA₂) islocated at the entrance to the active site. To study the roleof residue 31 in PLA₂, six mutant enzymes were produced by site-directedmutagenesis, replacing Leu by either Trp, Arg, Ala, Thr, Seror Gly. Direct binding studies indicated a three to six timesgreater affinity of the Trp31 PLA₂ for both monomeric and micellarsubstrate analogs, relative to the wild-type enzyme. The otherfive mutants possess an unchanged affinity for monomers of theproduct analog n-decylphosphocholine and for micelles of thediacyl substrate analog rac-l,2-dioctanoylamino-dideoxy-glycero-3-phosphocholine.The affinities for micelles of the monoacyl product analog n-hexadecylphosphocholinewere decreased 9–20 times for these five mutants. Kineticstudies with monomeric substrates showed that the mutants haveV_max values which range between 15 and 70% relative to the wild-typeenzyme. The V_max values for micelles of the zwitterionic substratel,2-dioctanoyl-sn-glycero-3-phosphocholine were lowered 3–50times. The K_m values for the monomeric substrate and the k_mvalues for the micellar substrate were hardly affected in thecase of five of the six mutants, but were considerably decreasedwhen Trp was present at position 31. The results of these investigationspoint to a versatile role for the residue at position 31: involvementin the binding and orientating of monomeric substrate (analogs),involvement in the binding of the enzyme to micellar substrateanalogs and possibly involvement in shielding the active sitefrom excess water.     

Compatibilization of a polyolefin blend through covalent and ionic coupling of grafted polypropylene and polyethylene. II. Morphology

The morphology of a polypropylene/polyethylene (PP/PE) blend and a maleic anhydride modified PP and PE (PPg/PEg) blend was studied. The initial morphology, at the extruder die, after the melt blending into a twin‐screw extruder, was first characterized. Then, the evolution of this initial morphology was followed after a injection molding operation, and during annealing in an oven at 200°C. The influence of the compatibilization of the blend by coupling reactions through covalent (with 1,12‐diaminododécane) and ionic reactions (with zinc acetate and sodium hydrogenocarbonate) was also investigated. At the extruder die, the viscosity ratio proved to be a determinant factor governing the dispersed phase diameter of the droplets, and as a second factor, the addition of small amounts of coupling‐agents together with (PPg/PEg) to (PP/PE) resulted in a decrease of the diameter of the droplets. The injection molding of these initial blends resulted in important coalescence and in an elongation of the dispersed phase. This was observed for the non compatibilized and also for some of the compatibilized blends. The ionic coupling showed a good stabilization of the morphology. Finally, the morphology of the non compatibilized blends was found to be instable when the material was annealed at 200°C. The average size of the dispersed phase increased. The coupling reactions delayed the occurrence of the coalescence about 5 min and limited its effects. The extent of the coarsening depended strongly on the composition of the blend and on the nature of the coupling. Still, the ionic agents appeared more effective. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2237–2244, 2004     

Nanostructuration of Unsaturated Polyester by All‐Acrylic Block Copolymers, 1 ‐ Use of High‐Molecular‐Weight Block Copolymers

Nanostructuration of maleate and orthophthalic unsaturated polyester (UP) resins was achieved by the use of high molecular weight amphiphilic PBA‐b‐P(MMA‐co‐DMA)₂ triblock copolymers. PBA is fully immiscible in cured UP resins, and the miscibility of P(MMA‐co‐DMA) random copolymers can be ensured with a minimum DMA content of 12 mol‐%. When using the triblock copolymers, fully transparent and nanostructured thermosets are obtained with a minimum DMA content in the outer blocks; the value of which is higher than 12 mol‐% and depends on the UP chemical structure. Finally, the fracture toughness of nanostructured thermoset was evaluated: for a triblock copolymer content as low as 5 wt.‐%, a 50% increase of K_Ic was obtained, as compared to the neat thermoset.