QM/MM molecular modelling on mutation effect of chorismate synthase enzyme catalysis.

Chorismate synthase (CS) catalyzes the conversion of 5-enolpyruvylshikimate-3-phosphate (EPSP) to chorismate which is a key intermediate in the biosynthesis of aromatic amino acids. CS enzyme is a new target for antibacterial drugs. Even though several reaction mechanisms have been proposed, the catalytic mechanism is still unclear. QM/MM adiabatic mapping calculations were performed in order to investigate roles of this enzyme. High-accuracy SCS-MP2/aVDZ/CHARMM27 calculations indicated that the reaction pathway has three steps; (i) proton transfer from reduced flavin mononucleotide (FMNH2) to D339, (ii) proton transfer from EPSP to FMNH- and (iii) phosphate elimination. Adiabatic mapping calculations indicated that H110 and R48 residues play essential catalyst roles for CS enzyme catalysis by transition state (TS) and product stabilizations via charge polarization and hydrogen bonding to EPSP and/or FMNH2. A high accuracy calculation - SCS-MP2/aVDZ/CHARMM27 method was employed to obtain the accurate reaction mechanism pathway and to evaluate the effect of amino acid residues in the active site on the enzyme catalysis. The potential energy barriers of the reactions of H110A and R48A were found to increase. The CS catalysis was consequently slowed down due to missing the TS and product stabilizations.

Impact of End-Tethered Polyhedral Nanoparticles on the Mobility of Poly(dimethylsiloxane).

A series of dumbbell-shaped nanocomposite materials of poly(dimethylsiloxanes) (PDMSs) and polyhedral oligomeric silsesquioxanes (POSSs) were synthesized through hydrosilylation reactions of allyl- and vinyl-POSS and hydride-terminated PDMS. The chemical structures of the dumbbell-shaped materials, so-called POSS-PDMS-POSS triblocks, were characterized by (1)H NMR and FT-IR spectroscopy. The molecular weights of the triblock polymers were determined by gel permeation chromatography (GPC). Their size was analyzed by small-angle neutron scattering (SANS) and pulsed-field gradient stimulated echo (PFG STE) NMR experiments. The impact of POSS on the molecular mobility of the PDMS middle chain was observed by using (1)H spin-spin (T2) relaxation NMR. In contrast to the PDMS melts, the triblocks showed an increase in mobility with increasing molecular weight over the range studied due to the reduced relative concentration of constraints imposed by the end-tethered nanoparticles. The triblock systems were used to compare the impact of tethered nanoparticles on the mobility of the linear component compared to the mobility of the polymer in conventional blended nanocomposites. The tethered nanoparticles were found to provide more reinforcement than physically dispersed particles especially at high molecular weights (low particle concentration). The physical blends showed an apparent percolation threshold behavior.