Superposition of these structures allows a common reaction mechanism for seryl-adenylate and Ap4A formation to be proposed. The bent conformation of the ATP and the position of the serine are consistent with nucleophilic attack of the serine carboxyl group on the alpha-phosphate by an in-line displacement mechanism leading to the release of the inorganic pyrophosphate. A second ATP molecule can bind with its gamma-phosphate group in the same position as the beta-phosphate of the original ATP. This can attack the seryl-adenylate with the formation of Ap4A by an identical in-line mechanism in the reverse direction. The divalent cation is essential for both reactions and may be directly involved in stabilizing the transition state.
The interparticle interactions in aqueous monoacid triglyceride nanosuspensions prepared by melt
homogenization are strongly influenced by the thickness distribution of the platelet-shaped particles.
Small-angle X-ray scattering and transmission electron microscopy investigations on the native, undiluted
tripalmitin dispersions stabilized with phospholipids and sodium glycocholate indicate that the dispersed
nanocrystals can form aggregates of particles which are oriented in the same direction (stacked lamellae)
with interparticle distances around 20 nm. The relative concentration of the particles in the stacks increases
with the total triglyceride concentration in a dispersion. The stacked lamellae almost disappear after
dilution with the aqueous phase or by increasing the temperature just below the onset of melting of the
tripalmitin nanocrystals. This observation indicates that the energy gained by the formation of a stack
is of the order of k
B
T. The tendency to form stacked lamellae is reduced for trimyristin and even more for
trilaurin suspensions, despite the fact that they also contain platelet-shaped nanocrystals.
The structure factor of a fragile glass-forming liquid orthoterphenyl was measured in the previously inaccessible intermediate q range between the conventional light scattering (LS) and small-angle x-ray scattering (SAXS) q ranges using the low-angle scattering beam line at the European Synchrotron Radiation Facility. At low q the structure factor exhibits an excess scattering and matches well the LS data. This excess scattering is due to long-range density fluctuations also observed in the isotropic component of scattered light. At high q the structure factor decays to a plateau corresponding to the isothermal compressibility in agreement with the conventional SAXS data. In the intermediate q range, the structure factor exhibits a power law q dependence which indicates that the excess scattering is due to fractal aggregates of denser domains.
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