9 research outputs found
Structurally Similar Triphenylphosphine-Stabilized Undecagolds, Au<sub>11</sub>(PPh<sub>3</sub>)<sub>7</sub>Cl<sub>3</sub> and [Au<sub>11</sub>(PPh<sub>3</sub>)<sub>8</sub>Cl<sub>2</sub>]Cl, Exhibit Distinct Ligand Exchange Pathways with Glutathione
Ligand
exchange is frequently used to introduce new functional
groups on the surface of inorganic nanoparticles or clusters while
preserving the core size. For one of the smallest clusters, triphenylphosphine
(TPP)-stabilized undecagold, there are conflicting reports in the
literature regarding whether core size is retained or significant
growth occurs during exchange with thiol ligands. During an investigation
of these differences in reactivity, two distinct forms of undecagold
were isolated. The X-ray structures of the two forms, Au<sub>11</sub>(PPh<sub>3</sub>)<sub>7</sub>Cl<sub>3</sub> and [Au<sub>11</sub>(PPh<sub>3</sub>)<sub>8</sub>Cl<sub>2</sub>]ÂCl, differ only in the number
of TPP ligands bound to the core. Syntheses were developed to produce
each of the two forms, and their spectroscopic features correlated
with the structures. Ligand exchange on [Au<sub>11</sub>(PPh<sub>3</sub>)<sub>8</sub>Cl<sub>2</sub>]Cl yields only small clusters, whereas
exchange on Au<sub>11</sub>(PPh<sub>3</sub>)<sub>7</sub>Cl<sub>3</sub> (or mixtures of the two forms) yields the larger Au<sub>25</sub> cluster. The distinctive features in the optical spectra of the
two forms made it possible to evaluate which of the cluster forms
were used in the previously published papers and clarify the origin
of the differences in reactivity that had been reported. The results
confirm that reactions of clusters and nanoparticles may be influenced
by small variations in the arrangement of ligands and suggest that
the role of the ligand shell in stabilizing intermediates during ligand
exchange may be essential to preventing particle growth or coalescence
Allosteric Interactions within Subsites of a Monomeric Enzyme: Kinetics of Fluorogenic Substrates of PI-Specific Phospholipase C
Two novel water-soluble fluorescein myo-inositol phosphate (FLIP) substrates, butyl-FLIP and methyl-FLIP, were used to examine the kinetics and subsite interactions of Bacillus cereus phosphatidylinositol-specific phospholipase C. Butyl-FLIP exhibited sigmoidal kinetics when initial rates are plotted versus substrate concentration. The data fit a Hill coefficient of 1.2–1.5, suggesting an allosteric interaction between two sites. Two substrate molecules bind to this enzyme, one at the active site and one at a subsite, causing an increase in activity. The kinetic behavior is mathematically similar to that of well-known cooperative multimeric enzymes even though this phosphatidylinositol-specific phospholipase C is a small, monomeric enzyme. The less hydrophobic substrate, methyl-FLIP, binds only to the active site and not the activator site, and thus exhibits standard hyperbolic kinetics. An analytical expression is presented that accounts for the kinetics of both substrates in the absence and presence of a nonsubstrate short-chain phospholipid, dihexanoylphosphatidylcholine. The fluorogenic substrates detect activation at much lower concentrations of dihexanoylphosphatidylcholine than previously reported