Silver nanoparticle-catalyzed Diels-Alder cycloadditions of 2'-hydroxychalcones.

Metal nanoparticles are currently being employed as catalysts for a number of classical chemical transformations. In contrast, identification of novel reactions of nanoparticles, especially toward the synthesis of complex natural products and derivatives, is highly underdeveloped and represents a bourgeoning area in chemical synthesis. Herein, we report silica-supported silver nanoparticles as solid, recyclable catalysts for Diels-Alder cycloadditions of 2'-hydroxychalcones and dienes in high yield and turnover number. The use of silver nanoparticle catalysts is further demonstrated by the total synthesis of the cytotoxic natural product panduratin A employing a highly electron-rich dienophile and Lewis acid sensitive diene.

Electrochemical evidence for multiple peroxidatic heme states of the diheme cytochrome c peroxidase of Pseudomonas aeruginosa.

The enzyme cytochrome c peroxidase from Pseudomonas aeruginosa and its catalytic mechanism were investigated using protein film voltammetry. Monolayers of the diheme bacterial enzyme were immobilized on both pyrolytic graphite edge and alkanethiol-modified Au electrodes. The redox couple associated with the low potential heme could be detected on both electrode surfaces at a reduction potential of -234 mV vs SHE. The midpoint potential displays a distinct pH dependence at acidic pH values, indicative of proton-coupled electron transfer. The nonturnover signal of the LP heme can be transformed into sigmoidal waves upon the addition of substrate. The midpoint potentials of the turnover signals were used to calculate Michaelis-Menten kinetics with a K(m) = 25 microM. Catalysis was inhibited with addition of cyanide (K(i) = 50 microM). These kinetic parameters are in good agreement with previously reported solution-based studies, indicating that the activity of the enzyme is unaffected by the immobilization on the electrode surface. The reduction potential of the catalytic wave clearly shows that the rate-limiting species during electrocatalysis differs from those previously reported for peroxidases, indicating that PFV may be used in the future to distinguish the requirement for reductive activation in bacterial cytochrome c peroxidases.