The search for new metal-catalyzed asymmetric reactions has provided some pascinating insights into the effects imposed on thc metal catalysts by chiral liganda. A practical consequence is the discovery of ligmd-accelerated catalysis (LAC). Thus, an existing catalyzed process is improved by the addition of a specific ligand, which leads to a faster. "ligand-accelerated" reaction. Both homogeneous and heterogeneok catalysts are known to exhibit this behavior. The concept is especially valuable i n reactions catalyzed by early transition metals, where dynamic ligand exchange processes require an efficient in situ self-selection of a highly reactive catalyst from a variety of thennodynamically dictatcd assemblies. Results of detailed mechanistic studies will be prcsented. and the significance of LAC phenomena in transformations catalyzed by early and late transition metals will be discussed.

Die Suche nach neuen metallkatalysierten, asymmetrischen Reaktionen hat zu faszinierenden Erkenntnissen darüber geführt, welchen Einfluß chirale Liganden auf Metallkatalysatoren ausüben. Eine praktische Konsequenz daraus war die Entdeckung der ligandenbeschleunigten Katalyse (ligand accelerated catalysis, LAC). Dabei wird ein katalytisch ablaufender Prozeß durch die Zugabe eines spezifischen Liganden so verbessert, daß die Reaktion schneller, „ligandenbeschleunigt”︁, abläuft. Sowohl für homogene als auch für heterogene Katalysatorsysteme ist dieses Verhalten bekannt. Das Konzept ist besonders wertvoll für Reaktionen, die durch frühe Übergangsmetalle katalysiert werden und bei denen wegen dynamischer Ligandenaustauschprozesse die effiziente In‐situ‐Selbstselektion eines hochreaktiven Katalysators aus einer Vielzahl thermodynamisch gebildeter Assoziate erforderlich ist. Die Ergebnisse von detaillierten mechanistischen Studien werden aufgeführt und die Bedeutung von LAC‐Phänomenen bei Umsetzungen diskutiert, die mit frühen und späten Übergangsmetallen katalysiert werden.

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A key ion-dependent folding unit within the hepatitis C IRES comprises the IIIef junction and pseudoknot. This region is also important in recruitment of the 40S ribosomal subunit. Here, circular dichroism is used to study the influence of metal ions on the structure and stability of this region. Comparison of the thermal stability of an IRES fragment encompassing subdomains IIIe/f and IV (named 3EF4) with that of a larger fragment also possessing subdomain IIId (3DEF4) indicates an additional stabilizing effect of Mg(2+) ions on the latter fragment. Magnesium and potassium ions stabilize both fragments through nonspecific counterion effects. The additional effect of magnesium on 3DEF4, observed in the absence or presence of 100 mM KCl, is attributed to a nonspecific but high-affinity site for metal ions created by a region of unusual high charge density. Subdomain IIId presumably participates in tertiary packing interactions that provide such a site. Viomycin binds to the full-length IRES and RNA fragments with K(d) values of 25-55 microM. Interestingly, viomycin binding to the two fragments is affected differently by Mg(2+); noncompetitive inhibition of binding to 3DEF4 is observed, whereas binding to 3EF4 is not impaired. Formation of a Mg(2+)-stabilized tertiary fold, involving subdomain IIId, may thereby hinder viomycin binding to 3DEF4 indirectly. Mutational and deletion studies locate viomycin binding within subdomains IIIe/f rather than within the pseudoknot. In pseudoknot mutants, Mg(2+) ions have different effects on viomycin binding and thermal stability, suggesting altered tertiary interactions involving subdomain IIId.

The concept of using equilibrium dynamics to provide for both protection and unveiling of latent functional groups at appropriate times in aqueous polymer colloid coatings designed for crosslinking only during film formation is introduced; the new functional monomer, 4-hydroxyethylsulfonylstyrene (HESS), readily undergoes emulsion copolymerization with acrylates to form stable latexes, followed by crosslinking by loss of water during film formation.