Abstract

Abstract

Abstract

Abstract

Although enzymes often incorporate molecular recognition elements to orient substrates selectively, such strategies are rarely achieved by synthetic catalysts. We combined molecular recognition through hydrogen bonding with C-H activation to obtain high-turnover catalytic regioselective functionalization of sp³ C-H bonds remote from the –COOH recognition group. The catalyst contains a Mn(μ-O)₂Mn reactive center and a ligand based on Kemp's triacid that directs a –COOH group to anchor the carboxylic acid group of the substrate and thus modify the usual selectivity for oxidation. Control experiments supported the role of hydrogen bonding in orienting the substrate to achieve high selectivity.

The Q_B quinone-binding site of photosystem II is an important target for herbicides. Two major classes of herbicides are based on s-triazine and phenylurea moieties. A small library of triazine and phenylurea compounds has been synthesized which have tail-like substituents in order to test the effects of charge, hydrophobicity and size of the tail on binding properties. It is found that a tail can be attached to one of the alkylamino groups of triazine-type herbicides or to the para position of phenylurea-type herbicides without loss of binding, provided that the tail is hydrophobic. This indicates that the herbicides must be oriented in the Q_B site such that these positions point toward the natural isoprenyl tail-binding pocket that extends out of the Q_B site. In turn, the requirement that the tail must extend out of the Q_B site constrains the size of the other herbicide substituents in the pocket. This is in agreement with the presumed orientation and fit of ligands in the Q_B site. When longer hydrophobic tails are used, the binding penalty that occurs upon adding a charged substituent at the distal end is reduced. This allows the use of a series of tail substituents possessing a distal charge as an approximate molecular ruler to measure the distance from the Q_B site to the aqueous phase. Even a 10-carbon alkyl chain still shows a 4-fold effect from the presence or absence of a distal charge. Such a chain does not appear to be long enough to extend from the bulk aqueous phase to the Q_B site because binding is completely lost when a large hydrophilic domain (PEG₄₀₀₀) is attached to the distal end. Longer tails are effective only if they are sufficiently hydrophobic. An effort was made to use tailed herbicides for affinity binding of photosystem II. It was found that hydrophobic linkers promote nonspecific binding, but careful choice of solvent conditions, such as the use of excess nonionic detergent well above its critical micelle concentration, might obviate this problem during affinity-binding applications. Copyright © 2001 John Wiley & Sons, Ltd.