Effects of tail-like substituents on the binding of competitive inhibitors to the Q(B) site of photosystem II

The QB 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 QB 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 QB 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 QB 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 QB 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 QB site because binding is completely lost when a large hydrophilic domain (PEG(4000)) 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.