Enzyme active site mimics based on TriAzaCyclophane (TAC)-scaffolded peptides and amino acid residues

Abstract

This thesis describes the scope and limitations of the application of TriAzaCyclophane (TAC)-scaffolded peptides or amino acid residues as enzyme active site mimics, as ligands in asymmetric catalysis and as hydrolysis catalysts attached to vancomycin. For the mimicry of functional group enzymes, of which serine hydrolases are well-known and -understood, both combinatorial approaches and the construction of complicated cage-like molecules containing the relevant components of the active site were followed. Results from these studies indicated that pre-organization of the arms that are attached to the TAC-scaffold is most likely required for the catalytic action that is caused by cooperation of the functional groups of the attached amino acid residues. Two attempts to achieve this goal are also described involving the construction of highly functionalized cryptand molecules. In one approach, the final synthetic step comprised of an amide-bond forming reaction, in the other the final double cyclization reaction was done by means of a copper-catalyzed alkyne-azide cycloaddition reaction. Mimicry of metalloenzyme active sites using similar approaches resulted in close structural mimicry of such active sites by three mimics based on a TAC-scaffold containing histidinyl imidazole and/or aspartate residues. Several spectroscopic and spectrometric techniques were used to characterize the coordination complexes of the ligands with copper(II). After this, these three mimics were used in two biomimetic reactions: (1) the reaction of Cu(I) with molecular oxygen and (2) the oxidation of olefins by Fe(II)-H2O2 chemistry. Although the first reaction revealed interesting and true biomimetic behavior of the mimics, their application in the last reaction gave results similar to the reaction without the ligands. Therefore, whereas the mimicry of serine hydrolases required complicated synthetic protocols in order to realize the needed pre-organization of catalytically active functional groups, mimicry of three important active sites of metalloenzymes was shown to be possible. In addition to the mimicry of enzyme active sites, these TAC-based mimics were used as ligands in asymmetric copper-catalyzed Diels-Alder and Michael addition reactions. Surprisingly, selectivity up to 50% enantiomeric excess for both reactions was observed. Subsequent tuning of the ligands showed that this enantioselectivity could be modulated by N- and C-terminal extensions. Also, the tris-histidine triad containing construct was conjugated to vancomycin in a mono- and tetravalent fashion. This resulted in a new type of antibiotics that could potentially decide the ongoing battle against resistance in favor of the invaded host.