Artificial Enzymes with Thiazolium and Imidazolium Coenzyme Mimics

Abstract

Hydrophobic thiazolium and imidazolium coenzyme mimics in the presence of modifiedpolyethylenimine enzyme mimics catalyze the benzoin condensation 2300-3300 times faster than the coenzyme mimics alone. Polycationic enzyme mimics provide not only a hydrophobic binding domain for coenzyme and substrate, but also electrostatic stabilization of anionic species that arise along the reaction pathway of the benzoin condensation.Thiamine diphosphate (Figure 1, 1) is a coenzyme for important enzymatic reactions, including pyruvate decarboxylase that converts pyruvate to acetaldehyde and transketolases that play a critical role in glucose degradation and in the Calvin cycle in photosynthesis. 1,2 It also condenses pyruvate to acetolactate in the biosynthetic path to valine. 3 In all these reactions, its role is to form a stable equivalent of an acyl anion. In 1957 we showed that the thiazolium ion -the most distinctive part of thiamine -can readily form an anion at C-2, species 2. 4 In a full paper in 1958 we pointed out the special stability 2 receives from resonance with a carbene form 3. 5 We showed that imidazolium ions 4 can also form such a resonance pair, 5 and 6.Addition of the 2/3 hybrid to pyruvic acid forms an adduct that can decarboxylate, ultimately releasing acetaldehyde and regenerating the 2/3 hybrid. The other biochemical roles played by thiamine diphosphate can be similarly understood. In 1973, Stetter expanded the synthetic usefulness of the thiazolium anions 2/3. 6 As numerous other advances have arisen, 7 it has become fashionable to refer to species such as 2/3 and 5/6 as "nucleophilic carbenes", although of course it is the zwitterion resonance form that makes it clear why these species are nucleophilic. rb33@columbia.edu. The role that thiazolium ions play in biochemistry is similar to the role that cyanide ion plays in the benzoin condensation, and indeed thiazolium ions can catalyze the benzoin condensation (Scheme 1), as can imidazolium ions. Kinetic studies supported this scheme. 8-10 The benzoin condensation was crucial in our earliest studies of thiazolium catalysis, and in many subsequent studies by us and others. NIH Public AccessPreviously, we mimicked the binding role played by the enzyme by attaching thiazolium ions to cyclodextrins. With β-cyclodextrin we saw evidence that the intermediate 7 was readily formed in water when benzaldehyde bound into the cyclodextrin cavity, but there was no room for the second benzaldehyde in the overall benzoin-forming mechanism. 11 The larger cavity of γ-cyclodextrin, linked to a thiazolium ion, permitted the second benzaldehyde to bind and couple with 7. 12 This catalyzed the benzoin condensation with a rate increase of ~7-fold contributed by the cyclodextrin. Pandit attached a thiazolium salt to polystyrene, and saw that this somewhat slowed the benzoin condensation. 13In the present work, we more closely mimic the enzyme/coenzyme system with modified polyethylenimines (PEIs) as enzyme surrogates, and hydrophobic thiamine analogs as correspond...

Cobalt-Catalyzed Alkenylation of Thiazoles with Alkynes via C-H Bond Functionalization

“…Grignard reagents except MeMgCl were prepared from the corresponding halides and Mg turnings and titrated before use. The following thiazole derivatives were prepared according to the literature procedure, and their spectral data showed good agreement with the reported data: 5-[2-(benzyloxy)ethyl]-4-methylthiazole, 12 5-phenylthiazole, 11c 5-(4-tolyl)thiazole, 11c 5-(4-methoxyphenyl)thiazole, 11c and 4-(4-methylthiazol-5-yl)benzonitrile. 11a …”

Investigation of the interaction of gold(III)–alkyldiamine complexes with l-histidine and imidazole ligands by 1H and 13C NMR, and UV spectrophotometry

“…It was reported by Marcon et al that Au(III) complexes binding to bovine serum albumin was through surface histidine or cysteine residues, and that imidazole side ring is the expected binding site of histidine along with thiols and thioether residues. The imidazole moiety that is present in vitamins, coenzymes and antibiotics, is reported as an important ligation site to Cu(II), Ni(II), Zn(II) and Pt(II) metal ions [22][23][24][25].…”

Transketolase reaction under credible prebiotic conditions

“…When a simple thiazolium salt 12 (Scheme 1), or thiamine, or TPP was used as the catalyst for the reaction of Dfructose with D-glyceraldehyde in phosphate buffer of pH 8, the pH we had used for previous thiazolium-catalyzed reactions (3,4), the reaction was very slow and less efficient than that of cyanide ion-catalyzed reaction (Scheme 1).…”