Development of a chemoenzymatic (-)-menthol synthesis

Abstract

Biocatalysis is an emergent research area for the development of efficient and sustainable synthesis processes. A crucial milestone for the better applicability of biocatalysts thereby consists of the increasing knowledge of the adaptability of enzymes for distinct synthetic needs like the conversion of specific molecular structures with defined selectivity. In addition, it is equally important to demonstrate that such novel catalysts are combinable among themselves and with established non enzymatic catalysts to enable unexplored synthetic routes. Using the example of the chemoenzymatic synthesis of (-)-menthol from citral, this work therefore addresses the development and applicability of such evolved enzyme catalysts for the synthesis of an industrially relevant molecule. In this complementary synthetic route inspired from an existing industrial process, a mixture of citral isomers is reduced to citronellal using an R-selective ene reductase. In a subsequent Prins reaction, the selective cyclization of R-citronellal to (-)-isopulegol is achieved by the application of an engineered squalene hopene cyclase variant. The final reduction to (-)-menthol proceeds by hydrogenation on a palladium catalyst. Especially the first catalytic step enables an immediate synthetic advantage in comparison to the currently performed industrial process. So far, no catalyst is applied converting both isomers of citral R-selectively at the same time. Both isomers have to be separated under high energy expenditure by distillation prior to reduction. No enzymatic catalyst is described displaying this reactivity yet. As, however, the opposite enantioconvergent S-selective citral reduction by ene reductases is known, the development of an enzyme catalyst constituted an attractive solution for this limitation. Hence, a focus of the work laid on the inversion of the S-selectivity of the citral reduction by NCR ene reductase from Zymomonas mobilis by enzyme engineering. The studies started by characterization of the citral reduction by NCR wild type. Next to the determination of the course of the reaction over time, semi empiric quantum mechanics calculations on the oxidative half reaction of this conversion were carried out. The calculations suggest a so far undescribed catalytic role of an arginine at position 224 for a facilitated hydride transfer and a more complex proton shift involving water molecules in the reaction. The subsequently performed engineering comprised the identification of selectivity determining amino acid positions W66, Y177, I231 and F269 in the active site of the enzyme followed by their variation in an iterative combinatorial fashion. In order to enable the analysis of the multitude of generated enzyme variants, a whole cell screening was developed using chiral gas chromatography. Thereby, the triple variant W66A/I231R/F269V was created converting E/Z-citral in the whole system to R-citronellal with an enantiomeric excess of 89 %. It could be determined that a cell induced citral isomerization leads to increased enantioselectivity in comparison to using purified enzyme. Especially for the influence of the selectivity determining positions W66 and I231 an increased understanding of structure function relations was achieved during the course of semi rational enzyme evolution by the separated analysis of single citral isomers and by supportive in silico analyses like docking and molecular dynamics simulations. The subsequent integration of the established variant A419G/Y420C/G600A of the squalene hopene cyclase from Alicyclobacillus acidocaldarius is remarkable catalyzing the Prins cyclization to (-)-isopulegol with an enantiomeric excess of 99 % and a diastereoselectivity of 90 %. In this context, the enzyme’s underlying Brønsted acid chemistry could be evolved towards the in nature unknown Prins reaction reactivity. In this work it could be shown that enzyme catalysts acquired by such chemical inspection can be implemented in application oriented synthetic routes. In combination with the developed selective ene reductase, the bienzymatic cascade to (-)-isopulegol was successfully performed and characterized. For the final reduction to (-)-menthol an established heterogeneous catalyst like palladium on charcoal could be applied under hydrogen atmosphere. This demonstrates nicely that novel biocatalysts can be combined with approved synthetic processes. With the attained insights, highly valuable (-)-menthol was made accessible for the first time by a chemoenzymatic cascade using an isomeric mixture of citral on preparative scale with 7 % isolated yield. This work not only highlights different strategies for the development of novel biocatalysts, but also contributes to their possible synthetic applicability in the synthesis of industrially relevant molecules.