Using deep eutectic solvents to improve the biocatalytic reduction of 2-hydroxyacetophenone to (R)-1-phenyl-1,2-ethanediol by Kurthia gibsonii SC0312

Highlights

• ChC/Bd can improve the catalytic rate of K. gibsonii SC0312 for asymmetric reduction of (R)-HAP.

• DESs are able to impact the growth, membrane permeability and membrane fatty acids of the strain.

• The effects of DESs on the structure of DNA were studied by circular dichroism spectra.

• A bioreaction system containing ChCl/Bd for the fabrication of (R)-PED was constructed.

Abstract

The effects of five deep eutectic solvents (DESs) on the production of (R)-1-phenyl-1,2-ethanediol from 2-hydroxyacetophenone catalyzed by Kurthia gibsonii SC0312 were investigated in this study. Of these, choline chloride/1,4-butanediol (ChCl/Bd) showed excellent biocompatibility and suitably increased the cell membrane permeability while having a little impact on the structure of DNA. Indeed, ChCl/Bd at the concentration of 2 % increased the catalytic rate of the cells by 22 %. The other DESs did not stimulate the catalytic capacity of the cells, despite some increases in the cell membrane permeability. Additionally, the conformation of DNA was visibly changed when adding the other examined DESs except for choline chloride/triethylene glycol. The DESs modified the fatty acid composition of cellular membrane, decreased the relative amount of iso-C14:0 and increased the relative amount of normal C15:0. Meanwhile, the DESs were able to improve the relative ratio of normal fatty acids to branched fatty acids. Finally, a highly efficient reduction of 80 mM 2-hydroxyacetophenone by K. gibsonii SC0312 in the ChCl/Bd-containing system was established, affording (R)-1-phenyl-1,2-ethanediol in 80 % yield and optical purity >99 % at 30 mg/mL wet cells. This work offers a promising approach for the preparation of (R)-1-phenyl-1,2-ethanediol from 2-hydroxyacetophenone using K. gibsonii SC0312.

Introduction

(R)-1-phenyl-1,2-ethanediol (PED) serves as a pivotal synthon for obtaining enantiopure medicines such as β-adrenergic blocking agents [1], which are suitable for treating cardiovascular disease and sympathetic nervous system disorder [[2], [3], [4]]. Up to date, some researchers have directed considerable attention towards biocatalytic approach for preparing enantiomeric PED owing to mild reaction conditions, high selectivity and environmental friendliness [5,6]. Enantiomerically pure (R)-PED is typically fabricated by asymmetric hydrolysis of epoxides, asymmetric resolution of racemic PED, or asymmetric reduction of prochiral ketones. Compared with enzymatic biocatalysis, whole cell biocatalytic reactions have characterized as extraordinarily superior features, including easy accessibility, strong resistance and cofactor regeneration [7,8]. Therefore, utilization of whole cell biocatalytic reaction for chiral PED production becomes potential and meaningful.

Deep eutectic solvents (DESs) generally encompass two or three structural components, which are usually sustainable, biodegradable or inexpensive, and form a eutectic mixture characterized by low toxicity, negligible volatility and biodegradation compared with conversional solvents, organic solvents and ionic liquids [[9], [10], [11], [12]]. Since the beginning of the 21th century, DESs [12,13] have been applied as novel green solvents in the bioconversion either as the main reaction medium [14,15] or as a co-solvent [[16], [17], [18]]. Some studies have introduced DESs as co-solvents to promote reaction efficiency [19], as exemplified by the use of choline chloride/urea (ChCl/U) and choline chloride/glycerol (ChCl/Gly) [17] to increase the initial reaction rate or the biocatalytic stability [20], or even to change the stereoselectivity of biocatalyst [21]. A moderate amount of DESs can stimulate biocatalytic reaction efficiency chiefly through increasing the solubility of a substrate [22], or changing enzymatic structure [23]. In comparison to enzyme biocatalysts, the cell membrane of whole cell biocatalysts protects intracellular enzyme from potential harm and hinders the contact between intracellular enzyme and extracellular substance. The impact of none-aqueous solvents (organic solvents, ionic liquids, and DESs, etc.) on whole cell biocatalysts is therefore more complex than on the enzyme biocatalysts. Prior researches have reported the effect of ionic liquids or DESs on the cellular membrane and confirmed that ionic liquids could pass through cellular membrane and interact with intercellular enzymes [[24], [25], [26]]. Some studies also found that DESs as a co-solvent would benefit the expansion of cellular membrane to enhance catalytic efficiency [[27], [28], [29]]. To our knowledge, however, little information is available regarding the change of fatty acid components in microbial cell membrane within DESs-containing systems. DNA is an important carrier of genetic information and the maintenance of its structure is of great significance to life activities. Calf thymus DNA (ctDNA) has often employed in studying the interactions with molecules, such as farrerol [30] and isoxazolcurcumin [31]. There were reports indicating the structure of DNA would be changed by DESs [32,33]. However, few studies have addressed the relationship between the DNA structure and the catalytic properties of whole cells. Moreover, the DESs for different biocatalytic reactions may be diverse and little information concerning the application of DESs in the reduction of 2-hydroxyacetophenone (HAP) to (R)-PED has hitherto been published.

We previously isolated a strain of Kurthia gibsonii SC0312 (K. gibsonii SC0312) from soil, which exhibited high enantioselectivity in the synthesis of chiral PED [34]. In this study, our aims were to investigate the impact of DESs on whole-cell catalytic properties on the reduction of HAP to (R)-PED by K. gibsonii SC0312, and establish an approach for preparing chiral PED based on the system containing a DES (Scheme 1). Firstly, the effects of DESs on the catalytic activity of the cells, metabolic activity of the cells, cell membrane permeability, fatty acid compositions of cellular membrane and DNA were investigated. Subsequently, we selected a DES as the co-solvent and tested the variation in catalytic properties with various reaction factors for asymmetric reduction of HAP by K. gibsonii SC0312.