Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions

Abstract

Pickering emulsion systems have emerged as platforms for the synthesis of organic molecules in biphasic biocatalysis. Herein, the catalytic performance was evaluated for biotransformation using whole cells exemplified for the dehydration of n-octanaloxime to n-octanenitrile catalysed by an aldoxime dehydratase (OxdB) overexpressed in E. coli. This study was carried out in Pickering emulsions stabilised solely with silica particles of different hydrophobicity. We correlate, for the first time, the properties of the emulsions with the conversion of the reaction, thus gaining an insight into the impact of the particle wettability and particle concentration. When comparing two emulsions of different type with similar stability and droplet diameter, the oil-in-water (o/w) system displayed a higher conversion than the water-in-oil (w/o) system, despite the conversion in both cases being higher than that in a "classic" two-phase system. Furthermore, an increase in particle concentration prior to emulsification resulted in an increase of the interfacial area and hence a higher conversion.

Keywords: Pickering emulsions; biphasic catalysis; enzyme catalysis; silica particles.

Scheme 1

Biocatalytic transformation of n‐octanaloxime to n‐octanenitrile carried out in Pickering emulsions (w/o and o/w) stabilised by silica particles of different hydrophobicity. Although the starting material and the final product are slightly soluble in the aqueous phase, this has not been included for simplicity.

Figure 1

a) Appearance of emulsions prepared with silica particles of different hydrophobicity (% SiOH) 1 month after preparation. Each emulsion is obtained upon mixing equal volumes of an aqueous phase containing E. coli cells containing OxdB dispersed in 50 mM K₂HPO₄/KH₂PO₄ buffer (pH 7) and an organic phase containing n‐octanaloxime dissolved in n‐dodecane (10 mM). Silica particles with a SiOH content between 100 % and 65 % were dispersed in the aqueous dispersion while silica particles with a SiOH content between 51 % and 15 % were dispersed in the organic phase. The concentration of the different components in the emulsion are: 0.082 wt. % n‐octanaloxime, 0.025 wt. % E. coli cells containing OxdB and 1.14 wt. % silica particles. Scale bar=1 cm. b) Plot of the average droplet diameter after 1 day and 1 month versus SiOH content for emulsions in (a).

Figure 2

Plot of the fraction of organic and aqueous phase resolved after 1 month versus SiOH content for emulsions in Figure 1.

Figure 3

a) Plot of the conversion of the reaction of n‐octanaloxime into n‐octanenitrile versus time for the emulsions in Figure 1 and control. The control is the two‐phase system comprising of a 0.44 g L⁻¹ E. coli cells (containing OxdB) dispersion in 50 mM K₂HPO₄/KH₂PO₄ buffer (pH 7) and a 10 mM solution of n‐octanaloxime in n‐dodecane. The emulsion type and % SiOH are given. b) Plot of the initial rate of the reaction (filled circles) and conversion of the reaction after 48 h (empty circles) versus SiOH content.

Figure 4

a) Appearance of o/w emulsions prepared at different concentrations of 65 % SiOH silica particles (with respect to emulsion) 1 month after preparation. Each emulsion is made by mixing equal volumes of an aqueous phase containing E. coli cells containing OxdB and silica particles dispersed in 50 mM K₂HPO₄/KH₂PO₄ buffer (pH 7) and an organic phase containing n‐octanaloxime dissolved in n‐dodecane (10 mM). The concentration of the different components in the emulsion are: 0.082 wt. % n‐octanaloxime and 0.025 wt. % E. coli cells containing OxdB. Scale bar=1 cm. b) Plot of the average droplet diameter versus particle concentration in the emulsions in (a) 1 day and 1 month after preparation. Inset: inverse of average droplet diameter as a function of the mass of 65 % SiOH silica particles in the aqueous dispersion.

Figure 5

Fraction of aqueous (empty circles) and organic (filled circles) phase resolved from the o/w emulsions in Figure 4 a prepared with different concentrations of 65 % SiOH silica particles 1 month after preparation.

Figure 6

a) Plot of the conversion of the reaction of n‐octanaloxime into n‐octanenitrile versus time for the o/w emulsions in Figure 4 a prepared with different concentrations of 65 % SiOH silica particles and control. The control is the two‐phase system comprising of a 0.44 g L⁻¹ E. coli cells (containing OxdB) dispersion in 50 mM K₂HPO₄/KH₂PO₄ buffer (pH 7) and a 10 mM solution of n‐octanaloxime in n‐dodecane. b) Plot of the initial rate of the reaction (filled circles) and conversion of the reaction after 48 h (empty circles) versus particle concentration in the emulsion.

Figure 7

Plot of the conversion of the reaction in the organic phase recovered after centrifugation (empty circles) and average droplet diameter (filled circles) versus the total interfacial area in the o/w emulsions prepared with different concentrations of 65 % SiOH silica particles (Figure 4 a). The dotted lines are a guide for the eye.