Rational Biocatalyst Design for a Cyanide‐Free Synthesis of Long‐Chain Fatty Nitriles from Their Aldoximes

Abstract

Dedicated to Dr. Yasuhisa Asano on the occasion of his 70th birthdayRecently, the capability of the aldoxime dehydratase from Bacillus sp. (OxdB) for the transformation of fatty aldoximes into fatty nitriles with impressive substrate loadings is reported. However, the substrate scope of this biocatalyst turned out to be limited in terms of the chain length with decanal oxime being the substrate with the longest well tolerated n-alkyl chain. Besides the increased bulkiness of the long-chain aldoximes, their strongly decreased water solubility represents a further hurdle for an efficient biotransformation. Addressing this challenge of an expanded substrate spectrum comprising long-chain fatty aldoximes, this work investigates the substrate solubility and enzyme kinetics in combination with molecular modeling in order to find an enzyme mutant being suitable for C12-to C16-aldoximes. Both, fatty aldoxime solubility in water and the active site of the wild-type enzyme OxdB are identified as critical issues for an efficient biotransformation of these substrates. The activity issue is addressed by a rational design of a mutant using a homology modeling as well as a molecular modeling software suitable for enzymes. With the resulting double mutant OxdB-F289A/L293A, this report can achieve successful biotransformations with the C12-to C16-aldoximes at substrate concentrations of 250 × 10 −3 to 1000 × 10 −3 m. For example, an excellent conversion of >99% is obtained with tetradecanal oxime. Practical applications: Fatty nitriles with a prolonged chain length of C12 or more are of high interest in industry due to their use for the production of fatty amines on large technical scale. As an alternative route, fatty nitriles can be generated from their aldoximes by means of an aldoxime dehydratase (Oxd) as biocatalyst. The conversion of long-chain fatty aldoximes, however, remained a challenge up to now. This work describes the optimization of the aldoxime dehydratase OxdB from Bacillus sp. for the dehydration of nonsoluble bulky fatty aldoximes. The created variant can convert long chain fatty aldoximes toward the corresponding nitrile as demonstrated for C12-to C16-nitriles. In addition, high conversion (of up to >99%) is achieved when operating at high substrate concentrations of up to 1000 × 10 −3 m, thus making this approach interesting for industrial applications.

Kinetic model of asymmetric dehydration of aldoxime catalyzed by immobilized OxdPsp in an organic solvent

“…31 C12- to C16-nitriles were prepared via the dehydration of the corresponding aldoximes catalyzed by OxdB with high conversion and substrate concentrations, leading to good industrial application prospects. 32 In a biphasic reaction system, nitriles and corresponding amides were prepared starting from aldoxime via aldoxime dehydration and nitrile hydration catalyzed sequentially by OxdF1 and nitrile hydratase with an isolation yield of approximately 62%. 33 The reaction starts with the oxidation of alcohols to aldehydes by sodium hypochlorite, and their subsequent condensation with hydroxylamine toward aldoximes.…”