Quantum mechanical study of Burkholderia cepacia lipase enantioselectivity

Chemoenzymatic synthesis of enantiomerically enriched diprophylline and xanthinol nicotinate

A concise chemoenzymatic route toward enantiomerically enriched active pharmaceutical ingredients (API) - diprophylline and xanthinol nicotinate - is reported for the first time. The decisive step is an enantioselective lipase-mediated methanolysis of racemic chlorohydrin-synthon acetate, namely 1-chloro-3-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)propan-2-yl acetate, performed under kinetically-controlled conditions on a preparative 500 mg-scale. The best results in terms of reaction enantioselectivity (E = 14) were obtained for the enantiomers resolution performed with lipase type B from Candida antarctica immobilized on acrylic resin (CAL-B, Novozym 435) suspended in homophasic acetonitrile-methanol mixture. The elaborated biocatalytic system furnished the key chlorohydrin intermediate (in 71% ee and 38% yield), which was then smoothly converted into enantioenriched active agents: (R)-(-)-diprophylline (57% ee) and (S)-(+)-xanthinol nicotinate (65% ee). To support the assignment of absolute configurations of EKR-products as well as to confirm the stereochemical outcome of the remaining reaction steps, docking studies toward the prediction of enantiomers binding selectivity in CAL-B active site as well as the respective chemical correlations with enantiomerically enriched analytical standards obtained from commercially available (R)-(-)-epichlorohydrin, were applied. In addition, single-crystal X-ray diffraction (XRD) analyses were performed for the synthesized optically active APIs furnishing by this manner a first crystal structures of nicotinic acid salt of xanthinol.

Investigating the influence of the interface in thiol-functionalized silver-gold nanoshells over lipase activity

We employed thiol-funcionalized AgAu nanoshells (AgAu NSs) as supports for the covalent attachment of lipases (BCL, Burkholderia cepacia lipase; PPL, pancreatic porcine lipase). Specifically, we were interested in investigating the effect of the nature/size of the spacer in AgAu NSs-functionalized organic thiols over the covalent attachment of lipases. The catalytic performance of AgAu-lipase systems was measured in the kinetic resolution of (R,S)-1-(phenyl)ethanol via a transesterification reaction. In comparison to free BCL, the lipase attached to AgAu NSs using a small spacer such as cysteamine or mercaptoacetic acid, with the largest spacer mercaptoundecanoic acid, had the fastest conversion rate. The recycling potential for BCL was investigated. After three reaction cycles, the enzyme activity was kept at around 90% of the initial value. The results described herein show that the size of the spacer plays an important role in optimizing lipase activities in metallic nanoshells as solid supports.

Influence of delta-functional groups on the enantiorecognition of secondary alcohols by Candida antarctica lipase B

The selectivity of acetylation of delta-functionalized secondary alcohols catalyzed by Candida antarctica lipase B has been examined by molecular dynamics. The results from the simulation show that a delta-alcohol functionality forms a hydrogen bond with the carbonyl group of Thr 40. This interaction stabilizes the tetrahedral intermediate and thus leads to selective acetylation of the R enantiomer. A stabilizing interaction of the delta-(R)-acetoxy group with the peptide NH of alanine 282 was also observed. No stabilizing interaction could be found for the delta-keto functionality, and it is proposed that this is the reason for the experimentally observed decrease in enantioselectivity. From these results, it was hypothesized that the enantioselectivity could be restored by mutating the alanine in position 281 for serine. The mutation was made experimentally, and the results show that the E value increased from 9 to 120.

Combined X-ray diffraction and QM/MM study of the Burkholderia cepacia lipase-catalyzed secondary alcohol esterification

To understand the origin of high enantioselectivity of Burkholderia cepacia lipase (BCL) toward secondary alcohol, (R,S)-1-phenoxy-2-hydroxybutane (1), and its ester (E1), we determined the crystal structure of BCL complexed with phosphonate analogue of S-E1 and accomplished a series of MM, MC, and QM/MM studies. We have found that the inhibitor in the S configuration binds into the BCL active site in the same manner as the R isomer, with an important difference: while in case of the R-inhibitor the H-bond between its alcohol oxygen and catalytic His286 can be formed, in the case of the S-inhibitor this is not possible. Molecular modeling for both E1 enantiomers revealed orientations in which all hydrogen bonds characteristic of productive binding are formed. To check the possibility of chemical transformation, four different orientations of the substrate (two for each enantiomer) were chosen, and a series of ab initio QM/MM calculations were accomplished. Starting from the covalent complex, we modeled the ester (E1) hydrolysis and the alcohol (1) esterification. The calculations revealed that ester release is possible starting with all four covalent complexes. Alcohol release from the BCL-E1 complex in which the S-substrate is bound in the same manner as the S-inhibitor in the crystal structure however is not possible. These results show that the crystallographically determined binding modes should be taken with caution when modeling chemical reactions.