Biocatalysis for the Synthesis of Active Pharmaceutical Ingredients in Deep Eutectic Solvents: State-of-the-Art and Prospects

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Zhang, Maria and Kara outlined the recent advances in the biocatalysis for the synthesis of active pharmaceutical ingedients in deep eutectic solvents. This is an emerging research field. In this review, classical examples are discussed. The limitations and the prospects are also discussed. The manuscript is well organized and written. From the review, the referee thinks related scientific workers can easily get many useful information and thus supports its publication after minor revision. 1) The application of deep eutectic solvents in non-biocatalytic synthesis should be commented in the introduction, related references should be cited. 2) Reviews on the application of deep eutectic solvents reported should also be cited in the introduction.

Author Response

Auth.1.1. Thanks for the comment. The relevant text has been added to the revised manuscript as “Specifically, chemical catalysis involving DESs has been developed primarily for organometallic chemistry and metal-catalyzed reactions.” in lines 137–139. The relevant references were cited as R52–R57.

Auth.1.2. Thanks for the remark. The relevant references have now been cited as R39, R40, R43, R50, R51 in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is a comprehensive overview on the biocatalytic synthesis of active pharmaceutical ingredients (APIs) using deep eutectic solvents (DES) as reaction media, to overcome some drawbacks of the already employed reaction systems. The constantly increasing interest toward DES and their applications in biocatalysis fully justify the idea of such an updated review, and I am sure that it will be very useful to scientists working in this field. The manuscript is well written and well organized, focusing on oxidoreductases and hydrolases, but also mentioning enzymes from other classes with potential involvement in the development of biotransformations in DESs. The main advantages and actual shortcomings are accurately discussed, and I also appreciate how the tables provide the essential information concerning the referenced works. Therefore, I recommend this excellent manuscript for publication, with only a few questions and recommendations.

1. Maybe a sugar-based polyol can be added as HBD in Figure 1, these compounds being noteworthy DES components, also mentioned in the text (line 115).

2. Also in the Introduction part, concerning the utilization of non-aqueous solvents as reaction media (lines 91-103) I recommend extending a little the discussion on “green solvents” (mentioned here as “biobased solvents”), as their development could represent a solution for several problems associated with solvents of petrochemical origin, not only toxicity.

3. Lines 162-164, adding water as cosolvent. The authors surely know that in esterifications catalyzed by lipases in DESs, the addition of relatively high amounts of water does not significantly affect the reaction equilibrium due to the advanced hydrogen bonding mentioned in the manuscript. A short discussion could be added.

4. Immobilization is considered essential for the development of biocatalytic processes, including APIs. In the manuscript, I found only in lines  510-511 and 524-525 the advantages of combining the utilization of immobilized enzymes or cells and DESs as reaction media. Maybe it could be discussed somewhere in a more general manner, also specifying (if there are available data) the immobilization method most suitable for combined utilization with DESs (in my opinion could be covalent bonding).

5. Are there biocatalytic processes in DES s already scaled-up to industrial level? If not, it can be mentioned as an objective in the Future perspectives section, together with the possible development of continuous processes (now impeded by the high viscosity and downstream processing issues, but maybe solutions will be found).

Author Response

Auth.2.1. Thanks for the suggestion. The representative sugar-based polyols, glucose and fructose, have now been added in the Figure 1 of the revised manuscript.

Auth.2.2. The discussion on biobased solvents has now been extended in the revised manuscript as in lines 104–108 “Especially, biomass-based solvents derived from renewable natural resources (e.g., plants and animals) become attractive for the large-scale production of a wide range of green solvents at acceptable prices. Compared to their counterparts, biomass-based solvents are safer and more effective from manufacture to application, and thus more sustainable.”

Auth.2.3. Thanks for the comment. The discussion has now been added to the revised manuscript in lines 185–188 “In addition, when water and DES components are part of the hydrogen bonding network, their reactivity is significantly reduced (Chem. Commun. 2008, 1235–1237). This avoids possible side reactions and unfavorable thermodynamic equilibria.”.

Auth.2.4. The discussion on enzyme immobilization has now been covered in the revised manuscript as shown in lines 202–207 “To adapt enzymes to harsh DES conditions, a variety of immobilization techniques (e.g., physical adsorption/deposition, entrapment, covalent attachment, and cross-linked enzyme aggregates) can be employed to generate robust immobilized biocatalysts that can be easily processed and reused in DES systems. On this basis, a number of biotranformations have been developed using immobilized whole cells or enzymes, which were outlined in detail in the following sections”.

Auth.2.5. Thanks for the question. To the best of our knowledge, the biocatalytic processes in DESs have not been scaled up to the industrial level yet, which has been envisaged in the last section of Summary and Future Perspectives in the revised manuscript.

Author Response File: Author Response.pdf