pH-Dependent Crystallization of 2-, 4-, 5-, and 6-Hydroxynicotinic Acids in Aqueous Media
Round 1
Reviewer 1 Report
Summary
The present article deals with the crystallization of hydroxynicotinic acid derivatives (the position of the hydroxyl moiety being varied around the pyridine ring) from solution at different pH. The crystallization outcome is characterized by X-ray powder diffraction and either optical or electronic (SEM) microscopy. It appears that a plethora of solid forms is obtained depending on the pH of the aqueous solution (by adding HCl or NaOH). Some solid forms could be identified but many others were demonstrating the large variety of solid forms that can be obtained. The authors also used the species distribution diagrams calculated from known protonation constants to interpret the different X-ray diffraction patterns measured.
My opinion
The present article is a good example that pH has to be considered in solution crystallization experiments especially when considering systems with acidic/basic properties (as HNAs derivatives dealt with in the present article). The species distribution diagrams as a function of pH introduced by the authors already show that several molecular structures can be obtained as a function of pH, each of them, having the ability to arrange differently at the solid state thus yielding polymorphs or hydrates. However, I think that the interpretation of the X-ray diffraction patterns is not always rigorous and biased by the knowledge the authors have on the species distribution diagrams, making them over-interpreting their data. I would recommend this article to be published after the authors have answered to my comments related to those over-interpretations.
General questions and comments.
1. My main comment is that the interpretation of the X-ray diffraction patterns does not follow the presence of the species mentioned by the authors. Here is one striking example but the authors should check the entire manuscript to avoid those errors.
For 2HNA, the authors say that for the solid obtained from a solution at pH = 10, might be consistent with a mixture of LH- and L2- species while the X-ray diffraction pattern at pH = 12 must correspond to L2- alone. However, if at pH = 12, we have L2- species alone we should find some peaks which are present in the X-ray diffraction pattern of the solid crystallized at pH = 10 as it is, according to the authors, a mixture between LH- and L2- species. I understand that many forms seem not to be known but the authors should be more rigorous in the interpretation of their X-ray diffraction patterns. If there is a mixture of two species which each crystallizes in their own crystal structure, the X-ray diffraction patterns should show the diffraction peaks of the two individual phases. To the contrary, if the two species crystallize in the same crystal structure (solid solution), we then should see the X-ray diffraction patterns of a unique solid phase. The authors are asked to carefully revise the interpretation of the displayed X-ray diffraction patterns knowing those two criteria.
2 The scale bars are not visible in the microscopy images given in the manuscript. The authors should provide them.
3. The authors use the term “diffractions” to designate the diffraction peaks but this term is not commonly used. The authors should use “diffraction peaks” or “reflections” which are more of use. 5. 4. “amorph” in the abstract should be replaced by “amorphous”.
5. In Figure 4 that displays the image of the crystals obtained from the crystallization experiments, there might be a mistake. The images at the left column are said to be the starting materials but for 2HNA and 6HNA, it is specified that the crystals have been grown from a solution at pH = 4. Is it correct? The authors should have a look at this possible misunderstanding.
Author Response
“Summary
The present article deals with the crystallization of hydroxynicotinic acid derivatives (the position of the hydroxyl moiety being varied around the pyridine ring) from solution at different pH. The crystallization outcome is characterized by X-ray powder diffraction and either optical or electronic (SEM) microscopy. It appears that a plethora of solid forms is obtained depending on the pH of the aqueous solution (by adding HCl or NaOH). Some solid forms could be identified but many others were demonstrating the large variety of solid forms that can be obtained. The authors also used the species distribution diagrams calculated from known protonation constants to interpret the different X-ray diffraction patterns measured.
My opinion
The present article is a good example that pH has to be considered in solution crystallization experiments especially when considering systems with acidic/basic properties (as HNAs derivatives dealt with in the present article). The species distribution diagrams as a function of pH introduced by the authors already show that several molecular structures can be obtained as a function of pH, each of them, having the ability to arrange differently at the solid state thus yielding polymorphs or hydrates. However, I think that the interpretation of the X-ray diffraction patterns is not always rigorous and biased by the knowledge the authors have on the species distribution diagrams, making them over-interpreting their data. I would recommend this article to be published after the authors have answered to my comments related to those over-interpretations.
General questions and comments.
- My main comment is that the interpretation of the X-ray diffraction patterns does not follow the presence of the species mentioned by the authors. Here is one striking example but the authors should check the entire manuscript to avoid those errors.
For 2HNA, the authors say that for the solid obtained from a solution at pH = 10, might be consistent with a mixture of LH- and L2- species while the X-ray diffraction pattern at pH = 12 must correspond to L2- alone. However, if at pH = 12, we have L2- species alone we should find some peaks which are present in the X-ray diffraction pattern of the solid crystallized at pH = 10 as it is, according to the authors, a mixture between LH- and L2- species. I understand that many forms seem not to be known but the authors should be more rigorous in the interpretation of their X-ray diffraction patterns. If there is a mixture of two species which each crystallizes in their own crystal structure, the X-ray diffraction patterns should show the diffraction peaks of the two individual phases. To the contrary, if the two species crystallize in the same crystal structure (solid solution), we then should see the X-ray diffraction patterns of a unique solid phase. The authors are asked to carefully revise the interpretation of the displayed X-ray diffraction patterns knowing those two criteria.”
R: Dear Reviewer, I revised the interpretation according to your kind feedback.
“2. The scale bars are not visible in the microscopy images given in the manuscript. The authors should provide them.”
R: I have inserted new sharper figures in the manuscript with the scale bars in the microscopy images and reference to the scale values in the figure caption.
“3. The authors use the term “diffractions” to designate the diffraction peaks but this term is not commonly used. The authors should use “diffraction peaks” or “reflections” which are more of use.”
R: I made changes in the manuscript according to your comment.
“4. “amorph” in the abstract should be replaced by “amorphous”.”
R: Thank you for this correction.
“5. In Figure 4 that displays the image of the crystals obtained from the crystallization experiments, there might be a mistake. The images at the left column are said to be the starting materials but for 2HNA and 6HNA, it is specified that the crystals have been grown from a solution at pH = 4. Is it correct? The authors should have a look at this possible misunderstanding.”
R: Dear reviewer, the left column of now Figure 6 has the SEM images of the hydroxynicotinic acids starting materials which were purified by (i) recrystallization (2 and 6 HNAs at pH ~ 4, which is the pH observed for the saturated solutions) and by (ii) sublimation (4 and 5 HNAs). Then, at the right column there are the SEM images of the selected samples from the pH dependent crystallization study. I altered the caption in order to make it clearer.
Reviewer 2 Report
Manuscript ID: crystals-2468705
Type of manuscript: Article
Title: pH Dependent Crystallization of the 2-, 4-, 5-, and 6-Hydroxynicotinic Acids in Aqueous Media
Authors: Aidan V. Johnson, M. Fátima M. Piedade, Catarina V. Esteves *
Comments:
The manuscript reports an experimental study of the crystallization of hydroxynicotinic acid isomers from water at different pH values. The topic of the article is interesting and relevant.
The only single crystal structure obtained was a complex of 4- hydroxynicotinic acid and HCl. For all other samples, only PXRD measures were possible. These PXRD spectra were interpreted based on the most general assumptions about the protonation/deprotonation states of the acid–base groups of each of the considered isomers at different pH values. A change in the structure of the PXRD spectrum with a change in pH was considered as evidence of the transition of the molecule to the next protonation/deprotonation state. The applicability of this approach requires justification.
For each isomer a PXRD spectrum of pure acid should be reported and discussed.
What are “recryst” samples? At what pH they were obtained?
What are Forms VIII, IV, I, and H-II? These Forms must be discussed in detail.
For each isomer, a final table should be drawn up, which clearly indicates which pH range of the solution allows each of the expected forms from LH3- to LH- to be obtained.
The nomenclature of the forms from LH3- to LH- in the text and on Figures must be the same.
“Crystals of different sizes and shapes were obtained, and crystalline aggregates observed, and amorphous solids were also formed. Such variety of solid forms demonstrates well the relevance of pH control during crystallization.” This conclusion is ridiculous.
Author Response
“Comments:
The manuscript reports an experimental study of the crystallization of hydroxynicotinic acid isomers from water at different pH values. The topic of the article is interesting and relevant.
The only single crystal structure obtained was a complex of 4- hydroxynicotinic acid and HCl. For all other samples, only PXRD measures were possible. These PXRD spectra were interpreted based on the most general assumptions about the protonation/deprotonation states of the acid–base groups of each of the considered isomers at different pH values. A change in the structure of the PXRD spectrum with a change in pH was considered as evidence of the transition of the molecule to the next protonation/deprotonation state. The applicability of this approach requires justification.
For each isomer a PXRD spectrum of pure acid should be reported and discussed."
R: Dear Reviewer, the PXRD spectra of the pure acids are shown in Figures 2 to 5. I altered the figures caption to clarify which are the PXRD spectra of the pure starting materials. Such spectra were also previously described and reported in Esteves, C.V. Hydroxynicotinic acids crystallisation and solubility systematic studies. New J. Chem. 2022, 46, 21124–21135.
"What are “recryst” samples? At what pH they were obtained?"
R: The figure label reads “Recryst.” as 2HNA and 6HNA were purified by recrystallization. At pH ~ 4, which was the pH observed for the saturated aqueous solutions.
"What are Forms VIII, IV, I, and H-II? These Forms must be discussed in detail."
R: Form VIII from 2HNA is the published form closer to the one obtained in this work. Form VIII was described in Santos, R. C.; Figueira, R. M.; Piedade, M. F. M.; Diogo, H. P.; Minas da Piedade, M. E. Energetics and structure of hydroxynicotinic acids. Crystal structures of 2-, 4-, 6-hydroxynicotinic and 5-chloro-6-hydroxynicotinic acids. J. Phys. Chem. B 2009, 113, 14291–14309. The number VIII was given in chronological order, as other authors obtained 2HNA forms previously (please see Table S2). The same line of thinking was used for the other isomers, and in the figures the forms closer to the ones obtained in this study were put near them.
"For each isomer, a final table should be drawn up, which clearly indicates which pH range of the solution allows each of the expected forms from LH3- to LH- to be obtained."
R: I included such table in the manuscript.
"The nomenclature of the forms from LH3- to LH- in the text and on Figures must be the same."
R: In the speciations the charges were omitted to simplify the reading of the figures.
"“Crystals of different sizes and shapes were obtained, and crystalline aggregates observed, and amorphous solids were also formed. Such variety of solid forms demonstrates well the relevance of pH control during crystallization.” This conclusion is ridiculous."
R: I tried to rewrite this conclusion.
Round 2
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
Dear Reviewer,
I made changes in the manuscript according to your comments very kindly provided directly on the pdf file.
Thank you for your time!
All the best.
Reviewer 2 Report
I have no more comments that would necessitate another review cycle.
Author Response
Dear Reviewer,
Thank you for your time!
Kind regards.
