Maxier’s comment on: Macromolecular Crowding Modifies the Impact of Specific Hofmeister Ions on the Coil–Globule Transition of PNIPAM

By Maxier Acosta Santiago

MAS blog image 18-05

While the recently discussed Cremer paper described efforts towards understanding the effect on the LCST of PNIPAM by salts of the Hofmeister’s series, Sakota’s article went somewhat deeper into the study of this phenomenon by taking in consideration molecular crowding. Sakota’s group decided to use a PEG polymer as a crowding agent. Crowding may affect the biding of anions from the Hofmeister’s series and the PNIPAM resulting in a change in the LCST. First, they studied the presence of PEG in a PNIPAM solution showing that the crowding agent reduces the LCST. Kosmotropic anions, that decrease the LCST, but chaotropes increase the LCST. For the Hofemeister series effect on LCST we can go back to Luis Prieto’s blog post and Cremer’s paper which explains this effect better.

When the article begins to look at the presence of PEG at different salt concentrations, they see a close correlation between the LCST and the Hofmeister series. Yet, for the chaotropes ClO4⁻ and SCN⁻, the presence of PEG lead to a larger increase in the LCST. From here they decide to apply different theories to explain the results. Within the examined theories they discuss around thermodynamics of the system. Their explanation evolves as follows, even if the organization via LCST of PNIPAM is not thermodynamically favorable, the overlapping excluded volumes of PEG and the PNIPAM particles increase the translational entropy of water molecules in solution, which makes the formation of the system possible.

Although this paper brings something new to the table to discuss (molecular crowding and LCST), I do have some concerns. When taking into consideration so many different factors like molecular crowding, salt, and the responsive system itself, we should look deeper into the behavior. To limit certain factors, they maintained certain constant concentrations throughout the paper. Yet, in the discussion, I feel they lacked additional experimental results or computational studies (using molecular and/or coarse-grained simulations) to support their theoretical thermodynamics analysis.


3 thoughts on “Maxier’s comment on: Macromolecular Crowding Modifies the Impact of Specific Hofmeister Ions on the Coil–Globule Transition of PNIPAM

  1. Rating (synopsis): 3/5
    Rating (figure): 4/5

    Reading this synopsis, I got kinda lost. The strange thing is that while I read it I kept feeling that everything seems to be well written, and yet, I didn’t understand almost anything until reading it a few times and then going back to the paper. Maybe it’s the way in which somethings are phrased. For example, my guess is that when it says: “When the article begins to look at the presence of PEG at different salt concentrations, they see a close correlation between the LCST and the Hofmeister series”, it means that for most of the salts, the LCST properties remain the same with or without PEG. Otherwise, I could hardly make sense of the next sentences. I’d also suggest dedicating a little more of the synopsis to the explanation of why PNIPAM is more affected by the presence of PEG when in presence of perchlorate and thiocyanate salts. It was very briefly discussed in the synopsis, but is a rather important aspect in the paper.
    The figure is nice, both esthetically and in terms of conveying the message of the paper. I think, however, that its rather difficult to understand the meaning of the text included in figure unless you’ve actually read the paper (not just the synopsis).

  2. Rating (synopsis): 4/5
    Rating (figure): 4/5

    The essence of molecular crowding presented in this article is highlighted in Maxier’s narrative. I would have highlighted the hydrophobicity studies of the solution with and without PEG and how does this affect the solubility of the polymer, which is important in the changes in LCST.

    The image constructed by Maxier is simple and easy to understand. I would change the representation of the kosmotrope interaction with PNIPAM. In the article, it is established that the chaotropic anions interact with the backbone of the polymer, not the other way around, and the representation suggests kosmotropes interact strongly with the polymer, which is not necessarily correct.

  3. Rating (synopsis): (4/5)
    Rating (figure): (4/5)

    In the first paragraph of the synopsis I did not understand very well what the article was about. However, when I read the second paragraph, the idea that Maxier wanted to project was clear to me. Once again the LCST playing an important role but this time controlling the concentrations of salts or caotropic agents. In general it was a bit difficult to understand.

    However, with the image I could understand the purpose of the article. Although I would have used another color for one of the lines in the graph so that the difference in the conditions of those reactions would be highlighted.

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