Miriam’s comment on: Reversible Changes in Solution pH Resulting from Changes in Thermoresponsive Polymer Solubility

By Miriam I. Otero Rivera
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In this article by Bergbreiter’s group, present the effect of a LCST event on the solution pH of a polymer like poly(N isopropylacrylamide) better known as PNIPAM. They present different experiments were they change pendant groups on PNIPAM and all of these resulting polymer derivatives had lower critical solution temperature (LCST) properties. In the experiments they show how after heating the solution to its LCST the polymer experience a dehydration and the polymer phase separates from solution. In several experiments, the presence of a cationic or anionic pendant groups, that have low or high pH values respectively, its stabilization is dependent of its solvation, and after the LCST event, the species becomes neutral since it is more stable in the dehydrated state resulting from the LCST phenomenon. These experiments show that changes in the environment of the polymers results in great changes in solution acidity and basicity. Also they show that these process of hydration and dehydration is reversible by cooling the solution they obtain the polymer back in solution and the pH was revert to its initial value, this process of heating and cooling was reversible for up to 100 cycles.
Similarly, the article studied the effect of the concentration of the polymer, and the mole % loading of the pendant group (carboxylic acid) in changing the bulk solution pH as a function of temperature. First they saw that if the pH of the solution is significantly different from the pKa of the carboxylic acid (4.76) there is no change in pH. They also saw that at lower mole % loading of carboxylic acid the cloud point curve is most narrow and that at lower concentration of the polymer the clouding curves are broader. Also, studies with solutions containing added salts (LiCl or LiBr) show that the solution’s ionic strength does not significantly affect the ΔpH, although the clouding curve’s onset changes. Finally, they performed experiments with kosmotropic salts, which lower the LCST below room temperature due to the Hofmeister effect, as a result the addition of these salts to solutions lead to the same behavior of changing the pH of the solution, and also the process was reversible. This experiment was visualized using the pH-indicator phenolphtalein. In this experiment they also used visible spectroscopy to show how  reaching  the LCST temperature produce an inflection, before a drastic change in absorbance.  This article show how properties in the pendant group of a polymers change pH of solution as a result of changes in solubility because of changes in temperature, showing the thermoresponsiveness of polymers. This article was one of relevance to our laboratory because we work with LCST events on supramolecular G-quadruplexes. This article can be used as a reference to know the resulting effects on solubility and pH of an LCST event on the systems with which we work in our lab.


Minelise’s comment on: Synthesis and Direct Observation of Thermoresponsive DNA Copolymers

By Minelise E. Rivera

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In this paper, Li and Schroeder use single molecule techniques to have a direct observation on DNA-PNIPAM copolymers. First, they synthetized DNA-PNIPAM copolymers using a two-step strategy based on polymerase chain reaction (PCR) for generating linear DNA backbones containing dibenzocyclooctyne-dUTP, then grafted thermoresponsive side branches (PNIPAM) onto DNA backbones using copper-free click chemistry. Subsequent single molecule fluorescence spectroscopy studies unveiled more clearly the molecular heterogeneity association with the stretching and relaxing of the polymer above and below their LCST. Their results showed that intramolecular conformational dynamics of DNA-PNIPAM copolymers are affected by properties of the branches like molecular weight, density, leading to a change in transition temperatures. In other words, the single molecule technique provided a better understanding in a molecular perspective of chemically heterogeneous and stimuli-response polymers.

As I read this paper and looked for information to better understand it, I was amazed by the details with which they worked with throughout their study. I would have thought of working better with a bunch of them instead of just single molecules. It didn’t cross my mind that someone was going to, not only synthesized the molecule, but also study its characteristics. It was very interesting to learn about the methods that they used for characterization and synthesis. It got me wondering if those methods were the only ones that would work in this case and why. But, what I think that was very useful for me is that I got to understand better the importance of the LCST and the role that it played in their system. It reminded me of our project in which the SGQ self-assembles into the SHS and how it is to study it and understand its influence in the SHS as it was important for the copolymers with which it was worked with in the paper.

Valeria’s comment on: Reversible Regulation of Thermoresponsive Property of Dithiomaleimide-Containing Copolymers via Sequential Thiol Exchange Reactions

By Valeria Burgos Caldero

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The main purpose of this article was to synthesize multi-responsive polymers that could be reversibly modified to adjust their LCST. Indeed, these researchers were able to develop a system in which multiple thiol exchanges were made, and in turn, they could determine how the thiols affected the transition temperature of the polymer. They used a copolymer containing P(TEGA) and DMMA. By performing transmission measurements at various temperatures, they concluded that as the thiol changed, the transition temperature of polymer varied depending on the resulting hydrophobicity. More polar functionalities increased the transition temperature and less polar ones decreased it. They were able to demonstrate the reversibility of the modifications since they managed to return to their original functionality after various thiol exchanges. Finally, they implemented a fluorescence signal to monitor the reaction progress. They found that thioglucose quenches the polymer’s fluorescence while making the compound soluble throughout the range of temperatures. With these findings, a wide range of possibilities were opened, since now, if you want a polymer for a specific type of function where a specific temperature response is needed, it is easily accessible by adding the corresponding thiol to the polymer solution. The mechanism of turning off the fluorescence may give access to reversible systems in aqueous conditions.
In general, I found it much simpler to prepare for this article than for the first one I presented. I feel that by doing these exercises of presenting scientific articles I have been acquiring maturity in the analysis process since it was difficult for me to understand articles in the beginning. Something that I found missing in the article is that they never explained the experimental procedure on how they achieved reversibility after adding different thiols to the same sample. I liked that they used common thiols, some of which we use in our research and others that maybe we could apply. In general, the article relates a lot to the research I’m doing with Diana. It could be useful to try to see the stimuli-responsive variations in the compounds that we are synthesizing. Maybe because it is related to variations in functionalities with thiols, similar to my own research, I found it more enjoyable to prepare the discussion and understand the material in the article.

Tang, 2016. Reversible Regulation of Thermoresponsive Property of Dithiomaleimide-Containing Copolymers via Sequential Thiol Exchange Reactions