Signature of hydrophobic hydration in a single polymer
Isaac T. S. Li & Gilbert C. Walker
Proc. Natl. Acad. Sci. USA 2011, 108, 16527. DOI: 10.1073/pnas.1105450108
By Luis M. Negrón Ríos
There is still a lot of progress to be made in theoretical studies (molecular simulations) regarding the understanding of the hydrophobic effect from small solutes to large proteins (protein folding). However, correlation between theoretical and experimental studies of hydrophobic hydration (ΔGhyd) data was still missing until the publication of this article of Li and Walker. According with theoretical studies, the ΔGhyd scale with the solvent accessible surface area (SASA) of molecules follows macroscopic interfacial thermodynamics (water surface tension). However, at the microscopic scale, macroscopic interfacial thermodynamics does not follow a correlation between ΔGhyd and SASA.
In this article, they explain this behavior by measuring the “hydrophobic signal” which is the temperature dependence of ΔGhyd. Using Atomic Force Microscopy (AFM), they performed single chain elastic stretching on hydrophobic polymers that form globules in presence of water. These hydrophobic polymers (polystyrene (PS), poly(4-tert-butylstyrene) (PtBS), and poly(4-vinylbiphenyl) (PVBP)) were deposited on a silicon surface. The force required to pull a single polymer to unravel the formed globule provides information of ΔGhyd of the exposed monomer in water. To obtain the temperature dependence of ΔGhyd by this procedure, they repeated this experiments thousands of times at different temperatures (25–80 °C). They divided their results, in three main findings: temperature, size and polymer/monomer dependence.
First, for each polymer ΔGhyd is strongly dependent on temperature and does not correlate with interfacial thermodynamics. To understand this explanation we have to separate the macroscopic (above 1 nm) and microscopic scale (below 1 nm). In the case of macroscopic scale, the temperature dependence of ΔGhyd follows a correlation described by interfacial thermodynamics, which means that ΔGhyd decreases with increasing temperature having a similar trend of water surface tension by increasing temperatures. For that reason, in the macroscopic scale, this tendency is dominated by “surface area” and is “enthalpy driven” as the number of disrupted hydrogen bonds in the large hydrophobic particle scale with the surface area. However, for small hydrophobic particles measuring less than 1 nm (microscopic scale), the ΔGhyd increases at low temperature by reaching a maximum and then decrease at high temperature. This behavior is predicted to scale with volume instead of surface area. According to the article, this particular increase of ΔGhyd at low temperatures results from the lowered entropy of the surrounding water molecules.
Second, results of temperature dependence of ΔGhyd at different sizes (backbone + side chain) are showed for the studies with PS (7.2 Å), PtBS (9.5 Å) and PVBP (11.4 Å). In these, they point out that for PS, the ΔGhyd increases “monotonically” with increasing temperature. In contrast, for PtBS and PVBP the ΔGhyd varies parabolically with temperature showing maxima at 55.1 °C and 47.8 °C, respectively. They state that this behavior is characteristic for hydrophobic solutes whose sizes are in a “crossover regime” between ΔGhyd of small (<7 Å) and large (>20 Å) solutes.
The third finding is the relationship of ΔGhyd in free monomer versus the monomer on a polymer. They found that the cost of hydrating free monomers is higher than hydrating monomers that are part of the polymer. This is because the monomers of the polymeric chain are stabilized by hydrophobic interactions with other monomers that are part of the chain. The high impact of all the presented results is a validation of theoretical predictions of temperature dependency of ΔGhyd and experimental evidence that the crossover length scale is in the order of 1 nm.
About the article, I have to agree with Garde and Patel’s comment that this study fills a gap between theoretical studies (molecular simulations) and experimental models. This article helps to see the hydrophobic effect in a scale aspect and the presented trends between ΔGhyd at different temperatures. There is a lot of work that have to be done to understand the hydrophobic effect in natural systems like folding in proteins. I understand that polymers are good candidates for these kinds of studies, but to address the challenge of hydrophobicity we need materials that can be good analogues to biological systems such as proteins. I like the article, not only because of the methodology used, but also because the implications of the reported findings makes a strong contribution to understand other aspects of water that are still elusive in terms of the hydrophobic effect. We can dedicate many years to the story of the hydrophobic effect, but like characters that appear in stories, there are a lot of parameters, anomalies and phenomena that will continue appearing in the story of this phenomenon.
Supramolecular Space 
There is a mistake in the picture, for the temperature dependence graph, the parabolic graph should be in the LEFT in Microscopic Scale. And the linear should be in the Right in Macroscopic Scale with a POSITIVE slope, not negative. Thanks and sorry for the mistake.
Here is the link for the CORRECT PICTURE:
http://imageshack.us/photo/my-images/827/blogpicturelmn201225.jpg/
I replaced the wrong picture with the corrected one.
In this article, Li and Walker shine light on the hydrophobic effect of large proteins and small solutes by studying hydrophobic hydration which, is seen to be temperature dependant. The group reported the first experimental data of temperature and size dependant hydrophobic hydration. By using an Atomic Force Microscope, and various hydrophobic polymers; polystyrene (PS), poly (4-tert-butylstyrene) (PtBS), and poly(4-vinylbiphenyl) (PVBP), they obtained the hydrophobic hydration at different temperatures. Their results were divided into three principal findings which explained the ΔGhyd by temperature, size and polymer (monomer) dependence.
Truly, I enjoyed the article and wish to learn more on how hydrophobic effects work in natural systems. About Manolo’s sypnopsis, it was well written and organized. It really helped me to further understand the article as he, very nicely, targeted the main findings presented in the paper.
Hydrophobicity plays an important role in the self-assembly of many systems. Hydrophobic hydration has to do with the minimization of the free energy of the water molecules near nonpolar surfaces. This study presents the effect of the temperature on hydrophobic hydration of various polymers as well as how their side chains affect the temperature dependence. To achieve their objectives, various techniques such as AFM to have the ΔGhyd by stretching the polymers. Then to study the temperature dependence they repeated the experiments many times at different temperatures. It was found that the hydration free energy is dependent on temperature and it does not follow interfacial thermodynamics. This means that the microscopic scale particles has an increase of the free energy at low temperatures and it then decreases at high temperatures because of the lowered entropy of the nearby water molecules. There is also a hydrophobic size effect and the hydration free energy of the monomer on a macromolecule is different from a free monomer. This study basically compliments theoretical studies and the experimental studies. Patience and careful reading is needed in my opinion to understand this article because it has a lot of information that leads to the next steps discussed in the article.
About the synopsis I believe that Manolo did a good job. It was well written in terms of spelling, grammar and logic. However, because of the nature of this article, a person with little or a general knowledge of chemistry couldn’t probably understand it. This is expected because yes they might know what hydrophobicity is, but in general people can’t imagine all of the effects that hydrophobicity has in self-assembly. The hydrophobic effect is when nonpolar molecules aggregate to exclude water molecules in aqueous media and these aggregation have a high impact on the structure of macromolecules. Depending on how the molecules interact, is the energy and stability that they are going to have. In general, the guidelines for the synopsis were followed because he discussed the significance of the article, why were those results important, etc. The picture is very attractive and it shows the main concepts of the article involving the free energy, temperature dependence and the size effect (microscopic and macroscopic sizes) differences in the results. For future synopsis I suggest incorporating a very brief example of the past experiment that led to this study, if it is available.
Usually we use past experiments to know the researcher motivation to do this project but it is very clear the lack of experimental data so I think that’s motivation enough to be creative. Yet there is always room for improvement.
I think the study of hydrophobicity is very much helpful in drug design as it induces the self-assembly of molecules. In this article, Li and Walker explained the hydrophobicity of hydrophobic molecules.
According to Li and Walker, the hydrophobic hydration free energy (ΔGhyd) depends on temperature, size of the solute (or polymer) and molecularity (monomer/polymer). In this temperature dependent ΔGhyd¬¬ does not follow interfacial thermodynamics and they used Atomic Force Microscopy (AFM) to scale the temperature dependence of ΔGhyd by pulling the single chain polymer globules in water.
They examined the practical results by using three different side chained poly styrenes (PS, PtBS and PVBP). In these, each single polymer ΔGhyd is dependent on temperature and does not correlate with SASA. Here depending on size, solute was divided in to micro scale (bellow 1nm radius of solute) and macro scale (above 1nm radius of solute). In macroscopic scale ΔGhyd follows ¬the correlation with SASA. In the case of PS-polymer the ΔGhyd ¬increases with the increase in temperature range from 25-80 oC, but in the case of PtBS and PVBP-polymers ΔGhyd initially increases with temperature and reaches to a maximum then it goes down in parabolic model. Here, the shifting of maximum position was observed towards lower temperature with increase in size of the solute. That means the formation of hydrogen bond around the solute is difficult with the increasing size of solute. Last but not least finding in this article is the ΔGhyd of monomers which is rather smaller when they are a part of the polymer, than the ΔGhyd of them independently, because in the part of the chain, the monomers are stabilized by hydrophobic interactions with adjacent monomers.
First of all I would like to thanks Manolo, he has provided lot of supporting information regarding this article. I think the synopsis was written well and the figures are very appropriate and scientific, in briefly I like it because this is very new subject for me but it’s not difficult to understand.
This article makes computational data into experimental data for the study of hydrophobicity. They explain how the free hydration energy changes for microscale and macroscale hydrophobic polymers. In the case of the microsacle polymers its hydrophobicity is dependent on the temperature (the higher the temperature the lower the hydration energy), but in the case of the macroscale polymers its hydrophobicity it dependent on the surface interaction. These polymers in the presence of water they become globules in comparison with the extended polymer, this results in higher entropy. This means that the lower the entropy, higher the hydration energy. This was futher studied experimentally with polystyrene, poly(4-tert-butylstyrene) and poly(4-vinylbiphenyl).
These results seemed very encouraging and new for the people working in these area, because, you not need to know much about these type of research to notice that there is not much experimental data for this (as they are constantly referring to theoretical data and not past experimentation). So I think hey took a leap into doing these successfully correlation. My initial tough was that the way that the paper is written was very….theorycal. After the introduction (theory) and the explanation of the experiment, the results section, instead of presenting and discussing them, they present them and keep on developing theory (as if I haven’t said that word enough). And the discussion section they go deeper (which is the point, no critique about that). But further re-reading cleared all that.
L. Negron did a great job explaining the article, he used terms that would be used in general chemistry. Gave his opinion about the novelty of the procedures and results. The picture seemed….politically correct and self-explanatory.
In this article Li and Walker report their advances toward the understanding of the hydrophobic effect. To accomplish this they use three hydrophobic polymers with differently sized aromatic side chains and measured the system by single molecule atomic force microscopy (AFM). As a result they find out that the hydration free energy, per monomer, depend on the temperature. Also, this temperature is dependent on the hydrophobic size of the polymer. And for last, they establish that corrections to the theoretical models are required do to the reduced hydration free energy, which are also effected by the hydrophobic interaction from neighboring units.
In my opinion the article was well written and very easy to follow, The figures were appealing and appropriate, but the captions needs a little more explanations. I really enjoy this article in specific because I feel that I have learned a lot of basic knowledge from it. But, as Manolo mention, that in “the story of the hydrophobic effect, but like characters that appear in stories, there are a lot of parameters, anomalies and phenomena that will continue appearing in the story of the hydrophobic effect” I believed that characters will keep appearing because the hydrophobic effect is a phenomena cause by the combination of many factors and as long as scientist concentrate in only on of them the hydrophobic effect will not be fully understood. I also believed that more important that identifying the factors that govern that phenomenon is the understanding the impact that shifting the equilibrium among those factors have on the system.
About Manolo’s synopsis It was strait to the point and most of all it covers the highlights of the article. Also, as well as in the article, the synopsis was as reading an educational article, since even a person out of the filed can understood and most of all learn the basic from. The picture, just as the article was very instructive.
This article is another step forward for Walker and Li regarding their unusual treatment of the problem of hydrophobicity. It deals with the temperature and size dependence of the ∆Ghyd of a homomeric polymer. The results show marked differences from what was predicted with SASA, highlighting the fact that hydrophobicity is different for molecules than for collections of them (even after pulling the polymerchain). The manuscript explains the experiments performed in a quick manner and is of appropriate size.
The authors measured the ∆Ghyd of thousands of polymer chains and normalized them by size. They did this for 3 types of polymer and varying the temperature. They also performed the experiment with out actually pulling the polymer, which gave them a baseline. I believe the conclusions are supported by their results. They were very careful when attempting to explain their results, particularly, the effects of size on the ∆Ghyd.
I have to admit that the narrative was not that engaging. While the article certainly covers an interesting topic it was really hard for me to read through the dense narrative. However, they did a good job explaining where the theory of hydrophobicity currently stands and how their results attempt to merge the macroscopic view of hydrophobicity and the microscopic view.
Manolo’s synopsis was, for the most part, well written. Some sentences like, for example, the last sentence of the first paragraph, could use some work. I think the synopsis is relatively easy to understand, even by people who don’t know much about hydrophobicity. Like Ana, Manolo did not follow the guidelines but he did not have them yet. I think Manolo’s picture is informative and actually useful to understand what the authors are doing. Manolo used a hyperlink to a commentary by other authors in the field, a nice detail. Manolo’s synopses are usually on point and don’t have any major problems other than some sentences. However, like I said for Ana, practice and appropriate proofreading can improve this.
Before I even read the paper, since the first look to the blog title, synopsis and cartoon I knew this was a tough article. It is good that the blog has a different title to make it more appealing and pedagogical. I’m not sure if this is due to the complexity of the subject or due to my lack of knowledge regarding the hydrophobic effect. I found the synopsis as difficult as the paper itself. From Manolos’ synopsis I clearly understood the impact of the article, “validation of the theoretical predictions of temperature dependency of ΔGhyd”. In my opinion the synopsis didn’t simplify the understanding of the article, since in principle the same phrasing were used. In the synopsis the second sentence may cause confusion due to the way in which the hydration free energy or ΔG of hydration is defined in an oversimplified form. Manolo used poorly defined terms repeatedly like for example: interfacial thermodynamics, “hydrophobic hydration”, “hydrophobic signal”, and “scale”. This last one should not be problematic but it gives me some problems to put in context. Does the authors means direct proportionality when they used it? Does the authors really “repeat the experiments thousand of times at different temperatures”? I think that the blogger should first focused in the biggest result of the article, explained it well in a concise manner, and then, if there is extra room, put some data details. The top portion of the cartoon was very useful but the “Implication” part was less explanatory. Although, it is very easy for me to make this critics but it will be difficult to implement in my next synopsis. Plus, in my opinion hydrophobicity is an abstract subject difficult to explain to general public. Usually, the smaller you go in atomic scale, the more difficult it is to explained to the none scientific community. Manolos’ link (send via e-mail) was very helpful to understand and learn more about the hydrophobic effect and about this article.
The article was difficult to digest; it gives me stomach problems since I start reading it. I have been in pain and taking strong anti acids for the last two days. A few things confuses me of this article but the one that confused me the most is, why in Fig.1 they show and talk about a protein if only three polymers are being study in the article? In contrast, Fig. 2C was of greatest help at the time of understanding the specific process that is being studied in the article.
Walker research is very interesting, I due like polymers and he has many interesting projects regarding them, specially the marine biofouling one. I believed biofouling is something that affects more the architectures of small tropical islands. I have been personally interested in this self-assembly processes since a few years ago and is the first time I see it is been directly attack in a research program.
In this article, Lee and Walker present to us some important findings regarding to hydrophobicity. Thanks to this article, it was possible to correlate computational data with experimental data in their studies regarding hydrophobic hydration. They used a small hydrophobic polymer, which formed globules in the presence of water, and then used Atomic Force Microscopy (AFM) to stretch these polymers. The force required to stretch the polymer gave them information about the hydrophobic hydration. They also studied the dependence temperature has on hydrophobic hydration and they found out that the behavior of large particles was different than for smaller particles. In a macroscopic scale (radius above 1nm), the hydrophobic hydration is largely dictated by the surface interactions. However for a microscopic scale (radios below 1nm), hydropobic hydration was dictated by temperature. The also perform studies with monomers, showing that for individual monomer the hydration energy was larger than for monomer in a polymer.
This article was really interesting, with a lot of important information regarding hydrophobicity. They had enough data o support their conclusions and findings. And I also agree with the blogger in the fact that they filled a gap between computational models and experimental data. However I did find this article a little hard to read. But they do a good job in explaining all of their data, and the figures in the article were really helpful in keeping track what was going on.
As for Manolo’s synopsis, I really did enjoy reading it. He did a great job in summarizing the most important aspects in the paper, and his synopsis helped me understand the article better. I think that a people possessing only a general knowledge in chemistry would have been able to understand the main idea of this article by reading this synopsis. The picture was really useful too.
As we all might have perceived in life sometimes size does matter and understanding hydrophobicity gets more challenging because of its length-scale dependence. Before the article of Li and Walker many theoretical/simulated analyses have been done to evaluate how factors such as temperature, pressure, size, etc impacts the hydrophobic effect. However, when compared to previous experimental attempts, one of the key features of Walker’s design is that he overcomes water solubility related problems by fixation of single chains of hydrophobic polymers on an AFM tip. In general, Walker’s unique experimental design allows indirect and controlled measurements correlating the force to pull the polymers with the free energy of hydration and how this is affected with temperature. These was done using three hydrophobic polymers in which variations on the size of their monomeric subunits significantly impacts its free energy and temperature dependence (Figure 3). From the experiments reported herein Li and Walker hit the jackpot by (a) validating the theoretical prediction that the hydrophobic hydration is dependent on the temperature of the system and (b) by experimentally determining the cross over length scale to be similar to the theoretical which is around 1 nm.
In general we can’t forget the complex challenges behind the design of platforms for experimental measurements to explore certain aspects of the hydrophobic effect. Even thought Li and Walker’s set up amazingly allows further exploration of many aspects on the hydrophobic effect, still more ideas and technologies have to be develop for a more validated and comprehensive perspective on the hydrophobic effect.
Manolos’s figure is useful because in a simplistic way incorporates a cartoon of the main experimental set up reported in the article and in the bottom part compliments the aspects of the hydrophobic effect that were discuss on the article. His synopsis is well organized and dividing the main findings in three sections helps the reader to eliminates possible confusions arising from reading the article. The synopsis is sometimes a bit dense since it includes various details that wouldn’t be necessary in a more overall view of the article. However I think he gave us the kind of for dummies version of the story through Garde and Patel’s extraordinary commentary on “unraveling the hydrophobic effect one molecule at a time”.
In this article, Li and Walker use hydrophobic hydropolymers as simplistic representations for hydrophobic hydration and collapse of proteins. As we’ve discussed before in our very own group meetings, polymers that are hydrophobic in aqueous media undergo a coil-to-globule shift, and, in the article they feature this as a resemblance to that of the hydrophobic hydration in proteins. Nevertheless, since they are poorly soluble in aqueous media, ensemble studies weren’t really a viable option to study this. Which is why they decided to use an alternative method to estimate the ΔGhyd of the extension process (when it shifts from the globule to the coil).
The way they went about this was through the use of Atomic Force Microscopy to study the temperature dependence of ΔGhyd . From their results they gathered that: (1) It takes less energy to hydrate the whole chain, as opposed to hydrating each monomer separately; (2) That particles that are greater than 1nm behave differently than particles smaller than 1nm ( 1 nm = the ΔGhyd scales with surface area and is enthalpically driven.
About the article as a whole, I think it’s a very good article, and it’s a very important one in the sense that one more stepping-stone towards understanding the intricacies of hydrophobicity. I think that they were so thorough with their findings, it’s not even funny. If you take a look in the Materials and Methods section, you can see that they collected around 150,000 force curves per polymer sample to ensure the statistics of their results. Experimentally, I think it was a very complete article and its length was justified by the amount of aspects they wanted to attend to in their paper. I think it was very well written and I did enjoy, however, I can understand why it wasn’t everybody’s favorite. The thing is, that at first glance, the amount of detail and attention that the paper contains is somewhat daunting. And maybe the first time reading the paper may go down like cough syrup, but I don’t think it would be any worse than that. It’s still a really nice article.
As for Manolo’s synopsis, I have to commend him on writing a good synopsis on this. His synopsis summarizes the article really well and it explains it with simple language, it helps understand the paper. I also think his picture was nice, because it summarized some of the most important aspects of the article. Maybe in a future occasion he could try making a less crowded picture, and even then, I think in this case it’s justified, because of all the things involved in the article.
I just noticed I wrote something wrong, in my comment, at the end of the second paragraph, I meant to say that ΔGhyd is predicted to scale with volume if the particle is smaller than 1nm; and if it’s larger than 1nm, it will scale with Surface area and will be enthalpically driven.
Hydrophobic interactions are a fundamental force behind self-assembly, yet elusive to understand. Manolo brings up a paper that represents an effort to unravel how these forces occur at the microscopic level. The parallelisms that can be drawn between theory and experimental data give even more importance to the study, as they support the theoretical models that have been developed long before.
The paper is one of those crucial-yet-dense papers, which gives as much of a hard time to read as they give important insight.
In each case the solubility reaches a temperature threshold for maximum solubility, however in the smallest of the studied polymers, this threshold is higher than what can be studied using AFM, and only decreasing solubility (increasing solvation free energy) is observed. However the two larger polymers show an inflexion in the T vs solvation energy plots, showing that after reaching a maximum temperature-decreased solubility, they become again more soluble. I understand that it is the same observation behind the LCST phenomenon observed in our system; though in our case, our compounds are mostly soluble in the room temperature range (the threshold for maximal solubility) and, upon increasing the temperature, they drop out of solution. The authors point out that this behavior, in which decreased solubility is observed until a threshold temperature is reached, represents a transition between the microscopic regime (in which solubility of the hydrophobe is reduced with temperature) and the macromolecular regime (solubility increases with rising temperature)
I feel this article shows us that there is a great need for practical systems that can experimentally describe the hydrophobic effect. To me, it is reminiscent of the molecular tweezers and balances that are now part of the fundamental research in non-covalent interactions and supramolecular chemistry. The paper’s narrative is good, though I understood the comment by Patel/Garde a lot more easily.
Manolo’s synopsis is very good, with some minor grammatical errors. I admit I got lost in the fourth paragraph, perhaps because it includes numerical details that can distract a bit. It’s not an easy topic, at least in my case, to understand and I think Manolo did a good job in trying to make it accessible to all of us. The picture (the most updated version) was actually a reference for me while reading the paper (having disussed it with Manolo at the lab before). I’d suggest he add the monotonically decreasing hydration energy graph under the macroscopic scale representation (in the blue square), to get a more complete picture (and also to make the picture more symmetrical 😛 )
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