Entropy and Solvation Layer

[sarah990123]

Hi guys! I'm just a little bit confused on how entropy relates to the solvation layer that forms during hydrophobic interactions in tertiary protein structures.

My textbook (and EK) say:
When water surrounds a hydrophobic molecule, the optimal arrangement of hydrogen bonds results in a highly structured shell, or solvation layer, of water around the molecule. The increased order of the water molecules in the solvation layer correlates with an unfavorable decrease in the entropy of the water. However, when nonpolar groups cluster together, the extent of the solvation layer decreases because each group no longer presents its entire surface to the solution. The result is a favorable increase in entropy.

That last bolded part is what's confusing me since this is what Kaplan's BC book is saying:
However, when a hydrophobic side chain is placed in aqueous solution, the water molecules in the solvation layer cannot form hydrogen bonds with the side chain. This forces the nearby water molecules to rearrange themselves into specific arrangements to maximize hydrogen bonding—which means a negative change in entropy, ΔS. Remember that negative changes in entropy represent increasing order (decreasing disorder) and thus are unfavorable.

So I'm just confused as to how entropy truly does affect the solvation layer since both of my prep sources are saying different things! Will it increase or decrease in hydrophobic interactions? Wondering if anybody could clear this up? Thanks ahead of time!!

---

Members don't see this ad.

 

[RaspberrySlushy]

Both books are actually saying the same thing just in different ways which is what's making it confusing.

Your textbook is more thorough as it talks about the before and after, so to speak. It's saying that before the hydrophobic portions associate they have more contact with water, forcing water to H-bond with itself in more ordered ways causing a decrease in entropy. This is what makes it unfavorable.

The excerpt from your Kaplan book is also talking about this part of things, that's why it's talking about a decrease in entropy.

Your textbook then goes on to talk about after the hydrophobic portions associate with each other, usually at the center of an aqueous protein. Their association together decreases their interaction with water so that water no longer has to make so many ordered H-bonds and is now freer and has more entropy. So now there is an increase in entropy and everything is good. This is the part that the bolded part from the textbook refers to.

So they are both describing the same concept but focusing on different parts. I do think the hydrophobic effect is one of those concepts where pictures really help.

 

[sarah990123]

@RaspberrySlushy ahh thanks for the clarification there! I tried searching online for an image to help solidify the concept and found this one which was pretty useful:

So I guess Kaplan just failed to mention the "after" like you mentioned which made things more confusing. Thanks again.

 

[drechie]

i dont understand how the picture explains the explanation

 

[BerkReviewTeach]

i dont understand how the picture explains the explanation

The six free waters (before solvation) had the freedom to migrate wherever they wished to go. After hydrating the solute, those waters are now bonded (weakly, but still bound) and are unable to move freely around. The six water molecules are now less random, so their entropy has decreased. The solute experiences an increase in entropy however, which clouds the example. However, all of this describes solute dissociation into an aqueous medium, which is not what the issue is in the original question.

The issue with protein folding involves the concept of solvent cage versus hydrophobic collapse. It stems from the historical argument of whether protein folding is driven more by hydrogen bonding (favoring enthalpy considerations) or whether it is driven more by hydrophobic interactions, which are thermodynamically aided by entropic changes in the water that surrounds a globular protein as well as the protein itself.

On one hand, a protein with exposed hydrophilic R-groups can form hydrogen bonds with the surrounding water, which some theorists believe drives the folding through enthalpy reasons. This particular structure allows for more entropic freedom for the protein itself.

On the other hand, other theorists believe that the hydrogen bonding between water and water is essentially the same as R-group-to-water, so there is no significant gain in enthalpy upon hydrating a protein and that a protein folding in such a way as to have its polar side-chains hydrogen bond to one another in its core will allow water molecules to be free to move around in solution. This improved entropic state for water drives the folding process, despite the protein itself becoming less random.

There is bound to be confusion here, because there are two opposing theories at work. So memorizing one book's definition versus another book's definition is not going to be as useful as recognizing the key features that support one theory versus the other theory.

 

[FuturePhysician101]

Hello, could someone please explain this answer tot his question to me? I do not understand how the solvation layer surrounding polar groups is in lesser extent to the salvation layer to non-polar group and hence, i still think it is "C" - but the key says the answer is B ?

 

[aldol16]

Hello, could someone please explain this answer tot his question to me? I do not understand how the solvation layer surrounding polar groups is in lesser extent to the salvation layer to non-polar group and hence, i still think it is "C" - but the key says the answer is B ?

You should start a new thread with your question since it's not really related to the topic of this really old thread. Anyway, think about why the solvation layer forms around non-polar groups. It's because the water cannot interact with the nonpolar groups and thus to preserve/maximize electrostatic interactions/stabilization, it must interact with itself in an ordered "shell." With polar groups, water can interact with these groups to increase stability and since water has two dipolar bonds, it can interact in several ways and doesn't need to form an ordered "shell" so to speak.

 

[nsen]

You should start a new thread with your question since it's not really related to the topic of this really old thread. Anyway, think about why the solvation layer forms around non-polar groups. It's because the water cannot interact with the nonpolar groups and thus to preserve/maximize electrostatic interactions/stabilization, it must interact with itself in an ordered "shell." With polar groups, water can interact with these groups to increase stability and since water has two dipolar bonds, it can interact in several ways and doesn't need to form an ordered "shell" so to speak.

the very definition of a solvation shell is that water molecules surround the solute. so if the group is polar, wouldn't water molecules arrange themselves properly around the solute creating a solvent layer. Non-polar groups diminish the shell as it does not interact with the water molecules thus C seems like the correct answer.

 

[aldol16]

the very definition of a solvation shell is that water molecules surround the solute. so if the group is polar, wouldn't water molecules arrange themselves properly around the solute creating a solvent layer. Non-polar groups diminish the shell as it does not interact with the water molecules thus C seems like the correct answer.

You have the concept of solvation shell backwards. A solvation shell is formed around things that do not solvate well, so like an oil. It doesn't form around things that solvate well because, well, since it's solvated, there's no such thing as a "shell." It's like someone dancing in the middle of a dance floor. Say it's a normal person with people around them. The people will be packed in close around them and everybody is having a good time. There's no "ordered shell" forming around them because everyone is kind of moving in and out freely, interacting with them. Now imagine that that person smells like ass. Now, nobody is going to want to approach them. So everybody else kind of just forms a circle around the person to minimize their odor. Now, you have order - a circle - that has formed around the person in the middle because nobody wants to interact with him/her.

When you solvate something in water, you're doing precisely this. When you introduce a polar group, all the waters around it want to interact with it due to the dipole interactions. The molecules around it will jostle each other and move in close to the polar group, all trying to dance with it. No shell forms. But say now you have a nonpolar group. None of the waters want to interact with it. The waters would much rather interact with themselves. So they isolate the nonpolar group in the middle and form a ring around it. That's the solvation shell.

 

[Tbrquestions]

The six free waters (before solvation) had the freedom to migrate wherever they wished to go. After hydrating the solute, those waters are now bonded (weakly, but still bound) and are unable to move freely around. The six water molecules are now less random, so their entropy has decreased. The solute experiences an increase in entropy however, which clouds the example. However, all of this describes solute dissociation into an aqueous medium, which is not what the issue is in the original question.

The issue with protein folding involves the concept of solvent cage versus hydrophobic collapse. It stems from the historical argument of whether protein folding is driven more by hydrogen bonding (favoring enthalpy considerations) or whether it is driven more by hydrophobic interactions, which are thermodynamically aided by entropic changes in the water that surrounds a globular protein as well as the protein itself.

On one hand, a protein with exposed hydrophilic R-groups can form hydrogen bonds with the surrounding water, which some theorists believe drives the folding through enthalpy reasons. This particular structure allows for more entropic freedom for the protein itself.

On the other hand, other theorists believe that the hydrogen bonding between water and water is essentially the same as R-group-to-water, so there is no significant gain in enthalpy upon hydrating a protein and that a protein folding in such a way as to have its polar side-chains hydrogen bond to one another in its core will allow water molecules to be free to move around in solution. This improved entropic state for water drives the folding process, despite the protein itself becoming less random.

There is bound to be confusion here, because there are two opposing theories at work. So memorizing one book's definition versus another book's definition is not going to be as useful as recognizing the key features that support one theory versus the other theory.

Where exactly is this covered in tbr books?

 

[JustinM88]

Hi guys! I'm just a little bit confused on how entropy relates to the solvation layer that forms during hydrophobic interactions in tertiary protein structures.

My textbook (and EK) say:
When water surrounds a hydrophobic molecule, the optimal arrangement of hydrogen bonds results in a highly structured shell, or solvation layer, of water around the molecule. The increased order of the water molecules in the solvation layer correlates with an unfavorable decrease in the entropy of the water. However, when nonpolar groups cluster together, the extent of the solvation layer decreases because each group no longer presents its entire surface to the solution. The result is a favorable increase in entropy.

I had to draw this out multiple times back when I was in Biochem to truly understand this paragraph's stuff. Also, I think plenty of students in my class were/are still confused about it since it's just one of those fairly confusing things. Anyways, I drew out this paragraph so maybe this visual will help. Let's say we add some oil into water like so (on the right is the initial situation):

upload pic

First, just know that the Universe always favors disorder/chaos/moreentropy.
The green area here represents ORDERED water molecules (aka the SOLVATION LAYER).
The yellow area here represents the HYDROPHOBIC molecule(s).
Notice that the SURFACE AREA actually decreases upon going from the right to the left here. As surface area decreases, the amount of ordered water molecules (the green area aka the solvation layer) decreases. As the amount of ordered water molecules decreases (less order!), entropy (disorder) increases...the Universe is happy