Decreased solvation layer has what effect on the entropy of water?

[Lyoness]

The entropy of proteins increase when they unfold, so if they are less restricted by the solvation layer due to a decrease in the layer, then that means the proteins can become more disordered and have an increase in entropy, right? Well what about water's entropy when the solvation layer decreases? Doesn't the entropy of water also increase because that means more water molecules are allowed to flow randomly and there is less arrangement because the salvation layer is decreased?

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[aldol16]

Decreased solvation shell = increased entropy of water. But an unfolded protein increases the solvation layer such that the increase in the solvation layer decreases the entropy of water more so than the corresponding increase in entropy due to unfolding. This means that entropy does not go up overall and thus it's unfavorable. That's why most proteins are folded with hydrophobic residues buried on the inside.

 

[Lyoness]

1) Is a negative entropy more favorable than a positive entropy? 2)If the proteins unfold, then even though the entropy is increased due to unfolding, the unfolding causes the solvation layer to increase therefore causing an increased arrangement of water leading to a decreased entropy, am I on the right track? Now on a question, how would we know if the entropy they are referring to is about water or protein? Because in this case, the proteins have increase in entropy since they are unfolding and the water has a decreased entropy so it's less random, how do we know what's the overall entropy? I apologize, I am a bit shaky on my gen chem.

 

[aldol16]

1) Is a negative entropy more favorable than a positive entropy? 2)If the proteins unfold, then even though the entropy is increased due to unfolding, the unfolding causes the solvation layer to increase therefore causing an increased arrangement of water leading to a decreased entropy, am I on the right track? Now on a question, how would we know if the entropy they are referring to is about water or protein? Because in this case, the proteins have increase in entropy since they are unfolding and the water has a decreased entropy so it's less random, how do we know what's the overall entropy? I apologize, I am a bit shaky on my gen chem.

There's no such thing as positive or negative entropy. Entropy is not a quantifiable value for most systems (you can actually quantify it for simple things using Boltzmann's equation, but for complex systems, it becomes prohibitively tough to quantify). We only measure the change in entropy - and it's that change that matters, not the absolute entropy of a system. So in that regard, it's more useful to think of entropy changes as either "more disorder" or "less disorder." The laws of thermodynamics tells us that in order for a process to be spontaneous, the entropic change must be positive (or zero for a reversible case but for all practical purposes, it must be positive). In other words, the world always tends towards disorder.

Yes, there are two entropic changes to deal with when we're talking about protein folding. When a protein unfolds into an extended conformation, its entropy increases because it can now sample a greater array of conformations, i.e. it's more disordered. Based on this fact alone, one would always expect a protein to unfold itself. But nature tells us that this is not the case. Why not? Because we have to consider the entropy of the system - in other words, the net entropy change caused by a process. In this case, the system is consisted of the protein and the waters surrounding it. Due to the increased solvation shell around the unfolded protein, the entropy of the water molecules decreases drastically. In this case, the entropy decrease of the water is greater in magnitude than the entropy increase of the protein, therefore rendering the net entropy change negative. Thus, it is not spontaneous.

Whenever a question asks you about entropy, you can assume it's talking about the entropy of the system, or net entropy, unless specifically stated otherwise. In fact, the MCAT is usually clear on this aspect - they will always ask you specifically what entropic change they are referring to.

 

[Professional Student]

The entropy of proteins increase when they unfold, so if they are less restricted by the solvation layer due to a decrease in the layer, then that means the proteins can become more disordered and have an increase in entropy, right? Well what about water's entropy when the solvation layer decreases? Doesn't the entropy of water also increase because that means more water molecules are allowed to flow randomly and there is less arrangement because the salvation layer is decreased?

I think this question may be forgetting that proteins are not a molecule with the same properties throughout the entire molecule. Proteins are composed of amino acids that can be either hydrophilic or hydrophobic. In fact, the hydrophilic or hydrophobic nature of amino acids is one of the main driving forces of protein folding. Hydrophobic amino acids are folded to the inside of the protein stucture so that water does not have acces to them, because that would be energetically unfavorable. Meanwhile, hydrophilic amino acids are folded to the outside of the protein where they are solvated and more energetically favorable. This is reversed for proteins that reside in the cell membrane, which is a hydrophobic environment. Hydrophobic amino acids are folded to the outside to be near their environment whereas hydrophilic amino acids are folded to the inside to create their own separate environment. The hydrophobic and hydrophilic properties of amino acids are why entropy is not always increased, and thus more favorable, for a misfolded or unfolded protein.

Amino acids can also be amphipathic, where the same amino acid can be both hydrophilic OR hydrophobic depending on the conditions of its location. But that's a story for another day.