Biochemistry : Hydrophobic Interactions

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Hydrophobic Interactions

Which amino acid would you expect to find in the core of a protein that is in a solution of water? 

Possible Answers:

Arginine

Threonine

Tryptophan

Serine

Correct answer:

Tryptophan

Explanation:

Proteins will behave similarly to phospholipids in water; the polar groups will form favorable interactions on the surface with water, while the hydrophobic groups will be in the core and away from the water molecules. Usually, amino acids with non-polar residues will be found in the core of proteins. Tryptophan has a nonpolar side chain, and will thus be found in the core of a protein that is in a aqueous environment. 

Example Question #2 : Hydrophobic Interactions

Which of the following explains why nonpolar molecules such as lipids spontaneously aggregate in water?

Possible Answers:

Ionic bonding; lipid molecules form strong ionic bonds with each other that they cannot form with water 

Enthalpy; lipid molecules absorb a tremendous amount of heat when they come to associate with each other rather than with water

Covalent bonding; lipid molecules form strong covalent bonds with each other that they cannot form with water

Hydrogen bonding; lipid molecules have stronger interactions with each other through hydrogen bonding than they do with water

Entropy; water molecules acquire more degrees of freedom as a result of nonpolar molecules forming one large aggregate from many smaller ones

Correct answer:

Entropy; water molecules acquire more degrees of freedom as a result of nonpolar molecules forming one large aggregate from many smaller ones

Explanation:

In aqueous solutions, lipid molecules are surrounded by a lattice-like ring of water molecules known as a clathrate shell. This locks up previously free water molecules in this state, which is not entropically favored. Though it is unavoidable that some water molecules will have to be robbed of some of their freedom of motion by forming at least one clathrate shell, the ideal scenario thermodynamically is the one in which the fewest water molecules are stuck in the shell as possible. As a result, lipid bubbles in aqueous solutions tend to go from many to one, as this results in the  clathrate shell with the fewest number of water molecules. In the process, many smaller clathrate shells are broken, and many water molecules are freed, thus increasing the entropy of the system. 

Example Question #2 : Hydrophobic Interactions

Which of the following is false about hydrophobic effects?

Possible Answers:

They function because hydrophobic groups clump together, so they do not break the hydrogen bonds in the surrounding water.

They can occur in a non-aqueous environment.

Cell membranes are held together in part by hydrophobic effects.

Hydrophobic groups are not precisely bonded to each other, but rather are held together because of a repulsion from water.

Generally, it is only nonpolar substances which exhibit hydrophobic effects.

Correct answer:

They can occur in a non-aqueous environment.

Explanation:

Hydrophobic effects require water to occur. The reason that hydrophobic groups tend to group together is that by doing so, the network of water molecules around them stays intact. There are no other special forces at play between hydrophobic groups. It is precisely the non-polar nature of hydrophobic groups that gives them their character; water molecules are polar. Cell membranes have a phospholipid bilayer with internal hydrophobic regions (the lipid tails), holding together the membrane.

Example Question #4 : Hydrophobic Interactions

Which of the following statements explains the overall change in entropy when a small amount of nonpolar solute is immersed in water?

Possible Answers:

Entropy decreases because the nonpolar solute has an affinity for itself and aggregates together.

Entropy decreases because the water must become more ordered in a hydrogen-bond network around the nonpolar molecules.

Entropy remains the same because there is no significant interaction between the water molecules and the nonpolar solvent.

Entropy increases because water molecules exclude the nonpolar solute in order to interact with each other and regain a higher state of disorder.

Correct answer:

Entropy increases because water molecules exclude the nonpolar solute in order to interact with each other and regain a higher state of disorder.

Explanation:

This is called the hydrophobic effect. Although initially the water molecules arrange themselves in clathrates and become more ordered, their exclusion of the hydrophobic/nonpolar solute is entropically driven and energetically favorable.

Example Question #1 : Hydrophobic Interactions

What is the major driving force for the formation of a phospholipid bilayer?

Possible Answers:

Hydrophobic interactions

ATP hydrolysis

Hydrogen bond formation

Covalent interactions

Nucleophilic attack

Correct answer:

Hydrophobic interactions

Explanation:

Phospholipids are amphipathic - in other words they are simultaneously hydrophobic and hydrophilic. They have hydrophobic carbon tails and hydrophilic head groups. Because the carbon chains are repulsed by water, phospholipids come together so that their carbon tails are touching and the polar heads face out in either direction. These hydrophobic interactions ultimately form a phospholipid bilayer.

Example Question #4 : Hydrophobic Interactions

How do hydrogen bonds compare in strength to covalent bonds, ionic bonds, and London dispersion forces?

Possible Answers:

Weaker than covalent bonds and London dispersion forces, but stronger than ionic bonds

Weaker than covalent and ionic bonds, but stronger than London dispersion forces

Stronger than covalent and ionic bonds, but weaker than London dispersion forces

Stronger than covalent bonds, London dispersion forces, and ionic bonds

Weaker than London dispersion forces and ionic bonds, but stronger than covalent bonds

Correct answer:

Weaker than covalent and ionic bonds, but stronger than London dispersion forces

Explanation:

Hydrogen bonds are the strongest of the intermolecular forces. However, that strength is little in comparison the strength of intramolecular forces, such as ionic and covalent bonds. The strongest of the listed forces is covalent bonds, followed by ionic bonds, hydrogen bonds, and then finally London dispersion forces.

Hydrogen bonds are important in biochemistry because of the incredible effect that they have on life due to their relative strength. But remember, this strength is not nearly as as strong as the covalent and ionic bonds, which actually hold atoms within the same molecule together. 

Note, hydrogen bonds can be either an intermolecular or an intramolecular force. A hydrogen bond is considered intramolecular if it is occurring between different molecules, and intermolecular if it is occurring within the same molecule.

Example Question #7 : Hydrophobic Interactions

Which of the following are hydrophobic molecules?

Possible Answers:

Nonpolar molecules

Ionic molecules

Polar molecules

Charged molecules

Molecules with two amino acids

Correct answer:

Nonpolar molecules

Explanation:

Hydrophobic molecules are nonpolar molecules - from the Greek "hydro-" water and "phobic" fearing. Examples of hydrophobic molecules are lipids.

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