Some books and references call it the hydrophobic interaction, but this is a bit of a misnomer. Except for the week Van Der Waals interactions discussed on the previous page, there is very little interaction between nonpolar or hydrophobic molecules. Yet when you put hydrophobic molecules (such as vegetable oil) into water they rapidly "associate" into a "ball" on the bottom or a round "puddle" on the water surface. Why do molecules that don't interact with each other very well hang out together when put into water? This is really a phenomenon of the water itself and NOT of the oil. A reminder of the properties of nonpolar and polar molecules
are uniform no matter which side you look = one side looks just like the other. There is no differential charge fronm one side to the other
have a side that is slightly positively charged while the other side is equally negatively charged
There is no way for the nonpolar molecules to interact with polar ones beyond weak Van Der Waals forces.
Since water is polar and would much rather interact with other polar molecules (other water molecules for instance) through polar interactions and hydrogen bonds than with hydrophobic molecules. Nonpolar molecules cannot satistfy the charge interactions with polar molecules. Therefore, water "ignores" any hydrophobic molecules and aligns more with its neighboring water moelcules. For those water molecules adjecent to hydrophobic molecules however, there is an unsatisfied H-bond or polar interaction. This generates a problem for the water molecule. In order to minimize the problems the hydrophic molecules get organized in a sphere or circle as this minimizes the surface area. With minimal surface area there are fewer water molecules with problems. The fewer with problems the better.
Let's say you got to a large party with a bunch of friends. This is that kind of party where everyone knows everyone and there is lot's to talk about. The dynamics of the party go someting like this. You all stand around if small groups talking (interacting). As time passes people move into and out of different groups so the interactions morph a bit but continue through the evening. At some point someone walks into the party. Noone knows this person and this he is not very talkative. Inevtitably, the new person will be seated in the corner eating chips being ignored by everyone. Later another person of similar personality arrives. What will happen? This new arrival will soon be seated next to the other chip eating outcast... not talking to each other while everyone else ignores them both.
Just to make the analogy clear... the interacting partiers are like water molecules. Interacting in small groups, changing partners, perhaps, but continuously interacting. The "nerds" that arrived later are like hydrophobic molecules. While they do not interact in a substantial way either with each other or with the partiers, they end up together in the corner.... not interacting
How does this apply to protein structure? You will recall that several of the amino acids have non-polar sidechains (LEU, ILE, VAL, PHE, ALA). water soluble proteins tend to have many of their hydrophobic amino acids in the center of the protein sequestered away from water. This is driven by this hydopphobic effect. In the previous module when the amino acid structures were introduced, there was a demonstration showing how the nonpolar groups tended to be on the inside of a proteins structure while the polar and charged amino acids tended to be exposed to water.