Homework_7_01-23-09_Rev_C&A - Biology 5A Homework 7...

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Biology 5A Homework 7 January 23-26, 2009 1. We know from lecture that hydrophobic interaction is the clustering of nonpolar molecules or groups together in water to minimize disruption of H-bonding between water molecules. This homework problem will help you understand how clustering reduces the disruption of water H-bonding. Water molecules are in a lower energy configuration, and thus a more favorable situation, when they can form a maximum number of hydrogen bonds. When methyl groups are placed in water, the adjacent water molecules are restricted in the number of hydrogen bonds they can form, because methyl groups cannot form hydrogen bonds. This restriction means that there are fewer configurations of these adjacent waters that permit hydrogen bonding. In particular, these adjacent waters must be configured so they make hydrogen bonds only towards the surrounding water, away from the methyl groups. This restricted configuration of the adjacent water molecules means that they are more ordered (have less entropy) than they would be if the methyl group was not there. Water molecules at the interface with the methyl group thus represent a thermodynamically unfavorable conflict: These interfacial water molecules would be in a lower energy configuration if they made more hydrogen bonds, but to make the most hydrogen bonds they must assume restricted configurations that reduce entropy, which runs against the Second Law of Thermodynamics. The larger the area of the interface between waters and methyl groups, the larger the amount of water that is limited in its hydrogen bonding, and the more unfavorable the situation. If something can happen to reduce the amount of interface, it will be spontaneous ( Δ G<0) to do so. How much interface is there along which the hydrogen bonding of water to
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Homework_7_01-23-09_Rev_C&amp;A - Biology 5A Homework 7...

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