Hydrogen bonding continued

Hydrogen bonding (Continued) of phenolic resins, 388-389 Hydrogenation, 208 Hydrolases, 82... 

Fig. 16. An unusual interrupted helix from subtilisin (residues 62-86), in which the helical hydrogen bonds continue to a final tum that is formed by a separate piece of main chain. Such interrupted helices (broken on one side of the double helix) are apparently a fundamental feature of nucleic acid structure as illustrated by tRNA, but are exceedingly rare in protein structure.

When water is between 0° and 100°C, the hydrogen bonds continually form and break. Below 0 degrees, there is less energy acting on the water molecules, the hydrogen bonds do not break, and a solid forms. Water is one of the only natural compounds that is less dense in solid form than it is in liquid form. This is why ice floats. 

As water is heated from 0 C to 4 C, hydrogen bonds continue to be broken and the molecules come closer and closer to one another. The volume goes on decreasing... 

As liquid water is heated above the melting point, hydrogen bonds continue to break. The molecules become even more closely packed, and the density of the liquid water continues to increase. Liquid water attains its maximum density at 3.98 °C. Above this temperature, the water behaves in a "normal" fashion Its density decreases as temperature increases. The unusual freezing-point behavior of water explains why a freshwater lake freezes from the top down. When the water temperature falls below 4 °C, the denser water sinks to the bottom of the lake and the colder surface water freezes. The ice over the top of the lake then tends to insulate the water below from further heat loss. This allows fish to survive the winter in a lake that has been frozen over. Without hydrogen bonding, all lakes would freeze from the bottom up and fish, small bottom-feeding animals, and aquatic plants would not survive the winter. The density relationship between liquid water and ice is compared in Figure 12-8 with the more common liquid-solid density relationship. 

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