Absolutely Small - Michael D. Fayer [101]
FIGURE 15.2. Four ethanol molecules that form a chain. The oxygens are almost black in the figure. An oxygen has two lone pairs in addition to the hydrogen and carbon bonded to it. The dashed lines show hydrogen bonds that go from the hydroxyl’s H on one ethanol to an oxygen lone pair on another ethanol.
WATER MAKES HYDROGEN BONDS
Back to why hydrogen bonds are necessary for life. Water, H2O, is a very small molecule. It has a molecular weight that is similar to that of oxygen (O2), nitrogen (N2), or methane (CH4), all of which are gases at room temperature. Water has a single oxygen bonded to two hydrogens. Like the situation in ethanol, the oxygen makes covalent bonds to the hydrogens, but in an O-H covalent bond, the electrons are not shared perfectly equally. In a water molecule, the oxygen withdraws some electron density from the H’s. A diagram of water showing this is: Hδ+—Oδ-—Hδ+. The slightly positive hydrogens on one water molecule are attracted to the slightly negative oxygens on another water molecule. One water molecule can make up to four hydrogen bonds.
A schematic illustration of water hydrogen bonding is shown in Figure 15.3. The central water molecule is making four hydrogen bonds with the surrounding four waters. The central water’s two hydroxyls are hydrogen bonded to two oxygens of other water molecules. And two other water molecules’ hydroxyls are forming hydrogen bonds with the oxygen of the central water. In contrast to the depiction in Figure 15.3, the hydrogen bonding does not stop with the five water molecules. The outer four water molecules will each make approximately four hydrogen bonds with other water molecules. The result is a hydrogen bonding network.
FIGURE 15.3. A central water molecule hydrogen bonded to four surrounding water molecules. The central water’s two hydroxyl hydrogens bond to two oxygens, and the central water’s oxygen accepts two hydroxyl bonds from two other water molecules.
At room temperature, there is enough heat so that hydrogen bonds between water molecules are continually breaking and new hydrogen bonds are formed with different water molecules. So the hydrogen bond network is not static. It is continually reforming and rearranging. The time scale for these hydrogen bond rearrangements has been measured with ultrafast infrared spectroscopy, and it is approximately 3 ps (picosecond, a trillionth of a second, 10-12 s).
Life is based on chemistry that occurs in water. Recent spacecraft sent to Mars are not looking directly for signs of past life, but rather for signs of past liquid water. Liquid water is so fundamental to life that its presence is a necessary and perhaps a sufficient condition for life to exist. Water’s amazing properties, which are essential to the biochemistry of life, are a result of its hydrogen bonding network structure and the ability of that structure to rearrange. Water’s properties allow it to accommodate a vast array of chemical processes necessary for life. For example, proteins fold in water. Proteins are the very large, extremely complex molecules that are responsible for most of the chemical processes in our bodies. When proteins are chemically produced by other proteins, initially they are not in the correct configuration to function. This is called unfolded. Proteins have regions that will readily form hydrogen bonds to water and regions that are more like hydrocarbon and do not want to mix with water. The protein rearranges its structure, folds, such that the hydrophilic (likes water)