Читать книгу Astrobiology - Charles S. Cockell - Страница 52

Finally, we come to hydrogen bonding, which plays an enormously important role in biological processes. It is found in molecules containing an OH group (a hydroxyl group) including water, ethanol, methanol, and numerous other molecules. Like van der Waals interactions, hydrogen bonding is weaker than ionic and covalent bonds. Hydrogen bonds are typically 10–100 times weaker than covalent bonds, depending on the molecules involved.

A hydrogen atom, having one electron, can be covalently bonded to only one atom. However, the hydrogen atom can involve itself in an additional electrostatic bond with a second atom of highly electronegative character, such as fluorine or oxygen. This second bond is a hydrogen bond.

How does hydrogen bonding work? Let's go through the basic points using water:

 The charge density in a covalent bond such as an OH bond is highly asymmetric, and the center of charge is much closer to the O atom, as we discovered when discussing the dipole in HCl. For the same reasons, the OH bond also has this dipole character.

 This leaves the net positively charged H atom behaving more like a lone, positively charged proton (H+).

 Other electrons on the O atom distribute themselves so as to minimize repulsion (Figure 3.15, green lobes). These electrons form lobes of electron density on the opposite side of the O atom to the OH bond.

 Another H2O molecule orients itself so that its positively charged H is now close to these negatively charged electrons.

Figure 3.15 Hydrogen bonding in water ice. The dotted lines show the hydrogen bonding. The green lobes are the electrons on oxygen that take part in the interaction with the hydrogen atom on other water molecules.

You can see the result of these interactions much more clearly in Figure 3.15, which shows the hydrogen bonding in water ice depicted in two dimensions.