Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Hydrogen bonds are widespread in biological systems and play a vital role in them

Hydrogen Bonds are not unique to Water. They readily form between any electronegative atom (typically oxygen or nitrogen) and a hydrogen atom covalently bonded to another electronegative atom within the same or a different molecule (Fig. 4-3). Hydrogen atoms covalently linked to strongly electronegative atoms, such as oxygen, always bear a partial positive charge and are therefore capable of forming hydrogen bonds. Conversely, hydrogen atoms covalently bound to carbon atoms, which lack significant electronegativity, carry no partial positive charge and consequently cannot form hydrogen bonds. This exact difference explains why butyl alcohol (CH₃CH₂CH₂CH₂OH)—whose molecule has a hydrogen atom bonded to oxygen and can thus form a Hydrogen bond with another butyl alcohol molecule—has a relatively high boiling point (+117 °C). In contrast, butane (CH₃CH₂CH₂CH₃), which is incapable of forming intermolecular hydrogen bonds because all of its hydrogen atoms are bonded to carbon, has a low boiling point (-0.5 °C). Several Examples of biologically important hydrogen bonds are illustrated in Fig. 4-4.

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Fig. 4-3. Hydrogen bonds. In bonds of this type, the hydrogen atom is shared asymmetrically between two electronegative atoms. The atom to which the hydrogen is covalently bonded acts as the hydrogen donor, whereas the electronegative atom of the other molecule serves as the acceptor. In biological systems, the electronegative atoms participating in hydrogen bonding are oxygen and nitrogen; carbon atoms participate in hydrogen bond formation only in rare instances. The distance between two electronegative atoms joined by a hydrogen bond typically ranges from 0.26 to 0.31 nm. Common types of hydrogen bonds are shown below.

One of the hallmark features of hydrogen bonds is that they are strongest when the relative orientation of the bonded molecules maximizes electrostatic interaction energy (Fig. 4-5). In other words, a hydrogen bond is highly directional, which allows it to hold the two bonded molecules or groups in a specific spatial orientation relative to each other. As we will see later, this precise property of hydrogen bonds plays a crucial role in stabilizing the highly ordered three-dimensional structures characteristic of PROTEIN AND NUCLEIC acid molecules, which contain numerous intramolecular hydrogen bonds (Chapters 7, 8, and 27).