Hydrogen bonds present a paradox in chemistry and biology. They are technically classified as weak forces, yet they are collectively responsible for many properties of life, including the structure of DNA and the unique behavior of water. A hydrogen bond is not a true chemical bond, like a covalent or ionic bond, but rather a powerful intermolecular attraction. This intermediate strength allows biological systems to be stable enough to hold shape but dynamic enough to change and react rapidly.
The Mechanism of Hydrogen Bonding
A hydrogen bond forms through a specialized type of electrostatic attraction. It requires a hydrogen atom covalently bonded to a highly electronegative atom, typically Fluorine, Oxygen, or Nitrogen. This covalent bond is highly polarized because the electronegative atom pulls the shared electrons toward itself. This unequal sharing creates a dipole moment, giving the hydrogen atom a partial positive charge (the donor). This partially positive hydrogen is then electrostatically attracted to a nearby electronegative atom (the acceptor), which has a partial negative charge and a lone pair of electrons. The resulting hydrogen bond is represented by a dashed line to distinguish it from a stronger covalent bond.
Hydrogen Bonds: The Strongest of the Weak
Hydrogen bonds are weak when compared to primary bonds that hold atoms together. Breaking a typical covalent bond, such as an O-H bond, requires about 464 kilojoules per mole (kJ/mol). In contrast, the energy required to break a single hydrogen bond is far lower, generally ranging from 10 to 40 kJ/mol. This difference places them in the category of non-covalent interactions.
However, hydrogen bonds are the strongest of all intermolecular forces, surpassing weaker attractions like van der Waals forces and simple dipole-dipole interactions. This intermediate strength makes them suited for biological functions, providing temporary, dynamic stability. They are strong enough to maintain molecular shape but weak enough to be easily broken and reformed by thermal energy in a cellular environment. This dynamic capability is fundamental to processes like DNA replication, where strands must temporarily separate and quickly re-anneal.
Structural Roles in Biological Molecules
The collective strength of hydrogen bonds gives water its remarkable properties, which are foundational to life. Water molecules form extensive, three-dimensional networks where each molecule can participate in up to four hydrogen bonds. This network gives water a high boiling point and high heat capacity, allowing it to absorb heat without drastic temperature changes, stabilizing cellular environments.
In proteins, hydrogen bonds are essential for creating the secondary structure that dictates the molecule’s final shape and function. These bonds form between the backbone atoms of the polypeptide chain, specifically between the oxygen atom of one amino acid and the hydrogen atom attached to the nitrogen atom of another. This regular pattern stabilizes the two most common protein motifs: the alpha-helix and the beta-sheet.
In genetic material, hydrogen bonds ensure the fidelity of the double helix structure. They precisely pair complementary nitrogenous bases: Adenine (A) pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) pairs with Cytosine (C) via three hydrogen bonds. The cumulative effect of these weak bonds holds the two DNA strands together. This structure is stable enough to store genetic information but allows the bonds to unzip easily during transcription and replication.
Factors That Disrupt Hydrogen Bonds
Because hydrogen bonds are relatively weak, they are susceptible to disruption by external forces.
Thermal Energy
The most common factor is the application of heat. Increasing the temperature causes molecules to move more vigorously, increasing their kinetic energy until it exceeds the energy holding the hydrogen bonds together. This process is seen when water boils, or when a protein unfolds in a process called denaturation.
Changes in pH
Changes in pH can interfere with hydrogen bonds by altering the charge distribution within a molecule. Introducing excess hydrogen ions (low pH) or hydroxide ions (high pH) changes the electrical environment, preventing the necessary partial positive and negative charges from aligning correctly to form the attraction.
Competing Solvents
The introduction of competing solvents can dismantle a hydrogen bond network. Polar solvents, such as urea or certain alcohols, can insert themselves into the existing network and form new hydrogen bonds with the donor and acceptor atoms. This competitive interaction disrupts the original structure by offering an alternative bonding partner.