Aqueous Chemistry: Water – Noncovalent Interactions and Hydrogen Bonding

Noncovalent Interactions Define the Structure and Function of Biomolecules

Overview of Noncovalent Bonding

Noncovalent interactions are essential for the structure and function of biomolecules, such as proteins, nucleic acids, and hormones. These interactions are generally weaker than covalent bonds but are crucial for dynamic biological processes.

• Noncovalent bonds include charge-charge, hydrogen bonding, van der Waals, and dipole interactions.

• They allow biomolecules to interact, recognize, and bind to each other transiently.

• Example: Human growth hormone binding to its cellular receptor is mediated by noncovalent interactions.

Covalent and Noncovalent Bond Energies

Relative Strengths of Bonds

Covalent bonds are much stronger than noncovalent bonds, which can be continually broken and reformed in biological systems.

• Covalent bond energies are typically >300 kJ/mol.

• Noncovalent bond energies range from 0.4 to 21 kJ/mol, depending on the type.

• 1 kcal = 4.184 J (energy conversion).

Type of Interaction	Approximate Energy (kJ/mol)
Charge-Charge	13 to 17
Hydrogen bond	2 to 21
Van der Waals	0.4 to 0.8

Types of Noncovalent Interactions

Classification and Examples

Noncovalent interactions are classified based on the nature of the interacting species. Each type plays a specific role in biomolecular structure and recognition.

Type of Interaction	Model	Example
Charge-Charge	Electrostatic attraction/repulsion between ions	Na+ and Cl-
Charge-Dipole	Ion interacting with a polar molecule	Na+ with H2O
Dipole-Dipole	Interaction between two polar molecules	H2O with H2O
Charge-Induced Dipole	Ion induces dipole in a nonpolar molecule	Na+ with benzene
Dipole-Induced Dipole	Polar molecule induces dipole in a nonpolar molecule	H2O with benzene
Dispersion (van der Waals)	Transient induced dipoles	Between nonpolar molecules
Hydrogen bond	Donor and acceptor atoms	O-H...O in water

Hydrogen Bonding

Nature and Importance of Hydrogen Bonds

Hydrogen bonds are a special type of dipole-dipole interaction, critical for the structure of water and biomolecules like proteins and nucleic acids.

• Hydrogen bond forms when a hydrogen atom covalently bonded to an electronegative atom (such as O or N) interacts with another electronegative atom.

• Creates a lower energy state and stabilizes molecular structures.

• Hydrogen bonds are directional and have specific donor and acceptor atoms.

Donor → Acceptor	Distance (Å)	Comment
O-H...O	2.8	H bond formed in water
N-H...O	2.9	Bonding of water to other molecules
N-H...N	3.0	Important in protein and nucleic acid structures
O-H...N	3.2	Weaker than above
C-H...O	3.7	Relatively weak

Hydrogen Bonding in Biological Molecules

Hydrogen bonds are abundant in biomolecules, stabilizing secondary and tertiary structures.

• Proteins: Hydrogen bonds between backbone amide and carbonyl groups stabilize alpha helices and beta sheets.

• Nucleic acids: Hydrogen bonds between base pairs hold the double helix together.

• H-bond donors: Typically N-H or O-H groups.

• H-bond acceptors: Typically O or N atoms with lone pairs.

Additional info:

• Noncovalent interactions are reversible and allow for dynamic biological processes such as enzyme-substrate binding and molecular recognition.

• Hydrogen bonds are essential for the unique properties of water, including its high boiling point and surface tension.