1. Hydrogen Bonds

1.1 Importance of Hydrogen Bonds

• Water accounts for ~70% of cell weight; most reactions occur in aqueous environments.
• Hydrogen bond (氢键): weak electrical attraction between a partially positive H and a partially negative atom (e.g., O or N).
• Hydrogen bonds explain:
  • Water’s high boiling point and surface tension.
  • Water being liquid at room temperature.
  • Stability of many biological molecules.

1.2 Hydrogen Bonds in Biological Molecules

• Not limited to water — occur whenever H in a polar covalent bond approaches O/N in another molecule.
• Stabilize protein folding (蛋白质折叠) and DNA double helix (DNA双螺旋).
• Continuous formation and breaking of hydrogen bonds → dynamic biological interactions.

1.3 Hydrophilic and Hydrophobic Substances

• Hydrophilic (亲水性): molecules with polar bonds; dissolve easily in water (e.g., sugars, DNA, RNA, proteins).
• Hydrophobic (疏水性): nonpolar molecules; avoid water and cluster together (e.g., lipids).

2. Types of Noncovalent Bonds

2.1 Overview

Noncovalent bonds are weaker than covalent bonds but crucial for biological specificity and molecular interactions.

2.2 Main Types of Noncovalent Bonds

• a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. | Stabilizes protein folding, DNA double helix, and enzyme-substrate recognition 酶-底物识别. | | Electrostatic attraction (静电吸引) | Attraction between opposite charges. Two scenarios:

• Ionic bonds (离子键): Strongest when atoms are fully charged (e.g., Na⁺ and Cl⁻).

• Partial-charge interactions: Weaker, occur between polar molecules with partially positive and negative regions. | Critical for protein-protein binding, enzyme-substrate docking, and interactions between charged biomolecules. Even though water reduces their strength, many weak electrostatic attractions together can still enable strong and specific molecular recognition. | | Van der Waals forces (范德华力) | Weak attractions from transient dipoles due to electron fluctuations

More details see below. | Stabilize protein-ligand 蛋白-配体 and membrane interactions | | Hydrophobic interactions (疏水作用力) | Nonpolar regions cluster away from water - strictly speaking, not a bond at all | Membrane formation, protein folding |

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Review: The 3+1 Types of Non-covalent Bonds

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