2.2.2 Intermolecular and Intramolecular Interactions

Introduction

Apart from chemical bonds, various types of weak interactive forces exist between molecules and between different parts of the same molecule. These are known as intermolecular and intramolecular forces respectively. These interactions are very important in determining the physical properties of different substances and play crucial roles in biology.

Definition

Intermolecular Forces: Forces that exist between different molecules.

Intramolecular Forces: Forces that exist within the same molecule (between different parts of the same molecule).

• Intermolecular = between molecules
• Intramolecular = within a molecule

Core Concept

• These interactions are of various types and various strengths.
• They are very important in determining the physical properties of different substances.
• They play crucial roles in biology, influencing:
• The structure of biological molecules
• The function of biological molecules
• The behaviour of biological molecules
• Various cellular processes
• Protein folding, double helical structure of DNA, structure of cell membrane, molecular recognition, etc., would never be possible without these inter- and intramolecular interactions.

Detailed Explanation

2.2.2.1 Hydrogen Bonding

Definition: Hydrogen bonding is a type of intermolecular force which occurs between a hydrogen atom, bonded to a highly electronegative atom (i.e., oxygen, nitrogen, or fluorine) and a lone pair of electrons present on a neighbouring electronegative atom.

In simple words: A hydrogen bond is an attraction between a hydrogen atom (attached to an electronegative atom) and another electronegative atom nearby.

Key Points:

• They may be:
• Intermolecular – Between atoms of different molecules
• Intramolecular – Between atoms of the same molecule

• Double helical structure of DNA is due to hydrogen bonds present between nitrogenous bases:
• Three hydrogen bonds between cytosine and guanine
• Two hydrogen bonds between adenine and thymine

- Protein-DNA interactions and various other biological processes also involve hydrogen bonding.

Examples of Hydrogen Bonding in Biology:

2.2.2.2 Hydrophobic Interactions

Definition: Hydrophobic interactions are noncovalent interactions found between nonpolar molecules or nonpolar regions of molecules in the presence of water.

In simple words: Hydrophobic interactions are the tendency of nonpolar molecules to stick together in water.

Key Points:

• The term "hydrophobic" literally means "afraid of water".
• Water is a polar molecule.
• Hydrophobic molecules or regions minimize their contact with water by clustering and coming together.
• This hydrophobic effect has several important biological implications:
• Protein folding
• Membrane formation
• Ligand binding
• Protein-protein interactions

Examples of Hydrophobic Interactions in Biology:

2.2.2.3 Hydrophilic Interactions

Definition: The term "hydrophilic" literally means "water-loving". The molecules which show hydrophilic interactions are called hydrophilic molecules, which are found to be polar in nature.

In simple words: Hydrophilic interactions are the tendency of polar molecules to interact with water.

Key Points:

• Hydrophilic molecules have partially positive and partially negative regions/poles due to the uneven distribution of mutual shared electrons between the bonded atoms.
• Hydrogen is a hydrophilic process in which, when an ionic substance is placed in water, it dissociates into positive and negative ions, which are then surrounded by water molecules.
• This process helps in dissolution and stabilization of hydrophilic substances in aqueous solutions.
• Hydrophilic amino acid residues on the surface of proteins interact with water and help in maintaining solubility and stability of proteins in cellular environments.

Examples of Hydrophilic Interactions in Biology:

• Phospholipids arrange in such a way that their hydrophilic heads point outward and interact with water.
• Their hydrophobic tails are oriented towards each other and away from water.

• DNA and RNA have phosphate groups in their backbone, which makes them hydrophilic in nature.
• This enables them to dissolve and function effectively in the aqueous environment inside the nucleoplasm and the cytoplasm.

In Enzymes: - Hydrophilic active sites on various enzymes enable them to interact with water and participate in biochemical reactions and to catalyse them.

Intermediate Level

Comparison of Intermolecular and Intramolecular Interactions

Comparison of the Three Types of Interactions

Hydrogen Bonding in Detail

• A hydrogen atom bonded to a highly electronegative atom (O, N, or F).
• A lone pair of electrons on a neighbouring electronegative atom.

• Water: H₂O molecules form hydrogen bonds with each other.
• DNA: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds).
• Proteins: α-helix and β-pleated sheet structures.

• DNA double helix
• Protein structure
• Water properties
• Enzyme-substrate binding

Hydrophobic Interactions in Detail

• Water molecules are polar and form hydrogen bonds with each other.
• Nonpolar molecules cannot form hydrogen bonds with water.
• Therefore, water molecules push nonpolar molecules together.
• This minimizes the contact between water and nonpolar molecules.

• Cell membrane: Phospholipid tails face inward, away from water.
• Protein folding: Nonpolar amino acids cluster in the interior of the protein.
• Oil droplets: Oil drops join together in water.

• Membrane formation
• Protein folding
• Protein-protein interactions
• Ligand binding

Hydrophilic Interactions in Detail

• Hydrophilic molecules are polar.
• They have partially positive and partially negative regions.
• They interact with water molecules through hydrogen bonding.
• Ionic substances dissociate into ions and are surrounded by water molecules.

• Cell membrane: Phospholipid heads face outward, interacting with water.
• Proteins: Hydrophilic amino acids on the surface interact with water.
• DNA and RNA: Phosphate groups in backbone are hydrophilic.

• Protein solubility
• Enzyme function
• DNA and RNA function
• Cell membrane structure

Biological Importance of These Interactions

Important Points

• 1. Apart from chemical bonds, various types of weak interactive forces exist between molecules and between different parts of the same molecule.
• 2. These are known as intermolecular and intramolecular forces respectively.
• 3. These interactions are of various types and various strengths.
• 4. They are very important in determining the physical properties of different substances.
• 5. They play crucial roles in biology, influencing:
• - The structure of biological molecules
• - The function of biological molecules
• - The behaviour of biological molecules
• - Various cellular processes
• 6. Protein folding, double helical structure of DNA, structure of cell membrane, molecular recognition, etc., would never be possible without these inter- and intramolecular interactions.
• 7. Hydrogen Bonding:
• - Occurs between a hydrogen atom bonded to a highly electronegative atom (O, N, F) and a lone pair of electrons on a neighbouring electronegative atom.
• - May be intermolecular or intramolecular.
• - DNA double helix: 3 H-bonds between C and G; 2 H-bonds between A and T.
• - Protein-DNA interactions and various other biological processes involve hydrogen bonding.
• 8. Hydrophobic Interactions:
• - Noncovalent interactions between nonpolar molecules or nonpolar regions of molecules in the presence of water.
• - "Hydrophobic" means "afraid of water."
• - Water is polar; hydrophobic molecules minimize contact with water by clustering together.
• - Important biological implications: protein folding, membrane formation, ligand binding, protein-protein interactions.
• 9. Hydrophilic Interactions:
• - "Hydrophilic" means "water-loving."
• - Hydrophilic molecules are polar in nature.
• - They have partially positive and partially negative regions.
• - When an ionic substance is placed in water, it dissociates into positive and negative ions, which are surrounded by water molecules.
• - Helps in dissolution and stabilization of hydrophilic substances.
• - Hydrophilic amino acid residues on protein surface interact with water and help maintain solubility and stability.
• - Cell membrane: hydrophilic heads point outward, interact with water; hydrophobic tails face each other, away from water.
• - DNA and RNA: phosphate groups in backbone are hydrophilic; enable dissolution and function in aqueous environment.
• - Hydrophilic active sites on enzymes enable them to interact with water and catalyse reactions.

Remember

• - Intermolecular = between molecules
• - Intramolecular = within a molecule
• - Hydrogen bonding = H + electronegative atom (O, N, F)
• - Hydrophobic interactions = nonpolar molecules cluster in water
• - Hydrophilic interactions = polar molecules interact with water
• - DNA: A–T (2 H-bonds), G–C (3 H-bonds)
• - Protein folding: Hydrophobic interactions
• - Membrane formation: Hydrophobic and hydrophilic interactions
• - Protein solubility: Hydrophilic interactions
• - Enzyme function: Hydrogen bonding and hydrophilic interactions

Common Mistakes

• 1. Confusing intermolecular and intramolecular – Intermolecular = between different molecules; Intramolecular = within the same molecule.
• 2. Thinking hydrogen bonds are covalent bonds – Hydrogen bonds are weak interactions, not covalent bonds.
• 3. Confusing hydrophobic and hydrophilic – Hydrophobic = afraid of water (nonpolar); Hydrophilic = water-loving (polar).
• 4. Thinking hydrophobic interactions are actual bonds – They are interactions, not chemical bonds.
• 5. Forgetting the number of hydrogen bonds in DNA base pairs – A–T = 2 hydrogen bonds; G–C = 3 hydrogen bonds.
• 6. Thinking hydrogen bonds only occur between water molecules – Hydrogen bonds also occur in DNA, proteins, and other biological molecules.
• 7. Confusing the role of hydrophobic and hydrophilic interactions in membranes – Hydrophobic tails face inward; hydrophilic heads face outward.
• 8. Thinking these interactions are not important – They are crucial for protein folding, DNA structure, membrane formation, and many other processes.
• 9. Forgetting that hydrogen bonds are weak – Hydrogen bonds are weak individually but strong in large numbers.
• 10. Thinking hydrophilic molecules cannot dissolve in water – Hydrophilic molecules dissolve in water because they are polar.
• 11. Forgetting the role of hydrophilic interactions in enzyme function – Hydrophilic active sites interact with water and participate in catalysis.
• 12. Thinking hydrophobic interactions only occur in membranes – They also occur in protein folding, ligand binding, and protein-protein interactions.

Quick Revision

• Apart from chemical bonds, various types of weak interactive forces exist between molecules and between different parts of the same molecule.
• These are known as intermolecular and intramolecular forces respectively.
• These interactions are of various types and various strengths and are very important in determining the physical properties of different substances.
• They play crucial roles in biology, influencing the structure, function, and behaviour of biological molecules and various cellular processes.
• Protein folding, double helical structure of DNA, structure of cell membrane, molecular recognition, etc., would never be possible without these inter- and intramolecular interactions.
• Hydrogen Bonding:
• A type of intermolecular force which occurs between a hydrogen atom, bonded to a highly electronegative atom (i.e., oxygen, nitrogen, or fluorine) and a lone pair of electrons present on a neighbouring electronegative atom.
• They may be intermolecular or intramolecular.
• Double helical structure of DNA is due to hydrogen bonds present between nitrogenous bases: three hydrogen bonds between cytosine and guanine and two hydrogen bonds between adenine and thymine.
• Protein-DNA interactions and various other biological processes also involve hydrogen bonding.
• Hydrophobic Interactions:
• Noncovalent interactions found between nonpolar molecules or nonpolar regions of molecules in the presence of water.
• The term "hydrophobic" literally means "afraid of water."
• As water is a polar molecule, the hydrophobic molecules or regions minimize their contact with water by clustering and coming together.
• This hydrophobic effect has several important biological implications like protein folding, membrane formation, ligand binding, protein-protein interactions, etc.
• Hydrophilic Interactions:
• The term "hydrophilic" literally means "water-loving."
• The molecules which show hydrophilic interactions are called hydrophilic molecules, which are found to be polar in nature.
• They have partially positive and partially negative regions/poles due to the uneven distribution of mutual shared electrons between the bonded atoms.
• Hydrogen is a hydrophilic process in which, when an ionic substance is placed in water, it dissociates into positive and negative ions, which are then surrounded by water molecules.
• This process helps in dissolution and stabilization of hydrophilic substances in aqueous solutions.
• Hydrophilic amino acid residues on the surface of proteins interact with water and help in maintaining solubility and stability of proteins in cellular environments.
• In the cell membrane, phospholipids arrange in such a way that their hydrophilic heads point outward and interact with water while the hydrophobic tails are oriented towards each other and away from water.
• DNA and RNA have phosphate groups in their backbone, which makes them hydrophilic in nature and thus enable them to dissolve and function effectively in the aqueous environment inside the nucleoplasm and the cytoplasm.
• Hydrophilic active sites on various enzymes enable them to interact with water and participate in biochemical reactions and to catalyse them.

60-Second Revision

• Intermolecular = between molecules; Intramolecular = within a molecule
• Hydrogen bonding = H + electronegative atom (O, N, F); DNA: A–T (2), G–C (3)
• Hydrophobic interactions = nonpolar molecules cluster in water; protein folding, membrane formation
• Hydrophilic interactions = polar molecules interact with water; protein solubility, enzyme function
• Warning: These are weak interactions, not covalent bonds. Hydrophobic = afraid of water; Hydrophilic = water-loving. Hydrogen bonds are important in DNA and proteins. Do not confuse intermolecular and intramolecular.