Water: The Solvent for Biochemical Reactions – Study Notes

Water: The Solvent for Biochemical Reactions

Intermolecular Forces of Attraction

Intermolecular forces of attraction (IMFA) are the forces that determine whether matter exists as a solid, liquid, or gas under specific conditions. These forces are crucial in understanding how molecules interact in biological systems.

• Definition: IMFA are the attractive forces between molecules, not within them (which are intramolecular).

• Principle: If IMFA are strong, molecules interact closely; if weak, they remain separate.

• "Like dissolves like": Polar molecules dissolve in polar solvents; nonpolar molecules dissolve in nonpolar solvents.

Polarity of Molecules

Molecular polarity refers to the uneven distribution of electrons within a molecule, resulting in regions of partial positive and negative charge. This property is fundamental to the behavior of water and many biological molecules.

• Polarity: Caused by differences in electronegativity between atoms in a molecule.

• Electronegativity: The tendency of an atom to attract electrons. The mnemonic "FONX" (Fluorine, Oxygen, Nitrogen, and halogens) helps remember the most electronegative elements.

• Example: Water (H2O) is a polar molecule due to the high electronegativity of oxygen compared to hydrogen.

Non-covalent Interactions

Non-covalent interactions are weak, reversible forces that play a critical role in the structure and function of biological molecules. They arise from differences in polarity and include several types:

• van der Waals (London Dispersion Forces):

  • Weakest IMFA; present between all molecules.

  • Result from temporary (induced) dipoles due to electron movement.

• Dipole-Dipole or Ion-Dipole Interactions:

  • Occur when two polar molecules or a polar molecule and an ion interact.

  • Can be attractive (opposite charges) or repulsive (like charges).

  • Primarily electrostatic in nature.

  • Example: Sodium ion (Na+) interacting with water molecules.

• Hydrogen Bonding:

  • A special type of dipole-dipole interaction involving hydrogen bonded to N, O, or F.

  • Strength of hydrogen bonding follows the trend: N–H < O–H < F–H (increasing strength with electronegativity).

  • Critical for the structure of DNA, proteins, and water's unique properties.

  • Example: Hydrogen bonds between water molecules or between base pairs in DNA.

Water (H2O)

Water is essential for life, serving as the medium in which biochemical reactions occur. Its unique properties stem from its molecular structure and polarity.

• Biological Importance:

  • Life originated and evolved in water-rich environments.

  • Organisms are typically 70–90% water.

  • Metabolic activities require cells to be at least 65% water.

• Cellular Context:

  • Cells are surrounded and filled with water (cytoplasm).

  • Water forms an aqueous solution for ions, proteins, and metabolites.

  • Polarity enables water to dissolve salts and polar molecules, facilitating transport and reactions.

  • Nonpolar substances require special transport mechanisms due to water's properties.

Summary Table: Types of Non-covalent Interactions

Type of Interaction	Description	Relative Strength	Biological Example
van der Waals (London Dispersion)	Temporary dipoles between all molecules	Weakest	Lipid tail interactions in membranes
Dipole-Dipole	Attraction between polar molecules	Intermediate	Protein folding
Ion-Dipole	Interaction between ion and polar molecule	Intermediate	Na+ in water
Hydrogen Bonding	H atom bonded to N, O, or F interacts with another electronegative atom	Strongest (among non-covalent)	DNA base pairing, water structure

Additional info: These foundational concepts are essential for understanding how water acts as a solvent in biochemical reactions, influencing molecular interactions, cellular structure, and metabolic processes.