Weak Interactions in Biological Systems

Types of Weak Interactions

Weak interactions are fundamental to the structure and function of biomolecules. They are non-covalent and reversible, allowing dynamic molecular processes.

• Hydrogen Bonds: Formed between a hydrogen atom covalently bonded to an electronegative atom (donor) and another electronegative atom (acceptor). - Donors: Typically N-H or O-H groups. - Acceptors: Atoms like O or N with lone pairs. - Strength: Strongest when bond angle is linear and distance is short. - Example: Base pairing in DNA.

• Ionic Interactions: Electrostatic attraction between oppositely charged ions. - Coulomb’s Law: Strength depends on charge, distance, and dielectric constant of the medium. - Effect of Water: Water’s high dielectric constant weakens ionic interactions. - Equation: where and are charges, is distance, is dielectric constant.

• Van der Waals Forces: Weak, short-range interactions between all atoms. - Dipole-Dipole: Between permanent dipoles. - Induced Dipole: Temporary dipoles induced in atoms/molecules. - London Dispersion: Fluctuating electron distributions create temporary dipoles. - Distance: Strength increases as atoms approach, but repulsion occurs if too close.

Distance Dependence: All weak interactions decrease rapidly with increasing distance.

Structure and Properties of Water

Chemical Structure

Water (H2O) is a polar molecule with a bent geometry, leading to a partial negative charge on oxygen and partial positive charges on hydrogens.

• Hydrogen Bonding: Each water molecule can form up to four hydrogen bonds.

• Emergent Properties:

  • High Specific Heat: Water absorbs heat without large temperature changes.

  • High Dielectric Constant: Makes water an excellent polar solvent.

  • Density: Liquid water is denser than ice due to less ordered hydrogen bonding.

  • Cohesiveness: Water molecules stick to each other (surface tension).

  • Adhesiveness: Water sticks to other polar/charged substances.

Hydrophilicity and Hydrophobicity

Solubility Prediction

Hydrophilic molecules are water-soluble due to polar or charged groups; hydrophobic molecules are insoluble due to nonpolar groups.

• Hydrophilic: Amino acids, sugars, ions.

• Hydrophobic: Lipids, hydrocarbons.

• Amphipathic: Molecules with both polar and nonpolar regions (e.g., phospholipids).

Common Functional Groups in Biochemistry

Functional Groups and Properties

• Methyl (–CH3): Nonpolar, hydrophobic.

• Methylene Bridge (–CH2–): Nonpolar, hydrophobic.

• Amino (–NH2): Basic, can accept protons.

• Hydroxyl (–OH): Polar, forms hydrogen bonds.

• Carboxyl (–COOH): Acidic, can donate protons.

• Carbonyl (–C=O): Polar, found in aldehydes and ketones.

• Phosphoryl (–PO32–): Highly polar, involved in energy transfer.

• Sulfhydryl (–SH): Can form disulfide bonds, important in protein structure.

Hydrophobic Effect and Self-Assembly

Hydrophobic Interactions

Hydrophobic molecules aggregate in water to minimize disruption of hydrogen bonding, increasing entropy of water.

• Micelles: Spherical structures formed by amphipathic molecules.

• Liposomes: Bilayer vesicles formed by amphipathic molecules.

• Bilayers: Double-layered structures, basis of cell membranes.

• Protein Folding: Driven by hydrophobic effect.

Condensation and Hydrolysis Reactions

Reaction Types

• Condensation: Two molecules join, releasing water (e.g., peptide bond formation).

• Hydrolysis: Water is used to break a bond (e.g., digestion of proteins).

Ionization of Water and pH Scale

Water Ionization

• Equation:

• pH:

• Strong Acids/Bases: Fully dissociate; pH calculated directly from concentration.

Ionization of Weak Acids and Acid Dissociation Constant

Ka and pKa

• Ka: Acid dissociation constant; higher Ka means stronger acid.

• pKa: ; lower pKa means stronger acid.

• Equation:

Henderson-Hasselbalch Equation

Acid/Base Ratio and pH

• Equation:

• Application: Used to determine ratio or percentage of acid/conjugate base.

• Charge State Prediction: Compare pH and pKa to predict ionization.

Weak Acid Titration Curves and Buffering

Titration and Buffer Capacity

• Titration Curve: Plots pH vs. amount of base added; inflection point at pKa.

• Buffering Range: Effective within ±1 pH unit of pKa.

• Buffer Capacity: Depends on concentration and pKa.

• Suitable Buffer: Choose weak acid with pKa near desired pH.

Physiological Buffers: Carbonic Acid/Bicarbonate System

Blood Buffering

• Equation:

• Role: Maintains blood pH; connects to cellular metabolism and membrane transport.

Polyprotic Weak Acids and Isoelectric Point

Amino Acids and Peptides

• Polyprotic Acid: Can lose more than one proton (e.g., amino acids).

• Charge Determination: Use pKa values and pH to predict charge state.

• Isoelectric Point (pI): pH at which molecule is electrically neutral.

• Equation for pI (for amino acids with two pKa's):

Surface Charge, pH, Solubility, and Ionic Strength

Macromolecules

• Surface Charge: Depends on pH and ionizable groups.

• Solubility: Influenced by ionic strength and charge.

• Macroions: Large molecules with multiple charges.

• Counterion Atmosphere: Cloud of oppositely charged ions affects solubility.

Additional Terms and Concepts

Definitions

• Hydration Shell: Layer of water molecules oriented around charged/dipolar solutes.

• Amphipathic: Molecule with both polar and nonpolar regions.

• Clathrate: Cage-like structure of water around hydrophobic groups.

• Conjugate Base: Species formed after acid loses a proton.

• Micelle: Spherical aggregate of amphiphilic molecules.

• Liposome: Bilayer vesicle formed from amphiphilic molecules.

• Bilayer: Double layer of amphiphilic molecules.

• Metabolic Acidosis/Alkalosis: Low/high pH due to metabolic buffering issues.

• Ampholytes: Molecules with both acidic and basic groups.

• Zwitterion: Molecule with both positive and negative charges, overall neutral.

• Macroion: Large molecule with multiple ionizable groups.

• Counterion Atmosphere: Surrounding cloud of small, oppositely charged ions.

Table: Functional Groups and Their Properties

Table: Types of Weak Interactions

Table: Buffering and Acid/Base Properties

Additional info: Academic context was added to expand brief points into full explanations, including equations, examples, and tables for clarity.