Unit 1: Principles of biophysical chemistry and Carbohydrates - Subjective Questions

Chemical bonds are the attractive forces that hold atoms together in molecules. In biomolecules, both strong covalent bonds and weak non-covalent interactions play crucial roles.

Weak Stabilizing Interactions:

• Hydrogen bonds: Electrostatic attraction between an electronegative atom and a hydrogen atom covalently bonded to another electronegative atom (N, O, F). Bond energy ≈ 2–8 kJ/mol. Vital in DNA base pairing and protein secondary structure.
• Ionic (electrostatic) interactions: Attraction between oppositely charged groups (e.g., and ). Also called salt bridges.
• Van der Waals forces: Weak, transient attractions arising from fluctuating dipoles between closely packed atoms (≈ 0.4–4 kJ/mol).
• Hydrophobic interactions: Tendency of non-polar groups to cluster together in aqueous environments to minimize contact with water, driven by entropy.

Significance:

• Individually weak but collectively strong.
• Provide specificity and reversibility essential for enzyme-substrate binding, protein folding, and nucleic acid structure.

pH is defined as the negative logarithm of the hydrogen ion concentration:

Derivation of Henderson-Hasselbalch Equation:

Consider a weak acid dissociating as:

The dissociation constant is:

Rearranging for :

Taking negative logarithm on both sides:

Therefore:

Usefulness in Buffer Action:

• When , then , giving maximum buffering capacity.
• Explains how buffers resist pH change on adding small amounts of acid or base.
• Helps in preparing buffers of desired pH by adjusting the salt-to-acid ratio.

A buffer is a solution that resists changes in pH upon the addition of small amounts of acid or base. It usually consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).

Example: Acetic acid/acetate buffer ().

Mechanism of Buffer Action:

• On adding acid (): The conjugate base neutralizes it:

• On adding base (): The weak acid neutralizes it:

Important Biological Buffers:

• Bicarbonate buffer () — main blood buffer.
• Phosphate buffer () — intracellular buffer.
• Protein buffers — via ionizable side chains.

Buffers are essential for maintaining physiological pH (≈ 7.4) required for enzyme activity and metabolic processes.

Bioenergetics is the quantitative study of energy transformations that occur in living organisms and the chemical processes underlying these transformations.

Laws of Thermodynamics:

• First Law (Law of Conservation of Energy): Energy can neither be created nor destroyed, only converted from one form to another.

where is heat absorbed and is work done. In cells, chemical energy is converted to mechanical, electrical, and osmotic work.

• Second Law: In any spontaneous process, the total entropy () of the universe increases.

Living systems maintain order (low entropy) by increasing the entropy of their surroundings.

Relevance to Biological Systems:

• Cells are open systems exchanging matter and energy with surroundings.
• Organisms maintain a steady state, not equilibrium (equilibrium = death).
• Coupling of exergonic and endergonic reactions drives life processes.

Gibbs Free Energy () is the amount of energy in a system available to do useful work at constant temperature and pressure. The change in free energy is given by:

where = enthalpy change, = absolute temperature, = entropy change.

Significance of :

• : reaction is spontaneous (exergonic).
• : reaction is non-spontaneous (endergonic).
• : system is at equilibrium.

Distinction between Exergonic and Endergonic Reactions:

The relationship with equilibrium constant is:

Coupled Reactions are those in which an energetically unfavorable (endergonic) reaction is driven by an energetically favorable (exergonic) reaction, sharing a common intermediate.

Principle:

If reaction 1 has and reaction 2 has , coupling is favorable when:

ATP as Energy Currency:

• ATP (Adenosine Triphosphate) contains high-energy phosphoanhydride bonds.
• Hydrolysis of ATP is highly exergonic:

Group Transfer:

• ATP transfers a phosphoryl group to substrates (phosphorylation) rather than merely hydrolyzing.
• Example: In glycolysis, ATP phosphorylates glucose to glucose-6-phosphate (coupled reaction).

Role:

• ATP links catabolism (energy release) and anabolism (energy consumption).
• Acts as the universal energy transducer in cells.

Reaction Kinetics is the study of the rate (speed) of chemical reactions and the factors affecting them, such as concentration, temperature, and catalysts.

Rate of Reaction: Change in concentration of reactant or product per unit time.

Zero-Order Reaction:

• Rate is independent of reactant concentration.

• Integrated form:
• Units of :
• Example: Enzyme-catalyzed reactions at saturating substrate concentration.

First-Order Reaction:

• Rate is directly proportional to reactant concentration.

• Integrated form:
• Units of :
• Example: Radioactive decay, many hydrolysis reactions.

Biological Energy Transducers are systems or molecules that convert one form of energy into another usable form to drive cellular processes.

Major Types:

• Chloroplasts (Photosynthesis):

  • Convert light energy into chemical energy (ATP and NADPH).
  • Site: thylakoid membranes; involves photosystems I and II.

• Mitochondria (Oxidative Phosphorylation):

  • Convert chemical energy of nutrients into ATP.
  • Electron transport chain generates a proton-motive force across the inner membrane.

• ATP Synthase:

  • A molecular motor that uses the proton gradient to synthesize ATP from ADP and .

• Muscle proteins (Actin-Myosin):

  • Convert chemical energy of ATP into mechanical work (movement).

• Rhodopsin (Vision):

  • Converts light energy into nerve impulses.

Common Principle: Most transducers rely on membrane potentials and electrochemical gradients to couple energy transformation efficiently (chemiosmotic theory).

Standard Free Energy Change () is the free energy change of a reaction under standard conditions: 1 M concentration of reactants and products, 298 K, and 1 atm pressure. In biochemistry, is used (pH 7).

Relationship with Equilibrium Constant:

For a reaction , the actual free energy change is:

At equilibrium, and . Therefore:

Rearranging:

Interpretation:

• If , then (reaction favors products).
• If , then (reaction favors reactants).
• If , then .

where and is temperature in Kelvin.

Stereoisomerism refers to isomers that have the same molecular formula and sequence of bonded atoms but differ in the three-dimensional orientation of their atoms in space. It arises in sugars due to the presence of asymmetric (chiral) carbon atoms.

A molecule with chiral centers has stereoisomers.

Types:

• Enantiomers:

  • Non-superimposable mirror images of each other.
  • Differ in configuration at all chiral centers.
  • Example: D-glucose and L-glucose.

• Epimers:

  • Differ in configuration at only one chiral center.
  • Example: D-glucose and D-galactose (differ at C-4); D-glucose and D-mannose (differ at C-2).

• Anomers:

  • Isomers that differ in configuration at the anomeric carbon (C-1 in aldoses) formed during ring closure.
  • Example: α-D-glucose and β-D-glucose.

D and L Notation: Based on the configuration of the highest-numbered asymmetric carbon relative to glyceraldehyde.

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis). General formula: .

Classification:

1. Monosaccharides (Simple sugars):

• Cannot be hydrolyzed into simpler sugars.
• Classified by number of carbons: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C).
• Classified by functional group: aldoses (aldehyde) and ketoses (ketone).
• Examples: Glucose, Fructose, Ribose, Galactose.

2. Oligosaccharides (2–10 units):

• Disaccharides (2 units): Sucrose, Maltose, Lactose.
• Trisaccharides: Raffinose.

3. Polysaccharides (>10 units):

• Homopolysaccharides (same monomer): Starch, Glycogen, Cellulose.
• Heteropolysaccharides (different monomers): Hyaluronic acid, Heparin.

Based on functional role:

• Storage: Starch, Glycogen.
• Structural: Cellulose, Chitin.

Based on reducing property:

• Reducing sugars: Glucose, Maltose, Lactose.
• Non-reducing sugars: Sucrose.

Glucose () is an aldohexose — a six-carbon sugar with an aldehyde group at C-1.

Structure:

• Open-chain (Fischer) form: Contains one aldehyde group and five hydroxyl groups with 4 chiral centers (C-2, C-3, C-4, C-5), giving stereoisomers.
• Cyclic (Haworth) form: The C-1 aldehyde reacts intramolecularly with the C-5 hydroxyl to form a stable six-membered pyranose ring (hemiacetal).
• Ring formation creates a new chiral center at C-1 (anomeric carbon), producing α and β anomers.

Mutarotation:

• Definition: The spontaneous change in optical rotation of a sugar solution until it reaches an equilibrium value.
• α-D-glucose has specific rotation and β-D-glucose has .
• On standing in solution, both interconvert through the open-chain form to reach an equilibrium value of .

Mutarotation confirms the existence of both open-chain and cyclic forms in equilibrium.

Aldoses vs Ketoses:

Reducing Sugars:

• Sugars that possess a free aldehyde or ketone group (or a free anomeric carbon) capable of reducing oxidizing agents like Fehling's or Benedict's solution.
• The anomeric carbon is not involved in glycosidic bond formation.
• Examples: Glucose, Fructose, Maltose, Lactose.

Non-Reducing Sugars:

• Sugars in which the anomeric carbons of both monosaccharides are linked in the glycosidic bond, leaving no free reducing group.
• Examples: Sucrose, Trehalose.

Test: Reducing sugars give a brick-red precipitate () with Benedict's reagent; non-reducing sugars do not react.

Disaccharides are formed by joining two monosaccharides through a glycosidic bond with the loss of a water molecule.

1. Maltose (Malt sugar):

• Composition: Two glucose units.
• Linkage: α-1,4-glycosidic bond.
• Reducing sugar (free anomeric carbon present).
• Formed during starch digestion; found in germinating grains.

2. Lactose (Milk sugar):

• Composition: Galactose + Glucose.
• Linkage: β-1,4-glycosidic bond.
• Reducing sugar.
• Present in milk; hydrolyzed by the enzyme lactase. Its deficiency causes lactose intolerance.

3. Sucrose (Table sugar):

• Composition: Glucose + Fructose.
• Linkage: α-1,β-2-glycosidic bond (both anomeric carbons involved).
• Non-reducing sugar.
• Main transport sugar in plants; hydrolyzed by invertase to give invert sugar.

Importance: Serve as dietary energy sources and transport/storage forms of carbohydrates.

All three are homopolysaccharides of glucose but differ in linkage, branching, and function.

1. Starch (Plant storage):

• Composed of two components:
  • Amylose: Linear chain of glucose with α-1,4 linkages (helical).
  • Amylopectin: Branched, with α-1,4 linkages and α-1,6 branch points every 24–30 units.
• Function: Energy storage in plants.

2. Glycogen (Animal storage):

• Highly branched glucose polymer with α-1,4 linkages and α-1,6 branches every 8–12 units (more branching than amylopectin).
• Stored in liver and muscle.
• Function: Energy reserve in animals; rapid glucose mobilization.

3. Cellulose (Structural):

• Linear polymer of glucose with β-1,4 glycosidic linkages.
• Forms strong fibers via hydrogen bonding between chains.
• Function: Structural support in plant cell walls.
• Not digestible by humans (lack of cellulase enzyme); serves as dietary fiber.

Comparison Table:

The hydrophobic effect refers to the tendency of non-polar (hydrophobic) molecules or groups to aggregate together in an aqueous environment, minimizing their contact with water.

Thermodynamic Basis:

• Water molecules around a non-polar solute form ordered, cage-like structures (clathrates), decreasing entropy.
• When non-polar groups cluster, ordered water is released, increasing the entropy of the system.
• This is entropy-driven; overall makes aggregation favorable.

Role in Protein Folding:

• Non-polar amino acid side chains (Val, Leu, Ile, Phe) cluster in the interior core of the protein, away from water.
• This drives the collapse of the polypeptide into a compact tertiary structure.

Role in Membrane Formation:

• Phospholipids have hydrophilic heads and hydrophobic tails.
• In water, they spontaneously arrange into a bilayer with tails inward and heads facing water.
• This self-assembly is the basis of all biological membranes.

High-Energy Compounds are molecules that liberate a large amount of free energy ( more negative than kJ/mol) upon hydrolysis of specific bonds.

Why ATP is a High-Energy Compound:

ATP contains two phosphoanhydride bonds that release large energy upon hydrolysis:

Reasons for high energy of hydrolysis:

• Electrostatic repulsion: Adjacent negative charges on phosphate groups repel each other; hydrolysis relieves this strain.
• Resonance stabilization: The released has more resonance forms than in ATP.
• Increased entropy and better solvation of products.

Other High-Energy Compounds (with ):

• Phosphoenolpyruvate (PEP): kJ/mol
• 1,3-Bisphosphoglycerate: kJ/mol
• Creatine phosphate: kJ/mol
• Acetyl-CoA (thioester bond): kJ/mol

ATP occupies an intermediate position, allowing it to act as a common energy carrier between high- and low-energy phosphate compounds.

Group Transfer Reactions are biochemical reactions in which a chemical group (such as phosphoryl, acyl, glycosyl, or amino group) is transferred from a donor molecule to an acceptor molecule.

General Form:

where is the transferred group.

Types of Group Transfers:

• Phosphoryl transfer: Transfer of group (e.g., by kinases using ATP).
• Acyl transfer: Transfer of acyl groups (e.g., Acetyl-CoA in fatty acid synthesis).
• Glycosyl transfer: Transfer of sugar residues (glycosyltransferases).
• Amino transfer: Transamination reactions (aminotransferases).

Group Transfer Potential:

• The tendency of a compound to donate a group is measured by its group transfer potential, expressed as the negative of of hydrolysis.
• ATP has high phosphoryl group transfer potential.

Significance in Coupling:

• Group transfer via common intermediates allows energy from exergonic reactions to drive endergonic ones.
• Example: Glucose + ATP → Glucose-6-phosphate + ADP couples ATP hydrolysis to glucose phosphorylation.
• This is the fundamental mechanism by which cells conserve and utilize energy.

Monosaccharides are the simplest carbohydrates that cannot be hydrolyzed further. General formula where to .

Structure:

• Contain a carbonyl group (aldehyde or ketone) and multiple hydroxyl groups.
• Possess one or more chiral (asymmetric) carbons, giving rise to optical isomerism.
• Classified by carbon number (triose, tetrose, pentose, hexose) and carbonyl type (aldose/ketose).

Properties:

• Colorless, crystalline, water-soluble solids with sweet taste.
• Optically active (rotate plane-polarized light).
• Reducing sugars (free carbonyl group).
• Undergo mutarotation in solution.

Formation of Ring (Hemiacetal) Structure:

• In aqueous solution, the carbonyl carbon reacts with a hydroxyl group of the same molecule (intramolecular reaction).
• For glucose (aldose): C-1 aldehyde reacts with C-5 hydroxyl to form a stable six-membered pyranose ring (hemiacetal).

• For fructose (ketose): C-2 ketone reacts with C-5 hydroxyl to form a five-membered furanose ring (hemiketal).
• Ring formation generates a new asymmetric center called the anomeric carbon, producing α and β anomers.

First-Order Reaction: The rate depends linearly on the concentration of one reactant.

Derivation of Integrated Rate Law:

Separating variables and integrating:

Derivation of Half-Life ():

Half-life is the time at which . Substituting:

Significance:

• Half-life of a first-order reaction is independent of initial concentration.
• Used to determine the stability and turnover of biomolecules (e.g., mRNA, proteins, drugs).
• Important in pharmacokinetics for calculating drug dosage intervals.
• Applied in radioactive decay and enzyme inactivation studies.