Summary of "2. Chemical Bonding and Molecular Interactions; Lipids and Membranes"
Course context and goals
This lecture introduces the molecules of life and the chemical principles needed to understand them. Over the coming classes the course will cover carbohydrates, amino acids/peptides, nucleosides/nucleic acids, phospholipids, polymerization, and supramolecular structures (micelles, liposomes, lipid bilayers).
Emphasis is placed on both covalent structure (the stable molecular framework) and non‑covalent interactions (weaker, dynamic forces that determine folding, assembly, binding, and function).
Scale and units
Important length units used in chemistry and biochemistry:
- picometer (pm) = 10^-12 m
- angstrom (Å) = 10^-10 m (commonly used by chemists/biochemists; 10 Å = 1 nm)
- nanometer (nm) = 10^-9 m
- micrometer (µm) = 10^-6 m
Overview of the molecules of life
Key molecular classes:
- Proteins
- Carbohydrates
- Nucleic acids (DNA/RNA)
- Lipids (especially phospholipids)
Representative structural ideas:
- 3D protein structures are folded and stabilized by non‑covalent forces.
- Carbohydrates function in energy storage and cell‑surface signaling.
- Double‑stranded DNA has a covalent backbone with non‑covalent base pairing.
- Lipid bilayers form cell boundaries and enable compartmentalization.
Composition of living systems
- Cells are roughly 75% water—non‑covalent interactions occur in an aqueous environment, making the hydrophobic/hydrophilic balance central.
- Most macromolecular mass is built from six elements: H, C, N, O, P, S (≈98% of cellular mass).
- Other important ions: Na+, K+, Mg2+, Ca2+ and trace transition metals (Fe, Cu, Mn, Zn, etc.). Very small amounts of elements like I and Se are physiologically essential.
Covalent bonding basics (refresher)
Covalent bonds between the primary biological elements are the main structural bonds of macromolecules. Typical valences and neutral bonding states:
- Carbon: forms 4 bonds (neutral with 4 covalent bonds)
- Nitrogen: typically 3 bonds + 1 lone pair (can be protonated to carry a positive charge)
- Oxygen: typically 2 bonds + 2 lone pairs (can be protonated or deprotonated)
- Sulfur: chemistry similar to oxygen (sulfhydryl and thiolate forms)
- Phosphorus: commonly found in oxidized forms (phosphate groups) that can have multiple oxygens and carry negative charges (critical in ATP, nucleic acids, phosphorylation)
Lone pairs on N and O are central to hydrogen bonding and electrostatic interactions.
Common functional groups in biology
- Hydroxyl (–OH)
- Carboxyl / carboxylate (–COOH / –COO–) — mostly deprotonated (negative) at physiological pH
- Amine / protonated amine (–NH2 / –NH3+) — often protonated (positive) at physiological pH
- Phosphate (–PO4 groups; often ionized) — key for energy storage (ATP), nucleic acid backbone, phosphorylation
- Sulfhydryl / thiolate (–SH / –S–)
- Composite/condensation-derived groups:
- Amide (peptide bond) — links amino acids into proteins; amide N is less basic (usually neutral) but can be an H‑bond donor
- Ester — links fatty acids to glycerol (triglycerides, phospholipids)
- Phosphate ester — links phosphate to alcohols (nucleic acids: phosphodiester backbone)
Non‑covalent interactions — importance and types
Non‑covalent bonds are weaker (roughly 1–10 kcal/mol) and reversible; they confer dynamics necessary for folding, binding, catalysis, and supramolecular assembly.
Typical energy comparison:
Covalent bonds (C–C, C–H): ~80–100 kcal/mol Non‑covalent interactions: generally 1–10 kcal/mol; ionic/hydrogen/hydrophobic interactions often fall in the ~2–10 kcal/mol range.
Primary non‑covalent forces and their features:
-
Ionic (electrostatic) interactions / salt bridges
- Between oppositely charged groups (e.g., –NH3+ and –COO–).
- Often the strongest non‑covalent interaction but highly environment‑dependent (weaker in water due to solvation; stronger in hydrophobic microenvironments).
- Energies variable (approx. 2–10 kcal/mol).
-
Hydrogen bonds
- Donors: H attached to electronegative atoms (O–H, N–H, sometimes S–H).
- Acceptors: atoms with lone pairs (O, N, sometimes S). Carbon‑bound hydrogens are not donors.
- Hydrogen bonding networks stabilize protein secondary/tertiary structure, DNA base pairing, and ligand binding.
-
Hydrophobic interactions (the hydrophobic effect)
- Nonpolar (C–H, C–C–rich) groups cluster to minimize contact with water; this is a major driver of protein folding and membrane formation.
- Amphipathic molecules (e.g., phospholipids, fatty acids) have hydrophobic tails and hydrophilic heads.
-
van der Waals (dispersion) forces
- Weak, short‑range interactions between induced dipoles; they contribute to molecular complementarity and tight packing.
Practical identification rules emphasized
- Hydrogen bonds: identify donors (H on O/N/S) and acceptors (lone pairs on O/N/S). Carbon‑bound hydrogens are not hydrogen‑bond donors.
- Ionic interactions: identify charged functional groups (protonation state depends on pH); opposite charges can attract unless screened by water.
- Hydrophobic interactions: spot long C–C and C–H chains/regions (fatty acid tails, nonpolar amino acids) that will cluster away from water.
- Line‑angle drawing conventions:
- Each line = bond; each vertex = carbon.
- O, N, P, S (non‑carbon heteroatoms) should be explicitly shown.
- Hydrogens bonded to heteroatoms (O, N) must be shown; hydrogens on carbons are usually omitted and implied.
Lipids — definition, types, functions, and supramolecular behavior
Definition: a diverse set of relatively small, predominantly hydrophobic molecules rich in C–C and C–H bonds; many are amphipathic (hydrophilic head + hydrophobic tail).
Major lipid types:
- Fatty acids: long hydrophobic tails + carboxylate head (saturated, cis‑unsaturated, or trans‑unsaturated)
- Triglycerides: glycerol + three fatty acid esters — energy storage
- Phospholipids: glycerol + fatty acids + phosphate-containing head group — membrane building blocks
- Steroids (cholesterol, steroid hormones such as estradiol) — signaling and membrane components
- Lipid-derived vitamins (e.g., retinol / vitamin A) — retinol is derived from carotene and is essential in vision (light‑induced isomerization changes shape and signals)
Amphipathic behavior and self‑assembly:
- Phospholipids self‑assemble in water into micelles, liposomes, or bilayers depending on tail composition and molecular shape.
- Lipid bilayers form cell membranes: semi‑permeable boundaries that compartmentalize the cell and enable concentrated biochemistry — a critical evolutionary development.
Health/physiology notes:
- Trans fats are linked to increased low‑density lipoprotein (LDL) particles (smaller, stickier, prone to arterial deposition) and a higher risk of coronary heart disease. Cis‑unsaturated fats (e.g., in olive oil) are associated with better health profiles.
- Cholesterol is transported in blood in lipoprotein particles (HDL vs LDL); particle properties (size, density, stickiness) determine atherogenic risk.
Key biochemical pH / protonation points
- Many functional groups change protonation with pH (carboxylates are deprotonated at neutral pH; amines are often protonated).
- Amide nitrogens (peptide bonds) are generally not protonated and are poor bases; they can act as H‑bond donors (N–H) and acceptors (carbonyl O).
- Phosphate groups are commonly ionized and carry negative charges; they are central to energy storage and transfer (ATP) and nucleic acid backbones.
Takeaways and next steps
- Be able to recognize covalent functional groups and non‑covalent interaction motifs in molecules and macromolecular assemblies.
- Recognize the special role of water and the hydrophobic effect in driving biological structure and interactions.
- Next lecture: lipids and membranes; upcoming coverage of amino acids/peptides/proteins and nucleic acid polymers.
- Suggested preparatory reading: textbook section 3.2.
Practical “how to” lists (from the lecture)
How to spot hydrogen bonds:
- Find H attached to O, N, or S (donor).
- Find nearby lone‑pair bearing atoms (O, N, S) as acceptors.
- If donor and acceptor are in reasonable proximity/geometry, a hydrogen bond can form.
How to identify ionic interactions:
- Determine likely protonation states at physiological pH (e.g., –COO–, –NH3+).
- Opposite charges close together can form electrostatic/salt‑bridge interactions (strength depends on solvent/environment).
How to read/produce line‑angle drawings:
- Lines = bonds; vertices = implicit carbons.
- Explicitly draw heteroatoms (O, N, P, S) and hydrogens attached to them.
- Deduce implicit hydrogens on carbons from valence rules.
Speakers / sources mentioned
- The lecture presenter (unnamed professor / lecturer)
- David S. Goodsell (molecular renderings — Scripps Research Institute) — image used to illustrate molecules of life
- “Dr. A” — mentioned in passing as giving problem sets
- Recommended textbook (section 3.2) for further preview
(End of summary.)
Category
Educational
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