🧬 Overall Theme of the Lecture

The lecture transitions from:

• Amino acids & modifications → to
• How proteins fold into 3D structures
• What forces stabilize folded proteins
• Energetics of folding
• Protein dynamics (motions across time scales)

The central question:

How does a linear polypeptide become a stable 3D structure?

1️⃣ Energy Landscape of Covalent Bonds

The lecture begins with a classical energy–distance curve:

When two atoms approach:

• Attraction lowers energy
• Repulsion increases sharply at short distance
• Minimum energy = equilibrium bond length

Key concepts:

• Bond length depends on atom type
• Double bonds are shorter and stronger than single bonds
• Bond strength increases with electron sharing

This establishes the contrast:

Covalent bonds define the primary structure Non-covalent interactions define folding

2️⃣ Disulfide Bonds

Although folding is mainly non-covalent, disulfide bonds (Cys–Cys) are covalent crosslinks that:

• Stabilize tertiary structure
• Reduce conformational entropy of unfolded state
• Increase overall stability

These are especially important in extracellular proteins.

3️⃣ Dielectric Constant and Shielding

A major concept introduced is dielectric shielding.

Dielectric constant (ε):

Interpretation:

• High ε (water) strongly shields charges
• Ionic interactions are much weaker in water
• Ionic interactions are stronger in protein core (low dielectric)

This explains:

• Why buried salt bridges can be stabilizing
• Why surface charge interactions are weaker in aqueous solution

4️⃣ Surface vs Core Electrostatics

The lecture emphasizes a difference:

In Protein Core:

• Low dielectric
• Ionic interactions stronger
• Hydrogen bonds stronger
• Water absent

On Surface:

• Water competes
• Charges often stabilized by solvent
• Salt concentration affects behavior

This distinction is critical in understanding stability.

5️⃣ Salt Effects in More Detail

Three regimes:

1. Low Ionic Strength

• Charges poorly shielded
• Proteins may attract each other
• Aggregation possible

2. Moderate Salt

• Surface charges shielded
• Increased solubility
• Stabilizing

3. High Salt (“Salting Out”)

• Salt binds water
• Less water available for solvation
• Protein precipitates

Important thermodynamic principle:

Solvent competition drives precipitation.

6️⃣ Ion–Ion vs Ion–Dipole vs Dipole–Dipole Energies

Energy hierarchy:

Important conceptual shift: As interactions get weaker, they also become shorter range.

This explains:

• Why van der Waals forces require close packing.
• Why tight packing is crucial in protein cores.

7️⃣ Van der Waals Radii and Contact Distance

Van der Waals radius = effective size of atom.

Contact distance = sum of radii.

If atoms are:

• Too far → no attraction
• At contact distance → optimal energy
• Too close → steep repulsion

This explains:

• Why proteins pack tightly
• Why steric clashes destabilize structures

8️⃣ Hydrogen Bond Network in Proteins

The lecture expands hydrogen bonding beyond backbone:

Possible donors/acceptors:

• Backbone carbonyl oxygen
• Backbone amide NH
• Ser, Thr, Tyr OH
• His imidazole
• Carboxylates
• Sulfur groups

Key point: Hydrogen bonds form networks, not isolated interactions.

They:

• Stabilize α-helices
• Stabilize β-sheets
• Stabilize tertiary packing

9️⃣ Peptide Bond Dipole

Each peptide bond has:

• Partial negative on oxygen
• Partial positive on nitrogen

This gives:

• Dipole moment
• Ability to engage in electrostatic alignment

This leads directly into the macro dipole of α-helices.

🔟 Hydrogen Bonds in Core vs Surface

Very important conceptual point:

In water:

• Breaking protein hydrogen bond is less costly
• Water can replace donor/acceptor

In core:

• No water
• Breaking H-bond creates unsatisfied polarity
• Highly destabilizing

This explains:

Why buried polar groups must form hydrogen bonds.

1️⃣1️⃣ Hydrophobic Collapse as Folding Driver

The lecture strongly emphasizes:

Hydrophobic effect is not:

• Just attraction between hydrophobic residues

It is:

• Entropy-driven water release

Steps:

1. Hydrophobic side chain in water → ordered water cage (entropy ↓)
2. Two hydrophobic groups associate
3. Ordered water released
4. Water entropy increases
5. ΔG becomes negative

This is the main folding force.

1️⃣2️⃣ Transfer Free Energy

The lecture discusses experimental measurements of:

Transfer of amino acids:

• From membrane (non-polar)
• To aqueous solution

Observation: Free energy correlates with surface area.

Larger hydrophobic side chain → larger transfer energy.

This quantitatively supports:

Surface area drives hydrophobicity.

1️⃣3️⃣ Entropy and Motion

Important conceptual section:

Proteins are not rigid.

Entropy contributions come from:

• Bond vibrations
• Side chain rotations
• Domain movements
• Water exchange
• Conformational fluctuations

Key idea: Flexibility contributes positively to entropy, which contributes to stability (via TΔS).

1️⃣4️⃣ Protein Dynamics Timescales

This establishes:

Folding and function are dynamic processes.

1️⃣5️⃣ Water Exchange and Experimental Methods

Water exchange timescales are relevant to:

• NMR hydrogen exchange experiments
• Mass spectrometry (H/D exchange)

This connects structural stability to experimental observables.

1️⃣6️⃣ Stability as a Balance of Forces

The lecture builds toward this idea:

Protein stability is not dominated by one interaction.

It is the sum of:

• Ionic interactions
• Hydrogen bonds
• Van der Waals packing
• Hydrophobic effect
• Entropic contributions
• Dynamics

And governed by:

Delta G = Delta H - TDelta S

🔬 Big Integrative Insight

Folding is favored because:

• Hydrophobic collapse increases water entropy
• Tight packing maximizes van der Waals contacts
• Hydrogen bond networks stabilize backbone
• Electrostatics fine-tune structure
• Dynamic motions contribute entropy

Proteins are:

• Not static
• Not rigid
• Not stabilized by a single force

They are stabilized by a careful thermodynamic balance.