If a 100-residue protein had only 2 conformations per residue, it would have:
2^{100} approx 10^{30}
possible conformations.
If it sampled each conformation in 10⁻¹³ s, it would take 10⁹ years to find the native state.
But real proteins fold in seconds to minutes.
Proteins do not search randomly. Folding is guided.
You asked:
H-bonding of secondary structure guides folding?
Yes — but with nuance.
In early folding:
These pre-formed secondary structures reduce conformational space.
⚠ Important: Hydrogen bonds do not drive folding alone — they guide structure formation within the larger thermodynamic landscape.
Hydrophobic collapse and entropy of water are equally critical.
Modern molecular dynamics simulations include:
These are encoded in force field equations.
Small proteins (~40 aa) can now fold in silico in milliseconds.
You asked:
Can guanidinium chloride replace hydrogen bonds?
Yes — partially.
Guanidinium chloride:
Water also forms hydrogen bonds — but water alone does not denature because:
Simple case:
N ightleftharpoons U
Only two states:
The sharp transition indicates:
🔁 Cooperative process Either folded or unfolded — no stable intermediates.
2-state process means 2 independent units that have different denaturation?
No.
Two-state = one cooperative unit.
If a protein has multiple domains:
You asked:
Protein unfolding by heating related to interactions vibrating?
Yes.
Heating increases molecular motion:
When thermal energy exceeds stabilizing interactions → unfolding occurs.
Delta G = Delta H - TDelta S
You asked:
Free energy positive → unfolded?
Yes — for folding reaction.
If ΔG_folding > 0 → folding is unfavorable → protein unfolds.
Typical folding ΔG: -20 ext{ to } -60 ext{ kJ/mol}
This equals only a few hydrogen bonds!
⚠ Important: Proteins are marginally stable.
Not directly.
Hydrogen bonds contribute to:
But ΔG depends on both:
You cannot look at hydrogen bonds alone.
Favorable:
Unfavorable:
Now the important correction:
You asked:
Unfolded → ΔG decreases because entropy bigger than enthalpy? But aromatic rings exposed reduce entropy?
This is where many students get confused.
There are two entropies involved:
1️⃣ Chain entropy
2️⃣ Water entropy
For folding:
Net result: Delta G < 0
You said:
Aromatic ring exposed → entropy decreases → unfolded unfavorable?
Correct locally for water.
But in unfolded state:
The full balance determines stability.
It is not dependent on one amino acid. It is a global thermodynamic balance.
In Differential Scanning Calorimetry (DSC):
We measure heat capacity vs temperature.
When protein unfolds:
Some of the temperature unfolds the protein?
Correct — but more precisely:
Energy input goes into:
The area under the peak = ΔH_unfolding
You asked:
Smaller protein → lower heat capacity? Why?
Because:
Heat capacity scales roughly with:
Smaller protein → smaller hydrophobic core → smaller heat capacity change
ΔH and ΔS are temperature dependent.
At melting temperature (Tm):
Delta G = 0
So: T_m = rac{Delta H}{Delta S}
Important concept from your file:
Proteins can also unfold at low temperature.
Because:
Thus:
This surprises many students.
Even if ΔG = -10 kcal/mol:
K = e^{-ΔG/RT}
At 298K:
→ ~10⁷ folded per unfolded
Small energy difference = huge equilibrium shift.
| Contribution | Favors Folding? |
|---|---|
| Hydrogen bonds | Yes |
| Electrostatics | Yes |
| Hydrophobic effect | Strong yes |
| Chain entropy | No |
| Water release entropy | Yes |
Net result: Small negative ΔG → strong folded population.
Protein folding is: