🧬✨ Lecture Summary — Protein Crystallization & X-ray Crystallography Basics

🌟 1. What controls protein crystallization?

Protein crystallization is very delicate because the same parameters that help crystals form can also cause:

• ❌ Amorphous precipitation
• ❌ Aggregation
• ❌ Fibrillation
• ❌ Denaturation

So crystallization requires fine tuning of solution conditions.

🔬 Important parameters you can change

🧂 Ionic strength (salting in vs reverse salting-in)

• Salting-in: adding salt increases protein solubility.
• Reverse salting-in: lowering ionic strength pushes proteins out of solution → promotes crystallization.

💧 Precipitants (lower water activity)

• Precipitants remove available water molecules.
• Proteins start interacting with each other instead of water → crystal contacts form.

⚙️ Chelating agents (e.g., Zn²⁺)

• Metal ions can form bridges between protein surface residues.
• This stabilizes intermolecular contacts → nucleation may begin.

🌡️ Temperature

• Some proteins crystallize at low temperature (~4°C)
• Others at room temperature (~20°C) → Always test multiple temperatures.

⚡ pH

• Changes protonation of titratable residues.
• Alters surface charge and interaction patterns → affects crystallization.

⚠️ Why protein must stay native

Crystallization requires:

✅ Well-folded protein ❌ Partially denatured protein disrupts crystal packing ❌ Fibrils stop crystal growth

Even small amounts of damaged protein can terminate crystal growth, preventing crystals large enough for diffraction.

⚖️ Kinetics vs Thermodynamics (Very Important Concept)

• Amorphous precipitation → kinetically favored (fast).
• Crystal formation → thermodynamically favored (slow but stable).

👉 Therefore, conditions must be changed slowly and controllably to obtain crystals.

📈 2. Phase diagram of crystallization

Axes:

• Protein concentration
• Precipitant concentration

Regions

🔑 Key idea

• First enter labile region → nucleation
• Then move into metastable region → growth

This trajectory must be controlled for successful crystallization.

💧 3. Vapor diffusion (main crystallization method)

Used in ~99.99% of experiments.

Methods

• Hanging drop
• Sitting drop

How it works

1. Mix protein + precipitant (often 1:1).
2. Drop sealed over reservoir with higher precipitant concentration.
3. Water evaporates from drop → concentration increases.
4. System moves through phase diagram → nucleation + growth.

Important dynamics:

• Drop volume decreases.
• Protein concentration first increases.
• After nucleation, protein concentration decreases as crystal grows.

💎 4. Why do we need crystals?

Because:

👉 A crystal is a repeating lattice of billions of molecules.

Single molecule scattering is too weak.

Crystal acts as:

⭐ “Single-molecule amplifier.”

This amplification makes diffraction measurable.

📦 5. Unit cell and crystal systems

A unit cell is defined by:

• Axes: a, b, c
• Angles: α, β, γ

From geometric constraints we get:

• 7 crystal systems
• 14 Bravais lattices

Examples of increasing symmetry:

Lattices may be:

• Primitive (P)
• Body-centered (I)
• Face-centered (F)
• Base-centered (C)

🔄 6. Symmetry operations in protein crystals

Allowed:

• ✅ Translation
• ✅ Rotation (2-, 3-, 4-, 6-fold)
• ✅ Screw axes (rotation + translation)

Not allowed:

• ❌ Mirror planes
• ❌ Glide planes
• ❌ Inversion

Reason:

👉 These would convert L-amino acids into D-amino acids, which is biologically impossible.

🧩 7. Space groups

• Mathematically: 230 space groups
• For chiral biomolecules: only 65 allowed

Space groups combine:

• Bravais lattice
• Point group symmetry

Example:

• C2 space group → 4 equivalent positions.

🧱 8. Asymmetric unit (Very Exam-Important)

Definition:

👉 Smallest part of crystal structure that cannot be generated by symmetry operations.

Process:

1. Determine asymmetric unit structure.
2. Apply symmetry → build unit cell.
3. Translate unit cells → full crystal lattice.

Also:

• Non-crystallographic symmetry can exist within the asymmetric unit.

☢️ 9. What are X-rays?

• Wavelength ≈ Å scale (~0.1 nm) → same as chemical bonds.
• Energy ≈ 10⁴–10⁵ eV.

History:

• Discovered by Wilhelm Röntgen (1895).
• Diffraction demonstrated by Friedrich, Knipping & Laue (1910).

⚡ 10. X-ray sources

🏠 In-house sources

• Sealed tube
• Rotating anode

Pros:

• Reliable
• Simple

Cons:

• Low intensity
• Fixed wavelength
• Need larger crystals

🌍 Synchrotron radiation

• Very high intensity
• Tunable wavelength
• Lower beam divergence
• Helps solve phase problem

🚀 Free electron lasers (XFEL)

• Extremely intense pulses
• Rare facilities (Hamburg, Stanford, Japan)
• Emerging technology.

🎯 11. Diffraction experiment setup

Components:

1. X-ray source
2. Collimator
3. Monochromator crystal
4. Sample crystal
5. Detector
6. Goniometer (rotates crystal → collect many diffraction spots)

This enables reconstruction of 3D structure.

⭐ Ultimate Take-Home Messages

• Protein crystallization = thermodynamic vs kinetic balance.
• Vapor diffusion controls movement through phase diagram.
• Crystal amplifies scattering signal.
• Only 65 space groups for proteins due to chirality.
• Goal of X-ray crystallography → solve asymmetric unit structure.
• High-intensity X-rays reduce crystal size requirements.