Introduction to Biophysical Techniques
Key concepts, common methods, and practical considerations
1. What Are Biophysical Techniques?
| Definition |
Purpose |
Typical Applications |
| Experimental methods that apply physical principles to study biological molecules and systems. |
Quantify structure, dynamics, interactions, thermodynamics, and mechanical properties of biomolecules. |
Protein folding, membrane protein analysis, nucleic‑acid binding, enzyme kinetics, drug‑target validation. |
2. Core Physical Principles Used
| Principle |
Observable |
Typical Instrumentation |
| Electromagnetism |
Absorbance, fluorescence, scattering, conductivity. |
UV‑Vis spectrophotometer, fluorometer, SPR sensor. |
| Thermodynamics |
Enthalpy (ΔH), entropy (ΔS), binding constants (KD). |
Isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC). |
| Mechanics |
Force, elasticity, viscosity. |
Atomic force microscopy (AFM), optical tweezers, magnetic tweezers. |
| Quantum Mechanics |
Electron spin, nuclear spin. |
Electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR). |
| Wave phenomena |
Diffraction, interference. |
X‑ray crystallography, cryo‑electron microscopy (cryo‑EM), small‑angle X‑ray scattering (SAXS). |
3. Major Technique Categories
3.1 Spectroscopic Methods
| Technique |
What It Measures |
Strengths |
Limitations |
| UV‑Vis Absorption |
Chromophore concentration, conformational changes. |
Simple, fast, inexpensive. |
Low structural detail. |
| Fluorescence (steady‑state & time‑resolved) |
Environment of fluorophores, binding, distance (FRET). |
High sensitivity, can be done in live cells. |
Photobleaching, requires labeling. |
| Circular Dichroism (CD) |
Secondary structure content of proteins, nucleic acids. |
Quick secondary‑structure estimate. |
Limited to low‑resolution information. |
| Raman & FTIR |
Vibrational modes → chemical bonds, secondary structure. |
Minimal sample preparation, can work in aqueous solution. |
Weak signal (Raman), water absorption (FTIR). |
3.2 Scattering & Diffraction
| Technique |
Sample State |
Information Obtained |
| X‑ray Crystallography |
Crystals |
Atomic‑resolution 3‑D structure. |
| Small‑Angle X‑ray Scattering (SAXS) |
Solution |
Low‑resolution shape, radius of gyration, oligomeric state. |
| Small‑Angle Neutron Scattering (SANS) |
Solution |
Contrast variation (e.g., deuterium labeling) for multi‑component complexes. |
| Cryo‑EM |
Vitrified particles |
Near‑atomic structures for large complexes; does not require crystals. |
3.3 Microscopy & Imaging
| Technique |
Resolution |
Typical Use |
| Atomic Force Microscopy (AFM) |
~0.1 nm (lateral) |
Surface topography, single‑molecule force spectroscopy. |
| Optical Tweezers |
~10 nm |
Manipulating and measuring forces on single biomolecules. |
| Total Internal Reflection Fluorescence (TIRF) Microscopy |
~200 nm axial |
Single‑molecule dynamics near surfaces. |
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