Methods module
Advanced vibrational methods
Beyond transmission FTIR: sampling geometries for difficult samples, nonlinear and time-resolved methods for dynamics and coupling, chiroptical IR, and engineered vibrational probes that read local electric fields inside binding sites.
1 · Sampling geometries
| Technique | Best for | Principle / note |
|---|---|---|
| Transmission | thin films, solutions, KBr/gas | Beer–Lambert; needs short path in water (~6–10 µm). |
| ATR-FTIR | aqueous, surfaces, opaque solids | Evanescent wave at a high-index crystal (Ge/diamond); depth ~0.5–2 µm, minimal prep. |
| DRIFTS | powders, catalysts | Diffuse reflectance; Kubelka–Munk linearises intensity; good for in-situ catalysis. |
| IRRAS / PM-IRRAS | monolayers on metals | Grazing-incidence surface selection rule; polarisation modulation gives orientation and suppresses gas-phase water/CO₂. |
| Microscopy / imaging | spatial chemistry | FPA detectors map composition across tissue/materials at diffraction-limited resolution. |
2 · Two-dimensional IR (2D-IR)
A vibrational analogue of 2D-NMR run with femtosecond pulses. It spreads the spectrum onto two frequency axes; off-diagonal cross-peaks appear between coupled modes, and the waiting time resolves dynamics on the picosecond timescale that the modes themselves report.
| Observable | Reads out |
|---|---|
| Cross-peak intensity | Vibrational coupling → through-bond/space connectivity, structural constraints. |
| Antidiagonal width | Homogeneous linewidth → fast bath fluctuations. |
| Waiting-time evolution | Spectral diffusion, chemical exchange, energy transfer (ps–ns). |
| Node-slope / CLS | Frequency–frequency correlation decay — H-bond and solvation dynamics. |
Where it matters here
Combined with isotope editing, 2D-IR turns the congested amide I band into residue-pair distance/coupling constraints — central to the secondary-structure and protein–ligand objectives.
3 · Time-resolved & difference IR
Reaction-induced difference spectra subtract two states so only the bonds that change survive — a handful of bands out of thousands. Triggered by light, mixing, T-jump or potential, and followed by rapid-scan, step-scan, or pump–probe detection from ns to ps.
- Enzyme mechanism — caged-substrate or pH-jump difference IR resolves protonation and bond changes along a catalytic cycle.
- Photoreceptors / pumps — bacteriorhodopsin, photosynthetic reaction centres, rhodopsins followed band-by-band through their photocycles.
- Ligand photolysis — the classic myoglobin–CO flash experiment: the bound vs docked C≡O stretch reports conformational substates.
4 · Vibrational circular dichroism (VCD)
The differential absorption of left- vs right-circularly-polarised IR. VCD band signs encode absolute configuration and solution-state conformation; comparison with computed VCD (DFT) is now a routine way to assign chirality without crystals — complementary to electronic CD and to the structural IR above.
5 · Sum-frequency generation (SFG)
A second-order nonlinear method that is forbidden in centrosymmetric bulk and therefore intrinsically interface-specific. SFG vibrational spectroscopy reports the orientation and ordering of molecules at buried and air/water interfaces — membrane lipids, interfacial water, adsorbed proteins — that bulk IR cannot isolate.
6 · Vibrational Stark-effect (VSE) probes
A narrow, isolated oscillator placed in a molecule acts as a built-in voltmeter: its frequency shifts linearly with the local electric field (the vibrational Stark effect). Calibrated probes turn an IR frequency into an electrostatic measurement inside a protein active site — directly serving the protein–ligand objective.
| Probe | Band (cm⁻¹) | Why it works |
|---|---|---|
| Nitrile (C≡N) | 2200–2260 | Narrow, isolated, large Stark tuning rate; introduced via unnatural amino acids / ligands. |
| Azide (N₃) | ~2100 | Strong, transparent-window absorber; sensitive to local field and H-bonding. |
| Thiocyanate (SCN) | 2050–2160 | Small label, genetically/chemically installable. |
| Carbon–deuterium (C–D) | 2100–2300 | Minimally perturbing; sits in the cellular silent window. |
| Metal carbonyl (M–CO) | 1900–2100 | Backbonding makes ν(CO) an exquisite reporter of electron density / field. |
Caveats
VSE assumes a linear field response and a known, environment-independent Stark tuning rate; H-bonding to the probe adds a non-electrostatic shift that must be separated. C≡N is the most studied but is itself a weak H-bond acceptor — interpret pocket fields with explicit controls.
References
- Hamm, P. & Zanni, M. Concepts and Methods of 2D Infrared Spectroscopy (Cambridge, 2011).
- Boxer, S. G. Stark realities, J. Phys. Chem. B (2009) — vibrational Stark-effect probes.
- Nafie, L. A. Vibrational Optical Activity: Principles and Applications (Wiley, 2011) — VCD.
- Kim, H. & Cho, M. Infrared probes for studying the structure and dynamics of biomolecules, Chem. Rev. (2013).