Building blocks · amino acids
Amino-acid IR data — all 20
Every standard amino acid shares the zwitterion backbone modes — νas(COO⁻) ~1600, νs(COO⁻) ~1410, NH₃⁺ deformations ~1505 — over which the side chain adds its own marker bands. The side-chain bands below (in H₂O) are the ones used to read protonation, H-bonding and local environment inside proteins.
Side-chain reporters worth knowing
Asp / Glu carboxyls switch between ~1716/1712 (protonated C=O) and ~1574/1560 + ~1402/1404 (ionised COO⁻) — direct pKa / H-bond probes. Arg guanidinium 1673/1633 and Lys ammonium 1629/1526 mark charged residues. Tyr 1518/1250 reports ring protonation; Asn/Gln add side-chain amide carbonyls that overlap amide I.
| Code | Amino acid | Class | Diagnostic bands (cm⁻¹) | Note | Measured ↗ |
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Measured amino-acid IR
Amino acids are involatile, so they are absent from the gas-phase NIST experimental database — these entries are reconstructions from literature positions. Real solid-state / aqueous FTIR for each residue is reachable via the Measured ↗ links above (SDBS, SpectraBase).
Simulate & compare
Tick residues to overlay their reconstructed spectra. Defaults compare an acidic (Asp), a basic (Arg) and an aromatic (Tyr) side chain.
Backbone amide modes for assembled chains are on the protein IR page; the full simulator with bases and backbones is at simulate.
References
- Barth, A. The infrared absorption of amino acid side chains, Prog. Biophys. Mol. Biol. 74 (2000) 141.
- Barth, A. Infrared spectroscopy of proteins, Biochim. Biophys. Acta 1767 (2007) 1073.
- Venyaminov, S. & Kalnin, N. Quantitative IR spectrophotometry of peptide compounds, Biopolymers 30 (1990) 1243.