Histadine

L-Histidine

Names
IUPAC name Histidine
Other names 2-Amino-3-(1H-imidazol-4-yl)propanoic acid
Identifiers
CAS Number	71-00-1 Y
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:57595 Y
ChEMBL	ChEMBL17962 Y
ChemSpider	6038 Y
DrugBank	DB00117 Y
ECHA InfoCard	100.000.678
IUPHAR/BPS	3310
KEGG	D00032 Y
PubChem CID	773
UNII	4QD397987E Y
InChI InChI=1S/C6H9N3O2/c7-5(6(10)11)1-4-2-8-3-9-4/h2-3,5H,1,7H2,(H,8,9)(H,10,11)/t5-/m0/s1 Y Key: HNDVDQJCIGZPNO-YFKPBYRVSA-N Y InChI=1S/C6H9N3O2/c7-5(6(10)11)1-4-2-8-3-9-4/h2-3,5H,1,7H2,(H,8,9)(H,10,11)/t5-/m0/s1 Key: HNDVDQJCIGZPNO-YFKPBYRVSA-N
SMILES O=C([C@H](CC1=CNC=N1)N)O O=C(O)[C@@H](N)Cc1c[nH]cn1
Properties
Chemical formula	C6H9N3O2
Molar mass	155.16 g·mol−1
Solubility in water	4.19g/100g @ 25 °C
Hazards
Safety data sheet	See: data page
NFPA 704	1 1 0
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Histidine (symbol His or H; encoded by the codons CAU and CAC) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH₃⁺ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO⁻ form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, longer-term studies have shown it is essential for adults also.

Histidine was first isolated by German physician Albrecht Kossel and Sven Hedin in 1896. It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.

The conjugate acid (protonated form) of the imidazole side chain in histidine has a pK_a of approximately 6.0. This means that, at physiologically relevant pH values, relatively small shifts in pH will change its average charge. Below a pH of 6, the imidazole ring is mostly protonated as described by the Henderson–Hasselbalch equation. When protonated, the imidazole ring bears two NH bonds and has a positive charge. The positive charge is equally distributed between both nitrogens and can be represented with two equally important resonance structures. As the pH increases past approximately 6, one of the protons is lost. The remaining proton of the now-neutral imidazole ring can reside on either nitrogen, giving rise to what are known as the N1-H or N3-H tautomers. The N3-H tautomer, shown in the figure above, is protonated on the #3 nitrogen, farther from the amino acid backbone bearing the amino and carboxyl groups, whereas the N1-H tautomer is protonated on the nitrogen nearer the backbone.