L-histidine (H)

Side chain polarity Reset
Display the imidazole group as Ball & Stick. At lower pH, this nitrogen is protonated, resulting in a positively charged imidazolium ion.
Compare the planar aromatic ring of His to the sidechain ring of Pro.

Some facts on L-histidine.

1. Abbreviation: His.
2. One-letter code: H.
3. Molecular mass: 155 daltons.
4. Side chain (or R-group) features:
   • Structural formula:
     
   • Formula: -CH₂-imidazole;
   • Category: aromatic; basic.
   • pK_a: 6.0.