Histidine

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Histidine (abbreviated as His or H)[1] Histidine, an essential amino acid, has as a positively charged imidazole functional group. it is the one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans and other mammals. It was initially thought that it was only essential for infants, but longer term studies established it is also essential for adult humans.[2]

Contents

Chemical properties

The imidazole sidechain of histidine has a pKa of approximately 6.0, and, overall, the amino acid has a pKa of 6.5. 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.

Aromaticity

The imidazole ring of histidine is aromatic at all pH values.[3] It contains six pi electrons: four from two double bonds and two from a nitrogen lone pair. It can form pi stacking interactions,[4] but is complicated by the positive charge.[5] It doesn't absorb at 280 nm in either state, but does in the lower UV range more than some amino acids.[6][7]

Biochemistry

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