The Twenty

Atlas/His

H

Histidine

His

BasicAromaticEssentialHydrophilicSide-chain pKa 6137 Da

Imidazole

  • Imidazole pKa ≈ 6.0, so it buffers near physiological pH.
  • The only amino acid whose charge can titrate in the physiological window.

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Structure of Histidine
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