Amino acids exist as zwitterions at physiological pH. A zwitterion by definition is a molecule with 2 (zwitter) ions, one positive and one negative for a net zero charge.
This video shows you how to quickly calculate amino acid charge at any given pH by helping you recognize when a given side chain is protonated or deprotonated.
(Watch on YouTube: Zwitterion and Amino Acid Charge. Click cc on the bottom right for video transcript)
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