I am wondering whether aspartic acid and glutamic acid at physiological pH are present in zwitterionic form? In my textbook it says that all aminoacids at $\textrm{p}H$ of 7.4 or 7 are in zwitterion form. I wonder if $\textrm{p}K_\textrm{a}$ values of side chains of aspartic acid is 3.65. There are also arginine, lysine, cysteine and tyrosine that will not have overall neutral charge at $\textrm{p}H = 7$.

  • $\begingroup$ What's unclear about it? -NH2 is protonated, both -COOH are deprotonated, so here's your zwitterion. $\endgroup$ Commented Oct 6, 2017 at 13:34
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    $\begingroup$ How can it be zwitterion if definition of zwitterion is that it has separated charges but it is overall neutral . $\endgroup$
    – Lind
    Commented Oct 6, 2017 at 13:37
  • $\begingroup$ Related on Biology SE: biology.stackexchange.com/q/23824. I believe the issue might be the meaning of present. It may not be the dominant form of the amino acid, but all of them should have some population of the zwitterion at biological pH. $\endgroup$
    – Tyberius
    Commented Oct 6, 2017 at 14:00
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    $\begingroup$ Then again, probably the guys who wrote the textbook still considered this a zwitterion despite the overall negative charge, or they forgot that some aminoacids have an additional acidic group in the side chain, or they just did not care. $\endgroup$ Commented Oct 6, 2017 at 14:09

1 Answer 1


The IUPAC goldbook defines a zwitterion as being overall neutral. Any other definition is thus contrary to the standard definition.


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