TBR GenChem Ch4, Q91

[LoLCareerGoals]

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All of the following amino acids have a neutral sidechain at pH = 7 except:
A. cysteine
B. histidine
C. lysine
D. tyrosine

In the passage all pKas for all those side chains are given:
-CH2C=CH-N+H=CH-NH- pKa = 6.05 (Histidine)
-CH2SH pKa = 8.36 (Cysteine)
-CH2C6H4OH pKa = 10.07 (Tyrosine)
-CH2CH2CH2CH2NH3+ pKa = 10.80 (Lysine)

Book's explanation:
Cysteine is neutral when protonated...Fine.
Histidine is neutral when deprotonated...What??? There is one positive nitrogen and one negative in what I understand as protonated form in the passage's table. If deprotted it is gonna be negative!
Tyrosine is neutral when protonated...Fine
Lysine is neutral when deprotonated...Fine.
What did I miss here?

 

[sazerac]

I think you are misinterpreting their table. You might want to look at the structure of the histidine side chain in Wikipedia, and think about how hard it would be to add or remove a proton.

A negative nitrogen atom is really rare, and extremely unlikely under biochemical conditions. I'd hate to think how basic an N(-) would be.

 

[LoLCareerGoals]

Oh so the book table is wrong. Cool thanks!

 

[BerkReviewTeach]

Oh so the book table is wrong. Cool thanks!

That dash following NH- is meant to represent a bond connecting back to C₁ of the histidine ring; it's not a negative charge. So while it's not necessarily a typo, it's a crappy way to draw histidine. It should have probably been more like:

-CH₂-C=CH-N⁺H=CH-NH
......|.......................|
......^{-------------------------}​