Amino Acid in Basic Solution-Whats the charge?

[Turkelton]

I came across a question, that asked; "What is the charge on an amino acid in a solution with a pH of 12?"

Answer choices are: A) -2
B) -1
C) 0
D) +1

The answer choice is either A or B since solution is basic, does anyone know (for sure) the answer to this one? Thanks!

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[161927]

It's not the same for every amino acid.

 

[Turkelton]

The amino acid is lysine, sorry I didn't include that info earlier.

 

[Alfred E Newman]

The amino acid is lysine, sorry I didn't include that info earlier.

its -1....and I bet you pondered this yesterday.

 

[sleepy425]

I came across a question, that asked; "What is the charge on an amino acid in a solution with a pH of 12?"

Answer choices are: A) -2
B) -1
C) 0
D) +1

The answer choice is either A or B since solution is basic, does anyone know (for sure) the answer to this one? Thanks!

it's B. for lysine, you have two amino groups and one carboxyl group. the pKa of a carboxyl group is low (2.5-4ish) and the pKa of an aliphatic amine is between 9.5ish and 11. At pH 12, this means that the carboxyl group is deprotonated (because the pH is much higher than the pKa) so it contributes a charge of -1. At pH 12, both amino groups will be deprotonated because the pH is higher than the pKa. Deprotonated amino groups are neutral. So you have -1+0+0 which is a total charge of -1.

the only way it could be -2 is if the amino acid is cysteine, tyrosine, aspartic acid, or glutamic acid. those amino acids would also be fully deprotonated at pH 12 and thus you'd have the neutral amino group, the negative charge on the carboxyl group, and then the negative charge on the side chain.