Tyrosine would NOT serve as an effective buffer at which pH

Tyrosine would NOT serve as an effective buffer at which pH?

A) 3.7

B) 6.5

C) 8.5

D) 10.3

E) 11.0

This is a practice exam question so I know the answer but I do not know how you arrive at that answer or why.

Solution

B) 6.5 is the pH at which the tyrosine would not serve as an effective buffer.

reason
The isoelectric point(pI) of tyrosine is 5.66 where it is electrically neutral(no net charge)and tyrosine becomes an amphoteric molecule thus it is not suitable as a buffer. Here 6.5 is the pH close to the pI so it will lower the buffering capacity of the aminoacid.

The tyrosine (Tyr Y) has pKa values 2.20, 9.11, 10.07 (phenol) where the amino acid remains at 50% protonated/deprotonated condition.At pH +/_ 1 of the pKa values the tyrosine would serve as an effective buffer.Thus pH 3.7, 8.5,10.3,11.0 the tyrosine will have the best buffering capacity.