# Can tyrosine form dianionic form since it has acidic carboxyl group and also hydroxyl group at the phenyl group?

##### 1 Answer

Mar 16, 2016

This is **tyrosine** at physiological

Recall that a

#"pH" = "pKa" + log \frac(["B"])(["BH"^(+)])#

#= "pKa" + log \frac(["A"^(-)])(["HA"])#

From the **Henderson-Hasselbalch equation**, we could prove that:

- If
#"pH"# #<# #"pKa"# ,#log \frac(["B"])(["BH"^(+)]) < 0# and thus#["BH"^(+)] > ["B"]# , or#log \frac(["A"^(-)])(["HA"]) < 0# and thus#["HA"] > ["A"^(-)]# , because#log b# where#0 < b < 1# is negative. - If
#"pH"# #># #"pKa"# ,#log \frac(["B"])(["BH"^(+)]) > 0# and thus#["BH"^(+)] < ["B"]# , or#log \frac(["A"^(-)])(["HA"]) > 0# and thus#["HA"] < ["A"^(-)]# , because#log b# where#b > 1# is positive.

In examining the possible

#"pH" < 2.2# #2.2 < "pH" < 9.21# #9.21 < "pH" < 10.46# #"pH" > 10.46#

Now let's see how that turns out.

- The carboxyl is neutral, the
#"NH"_2# is#"NH"_3^(+)# , and the phenol is neutral. This has a net#+1# charge. - The carboxyl is
#"COO"^(-)# , the#"NH"_2# is#"NH"_3^(+)# , and the phenol is neutral. This is net neutral. The isoelectric point,#"pI"# , is at about#"pH" = 5.65# . - The carboxyl is
#"COO"^(-)# , the#"NH"_2# is neutral, and the phenol is neutral. This has a net#-1# charge. - The carboxyl is
#"COO"^(-)# , the#"NH"_2# is neutral, and the phenol is#"PhO"^(-)# . This has a net#-2# charge.

So yes, that can happen, but only at