Using the Pka' Values for glutamic acid (pk1= 2.19, pk2= 9.67, pkR= 4.25) indicate the ionic form which predominates at: a) pH 1.0 b)pH 7.0 c) pH13 d) What is the net charge of the predominant form at each of these pH's?

1 Answer
Feb 12, 2018

#"a)+1" " b)-1" " c)-2"#

Explanation:

Glumatic acid is an alpha #alpha-# amino acid with the with the formula #color(darkgreen)("C"_5"H"_9"O"_4"N"#.

It is usually abbreviated as #"Glu or E"# in biochemistry. Its molecular structure could be idealized as#" HOOC-CH"_2-"COOH"#, with two carboxyl groups -COOH and one amino group -#"NH"_2#

It contains an #color(red)(alpha - "group"# which is the protonated #color(blue)("NH"_3^+# present when this acid is at its isoelectric point and in acidic medium

It contains an #color(red)(alpha-"carboxylic acid group"# which is the deprotonated #color(purple)("COO"-# present in #"basic and at"" its isoelectric point"#

upload.wikimedia.org

Before the dissociation takes a reaction within the solution takes place.
Formation of the zwitterion
This is the zwitterion of this acid. A zwitterion is a compound with no overall electrical charge, but which contains separate parts which are positively and negatively charged. Protonation of the #color(blue)(alpha - "group"# is followed by deprotonation of #color(red)(alpha-"carboxylic acid group"#

Using the #"pKa's"# to find out the isoelectric point

#("pKa"_1 + "pKa"_2 + "pKR")/3 = (2.19 + 9.67 + 4.25)/3 = 5.37#

This gives us an assumption that at #"pH"=1# the solution will be positive , at #"pH" = 7# it'll be negative and at #"pH" = 13# it will may yield a charge of #-2#

pH = 1

Now that we know the solution will be positive at #"pH"=1#.Let us determine the reaction.

enter image source here
The total charge of #color(maroon)("1,3‐dicarboxypropan‐1‐aminium")# is #+1#

In the second case when #"pH"= 7# the solution is negative but not as negative as #"pH"=13#.

pH = 7

First look at the amino acid carefully.

Did you notice another carboxyl acid which can also be negative due to acid base dissociation? .

At #"pH"=7 # all the possible acid base dissociation will take place.

enter image source here

Here also Protonation of the #color(blue)(alpha - "group"# is followed by deprotonation of #color(red)(alpha-"carboxylic acid group"# and in addition to that the additional #"R"-"group"# will also dissociate.

Commonly known as glutamate monoanion it's #"IUPAC"# name is #color(maroon)("2‐azaniumylpentanedioate")#

#"Total charge" = "COOH"^-) + "NH"_3^+ + "COOH"^-#

#= -1 + 1 -1 = -1#

pH = 13

At #"pH" = 13##color(blue)(" NH"_2 color(white)(lll)cancel(rarr)color(white)(lll) "NH"_3^+)#

Because as soon as the #color(purple)("COOH"# dissociates to give #"H"^+# it immediately becomes #"H"_2"O"# because of the increased presence of #"OH"^-# in basic solutions
 pH = 13

This is commonly known as glutamate(2‐) or glutamate dianion but the preferred #"IUPAC"# name is #color(maroon)("2‐aminopentanedioate")#

#"Total charge" = color(purple)("COO"^(-) + "COO"^-)#

#-1 -1 = -2#