Question

Can someone please help EXPLAIN this to me? I don't just want answers, I need EXPLANATIONS! I will mark it thumbs down if it is not explained to me and just given the answer. Thanks! Even if you could answer a few of them. Please, you'd be saving my grade. Just explain it.

The curve can be remade by putting pH on y-axis and Equivalent of hydroxide on x-axis as:

Now, as the curve has three vertical lines, it means it has three equivalence points (indicated on curve by C,E and G) each for a proton donor site and hence, amino acid is triprotic, i.e. it can donate three protons till it reaches its completely deprotonated stage.

If three proton donating sites are there, then there would be three Ka and hence three pKavalues (indicated on curve by B, D and F).

The pKa values are identified by the region where pH remains almost constant.

At pH = pKa of a group, there is equal (50% each) concentration of both protonated and deprotonated form of that group.

As the carboxy terminal has lowest pKa and amino has highest pKa, so,

 pKa Associated group ~ 2.0 (B) terminal carboxyl ~ 6.0 (D) side chain ~ 10.0 (F) terminal amino

As side chain pKa (D) is less than 6.0 so, it is Histidine.

Side chain of histidine has side chain amino group inside the imidazole ring and has pKa of slightly less than seven.

Now, at various pH, the charges on different groups and net charge on amino acid can be represented as:

 pH range Charge on terminal carboxyl Charge on terminal amino Charge on side chain amino Overall charge 1.00-3.99 0 +1 +1 +2 4.00 -0.5 (pKa) +1 +1 +1.5 4.01-5.99 -1 +1 +1 +1 6.00 -1 +1 +0.5 (pKa) +0.5 6.01-9.99 -1 +1 0 0 10.00 -1 +0.5 (pKa) 0 -0.5 10.01-14.00 -1 0 0 -1

Isoelectric point (pI) : pH at which overall charge is zero.

So, pI = (6.01+9.99)/2 = 8.0

a) One letter code for Histidine : H

b) +2 net charge is at point : A

c) Average net charge is 1.5 : C

d) Half of amino groups ionized : E

e) pH equals pKa of carboxyl : B

f) pH equals pKa of amino : F

g) maximum buffering capacity : D

h) net charge zero : E

i) Carboxyl completely titrated (first equivalence point) : C

j) Amino completely titrated (third equivalence point) : G

l) Net charge -1 : G

m) +1 and 0 as 50:50 mixture : D

n) isoelectric point : E

o) End of titration : G

p) Worst regions for buffering : A, C, E, G.

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