The dissociation constant for a previously uncharacterized racemic acid discover

Question

The dissociation constant for a previously uncharacterized racemic acid discovered in a meteor have been determined to be pk1=2.0, pk2=3.5, pk3=6.3, pk4=8.0, pk5=9.8, and pk7=10.9.

a. What carboxyl or functional group would you expect to be associated with each dissociation?

ANSWER: pk1-pk3 are carboxyl and the rest are amino groups

b. What would be the approximate net charge on this amino acid at pH 2?

ANSWER- minus 1

c. What would be its approximate net charge at pH 6.3?

ANSWER- plus 0.5

Can someone help explain how to get these answers?

Explanation / Answer

pk1=2.0, pk2=3.5, pk3=6.3, pk4=8.0, pk5=9.8, and pk7=10.9.

PI =( pk1 + pk2 + pk3 +pk4 + pk5 + pk7 )/ 6

    = 6.75

so isoelectric point = 6.75

below 6.75 it is in acidic medium above 6.75 it is in basic medium

in acidic medium amino acid exist as a anion .

in basic medium amino acid exist as a cation.

(a)

pk1=2.0, pk2=3.5, pk3=6.3 are lesser then pI = 6.75 . so at these pK values these are in acidic medium.

so the resultant charge on amino acid is negative charge. here carboxylic functional group is present.

pk4=8.0, pk5=9.8, and pk7=10.9. these are above pI value .that means these are in presence of basic medium .

in basic medium only amino group is present .

(b)at pH = 2 means it is acidic medium . net charge in acidic medium negative charge

it is -1

(c) at pH = 6.3 means it is in basic medium in basic medium net charge on amino acid is positive charge .

at difference in pI-pH = 6.75-6.3 = 0.45

net charge on amino acid +0.45

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