Question

Please explain! I know the answers already but I do not understand how to find them.

H3 ОН (10) O(2) Question 2 (24 pts) This small protein has just the 6 ionizable side chains together with N and C termini (pK values in parentheses). 0.1 moles of the protein, in the ionic form shown, is dissolved in 1 L of pure water ("the(9H3N original solution"). Please answer the following questions COO a) what is the approximate pH of the original solution? (circle ONLY ONE number or circle "CP" cannot predict) SH 7 1 21 3 41 516171891 10 11 12 13 14 CP (5 HOOOC b) what is the approximate pl of this protein (circle ONLY ONE number - or circle "CP" - cannot predict) COOH (4 11 213 415161718191 1011 12 13 14 CP Non-graded workspace c) how much KOH or HCI (circle one) in moles would you need to get to from the original solution to these pH values pH 4 mol KOH or HCI pH = 6 mol KOH or HCI pH=10 mol KOH or HCI d) The side chain with a pK of 2 has a lower pK than a comparable side chain in the structure Give the 1 letter code for this amino acid And suggest why this side chain pK is unusually low 2

Answer 1

a) The approximte pH of the solution would be near 3 .

This can be predicted as pH=pK when half of the ion is ionised since a -COOH group is ionised completely it shows that pH is certainly above 2 and below 4 because other -COOH group having a pK of 4 is unionised showing that pH is in between 2 and 4 i.e. 3.

it can also be shown by Henderson-Hasselbach equation

pH = pK + log ([salt]/[acid])

(i) when acid is dissociated , conc.of salt is higher than acid and pH> pK

(ii) when acid is undissociated , conc. of salt is lower then acid and pH<pK

b) To calculate Isoelectric point just follow two steps

Step 1- Estimate the pH at which protein is neutral

Step 2- Find the average of the two pK values which are just above and below the pH at which protein is neutral

For this question if pH= 6 , then the protein will be neutral because all the three -COOH group will be ionised and will have negative charge and all three amino group will have positive charge thereby protein is neutral.

pK just below 6 = 5 of -COOH group

pK just above 6= 7 of -SH group

The average of 5 and 7 is 6, so 6 is the isoelectric point of the protein.

c) (i) For pH=4

Since pH of original solution is 3 to increase pH upto 4 , 1 mole of KOH is added

(ii) For pH = 6

3 moles of KOH is added to make the pH of the solution 6

(iii) For pH= 10

7 moles of KOH is added to make pH of the solution 10

d) The side chain having a lower pK value of 2 can be Aspartate it has a much lower pK value than other because it does not have more electron donating groups . The presence of electron donating group decreases the acidity and increases the pK . As electron donating group is not present it is more acidic and has a lower pK value