I need help! I understand part A. I think I can get the right answer for part B.
ID: 146363 • Letter: I
Question
I need help!
I understand part A. I think I can get the right answer for part B. But I don't know how to get part C.
1. For part B, should I just draw all 4 forms (from part A) and label each arrows with the given pka's?
2. So how do you know which pka goes to which arrows? (does the lowest pka go in between of the positive ones?)
3. For part C, I don't know how and why that form is the predominant at pH 7.4 . I looked it up on lecture note and I wrote that there's no net charge at intermediate pH. Is it what the answer is?
Thank you!
23. Three pKa values are associated with the amino acid histidine: pK1.80, pK2 9.33, and pKaR-6.04. Answer the following questions regarding the ionization states of His. A) How many forms of His are possible over the pH range 0-14? 3+14 Frm B) What is the isoelectric point (pI) of His? 3 pts 7.33 Hs N 7o C) Draw the predominant form ofHis at pH 7.4· 4B @ 3 pts 0Explanation / Answer
Your part (A) is correct.
Your part (B) is also correct, an easy way to find out about which pka goes to which arrow is by ionizing the H+ ion at high pka. You have 4 His forms and 3 pka's, pkar is the intermediate pka so it goes between those two pka's. For the transition of His from first to second form at a low pka1 it loses 1 H+, from second to third form it loses another H+ from NH at a medium Pkar, for third to fourth form it loses H+ ion even from the NH3+ group which is least acidic because of a very high Pka2.
Isoelectric point is the point where maximum no. of neutral forms of an amino acid occurs. In such cases we take the pka before and after the neutral form and divide by 2, which you did.
Forr part (c), to get the predominant form we see between which two pka's our pH lies.
7.4 lies between 6.04 and 9.33, at a pH> pKa (7.4 > 6.04 )the amino acid is deprotonated and the more predominant form will be that of the next pka, that is, at 9.33, for which you've drawn the correct structure.
If I have cleared your doubts then please give this answer a good rating, Thank You.