Consider the amino acid cysteine. Earlier in the semester you were told that pK1

Question

Consider the amino acid cysteine. Earlier in the semester you were told that pK1 (the pKa of the ?-carboxyl group) was 1.92. Similarly, you may recall that pK2(the pKa of the ?-amminium group) was 10.70. Finally, you may recall that pKR (the pKa of the side chain thiol) was 8.37. The value of pK1 is so far below the pKavalues of the two other groups that we can ignore it as far as microscopic pKas are concerned. But pK2 and pKR are close enough that we should consider the microscopic pKas as indicated in the figure below. In this scheme, KR = KA + KB and K2 = KCKD/(KC + KD). Also, KRK2 = KAKC = KBKD. (Note that these relations are similar to what we saw earlier with enolase, but the terms have been modified to reflect the terminology used to describe amino acid pKas.) If an experimentalist told you that she had determined the value of pKA to be 8.86, then what would be the value of pKC to the nearest hundredths? Caution: unlike the previous microscopic pKa problems we did where we were dealing with symmetrical molecules and we could therefore assume that KA = KB and KC = KD, we cannot make that assumption with this molecule.

Part B - pKB of Cysteine

Consider the information in Part A, and, once again, if an experimentalist told you that she had determined the value of pKA to be 8.86, then what would be the value of pKB to the nearest hundredths?

Part C - pKD of Cysteine

Consider the information in Part A, and, once again, if an experimentalist told you that she had determined the value of pKA to be 8.86, then what would be the value of pKD to the nearest hundredths?

Explanation / Answer

If pKA = 8.86 then

pKC = 10.36

pKB = 8.85

pKC = 10.36

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