pH Calculation

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The Henderson Hasselbalch equation is pH =pKa + log(A/HA). The numerator, A, is a form with one less proton than HA. a) pH = pKa + log (A/HA). NEVER use the pI where the pKa is called for! You must remember that pI and pKa haveopposite meanings. Always insert the pKavalue toward which you are moving. Hereyou are starting at the isoelectric point and adding acid so youwill be moving left from the zero charge form, or toward 6.0. To determine Aand HA: you will be adding a proton to 10% of the dipeptide, and leaving theother 90% unchanged, so A/HA should equal 9/1 or 0.9/0.1 (and NOT 10/1!!). Remember that A + HA = original concentration ofamino acid or peptide.pH = 6.0 + log (0.9/0.1) = 6.0 + log 9 =6.95 b) Here you are moving to the right, deprotonating 10% of the availabledipeptide, and leaving 90% unchanged (as HA).pH = 9.9 + log (0.1/0.9) = 9.9 – log 9 =8.95 c) Here you are moving farther to the left– protonating half of the peptide, and leaving the other half unchanged.pH = 6.0 + log (0.5/0.5) = 6.0 + log 1 =6.0 d) Here you move beyond 6.0. The first mole completely protonates thefirst ionizeable group, taking the compound from the zero to the (+1) chargeform, and then the second 1/2 mole of acid moves it toward 1.8.pH = 1.8 + log (0.5/0.5) = 1.8 + log 1 = 1.8

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