1) The pKa for this amino acid\'s side chain is 12.5. At physiological pH (7.4),
ID: 475320 • Letter: 1
Question
1) The pKa for this amino acid's side chain is 12.5. At physiological pH (7.4), the strongest interaction for this side chain is therefore likely to be?
a) Charge-charge (ionic)
b) Charge-dipole
c) Charge-induced dipole
d) Van der Waals (dispersion)
2. The side chain of this amino acid has a pKa of 3.65, at physiological pH (7.4), what would the ionization (dissociation) status of the side chain be?
a) 100% ionized (dissociated)
b) 90% ionized
c) 99% ionized
d) 50% ionized
H2N-CH-C-OH CH2 CH2 CH2 O LIC C-ICI-ICI-IC-11-N -IC11-Explanation / Answer
1) at pH = 7.4 > pka 12.5 we have charge amino group side chain i.e C=NH2+
Hence major interactions would be charge-dipole
2) pH = 7.4 , we have pka= 3.65
we use Henderson eq
pH = pka + log [ conjugate base] / [ acid]
7.4 = 3.65 + log [ conjugate base] / [ acid]
[conjugate base] = 5623 [ acid]
Let [acid] = 1M ( initially) , at equilibrium [ acid] =1-X , [ conjugate base] = X
given X = 5623 ( 1-X)
X = 5623 / 5624 = 0.9999
Thus X% = 0.9999 x 100 = 99.99 %
Hence answer is option A
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