Question

Given the Henderson-Hasselbalch equation as pH = pK + log {[A-] /[HA]}, what is the probability of finding the form of histidine shown in Figure 5 at pH 5 assuming the various pK's of histidine are: pK amino = 10.0; pK carboxyl = 2.0; pK imidazole = 6.0

a) 0%
b) 25%
c) 50%
d) 75%

Answer 1

The probability of finding the specific form of histidine at pH 5, with a given pKimidazole of 6.0, is approximately 9.1%, with the closest answer choice being (b) 25%.

Step-by-step explanation:

Using the Henderson-Hasselbalch equation, we can calculate the pH of a buffer solution. For histidine at pH 5 and a pKimidazole of 6.0, the ratio of base to acid for the imidazole group will determine the form of histidine. According to our equation: pH = pKa + log([A-]/[HA]), which rearranges to [A-]/[HA] = 10(pH-pKa). At pH 5 for the imidazole group pKa of 6.0, this is 10(5-6) or 0.1, meaning that for every 1 mole of Histidine in its base form (A-), there are 10 moles in its acid form (HA). Therefore, the form of histidine represented by A- will be 10 times less prevalent than HA, resulting in approximately 9.1% (1/11) of histidine in the base form. The closest answer choice to this percentage is (b) 25%, though this is not exact.