Question

A student wants to measure the activity of a particular enzyme at pH 6.8. Which buffer system based on one of the acids is appropriate ? MES ka 8.13 x 10-7, PIPES ka 1.74 x 10-7, MOPS ka 7.08 x 10-8, HEPES ka 3.39 x 10-8 or Tris ka 8.32 x 10-9

Answer 1

Given the K_a of the buffers, find the pK_a using the relation

pK_a = -log K_a

Acid	Ka	pKa = -log Ka
MES	8.13*10-7	-log (8.13*10-7) = 6.0899 ≈ 6.09
PIPES	1.74*10-7	-log (1.74*10-7) = 6.7594 ≈ 6.76
MOPS	7.08*10-8	-log (7.08*10-8) = 7.1499 ≈ 7.15
HEPES	3.39*10-8	-log (3.39*10-8) = 7.4698 ≈ 7.47
Tris	8.32*10-9	-log (8.32*10-9) = 8.0799 ≈ 8.08

The desired pH of the enzyme is 6.8. For all practical purposes, a buffer is so chosen such that (pK_a – 1) < pH < (pK_a + 1) , i.e, the pK_a of the buffer must be within 5.8-7.8. All the buffers except Tris satisfy this condition and hence, all of buffers can be used. However, the closer the pK_a of the buffer to the desired pH, the better is the choice of the buffer. Therefore, PIPES appears to be the best buffer for the analysis of the enzyme.