Question

1) Creatine phosphate is produced by creatine kinase which transfers a phosphate group from ATP to creatine to generate ADP and creatine phosphate. Calculate the ΔG°’ for the coupled reaction. (I calculated it to be -12.5 kJ/mol but I am not sure if this is right)

2) Calculate the ΔG for the reaction described above. The concentrations of creatine phosphate, creatine and ADP are 0.020 mM and the concentrations of the orthophosphate ion and ATP are 20 mM. Temperature is 37°C and pH is 7.6 (I know which equation to use but where does the orthophosphate ion come in?)

3) A common buffer used is 100 mM EPPS (pH 8.5). Describe the preparation of 100.0 mL of this buffer using NH4OH (13.8 M concentration). The pKa of EPPS is 8.00 and the molecular mass is 252.33 g/mol.

Answer 1

3. According to the given data:

The no. of millimoles of buffer = (100 mmol/1000 mL) * 100 mL = 10 mmol (since 1 mM =1 mmol/L and 1 L = 1000 mL)

i.e. n_A^- + n_HA = 10 .......... equation 1, where n = no. of mmol

According to the Henderson-Hasselbalch equation, we can write as shown below.

pH = pKa + Log(n_A^-/n_HA)

Here, pH = 8.5, pKa = 8

i.e. 8.5 = 8 + Log(n_A^-/n_HA)

i.e. Log(n_A^-/n_HA) = 8.5 - 8 = 0.5

i.e. n_A^-/n_HA = 3.1623 ........ equation 2

From equations 1 and 2, we can write as follows.

3.1623 n_HA + n_HA = 10

i.e. 4.1623 n_HA = 10

i.e. n_HA = 2.4025 mmol

i.e. n_A^-= 10 - 2.4025 = 7.5975 mmol

The concentration of base used = 13.8 M

i.e. The volume of A^- solution = 7.5975 mmol / (13.8 mmol/mL) = 0.55 mL

Hence, the volume of HA solution = 99.45 mL

Hence the given buffer can be prepared by treating the 7.5975 mmol (= 99.45 mL) of the acid EPPS with 2.4025 mmol (=0.55 mL) of NH₄OH.