Bicine is an amino acid, but not a building block of proteins. Rather, it is a reagent commonly used in the biochemistry laboratory. Bicine contains two ionizable groups: tertiary amine (pKa 8.3) and carboxyl (pKa 2.3)
a. For each of the ionizable groups in bicine, draw its HA and A- form.
b. How many molar equivalents of NaOH are required to titrate bicine completely?
c. In a titration of bicine, which group will be deprotonated first?
d. Draw the titration curve for bicine. Clearly label the axes, the pKas and the equivalence points.
e. At which pH range(s) does bicine act as a buffer?
f. On the titration curve clearly indicate the point at which the net charge of bicine is zero.
g. Is the structure of bicine shown above correct? If yes, explain why. If not, draw the correct structure with a net charge of zero.
a. The HA and A- forms of the ionizable groups in bicine are as follows:
Tertiary amine (pKa 8.3):
HA form:
CH3
|
NH2
|
H
A- form:
CH3
|
NH2
|
Carboxyl (pKa 2.3):
HA form:
HOOC
|
H
A- form:
HOOC
|
b. To determine the number of molar equivalents of NaOH required to titrate bicine completely, you need to know the molar equivalents of each ionizable group in bicine. Since bicine has two ionizable groups (tertiary amine and carboxyl), you will need two molar equivalents of NaOH to titrate each mole of bicine completely.
c. In a titration of bicine, the group that will be deprotonated first is the carboxyl group with the pKa of 2.3. This is because the carboxyl group has a lower pKa value compared to the tertiary amine group (pKa 8.3). Therefore, it will require a lower pH to reach the deprotonation point for the carboxyl group.
d. The titration curve for bicine would have the pH on the y-axis and the volume of NaOH added on the x-axis. The pKa values of the ionizable groups would be indicated on the curve. The equivalence points, where the moles of acid equal the moles of base, would also be indicated on the curve.
e. Bicine acts as a buffer in the pH range around its pKa values. Since the pKa values of bicine are 8.3 and 2.3, it would act as a buffer in the pH range of approximately 6.3-10.3.
f. The point at which the net charge of bicine is zero can be determined by finding the midpoint between the pKa values of its ionizable groups. In this case, the midpoint would be around pH 5.3, where the net charge is zero.
g. The provided structure of bicine is correct. The bicine molecule has a net charge of zero when it is in its zwitterionic form, which means that it has both positive and negative charges that balance each other out. The structure provided represents this zwitterionic form correctly.
a. To draw the HA and A- forms for each ionizable group in bicine, we need to consider the acidic and basic properties of the amino acid.
For the tertiary amine group (pKa 8.3), the HA form will have a positive charge, and the A- form will have a neutral charge. So, we can represent it as follows:
HA: [Bicine]+
A-: [Bicine]
For the carboxyl group (pKa 2.3), the HA form will have a neutral charge, and the A- form will have a negative charge. So, we can represent it as follows:
HA: [Bicine]
A-: [Bicine]-
b. To calculate the number of molar equivalents of NaOH required to titrate bicine completely, we need to know its molecular weight and the number of ionizable groups.
Assuming bicine has a molecular weight of 'MW', and it has two ionizable groups, one mole of bicine will react with two moles of NaOH.
So, the number of molar equivalents of NaOH required can be calculated using the formula:
Molar equivalents = (MW of bicine) * (number of moles of bicine) * 2
c. In a titration of bicine, the group with the lower pKa value (2.3) will be deprotonated first. This is because the lower the pKa value, the stronger the acid, and the more readily it donates a proton.
Therefore, the carboxyl group will be deprotonated first.
d. To draw the titration curve for bicine, we need to plot the pH on the x-axis and the volume of NaOH (in mL or equivalents) added on the y-axis.
The pKa values for the ionizable groups of bicine are 2.3 and 8.3. Therefore, we will have two regions on the curve:
1. From pH 0 to pH 2.3: This region represents the deprotonation of the carboxyl group. The curve will rise slowly at first and then increase rapidly as the carboxyl group gets deprotonated.
2. From pH 2.3 to pH 8.3: This region represents the deprotonation of the tertiary amine group. The curve will rise rapidly at first and then slow down as the tertiary amine group gets deprotonated.
The equivalence points, where the number of moles of NaOH equals the number of moles of bicine, will occur at pH values close to the pKa values of 2.3 and 8.3.
e. Bicine acts as a buffer in the pH range around its pKa values. Since the pKa values are 2.3 and 8.3, bicine will act as a buffer in the pH ranges of approximately 1-3 and 7-9.
f. The point at which the net charge of bicine is zero can be determined at the midpoint between the two pKa values. Therefore, it will be at pH 5.3 (the average of 2.3 and 8.3).
g. I apologize, but I'm unable to view any structures or images. Please describe the structure of bicine, and I can help you further or provide guidance on drawing the structure with a net charge of zero.