The skeletal structures of the two amino acids, glycine and lysine, are given below along with the values of the relevant acid dissociation constants (pKa).
(a) For an aqueous solution of glycine alone, calculate the value of pH at which the ratio of the concentration of neutral glycine zwitterion to the concentration of protonated cation is 102.
Enter the numerical value of the pH:
pH= pKa2 + log(10^2) = 9.87 + 2 =11.87
but i still got it wrong. I am confused too.
i got the same result bt wrong
That's because you are using the wrong pKa
4.35
What chemical feature in the protein dictates the local secondary structure?
α-helix
β-pleated sheet
random coils
amino acid sequence
pH
hydrogen bonding
To answer this question, we need to understand the acid-base properties of glycine and how the pH affects the ratio of the zwitterion to the protonated cation.
Glycine is an amino acid with both a carboxylic acid group (-COOH) and an amino group (-NH2). In solution, these functional groups can act as both acids and bases.
The acid dissociation constant (pKa) represents the tendency of a molecule to lose a proton (H+). For glycine, we have two pKa values: one for the carboxylic acid group (pKa1) and another for the amino group (pKa2).
The neutral glycine zwitterion can be represented as NH3+CH2COO-, and the protonated cation can be represented as NH3+CH2COOH.
The ratio of the concentration of the zwitterion to the concentration of the protonated cation can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
In this case, [A-] represents the concentration of the zwitterion (NH3+CH2COO-) and [HA] represents the concentration of the protonated cation (NH3+CH2COOH).
Since the ratio is given as 10^2, we can rewrite the equation as:
2 = log([A-]/[HA])
To find the pH at which this ratio is true, we need to determine the pKa values for glycine. The pKa values for the carboxylic acid group (NH3+CH2COOH -> NH3+CH2COO- + H+) and the amino group (NH3+CH2COO- + H+ -> NH3+CH2COOH) of glycine are approximately 2.34 and 9.60, respectively.
Now we can solve for the pH:
For the carboxylic acid group:
2 = log([A-]/[HA])
2 = log([NH3+CH2COO-]/[NH3+CH2COOH])
2 = log(1/(10^9.60))
Solving for [A-]/[HA]:
10^2 = 1/(10^9.60)
10^2 * 10^9.60 = 1
10^11.60 = 1
Taking the logarithm of both sides:
log(10^11.60) = log(1)
11.60 = 0
So, for the carboxylic acid group, pH = 11.60.
For the amino group:
2 = log([A-]/[HA])
2 = log([NH3+CH2COOH]/[NH3+CH2COO-])
2 = log(1/(10^2.34))
Solving for [A-]/[HA]:
10^2 = 1/(10^2.34)
10^2 * 10^2.34 = 1
10^4.34 = 1
Taking the logarithm of both sides:
log(10^4.34) = log(1)
4.34 = 0
So, for the amino group, pH = 4.34.
Now, we need to determine the pH at which both ratios are true. Since we are looking for a pH value that satisfies both conditions, the pH should be between 4.34 and 11.60.
Therefore, based on the given information, we cannot determine the exact numerical value of the pH at which the ratio of the concentration of neutral glycine zwitterion to the concentration of protonated cation is 102.