Question

Calculate the average charge on arginine when ph=9.20. (hint : find the average charge for each ionizable group and sum these together.)

Answer 1

The average charge on arginine at pH 9.20 is calculated by adding up the charges of its three ionizable groups, resulting in an average charge of 0.61.

Step-by-step explanation:

To calculate the average charge on arginine at pH 9.20, we need to consider the pKa values of its ionizable groups and use the Henderson-Hasselbalch equation. Arginine has three ionizable groups: the amino group (with a pKa around 9), the carboxyl group (with a pKa around 2), and the guanidinium group (with a pKa around 12.5). At pH 9.20, the amino group will mostly be protonated, the carboxyl group will be deprotonated, and the guanidinium group will be protonated.

For the amino group:

Charge = 1 / (1 + 10(pH-pKa))

Charge ≈ 1 / (1 + 10(9.20-9))

Charge ≈ 0.61

For the carboxyl group:

Charge = -1 / (1 + 10(pKa-pH))

Charge = -1 / (1 + 10(2-9.20))

Charge ≈ -1

For the guanidinium group:

Charge = 1 / (1 + 10(pH-pKa))

Charge ≈ 1 / (1 + 10(9.20-12.5))

Charge ≈ 1

The average charge on arginine at pH 9.20 is the sum of the charges of all ionizable groups:

Average Charge = Charge on amino group + Charge on carboxyl group + Charge on guanidinium group

Average Charge = 0.61 - 1 + 1

Average Charge = 0.61

Answer 2

Arginine is a basic aminoacid, because it has two amino groups and one acid group. At a low pH, every ionizable group is protoned. At a little higher pH, the acid group looses its proton. A little higher pH, one amino group looses its proton. At a very high pH, all ionizable groups are not protoned.

Pkas

pka1 = 1.82 pka2 = 8.99 pka3 = 12.48 So 9.20 is higher tan the second pKa and lower than the third pka. This means the acid has already lost its proton, and one of the aminos too, but the second amino hasn’t. When an acid is not protoned, it has a negative charge. When an amino is not protoned, it’s neutral. When an amino is protoned, it has a positive charge. So this amnino acid has one positive charge (one of the aminos) and one negative charge (the acid), what makes it neutral.