Question

Calculate the net charge of arginine (pK1=2.17, pK2=9.04, pK3=12.48) at pH 6.0. In order to calculate the net charge you must first calculate the percent “charged” for each ionizable functional group in the chemical structure of arginine. Use four significant digits in your calculation of the net charge.

Answer 1

Ans. Note the following point-s

            1. The N-terminal (pKa2) and R-group (pKa3) of Arginine (basic amino acid) act as basic groups. These groups are positively charge (in this case) or neutral but never negatively charged.

            2. The C-terminal of amino acids is an acidic group. It is either negative charged or neutral but never positively charged.

            3. Calculate total +ve charge % by summing up the charges on N-terminal and R-group.

            4. Calculate total -ve charge on the on the C-terminal group.

            5. Add total +ve and total -ve charges to get the NET charge.

# Result:

            Net Charge = Total +ve charge + Total -ve charge

                                    = (99.9089% + 100.0000%) + (-99.9852%) = + 99.9237%

On a relative (fraction) scale, the % value can be written as + 0.999237.

So,

            Net charge = + 0.999237 = + 0.9992 (four significant figures)