Question

HI, woul you please expaline why the correct answer is (0) for this quesiton although we have ASP which is a negative charge?

Q: the plypeptide Leu- Lys- Val- Ser- Gly- Thr- Asp- Ala has what charge at Ph 6.5?

a) -5 b) -2 c) 0 d) 2 e)4

Thank you !

Answer 1

The pKa of amino acids represents the pH of the medium at which they bear no net charge. Thus, the pKa of an amino acids largely depends upon the pH of the medium and slight variations in the pH of the medium causes changes in the protonation state of the amino acid, thus its pKa value.

Here, it must be carefully noted that the polypeptide shown in the information contains a mixture of acidic, basic and neutral amino acids. These amino acids, thus, bear a different net charge at the same pH. While acidic amino acids will tend to bear a net negative charge, basic amino acids will tend to bear a net positive charge at slightly acidic pH. Thus, the overall pKa value for this polypeptide should add up to zero and hence, it suggests that the polypeptide is stable at slightly acidic pH.