Discussion Forum

1)

The substituents $R_{1}$ and $R_{2}$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at pH=7.0?

Answer:

Option B

Explanation:

The amino acid remains completely in Zwitterionic form at its isoelectric point. Amino acids with the additional acidic group have their isoelectric pH less than 7.0 and increasing pH above the isoelectric  point makes them anionic. On the other hand, amino acids with the additional basic groups have their isoelectric pH greater than 7.0 and decreasing pH below the isoelectric point (by adding acid solution) makes them cationic. The given peptide with following R₁ and R₂ are basic, will remain Protonated (cationic) at PH = 7.0.