Final answer:
The principal form of arginine at pH 11.0 is HArg, as the pH is higher than the pKa2 but lower than the pKa3, indicating that the amino acid has lost protons from its two more acidic groups but retains the proton on the amine group in its side chain.
Step-by-step explanation:
The student's question is related to the principal form of arginine at a specific pH level. Specifically, we are asked what the principal form of arginine is at pH 11.0, given the pKa values of its different forms: H₃Arg¹⁺ (pKa1 = 1.82), H₂Arg⁺ (pKa2 = 8.99), and HArg (pKa3 = 12.1).
To determine the principal form of arginine at pH 11.0, we must compare the pH of the solution with the pKa values of arginine:
If the pH is lower than a pKa value, the form with more protons predominates.If the pH is higher than a pKa value, the form with fewer protons predominates.
Given these rules and the fact that pH 11.0 is higher than pKa2 (8.99) but lower than pKa3 (12.1), we can deduce that HArg will be the predominant form of arginine at pH 11.0. This is because it has lost protons corresponding to the first two acidic groups (-NH₃⁺ and the -COOH), but not the third one. The amine group in the side chain remains protonated (as -NH₃⁺), making HArg the major species present at that pH level.