The amino acid methionine, HOOC-CH(CH₂CH₂SCH₃)NH₃⁺,Has pK_a1 = 2.2 and pK_a2 = 9.1.If this amino acid is represented by H₂L⁺,The major species at pH 6 is

Question

Quizplus · Accepted Answer

At pH 6, the amino acid will be protonated once and exist as HL. The pKa of the carboxylic acid functional group is ~2.2, so at pH 6 it will be protonated (+) and the amino group will be in its basic form (-). Therefore, the predominant species will be HL. The other options are not the major species because the neutral form of the amino acid (H₂L⁺) will only be present at a pH near its pKa of 9.1, and the fully deprotonated form (LF1F1F1S1^-F1F1F10) will not be present at a pH of 6.

The Amino Acid Methionine, HOOC-CH(CH2CH2SCH3)NH3+,Has PKa1 = 2

The Amino Acid Methionine, HOOC-CH(CH₂CH₂SCH₃)NH₃⁺,Has PK_a1 = 2