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 is protonated and exists as the species HL. This is because at pH 6 (which is below the pKa of the amino group), the amino group will be protonated and positively charged (NH3+), while the carboxyl group will remain deprotonated and negatively charged (COO-). Therefore, the amino acid will exist in its protonated form, which is HL. The other options involve species that would result from deprotonation of the amino group or protonation of the carboxyl group, which are not relevant at this pH.

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

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