Question

Draw a structural formula for the form of each amino most prevalent at \(\mathrm{pH} 10.0\). (a) Leucine (b) Valine (c) Proline (d) Aspartic acid

Short answer

Answer: At pH 10.0, the most prevalent forms of the amino acids are: (a) Leucine: (NH2)(CH)₂(CH)₂(CH₂COO⁻)COO⁻ (b) Valine: (NH2)(CH)₂(CH(CH₃)₂)COO⁻ (c) Proline: (NH2)(CH)₂(CH)₂ - COO⁻, with the amino and carboxyl groups connected through a five-membered ring structure (d) Aspartic acid: (NH2)(CH)₂(CH₂COO⁻)COO⁻

Step-by-step solution

(Step 1: Determine ionization states of amino and carboxyl groups)

At \(\mathrm{pH} 10.0\), the amino and carboxyl groups of all amino acids will be deprotonated due to high \(\mathrm{pH}\). The amino group protonated form has a \(\mathrm{pK_a \approx 9.0}\), while the carboxyl group protonated form has a \(\mathrm{pK_a \approx 2.0}\). Since \(\mathrm{pH}10.0\) is greater than both \(\mathrm{pK_a}\) values, the amino group will be neutral 'NH\(_2\)', and the carboxyl group will be negatively charged 'COO\(^-\)'.

(Step 2: Consider any ionizable side chain groups)

For Leucine, Valine, and Proline, their side chains do not have ionizable groups. For Aspartic acid, its side chain contains a carboxyl group with a \(\mathrm{pK_a \approx 4.0}\). At \(\mathrm{pH}10.0\), this carboxyl group will also be deprotonated and negatively charged.

(Step 3: Draw the structural formulas of the amino acids in their most prevalent form at \(\mathrm{pH} 10.0\))

(Keep in mind the charges and ionization states of the amino and carboxyl groups, as well as Aspartic acid side chain) (a) Leucine: (NH\(_2\))(CH)\(_2\)(CH)\(_2\)(CH\(_2\)COO\(^-\))COO\(^-\) (b) Valine: (NH\(_2\))(CH)\(_2\)(CH(CH\(_3\))\(_2\))COO\(^-\) (c) Proline: (NH\(_2\)(CH)\(_2\)(CH)\(_2\) - COO\(^-\), where the amino and carboxyl groups are connected through a five-membered ring structure) (d) Aspartic acid: (NH\(_2\)(CH)\(_2\)(CH\(_2\)COO\(^-\))COO\(^-\)