Question

Place each charge form of alanine under the pH condition where it would be the predominant form. The pKa values for the carboxyl group and amino group of alanine are approximately 2.3 and 9.7, respectively.Note: If you answer any part of this question incorrectly, a single red X will appear indicating that one or more of the items were placed incorrectly.(A) pH < 1 for H3C-C(H)(NH3+)-COO-(B) pH = pl for H3C-C(H)(NH2)-COO- (C) pH > 11 for H3C-C(H)(NH2)-COOH(D) Does not occurs in any significant pH (for H3C-C(H)(NH3+)-COOH)

Answer 1

(A) pH < 1 the predominant form is the cation: H3C-C(H)(NH3+)-COOH

(B) pH = pl the predominant form is the zwitterion H3C-C(H)(NH3+)-COO-

(C) pH > 11 the predominant form is the anion: H3C-C(H)(NH2)-COO-

(D) Does not occurs in any significant pH: H3C-C(H)(NH2)-COOH

Explanation:

Amino acids are bifunctional because they have an amine group and a carboxyl group. The amine group is a weak base and the carboxyl group is a weak acid, but the pKa of both groups will depend on the whole structure of the amino acid. Also, every amino acid has an isoelectric point (pI), which means the pH were the predominant form of the amino acid is the zwitterion. The structure of the alanine (CH3CH2NH2COOH) shows it has the carboxyl group at C1 with a pKa1 of 2.3 and the amino group at C2 whit the pKa2 of 9.7. The isoelectric poin (pI) of Alanine is 6. Consequently, the protonation of the molecule will depend on the pH of the solution. There are three possibilities:

1) If the pH is under the pKa of the carboxyl group (2.3) the predominant form will be with the amino group protonated, forming a cation (CH3CH(NH3+)COOH).

2) If the pH is between pKa1 (2.3) and pKa2 (9.7) the predominant form will be the zwitterion (CH3CH(NH3+)(COO-)).

3) If the pH is upper the pKa2 of the amino group (9.7) the predominant form will be with the carboxyl group deprotonated, forming an anion (CH3CHNH2(COO-)).