Answer to Question #85552 in Organic Chemistry for Abdu

Question

An unknown aminoacid has following pk1-2 pk2-3.5 pk3-6.3 pk4-8 pk5-9.2 pk-6-10.8
What will be the next charge on aminoacid at pH 2&6.3

Answer 1

The pK values demonstrate that the amino acid has two acidic groups (pK1 and pK2) and four basic groups (pK3, pK4, pK5, pK6). Each pK equals the pH value at which half of all the groups are protonated whereas all other groups are completely protonated (if their pK is higher than pH value) or deprotonated (if their pK is lower than pH value).

At pH 2, the first group carries charge -0.5 (half of the groups are charged, and another half is uncharged), whereas all other groups are completely protonated. The second acidic group carries no charge (0), whereas all other groups are positively charged (+1). As a result, at pH 2, the net charge of the amino acid is (-0.5) + (0) + 4 × (+1) = +3.5.

At pH 6.3, two acidic groups are completely deprotonated and carry a negative charge (-1). The third group carries charge +0.5 (half of the groups are charged, and another half is uncharged) whereas three other groups are protonated and are positively charged (+1). As a result, at pH 6.3, the net charge of the amino acid is 2 × (-1) + (+0.5) + 3 × (+1) = +1.5.

Answer: at pH 2, the net charge is +3.5; at pH 6.3, the net charge is +1.5.