[figure] Choose the correct option for structures of A and B, respectively
- [figure] and [figure]
- [figure] and [figure]
- [figure] and [figure]
- [figure] and [figure]
Answer
(3) [figure] and [figure]
Solution
The compound is valine, an α-amino acid with a basic -NH2 group and an acidic -COOH group; its isoelectric point is about 6.
At pH = 2 (strongly acidic, pH < pI), H+ is added to -NH2 and -COOH stays un-ionised, so the amino acid exists as the cation:
A = [figure]
At pH = 10 (basic, pH > pI), -COOH loses H+ and -NH2 stays free, so the amino acid exists as the anion:
B = [figure]
These are the structures given in option (3).
Answer: (3)