Leucine structure

The systematic name of leucine is L-2-amino-4-methylpentanoic acid, and its structure is
(CH3)2CH-CH2-CH(NH2)-COOH

The -NH2 group is weakly basic and may accept a proton to give -NH3 cation.
The -COOH group is weakly acidic and may release a proton to give -COO- anion.
Therefore, leucine does not exist as above molecular structure in aqueous solution, but it exists in three different ionic forms as below:
A: (CH3)2CH-CH2-CH(NH3 )-COOH (cationic form)
B: (CH3)2CH-CH2-CH(NH2)-COO- (anionic form)
C: (CH3)2CH-CH2-CH(NH3 )-COO- (zwitterion)
In a solution of any pH, all of the above three forms exist. However, their relative amounts depend on the pH of the solution.

At pH 2, all of A, B and C exist in the aqueous solution. The solution is acidic. Therefore, most of the -NH2 groups are protonated to form -NH3 cations. Therefore, leucine exists mainly as the cationic form A ((CH3)2CH-CH2-CH(NH3 )-COOH).

At pH 5.98, all of A, B and C exist in the aqueous solution. It has been found that the isoelectric point of leucine is pH 5.98. An amino acid exists mainly as zwitterions at its isoelectric point. Therefore, leucine exists mainly as the zwitterions C ((CH3)2CH-CH2-CH(NH3 )-COO-) at pH 5.98.

At pH 9.24, all of A, B and C exist in the aqueous solution. The solution is alkaline. Therefore, most of the -COOH groups are deprotonated to form -COO- anions. Therefore, leucine exists mainly as the anionic form B ((CH3)2CH-CH2-CH(NH2)-COO-).