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Amino acids are categorized into different types, and one of them is the essential amino acid. These amino acids serve many purposes. The essential amino acid isoleucine is employed for energy regulation.

When the pH value is smaller than pI, more hydrogen ions are found in the solution. The hydrogen ions come close to the carboxylate ion, comprising a negative charge leading to protonation.

So, the overall charge of the amino acid will be +1.

(a)The amino acid, threonine has one carboxylic acid functional group and an amino functional group. The pH value of 1 corresponds to the acidic functional group. The protonation of the nitrogen atom happens at the amino group.

Hence, the overall charge of the compound threonine is +1. The structure can be given as:

Structure of threonine at pH=1

(b) Methionine has one carboxylic acid group and one amino group. The overall charge is +1 on the methionine amino acid. The structure of the compound at pH=1 can be given as:

Structure of methionine at pH=1

(c)Aspartic acid has two carboxylic acid functional groups and one amino functional group. The overall charge is +1 on aspartic acid. The structure of the compound at pH=1 can be given as:

Structure of aspartic acid at pH=1

(d)Arginine has one carboxylic acid functional group and three amine functional groups. The overall charge is +4 on arginine. The structure can be given as:

Structure of arginine at pH=1