Identify the amino acid, NOT having nitrogen atom in a side chain.

The structures of the side chains (R groups) for the given amino acids are as follows:

Asparagine: The side chain is $-CH_2CONH_2$, which contains an amide nitrogen.

Histidine: The side chain contains an imidazole ring, which has two nitrogen atoms.

Tryptophan: The side chain contains an indole ring, which has a nitrogen atom.

Threonine: The side chain is $-CH(OH)CH_3$, which contains only carbon, hydrogen, and oxygen atoms. It does not contain any nitrogen atom.

Therefore, Threonine is the amino acid that does not have a nitrogen atom in its side chain.

Answer: Threonine