As mentioned in the introduction, -photo-leucine is a synthetic derivative of the -Leucine amino acid. -photo-leucine is characterized by the presence of a diazirine ring linked to the R radical of the original amino acid. This cyclopropene ring-shaped molecule is constituted of a carbon atom attached to two nitrogen atoms through a covalent single bond. These two nitrogen atoms are simultaneously connected to each other by a double covalent bond. The diazirine carbon is located in the position where theoretically the 2nd carbon atom of the R radical of -leucine would be, linked up with the 1st and 3rd carbon of this theoretical R radical.  
The diazirine ring confers to the photo-leucine its photoreactive property. When irradiated with UV light, it splits releasing nitrogen in gas form and leaving an unbound carbon atom (see Diazirine). In protein-protein interactions (PPI), this atom is attached to the complex formed by the two proteins susceptible of being under study.
The rest of the amino acid has indeed the same structure as the original -leucine molecule, which includes, as every amino acid, an amino group and a carboxyl group bonded to an α-carbon, and a radical that is attached to this carbon atom. The R chain, contains, in this case, a diazirine ring and two extra carbon atoms connected each to the diazirine carbon as it has been previously mentioned. 
For use in biology experiments, only the -enantiomer of the photo-leucine amino acid is synthesized, so that it can substitute for natural -leucine. (Natural proteins consist only of -amino acids; see homochirality.)