The side-chain of Fmoc-L-Lys(Mtt)-OH is protected by Mtt group which is very acid labile and it can be selectively removed with mild or low concentration of acid conditions, making this an excellent Lysine derivative for the synthesis of branched peptides or lysine side-chain modified peptides.
"Fmoc-Leu-OH is isomeric with Fmoc-Ile-OH. Like isoleucine, leucine has branching in the sidechain. The branching is at the gamma carbon instead of the beta carbon as in isoleucine. PPARγ ligand that induces insulin sensitization, but not adipogenesis."