Multicomponent reactions for covalent lysine labelling

In multicomponent reactions (MCRs) at least three reactants are converted into the desired products in a single step, with high atom efficiency.
In covalent protein labelling, small molecules are covalently attached to an amino acid side chain of a given target protein. The most common targeted residue was cysteine, but recently, lysine labelling became prevalent. Lysine is present in large quantities in the proteome and at protein-protein and protein-RNA interaction surfaces, thus playing a key role in the discovery of functional binding sites and the identification of chemical starting points.

The ε-amino group of lysine reacts with aldehydes forming imines, and, notably, that is the elementary step of several MCRs. Thus, in our research, we aimed to investigate the labelling of N-α-acetyl-L-lysine in MCRs, in aqueous medium by HPLC-MS. The selected reactions were in situ reduction, Strecker-, Henry-, Kabachnik-Fields-, A3-, Mannich-, Diels-Alder-, Ugi-3CR- and Ugi-4CR reactions.

The formation of the imine proceeds generally weakly, however, the subsequent reaction steps improve the conversion, the best performing reactions were the Strecker- and the Ugi-3CR.

With simple aldehydes, the formation of open-chain α-amino compounds was detected. o-Formylphenylboronic acids were the most effective in all the applied reactions. o-Ethynyl, o-isocyano, o-formyl and o-carboxy benzaldehydes led to the formation diverse five and six-membered heterocycles.

In summary we showed that lysine might be susceptible to a wide set of diverse MCRs enabling the complex modification of protein targets on their surfaces in one step.