Interactions of methylglyoxal with methylamine

Ab initio quantum mechanical calculations are used to study the interactions of the aldehydic group of methylglyoxal with the NH2 groups of protein side-chains, using methylamine as a representative molecule. The hydrogen-bonding interaction, C = O...H - N, results in an electronic charge transfer from methylglyoxal to methylamine in both the ground and first excited triplet states. In this latter state a slight possibility is found for the H atom in the hydrogen bond to tunnel from methylglyoxal to methylamine, leading to the possible formation of two free radical fragments. The approach of methylamine to methylglyoxal in the stacked conformation C...N to form a hemiacetal, associated with electron charge transfer from methylamine to methylglyoxal, is energetically unfavourable in vacuum. The concomitant tunnelling of a proton from a proton-donating solvent molecule to the aldehydic oxygen of methylglyoxal is shown to make this approach favourable. The relative stability of the keto and enol forms of methylglyoxal is also investigated, the keto form being found the more stable in vacuum.