Reactions of a modified lysine with aldehydic and diketonic dicarbonyl compounds: an electrospray mass spectrometry structure/activity study

Abstract

The phenomenon known as non-enzymatic glycation is described as the reaction of reducing sugars with basic amino groups of proteins and nucleic acids, as well as with simple amines, without enzyme mediation. Non-enzymatic model glycation reactions that make use of low-molecular-weight compounds make an important contribution in the elucidation of glicated processes in vitro and in vivo. Four alpha-dicarbonyl compounds, aldehydic (glyoxal, methylglyoxal and phenylglyoxal) and ketonic (diacetyl), were reacted with the modified amino acid N(alpha)-acetyl-L-lysine (AcLys) in an attempt to establish structure/activity relationships for the reactivity of alpha-dicarbonyls with the amine compound. Electrospray ionization mass spectrometry (ESI-MS) combined with tandem mass spectrometry (MS/MS) and collision-induced dissociation (CID) was used to identify and characterize reagents, intermediates and reaction products. The formation of dicarbonyl-derived lysine dimers was observed exclusively. Especially, attention is drawn to alkyl- (asymmetrical dicarbonyl systems) and carboxyl- (glyoxal system) substituted imidazolium ions, at ring position 2. The main differences observed in the reactions studied were related to the reactivity with the diimine intermediate. This intermediate can react either with a non-hydrated dicarbonyl molecule at the aldehydic carbonyl, or with a mono-hydrated one at the ketonic carbonyl, particularly for asymmetrical dicarbonyls. For 2-carboxyl-substituted imidazolium ion (glyoxal reaction), besides the usual keto-enol rearrangement from the diol group, an alternative reaction pathway (proton abstraction) appears to contribute also for the imidazolium ring-closure process. Moreover, the formation of imidazolium ring structures can depend on several factors, namely, the presence (or absence) of electron donor substituents at the formed diol, the degree of stability of the new electrophile generated and/or the equilibrium concentration of the non- and mono-hydrated dicarbonyl forms in solution, the last being particularly important for asymmetrical dicarbonyls. The results reported reveal the complexity of reactivity as well as the diversity of imidazolium molecular structures.