Mass spectrometric studies of the reaction of a blocked arginine with diketonic α-dicarbonyls

Abstract

The modification of arginine residues by diketonic α-dicarbonyls, in structural proteins and enzymes studies, is a process known for decades. The chemistry of these reaction processes is, however, not fully understood. Moreover, modification of arginine residues by reaction with α-dicarbonyls in glycation has also not been completely elucidated. Aspects related to the early stages of the condensation of arginine with one dicarbonyl molecule, leading to the formation of dihydroxyimidazolidines and their dehydrated forms, in particular, are here approached in more detail. Taking into consideration the usually rapid kinetics involved in the formation of the early reaction product species, we decided to use fast, sensitive and selective analytical techniques, such as electrospray ionization mass spectrometry (ESI–MS) and tandem mass spectrometry (ESI–MSⁿ) to monitor the reactions of a blocked arginine (acetyl-arginine) with several selected diketonic α-dicarbonyls, to identify and characterize the mentioned transient species and to probe the reaction mechanism involved. Compounds grouped into two different classes according to their structural similarity were identified, namely acetyl-dihydroxyimidazolidines and acetyl-bis(dihydroxyimidazolidines), together with their dehydrated species. The former compounds are known to exist in solution. The reactivity of acetyl-bis(dihydroxyimidazolidines) seems to be different from that of acetyl-dihydroxyimidazolidines. To note that dehydration appears to be reinforced in acetyl-bis(dihydroxyimidazolidines) chemistry with respect to acetyl-dihydroxyimidazolidine chemistry, while both structurally related compounds involve mostly dihemiaminals reactivity. Two different ion structures are proposed for single dehydrated acetyl-bis(dihydroxyimidazolidines), concerning the two more symmetrical and two more asymmetrical dicarbonyls reacted. In acetyl-bis(dihydroxyimidazolidines) formation, we concluded that the importance of single dehydration relies on the rapid minimization of sterics and energetics of the reaction moieties formed. These reactions occur also in a selective way, regarding the two compound structures proposed for single dehydrated acetyl-bis(dihydroxyimidazolidines). Further considerations are also established for the formation of single dehydrated acetyl-bis(dihydroxyimidazolidines). An explanation for the reversible nature of the reaction of arginine with diketonic dicarbonyls is also provided. This study reinforces the potential of the fast, sensitive and selective electrospray ionization mass spectrometry techniques for the investigation of transient species and their mechanistics, that might otherwise not be feasible by means of the most commonly used spectroscopic techniques.