From ascorbic acid to furan molecules: a theoretical and experimental study on the gas phase acid catalyzed degradation of Vitamin C

Degradation of L-ascorbic acid (L-AA) occurs into two types of reactions, named the non-oxidative and the oxidative. The main difference between these two pathways is that furfural is more easily produced through the former. It should be noted that the expression non-oxidative refers solely to the nature of the initial step, since subsequent transformations may involve various oxidation steps. Indeed, the oxidative pathway describes the reaction which involves as an initial step the oxidation of L-ascorbic acid to dehydro-L-ascorbic acid. Likewise, the non-oxidative pathway relates to the direct decomposition of L-ascorbic acid with exclusion of dehydro-L-ascorbic acid as an intermediate structure. In1995, furan and its derivatives were classified by the International Agency on Cancer Research (IARC) in the group 2B, as possibly carcinogenic to humans. Since the first report in 1933, the formation of furfural from L-AA in strong acid media has been confirmed by many workers and some reaction mechanisms for the formation of furfural from L-AA have been proposed. However, none of them seems to be acceptable as the mechanism taking place in ordinary food stuffs. Here we report on the gas-phase investigation performed by a joined mass spectrometric and theoretical approach on the acid catalysed mechanism for the formation of furan compounds in the non-oxidative degradation of L-AA. According to this approach, gaseous protonated ascorbic acid ions, [C6H8O6]H+, at m/z 177, were generated by Electrospray Ionization Mass Spectrometry of an ascorbic acid solution. The [C6H8O6]H+ ionic reactants at m/z 177 were previously structurally characterized as the ascorbic acid molecule protonated at the O2 carbonyl oxygen atom.(1) They were subjected to collisionally activated decomposition (CAD) in order to induce the gas phase unimolecular degradation pathway of protonated ascorbic acid. The degradation pathway emerging from the CAD mass spectrum of the precursor ion at m/z 177 shows a twofold dehydration step, 177->159 and 159 ->141 followed by the elimination of an HCOOH/CO2 moiety, 141->95/97, leading to the formation of furanic products. Energy Resolved CAD mass spectra allowed to obtain informations on the relative energies of degradations steps. Experimental results were compared with those of a theoretical investigation performed at B3LYP/6-31+G(d,p) level of theory highlighting the most favourable decomposition pathway. The mechanism leading to furan compounds involves dehydration, hydrolysis of the lactone ring followed by decarboxylation.