Vitamin C: an experimental and theoretical study on the gas-phase structure and ion energetics of protonated ascorbic acid

Furan products have been known for long ti me as food constituents but, since 1995, they received increasing attention after the publication of report 2B by the International Agency on Cancer Re­ search (IARC). Many studies have dealt with the precursors of furan in foods and identified ascorbic acid (AA) among one ofthe major sources. In order to investigate the gas phase mechanisms of the acid catalyzed degradation of ascorbic ac­ id (AA) to furan molecule we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investi­ gation at the B3LYP/6-31+G(d,p) level oftheory. According to this joint experimental and theoretical approach, the gaseous reactant species, the protonated ascorbic acid,[C6HS06]H+ ions, were generated by electrospray ionization (ESI) of a 10-3 M H20/CH30H (1: l) ascorbic acid solution . In order to structurally characterize the gaseous [C6Hs06]H+ ionic reactants, and to rationalize their gas phase degradation mechanism, we engaged in the experimental and theoretical estimate of the unknown gas phase basicity and proton affinity of AA. Indeed, in this paper, we reported the proton affinity and the gas phase basicity of ascorbic acid measured by the extended Cooks's kinetic method and estimated by computational methods at the B3L YP/6-31 +G( d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (02) of AA as the preferred protonation site. From the experimental proton affinity (P A) of 875.0 ± 12 kJ mOri and protonation entropy i1Sp (108.9 ± 2 J mol') a GB value of AA of 842.5 ± 12 kJ mOri kJ mOri at 298 K was ob­ tained, which is in agreement with the value issuing from quantum mechanical computations. Following this study on the structure and energetics of the gaseous reactant ion, an ongoing investi­ gation will deal with the whole degradation mechanism of protonated ascorbic acid to furan mole­ cule in the gas phase by means of experimental and theoretical methods.