High Production of Small Organic Dicarboxylate Dia

Authors: Yonghui Dong Graziano Guella Fulvio Mattivi Pietro Franceschi

Publish Date: 2015/01/17

Volume: 26, Issue: 3, Pages: 386-389

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Abstract

A significant production of gasphase dicarboxylate dianions has been observed in standard ESI and DESI during the analysis of small organic dicarboxylic acids under moderate or highly alkaline conditions In ESI this can be attributed to an excess of hydroxyl ions OH– which favor the formation of an high amount of dianions in solution contemporarily trapping the potential counter ions during the ESI process The results obtained in DESI highlight the role of the surface in trapping the counterions during desorption process and determining the ultimate nature of the observed gasphase ionsSmall organic dicarboxylic acids play an important role in many biological systems In the case of plants they are key intermediates in carbon metabolism and may be present in high concentrations—often stored as K+ salts 1—with important implications for the production of beverages like juices or wineSince the pKa2 values in water of dicarboxylic acids H2DCA such as succinic glutaric adipic malic and tartaric acids are lower than six 2 3 in alkaline aqueous solutions they are expected to be present mostly as dicarboxylate dianions DCA2– with relatively minor amounts of the monoprotonated HDCA– species Despite this small DCA2– species are rarely and barely observed in the gas phase and it is speculated that this is due to the absence of the interactions with surrounding media ie counterions or solvation This type of interaction is always required to stabilize the charges 4 5 6 From a general point of view DCA2– species can disappear through three main mechanisms 1 protonation during solvent evaporation leading to HDCA– 2 one electron loss leading through an intermediate radical anion to a decarboxylated singly charged radical anion 7 8 or 3 dissociation into singlycharged negative ionsFor a given carbon backbone the stability of DCA2– in the gas phase can be increased by introducing additional functional groups which allow charge delocalization and/or favor intramolecular hydrogen bonding as in the case of tartaric acid 7 This multifaceted behavior coupled with a relatively simple structure has made H2DCA an ideal system for studying fundamental molecular phenomena like Coulomb repulsion 5 and solutesolvent interactions 6 9 From an experimental point of view ESI should be the favored technique for producing gas phase DCA2– but it has always been quite difficult to produce these ionic species in significant amounts 10 at least for H2DCA with low molecular weight In particular a recent study by Tonner et al 7 shows that tartaric dianions TA2– can be produced by ESI although not very efficiently only under restricted experimental conditions whereby their spontaneous thermodynamic decomposition via mechanism b is somewhat hindered These calculations indeed indicate that TA2– is metastable with respect to dissociation into the decarboxylated radical anion + CO2 + e–a DESI images of tartrate in grapevine leaf petiole M – 2H2 denotes doubly charged tartrate dianions TA2– M – H– singly deprotonated ion HTA– and M – 2H + K – potassium adduct ions KTA– b Representative spectrum of tartrate in grapevine leaf petiole The insert shows the halfinteger isotope spacing between the ions with molecular formulas 12C4H4O6 2– at m/z 7400 and 12C3 13CH4O6 2– at m/z 7450DCA2– ion yields as observed in ESI and DESI ionsources for K2DCA aq solutions DCA2– ion yields were calculated as the ratio of M – 2H2 abundances to those of their respective M – H– singlycharged ions Values represent mean ± SD n = 6 ‘31’ represents salts obtained from H2DCA using three molar equivalents 31 of KOH whereas ‘21’ represents salts obtained from H2DCA using two molar equivalents 21 of KOH The representative mass spectra shown in the plots are DESI spectra at ‘31’ Results for H2TrA are not shown here as no detectable signal for TrA2 was found either in DESI or in ESI spectra

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