Effects of Residual Water in a Linear Quadrupole Ion Trap on the Protonation Sites of 4-Aminobenzoic Acid

Abstract

In solution, the most basic site in 4-aminobenzoic acid is the amino nitrogen, while the carboxylic acid oxygen is the most basic site in the gas phase. However, the protonation site in the gas phase has been demonstrated to depend on the ionization solvents when ionized using positive ion mode electrospray ionization (ESI). In many of these studies, collision-activated dissociation (CAD) was used to differentiate the protomers. To explore the influence of different CAD conditions on the manifested protonation site, 4-aminobenzoic acid dissolved either in 1:1 acetonitrile−water or 3:1 methanol−water was ionized by ESI and subjected to three different CAD experiments in a linear quadrupole ion trap/orbitrap mass spectrometer. Based on in-source CAD (ISCAD) and beam-type medium-energy CAD (MCAD), the proton resided on the nitrogen atom (N-protomer) when acetonitrile−water was used as the solvent system but on the oxygen atom (O-protomer) when methanol−water was used. Interestingly, a predominant Nprotomer was observed when CAD was performed in the linear quadrupole ion trap (ITCAD), irrespective of the solvents used, in disagreement with literature. This unexpected result is rationalized based on the formation of long-lived water clusters of varying sizes for the protomers in the quadrupole ion trap due to residual water, low ion kinetic energies, long ion storage times, and relatively high pressure. Further, addition of extra water into the quadrupole ion trap resulted in nearly identical protomer distributions for both protomers. Therefore, this distribution must be near the equilibrium distribution caused by the presence of water clusters of varying sizes, some favoring the N-protomer and others the O-protomer.

Impact of Ambient Vapors Present in an Electrospray Ionization Source on Gas-Phase Ion Structures

“…As Seo et al have suggested, the proton relocation is inhibited by acetonitrile. We propose that acetonitrile forms a strong complex with N -protonated PABA and thereby prevents the transformation of the N -protomer to the gas-phase favored O -protomer . In fact, we stipulated previously that the kinetic trapping , of the energetically less favored N -protomer is a gas-phase phenomenon .…”

Collision-assisted stripping for determination of microsolvation-dependent protonation sites in hydrated clusters by cryogenic ion trap infrared spectroscopy: the case of benzocaineH+(H2O)n

“…3,4 In general, the protonation site of an organic molecule highly depends on the surrounding solvent. 6–22 This solvent-dependent behavior is particularly prominent for molecules with multiple basic sites. For example, benzocaine (BC), which has both carbonyl and amino groups, is most stable in its NH 2 -protonated form (N-protomer, H + BC( N )) in polar solvents, whereas the CO-protonated form (O-protomer, H + BC( O )) is most stable in the gas phase (Fig.…”

“…14,15 This solvent-dependent protonation-site switching is not only found in BC but prevails in other (bio-)organic molecules. 16–33 Much effort has been devoted to microsolvated clusters, in which an ion is solvated by a few to several hundreds of molecules, 14,34–36 because the degree of solvation can be tuned by controlling the type and numbers of solvent molecules.…”

Effects of Analyte Concentration on the Protonation Sites of 4-Aminobenzoic Acid upon Atmospheric Pressure Chemical Ionization As Revealed by Gas-Phase Ion–Molecule Reactions

“…This study focuses on the possible influence of the concentration of 4-aminobenzoic acid on the manifested protonation sites after APCI in a linear quadrupole ion trap (LQIT) mass spectrometer when using two different solvent systems. While CAD has been commonly used for the structural elucidation of the protomers of 4-aminobenzoic acid, , this method cannot be used to differentiate the protomers in LQIT mass spectrometers because residual water present in the ion trap can change the protonation site . Thus, gas-phase ion–molecule reactions between trimethoxymethylsilane (TMMS) and protonated 4-aminobenzoic acid were employed instead.…”

“…While CAD has been commonly used for the structural elucidation of the protomers of 4-aminobenzoic acid, 15,26 this method cannot be used to differentiate the protomers in LQIT mass spectrometers because residual water present in the ion trap can change the protonation site. 27 Thus, gas-phase ion− molecule reactions between trimethoxymethylsilane (TMMS) and protonated 4-aminobenzoic acid were employed instead. TMMS has been reported to react with protonated hydroxylamines and protonated carboxylic acids to generate different products: a stable adduct is generated for protonated hydroxylamines while an adduct that has lost a methanol molecule is observed for protonated aliphatic and aromatic carboxylic acids.…”