Solvent-Mediated Proton-Transfer Catalysis of the Gas-Phase Isomerization of Ciprofloxacin Protomers

Abstract

Understanding how neutral molecules become protonated during positive-ion electrospray ionization (ESI) mass spectrometry is critically important to ensure analytes can be efficiently ionized, detected, and unambiguously identified. The ESI solvent is one of several parameters that can alter the dominant site of protonation in polyfunctional molecules and thus, in turn, can significantly change the collision-induced dissociation (CID) mass spectra relied upon for compound identification. Ciprofloxacina common fluoroquinolone antibioticis one such example whereby positive-ion ESI can result in gas-phase [M + H]+ ions protonated at either the keto-oxygen or the piperazine-nitrogen. Here, we demonstrate that these protonation isomers (or protomers) of ciprofloxacin can be resolved by differential ion mobility spectrometry and give rise to distinctive CID mass spectra following both charge-directed and charge-remote mechanisms. Interaction of mobility-selected protomers with methanol vapor (added via the throttle gas supply) was found to irreversibly convert the piperazine N-protomer to the keto-O-protomer. This methanol-mediated proton-transport catalysis is driven by the overall exothermicity of the reaction, which is computed to favor the O-protomer by 93 kJ mol–1 (in the gas phase). Conversely, gas phase interactions of mobility-selected ions with acetonitrile vapor selectively depletes the N-protomer ion signal as formation of stable [M + H + CH3CN]+ cluster ions skews the apparent protomer population ratio, as the O-protomer is unaffected. These findings provide a mechanistic basis for tuning protomer populations to ensure faithful characterization of multifunctional molecules by tandem mass spectrometry.

Deciphering and investigating fragment mechanism of quinolones using multi‐collision energy mass spectrometry and computational chemistry strategy

“…Therefore, besides O‐isomers, N‐isomers will also have great proportion in ESI and need to be considered. Literature 31,34 and the MS experiment of quinolone methanol solution confirm this phenomenon.…”

“…Analysis of the daughter fragment structures of the aforementioned three precursor ions confirmed the fragmentation processes of five quinolones, as shown in Figure 3 and Figures S6–S9 (supporting information). Previous studies indicate that O‐ and N‐species would yield [M + H‐H 2 O] + and [M + H‐CO 2 ] + , respectively 31–34 . Figure 2 and Figures S2–S5 (supporting information) show that the ratios of [M + H‐H 2 O] + to [M + H‐CO 2 ] + in the quinolone–MeOH system were higher than those in the quinolone–ACN system.…”

Impact of Ambient Vapors on Spectra of 4-Nitroaniline Recorded under Atmospheric Solids Analysis Probe (ASAP) Mass Spectrometric Conditions

“…Once activated, the proton-bound complex of acetonitrile and 4-nitroaniline dissociates to form the amino protomer, which upon further activation loses • NO 2 by a homolytic cleavage to form the distonic radical cation of aniline ( m / z 93; Scheme ). The affinity of acetonitrile to form a proton-bound complex specifically with the amino group of 4-nitroaniline is not a surprise because other compounds with N–H functionality such as para -aminobenzoic acid and ciprofloxacin are known to behave in a similar manner. − …”

Hydration-induced protomer switching in p-aminobenzoic acid studied by cold double ion trap infrared spectroscopy

“…19,41,42 On the other hand, the bridged structure becomes most abundant in higher hydrated clusters (n = 7). This suggests that the Grotthuss mechanism 13,26,43,44 may also be involved in the proton transfer, especially in larger hydrated clusters. However, a clear conclusion that can be drawn here for the smaller clusters (n r 6) is that if the Grotthuss mechanism plays a role in the proton transfer, it would first require the formation of energetically highly unfavourable structures.…”

“…1–4 In addition, some molecules vary their preferred protonation site depending on the surrounding environment such as solvent or counter ion. 5–29 Such protonation site-switching obviously has a huge effect on the chemical reactivity and biological activity of the molecule. 12,28…”