Charge Retention/Charge Depletion in ESI-MS: Theoretical Rationale

Abstract

Gas phase modification in ESI-MS can significantly alter the charge state distribution of small peptides and proteins. The preceding paper presented a systematic experimental study on this topic using Substance P and proposed a charge retention/charge depletion mechanism, explaining different gas- and liquid-phase modifications [Thinius et al. 10.1021/jasms.9b00044J. Am. Soc. Mass Spec.2020]. In this work, we aim to support this rational by theoretical investigations on the proton transfer processes from (multiply) charged analytes toward solvent clusters. As model systems we use small (di)­amines as analytes and methanol (MeOH) and acetonitrile (ACN) as gas phase modifiers. The calculations are supported by a set of experiments using (di)­amines, to bridge the gap between the present model system and Substance P used in the preceding study. Upon calculation of the thermochemical stability as well as the proton transfer pathways, we find that both ACN and MeOH form stable adduct clusters at the protonation site. MeOH can form large clusters through a chain of H-bridges, eventually lowering the barriers for proton transfer to an extent that charge transfer from the analyte to the MeOH cluster becomes feasible. ACN, however, cannot form H-bridged structures due to its aprotic nature. Hence, the charge is retained at the original protonation site, i.e., the analyte. The investigation confirms the proposed charge retention/charge depletion model. Thus, adding aprotic solvent vapors to the gas phase of an ESI source more likely yields higher charge states than using protic compounds.

Computational analysis of the proton‐bound acetonitrile dimer, (ACN)2H+

“…Early theoretical investigations by Deakyne et al 34 suggested an asymmetric equilibrium structure of the dimer to be energetically slightly more favorable than the symmetric one. However, as already shown in a previous publication, 20 we cannot support their results with the applied model chemistry.…”

“…The second vibration that we will focus on is vibration 6. This mode corresponds to the asymmetric movement of the H + between the two stationary ACN molecules and was already found to be poorly described by the harmonic treatment of the normal mode analysis 20 . Its potential curve is better described by a quartic function, which is investigated further below.…”

“…In this contribution we investigate the especially difficult case of the proton‐bound acetonitrile dimer, (ACN) 2 H + , i.e., its internal degrees of freedom, which are necessary to compute the partition functions and respective thermochemical data. In several publications the experimental observation of this dimer has been reported 15–18,20 but there is little information from a theoretical point of view 20,34 . We show that a computational approach to reasonable thermochemical data, even for such simple structures, is rather challenging and necessitates more advanced techniques.…”

“…Even in ion mobility spectrometry (IMS), clusters play an important role in the separation process 30–33 . In previous publications 19,20 we have shown the pivotal role of prolonged cluster chemistry with gas‐phase additives that strongly enhance or decrease ion signals and alter the observed charge state distribution of peptides or small multiply charged diamines. Despite its importance, in particular regarding ion generation and suppression, often little attention is paid to the cluster chemistry in API sources 5 …”

“…Neutral analytes readily attach to these clusters to form mixed solvent‐analyte clusters 11 . At this point thermodynamics and reaction dynamics essentially determine whether the analyte is protonated to obtain a [M + H] + signal, the mixed cluster remains, or the analyte is not protonated at all 2,19,20 . This includes parameters such as solvent concentration, pressure, ion temperature, through electric field strengths in ion transfer and the mass analyzer, 11 as well as the shape of the potential energy surface (PES) 20,21 .…”