The Charge-State and Structural Stability of Peptides Conferred by Microsolvating Environments in Differential Mobility Spectrometry

Abstract

The presence of solvent vapor in a differential mobility spectrometry (DMS) cell creates a microsolvating environment that can mitigate complications associated with field-induced heating. In the case of peptides, the microsolvation of protonation sites results in a stabilization of charge density through localized solvent clustering, sheltering the ion from collisional activation. Seeding the DMS carrier gas (N2) with a solvent vapor prevented nearly all field-induced fragmentation of the protonated peptides GGG, AAA, and the Lys-rich Polybia-MP1 (IDWKKLL­DAAKQIL-NH2). Modeling the microsolvation propensity of protonated n-propylamine [PrNH3]+, a mimic of the Lys side chain and N-terminus, with common gas-phase modifiers (H2O, MeOH, EtOH, iPrOH, acetone, and MeCN) confirms that all solvent molecules form stable clusters at the site of protonation. Moreover, modeling populations of microsolvated clusters indicates that species containing protonated amine moieties exist as microsolvated species with one to six solvent ligands at all effective ion temperatures (T eff) accessible during a DMS experiment (ca. 375–600 K). Calculated T eff of protonated GGG, AAA, and Polybia-MPI using a modified two-temperature theory approach were up to 86 K cooler in DMS environments seeded with solvent vapor compared to pure N2 environments. Stabilizing effects were largely driven by an increase in the ion’s apparent collision cross section and by evaporative cooling processes induced by the dynamic evaporation/condensation cycles incurred in the presence of an oscillating electric separation field. When the microsolvating partner was a protic solvent, abstraction of a proton from [MP1 + 3H]3+ to yield [MP1 + 2H]2+ was observed. This result was attributed to the proclivity of protic solvents to form hydrogen-bond networks with enhanced gas-phase basicity. Collectively, microsolvation provides analytes with a solvent “air bag,” whereby charge reduction and microsolvation-induced stabilization were shown to shelter peptides from the fragmentation induced by field heating and may play a role in preserving native-like ion configurations.

The hitchhiker's guide to dynamic ion–solvent clustering: applications in differential ion mobility spectrometry

“…Upon computing DLPNO-CCSD(T) electronic energies and DFT thermochemical corrections, the populations of each microsolvated cluster size can be calculated from the Gibbs energy of association (Δ G ass ). 69 Δ G ass is defined by eqn (5), where G 0 is the Gibbs energies of the bare ion ( G 0 ), G solv is the Gibbs energy of the bare solvent ligand, and G n is the Gibbs energy of the microsolvated cluster containing n solvent ligands.Δ G ass, n ( T ) = G n ( T ) − [ G 0 ( T ) + n · G solv ( T )]One can use the temperature dependence of Δ G ass to calculate the population ( ρ ) of a microsolvated cluster of size n as a function of ion temperature using eqn (6), where N is the gas particle density, R is the gas constant, G ( k ) ass, n ( T ) is the Gibbs energy of the k th isomer of the cluster consisting of n solvent molecules at temperature T , and [M] is the concentration of the solvent modifier.The temperature-dependent population of each microsolvated cluster size will depend on the magnitude and sign for the respective Δ G ass, n , which is determined by the thermodynamic stability of the microsolvated cluster (enthalpic contribution) and the entropic penalty that must be paid to form the microsolvation network (Δ G ass = Δ H ass − T Δ S ass ). Entropic contributions dictate the thermodynamics of cluster formation at higher field strengths since ion T eff increases quadratically with field strength ( vide infra ).…”

“…180 Td. 69 Since the DMS cell operates at atmospheric pressure, ion–neutral collisions will occur every 0.1 to 1 nanosecond during each SV duty cycle. 78 The rapid collision frequency suggests that any change to the ion's internal energy induced by a collision event will be (1) equilibrated throughout all molecular degrees of freedom, 95 and (2) will respond almost instantaneously with respect to changes in the SV field strength.…”

“…This could be especially useful in preserving native-like ion configurations of peptides/proteins under the extreme conditions of a DMS device without modifying the charge distribution. 69,150…”

“…The details of these PES mapping methods can be found in the associated publications. [66][67][68][69] PES mapping is typically performed using molecular mechanics to identify possible low energy cluster structures, [70][71][72] which are subsequently refined using higher levels of theory (see Fig. 3).…”

“…Upon computing DLPNO-CCSD(T) electronic energies and DFT thermochemical corrections, the populations of each microsolvated cluster size can be calculated from the Gibbs energy of association (DG ass ). 69 DG ass is defined by eqn (5), where G 0 is the Gibbs energies of the bare ion (G 0 ), G solv is the Gibbs energy of the bare solvent ligand, and G n is the Gibbs energy of the microsolvated cluster containing n solvent ligands.…”