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

The Gas Phase Protonation Sites of Six Naturally Occurring Nicotinoids

The gas phase protonation sites of six naturally occurring nicotinoids, namely nicotine (NIC), nornicotine (NOR), anabasine (ANB), anatabine (ANT), cotinine (COT), and myosmine (MYO), consisting of a common Pyridine and differing non-Pyridine rings, have been determined for the first time at the physiological temperature from cryogenic ion trap infrared spectroscopy and electronic structure calculations. The protonation site on either of these two rings is related to the nicotinoid's biological activity. At room temperature, NIC is a mixture of Pyridine and Pyrrolidine (non-Pyridine) protomers, whereas NOR, ANB, ANT, and COT are pure Pyridine protomers and finally MYO is mostly a Pyroline (non-Pyridine) protomer. The nearly planar structure of MYO-H+, induced by the presence of a conjugated π system and confirmed from calculations and the UV absorption spectra, breaks from the trends observed for NIC, NOR, and ANB, since its structure is drastically different from the structures of the other nicotinoids.

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation

Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (∼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 ← S0 and S3 ← S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.

The impact of solvation on the structure and electric field strength in Li+GlyGly complexes

To scrutinise the impact of electric fields on the structure and vibrations of biomolecules in the presence of water, we study the sequential solvation of lithium diglycine up to three water molecules with cryogenic infrared action spectroscopy. Conformer-specific IR-IR spectroscopy and H2O/D2O isotopic substitution experiments provide most of the information required to decipher the structure of the observed conformers. Additional confirmation is provided by scaled harmonic vibrational frequency calculations using MP2 and DFT. The first water molecule is shown to bind to the Li+ ion, which weakens the electric field experienced by the peptide and as a consequence, also the strength of an internal NH⋯NH2 hydrogen bond in the diglycine backbone. The strength of this hydrogen bond decreases approximately linearly with the number of water molecules as a result of the decreasing electric field strength and coincides with an increase in the number of conformers observed in our spectra. The addition of two water molecules is already sufficient to change the preferred conformation of the peptide backbone, allowing for Li+ coordination to the lone pair of the terminal amine group.

Ion Spectroscopy Reveals Structural Difference for Proteins Microhydrated by Retention and Condensation of Water

Protein ubiquitin in its +7 charge state microhydrated by 5 and 10 water molecules has been interrogated in the gas phase by cold ion UV/IR spectroscopy. The complexes were formed either by condensing water onto the unfolded bare proteins in a temperature-controlled ion trap or by incomplete dehydration of the folded proteins. In the case of cryogenic condensation, the UV spectra of the complexes exhibit a resolved vibrational structure, which looks similar to the spectrum of bare unfolded ubiquitin. The spectra become, however, broad-band with no structure when complexes of the same size are produced by incomplete dehydration under soft conditions of electrospray ionization. We attribute this spectroscopic dissimilarity to the structural difference of the protein: condensing a few water molecules cannot refold the gas-phase structure of the bare ubiquitin, while the retained water preserves its solution-like folded motif through evaporative cooling. This assessment is firmly confirmed by IR spectroscopy, which reveals the presence of free NH and carboxylic OH stretching vibrations only in the complexes with condensed water.

Transition from vehicle to Grotthuss proton transfer in a nanosized flask: cryogenic ion spectroscopy of protonated p-aminobenzoic acid solvated with D2O

Proton transfer (PT) is one of the most ubiquitous reactions in chemistry and life science. The unique nature of PT has been rationalized not by the transport of a solvated proton (vehicle mechanism) but by the Grotthuss mechanism in which a proton is transported to the nearest proton acceptor along a hydrogen-bonded network. However, clear experimental evidence of the Grotthuss mechanism has not been reported yet. Herein we show by infrared spectroscopy that a vehicle-type PT occurs in the penta- and hexahydrated clusters of protonated p-aminobenzoic acid, while Grotthuss-type PT is observed in heptahydrated clusters, indicating a change in the PT mechanism depending on the degree of hydration. These findings emphasize the importance of the usually ignored vehicle mechanism as well as the degree of hydration. It highlights the possibility of controlling the PT mechanism by the number of water molecules in chemical and biological environments.

Switching of Protonation Sites in Hydrated Nicotine via a Grotthuss Mechanism

The switching of the protonation sites in hydrated nicotine, probed by experimental infrared (IR) spectroscopy and theoretical ab initio calculations, is facilitated via a Grotthuss instead of a bimolecular proton transfer (vehicle) mechanism at the experimental temperature (T = 130 K) as unambiguously confirmed by experiments with deuterated water. In contrast, the bimolecular vehicle mechanism is preferred at higher temperatures (T = 300 K) as determined by theory. The Grotthuss mechanism for the concerted proton transfer results in the production of nicotine's bioactive and addictive pyrrolidine-protonated (Pyrro-H+) protomer with just 5 water molecules. Theoretical analysis suggests that the concerted proton transfer occurs via hydrogen-bonded bridges consisting of a 3 water molecule "core" that connects the pyridine protonated (Pyri-H+) with the pyrrolidine-protonated (Pyrro-H+) protomers. Additional water molecules attached as acceptors to the hydrogen-bonded "core" bridge result in lowering the reaction barrier of the concerted proton transfer down to less than 6 kcal/mol, which is consistent with the experimental observations.

Formation of Eigen or Zundel Features at Protonated Water Cluster-Aromatic Interfaces

Interfacial interactions of protonated water clusters adsorbed at aromatic surfaces play an important role in biology, and in atmospheric, chemical and materials sciences. Here, we investigate the interaction of protonated water clusters ((H+ H2 O)n (where n=1-3)) with benzene (Bz), coronene (Cor) and dodecabenzocoronene (Dbc)). To study the structure, stability and spectral features of these complexes, computations are done using DFT-PBE0(+D3) and SAPT0 methods. These interactions are probed by AIM electron density topography and non-covalent interactions index (NCI) analyses. We suggest that the excess proton plays a crucial role in the stability of these model interfaces through strong inductive effects and the formation of Eigen or Zundel features. Also, computations reveal that the extension of the π-aromatic system and the increase of the number of water molecules in the H-bounded water network led to a strengthening of the interactions between the corresponding aromatic compound and protonated water molecules, except when a Zundel ion is formed. The present findings may serve to understand in-depth the role of proton localized at aqueous medium interacting with large aromatic surfaces such as graphene interacting with acidic liquid water. Besides, we give the IR and UV-Vis spectra of these complexes, which may help for their identification in laboratory.

Microhydration of the Pyrrole Cation (Py+) Revealed by IR Spectroscopy: Ionization-Induced Rearrangement of the Hydrogen-Bonded Network of Py+(H2O)2

Microhydration of heterocyclic aromatic molecules can be an appropriate fundamental model to shed light on intermolecular interactions and functions of macromolecules and biomolecules. We characterize herein the microhydration process of the pyrrole cation (Py⁺) by infrared photodissociation (IRPD) spectroscopy and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Analysis of IRPD spectra of mass-selected Py⁺(H₂O)₂ and its cold Ar-tagged cluster in the NH and OH stretch range combined with geometric parameters of intermolecular structures, binding energies, and natural atomic charge distribution provides a clear picture of the growth of the hydration shell and cooperativity effects. Py⁺(H₂O)₂ is formed by stepwise hydration of the acidic NH group of Py⁺ by a hydrogen-bonded (H₂O)₂ chain with NH···OH···OH configuration. In this linear H-bonded hydration chain, strong cooperativity, mainly arising from the positive charge, strengthens both the NH···O and OH···O H-bonds with respect to those of Py⁺H₂O and (H₂O)₂, respectively. The linear chain structure of the Py⁺(H₂O)₂ cation is discussed in terms of the ionization-induced rearrangement of the hydration shell of the neutral Py(H₂O)₂ global minimum characterized by the so-called "σ-π bridge structure" featuring a cyclic NH···OH···OH···π H-bonded network. Emission of the π electron from Py by ionization generates a repulsive interaction between the positive π site of Py⁺ and the π-bonded OH hydrogen of (H₂O)₂, thereby breaking this OH···π hydrogen bond and driving the hydration structure toward the linear chain motif of the global minimum on the cation potential.

Microhydrated clusters of a pharmaceutical drug: infrared spectra and structures of amantadineH+(H2O)n

Solvation of pharmaceutical drugs has an important effect on their structure and function. Analysis of infrared photodissociation spectra of amantadineH⁺(H₂O)_n=1-4 clusters in the sensitive OH, NH, and CH stretch range by quantum chemical calculations (B3LYP-D3/cc-pVTZ) provides a first impression of the interaction of this pharmaceutically active cation with water at the molecular level. The size-dependent frequency shifts reveal detailed information about the acidity of the protons of the NH₃⁺ group of N-protonated amantadineH⁺ (AmaH⁺) and the strength of the NH⋯O and OH⋯O hydrogen bonds (H-bonds) of the hydration network. The preferred cluster growth begins with sequential hydration of the NH₃⁺ group by NH⋯O ionic H-bonds (n = 1-3), followed by the extension of the solvent network through OH⋯O H-bonds. However, smaller populations of cluster isomers with an H-bonded solvent network and free N-H bonds are already observed for n ≥ 2, indicating the subtle competition between noncooperative ion hydration and cooperative H-bonding. Interestingly, cyclic water ring structures are identified for n ≥ 3, each with two NH⋯O and two OH⋯O H-bonds. Despite the increasing destabilization of the N-H proton donor bonds upon gradual hydration, no proton transfer to the (H₂O)_n solvent cluster is observed up to n = 4. In addition to ammonium cluster ions, a small population of microhydrated iminium isomers is also detected, which is substantially lower for the hydrophilic H₂O than for the hydrophobic Ar environment.

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

Para-Aminobenzoic acid (PABA) is a benchmark molecule to study solvent-induced proton site switching. Protonation of the carboxy and amino groups of PABA generates O- and N-protomers of PABAH⁺, respectively. Ion mobility mass spectrometry (IMS) and infrared photodissociation (IRPD) studies have claimed that the O-protomer most stable in the gas phase is converted to the N-protomer most stable in solution upon hydration with six water molecules in the gas-phase cluster. However, the threshold size has remained ambiguous because the arrival time distributions in the IMS experiments exhibit multiple peaks. On the other hand, IRPD spectroscopy could not detect the N-protomer for smaller hydrated clusters because of broad background due to annealing required to reduce kinetic trapping. Herein, we report the threshold size for O → N protomer switching without ambiguity using IR spectroscopy in a double ion trap spectrometer from 1300 to 1800 cm^-1. The pure O-protomer is prepared by electrospray, and size-specific hydrated clusters are formed in a reaction ion trap. The resulting clusters are transferred into a second cryogenic ion trap and the distribution of O- and N-protomers is determined by mid-IR spectroscopy without broadening. The threshold to promote O → N protomer switching is indeed five water molecules. It is smaller than the value reported previously, and as a result, its pentahydrated structure does not support the Grotthuss mechanism proposed previously. The extent of O → N proton transfer is evaluated by collision-assisted stripping IR spectroscopy, and the N-protomer population increases with the number of water molecules. This result is consistent with the dominant population of the N-protomer in aqueous solution.

Gentle nano-electrospray ion source for reliable and efficient generation of microsolvated ions

We present herein the design of a nano-electrospray ion source capable of reliable generation of large quantities of microsolvated ions. The source is based on a triple molecular skimmer scheme and can be quickly tuned to generate bare ions or their ionic complexes with up to more than 100 solvent molecules retained from solution. The performance of this source is illustrated by recording the mass spectra of distributions of ionic complexes of protonated water, amino acids, and a small protein ubiquitin. Protonated water complexes with more than 110 molecules and amino acids with more than 45 water molecules could be generated. Although the commercial ion source based on the double ion funnel design with orthogonal injection, which we used in our laboratory, is more efficient in generating ions than our triple skimmer ion source, they both exhibit comparable short-term stability in generating bare ions. In return, only the new source is capable of generating microsolvated ions.

Structure of Gas Phase Monohydrated Nicotine: Implications for Nicotine's Native Structure in the Acetylcholine Binding Protein

We report a joint experimental-theoretical study of the never reported before structure and infrared spectra of gas phase monohydrated nicotine (NIC) and nornicotine (NOR) and use them to assign their protonation sites. NIC's biological activity is strongly affected by its protonation site, namely, the pyrrolidine (Pyrro-NICH⁺, anticipated active form) and pyridine (Pyri-NICH⁺) forms; however, these have yet to be directly experimentally determined in either the nicotinic acetylcholine receptor (nAChR, no water present) or the acetylcholine-binding protein (AChBP, a single water molecule is present) but can only be inferred to be Pyrro-NICH⁺ from the intermolecular distance to the neighboring residues (i.e., tryptophan). Our temperature-controlled double ion trap infrared spectroscopic experiments assisted by the collisional stripping method and high-level theoretical calculations yield the protonation ratio of Pyri:Pyrro = 8:2 at 240 K for the gas phase NICH⁺···(H₂O) complex, which resembles the molecular cluster present in the AChBP. Therefore, a single water molecule in the gas phase enhances this ratio in NICH⁺ relative to the 3:2 for the nonhydrated gas phase NICH⁺ in a trend that contrasts with the almost exclusive presence of Pyrro-NICH⁺ in aqueous solution. In contrast, the Pyri-NORH⁺ protomer is exclusively observed, a fact that may correlate with its weaker biological activity.