Drift Cell Research Articles

Laser ionization of H2S and ion-molecule reactions of H3S+ in laser-based ion mobility spectrometry and drift cell time-of-flight mass spectrometry

The detection of hydrogen sulfide (H2S) by 2 + 1 resonance-enhanced multi-photon ionization (REMPI) and the application of H2S as a laser dopant for the detection of polar compounds in laser ion mobility (IM) spectrometry at atmospheric pressure were investigated. Underlying ionization mechanisms were elucidated by additional studies employing a drift cell interfaced to a time-of-flight mass spectrometer. Depending on the pressure, the primary ions H2S(+), HS(+), S(+), and secondary ions, such as H3S(+), were observed. The 2 + 1 REMPI spectrum of H2S near λ = 302.5 nm was recorded at atmospheric pressure. Furthermore, the limit of detection and the linear range were established. In the second part of the work, H2S was investigated as an H2O analogous laser dopant for the ionization of polar substances by proton transfer. H2S exhibits a proton affinity (PA) similar to that of H2O, but a significantly lower ionization energy facilitating laser ionization. Ion-molecule reactions (IMR) of H3S(+) with a variety of polar substances with PA between 754.6 and 841.6 kJ/mol were investigated. Representatives of different compound classes, including alcohols, ketones, esters, and nitroaromatics were analyzed. The IM spectra resulting from IMR of H3S(+) and H3O(+) with these substances are similar in structure, i.e., protonated monomer and dimer ion peaks are found depending on the analyte concentration.

Probing Ni+ metastable state production in sputtering glow discharges with the use of diagnostic reactions

The gas-phase reactions of ground and excited state Ni+ with CH3X and CF3X (X=Cl, Br) have been examined in a selected ion drift cell apparatus. Ni+ in both the 3d9 (ground state) and 3d84s1 (excited state) configurations was prepared in a sputtering glow discharge utilizing either Ne or Ar as the working gas. Configuration-specific product formation was determined using ion mobility spectrometry in combination with mass spectra collected under different ionization conditions. While the 2D ground state can be assigned to the 3d9 configuration, these results indicate several excited states may contribute to the 3d84s1 configuration. Under the near-thermal conditions of the drift cell, Ni+(2D) produces association products with all four reactant neutrals and NiBr+ from the brominated species. Dependent on the reactant neutral, excited states contributing to the 3d84s1 Ni+ configuration participate in both charge- and halogen atom transfer. Overall energetic requirements limit access to these product channels to those states from which products are formed exothermically. Observed products are consistent with the presence of excited states as high as 2P. Additionally, configuration-specific kinetic measurements suggest that unreactive states are present within the 3d84s1 configuration. Experiments conducted with Ni+ produced in Ar and Ne discharges suggest that excited state Ni+ distributions are not dependent on the identity of the discharge gas, but that the proportion of reactive excited states may be influenced by other discharge parameters. These results are consistent with an ionization/excitation mechanism involving energetic electrons within the discharge.

Mutual influences of halogenated compounds during atmospheric pressure chemical ionization

Sensor techniques and especially ion mobility spectrometry (IMS) play an ever growing role in environmental monitoring. Due to the sensitive detection of halogenated compounds IMS is predestinated for this application field. However, there are many influencing factors which can affect the signal response of a certain compound, i.e. temperature, humidity and cross sensitivity. In this study we investigate the mutual influence of halogenated hexanes (C 6 H 13 X) regarding to their simultaneous detectability and signal response in ion mobility spectra. It is known from literature that the atmospheric pressure chemical ionization of halogenated compounds precedes through dissociative electron transfer. In the negative measurement mode product ions in the form of (H 2 O) n X − can be detected. We quantified the decrease of signal response of the measured halogenated target compounds in dependency of the concentration of the affecting substance provided. We observed that the influence on the signal intensity of halogenated substances increases in the following order of halogenated admixtures: F < Cl < Br < I. Furthermore it is shown that the temperature of the drift cell can be used to influence the extent of cross sensitivity.

The effect of salt on the conformations of three model proteins is revealed by variable temperature ion mobility mass spectrometry

Three ‘model’ proteins of varying molecular mass: lysozyme, cytochrome C, and BPTI are incubated with sodium iodide and following nano-spray ionisation, their gas-phase conformations are determined using drift tube ion mobility mass spectrometry (DT IM-MS). Ion mobility measurements were carried out on these proteins with helium as the buffer gas at three different drift cell temperatures – ‘ambient’ (300K), ‘cold’ (260K) and ‘hot’ (360K). Significant levels of adducted iodide are observed on all three proteins with the number of iodides correlating to the number of available basic sites. The conformational space occupied by each protein is altered in the presence of salt, this is exhibited by a reduction in the intensities of adducted species for high and low charge states for each protein. This ‘salting in’ reduces the CCS spread for lysozyme and BTPI to converge on the values calculated from the crystal structure data. A change in conformation of all proteins is observed as a function of charge state, which is attributed to coulombically driven unfolding. Thermally induced unfolding (which is observed at both cold and hot temperatures) is minimised in the presence of adducted iodide. This is the first report of ‘cold denaturisation’ for proteins in the gas phase and suggests that this effect maybe intrinsic to the protein fold.

Ionization, Transport, Separation, and Detection of Ions in Non-Electrolyte Containing Liquids

Liquid phase ion mobility spectrometry (LPIMS) has the potential to be miniaturized such that it can be incorporated into chip based technology, providing higher performance in terms of both detection sensitivity and resolving power than is currently available by other separation technologies such as gas phase IMS, chromatography, or electrophoresis. This work presents modeling, simulation, and experimental investigations to characterize the mobility of ions in a liquid phase. This study included the ionization, transfer, separation, and detection of ions in non-electrolyte liquids. Using a resistive glass tube, mobility spectra were obtained by pulsed ionization for several different analytes, namely, tetramethylammonium chloride, tetrabutylammonium chloride, and dimethyl methylphosphonate (DMMP). Ion separation was demonstrated by separating solvent ions from the ions generated from the test compounds. Simulation and theoretical resolving power calculations were made to validate the experimental mobility measurements. A parametric study on the dependence of IMS resolving power on drift length, voltage across drift cell, and pulse width determined the requirements for designing a miniaturized IMS system, approximately the centimeter scale, with high performance, resolving power approaching 100 or higher. Mobility spectra are used for the first time to determine the diffusion coefficients of ions in a liquid.

Ion Mobility Mass Spectrometry of Peptide Ions: Effects of Drift Gas and Calibration Strategies

One difficulty in using ion mobility (IM) mass spectrometry (MS) to improve the specificity of peptide ion assignments is that IM separations are performed using a range of pressures, gas compositions, temperatures, and modes of separation, which makes it challenging to rapidly extract accurate shape parameters. We report collision cross section values (Ω) in both He and N(2) gases for 113 peptide ions determined directly from drift times measured in a low-pressure, ambient temperature drift cell with radio-frequency (rf) ion confinement. These peptide ions have masses ranging from 231 to 2969 Da, Ω(He) of 89-616 Å(2), and Ω(N(2)) of 151-801 Å(2); thus, they are ideal for calibrating results from proteomics experiments. These results were used to quantify the errors associated with traveling-wave Ω measurements of peptide ions and the errors concomitant with using drift times measured in N(2) gas to estimate Ω(He). More broadly, these results enable the rapid and accurate determination of calibrated Ω for peptide ions, which could be used as an additional parameter to increase the specificity of assignments in proteomics experiments.

Stepwise Association of Hydrogen Cyanide and Acetonitrile with the Benzene Radical Cation: Structures and Binding Energies of (C6H6•+)(HCN)n, n = 1–6, and (C6H6•+)(CH3CN)n, n = 1–4, Clusters

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes associated with the stepwise association of HCN and CH(3)CN molecules with the benzene radical cation in the C(6)H(6)(•+)(HCN)(n) and C(6)H(6)(•+)(CH(3)CN)(n) clusters with n = 1-6 and 1-4, respectively. The binding energy of CH(3)CN to the benzene cation (14 kcal/mol) is stronger than that of HCN (9 kcal/mol) mostly due to a stronger ion-dipole interaction because of the large dipole moment of acetonitrile (3.9 D). However, HCN can form hydrogen bonds with the hydrogen atoms of the benzene cation (CH(δ+)···NCH) and linear hydrogen bonding chains involving HCN···HCN interaction. HCN molecules tend to form externally solvated structures with the benzene cation where the ion is hydrogen bonded to the exterior of HCN chains. For the C(6)H(6)(•+)(CH(3)CN)(n) clusters, internally solvated structures are formed where the acetonitrile molecules are directly interacting with the benzene cation through ion-dipole and hydrogen bonding interactions. The lack of formation of higher clusters with n > 4, in contrast to HCN, suggests the formation of a solvent shell at n = 4, which is attributed to steric interactions among the acetonitrile molecules attached to the benzene cation and to the presence of the blocking CH(3) groups, both effects make the addition of more than four acetonitrile molecules less favorable.

State-Specific Reactions of Cu+(1S, 3D) with CH3X and CF3X (X = Cl, Br, I): Exploring the Influence of Dipole Orientation on Association and C–X Bond Activation

The reactions of gas-phase Cu(+)((1)S) and Cu(+)((3)D) with CF(3)X and CH(3)X (X = Cl, Br, and I) have been examined experimentally using the drift cell technique at 3.5 Torr in He at room temperature. State-specific product channels and overall bimolecular rate constants for depletion of the two Cu(+) states were determined using electronic state chromatography. The results showed that Cu(+)((1)S) participates exclusively in association with all of these neutrals, whereas, depending on the neutral, Cu(+)((3)D) initiates up to three bimolecular processes, resulting in the formation of CuX(+), CuC(H/F)(3)(+), and C(H/F)(3)X(+). Possible structures for the singlet association products were explored using density functional methods. These calculations indicated that Cu(+) preferentially associates with the labile halogen (Cl, Br, I) with all neutrals except CF(3)Cl, for which a "backside" geometry occurs in which Cu(+)((1)S) is weakly bound to the -CF(3) end of the molecule. All products observed on the triplet reaction surface can be understood in terms of either known or calculated thermochemical requirements. Product distributions and overall reaction efficiencies for C-X bond activation (X = Br, I) through Cu(+)((3)D) suggest that the orientation of the neutral dipole has little or no effect in controlling access to specific product channels. Likewise, second-order rate constants for reactions with X = Br and I indicate efficient depletion of Cu(+)((3)D) and do not exhibit the dramatic variations in reaction efficiency previously observed with CH(3)Cl and CF(3)Cl. These results suggest that C-X bond activation proceeds through a bond-insertion mechanism as opposed to direct abstraction.

Near-Thermal Reactions of Au+(1S,3D) with CH3X (X = F,Cl)

Reactions of Au(+)((1)S) and Au(+)((3)D) with CH(3)F and CH(3)Cl have been carried out in a drift cell in He at a pressure of 3.5 Torr at both room temperature and reduced temperatures in order to explore the influence of the electronic state of the metal on reaction outcomes. State-specific product channels and overall two-body rate constants were identified using electronic state chromatography. These results indicate that Au(+)((1)S) reacts to yield an association product in addition to AuCH(2)(+) in parallel steps with both neutrals. Product distributions for association vs HX elimination were determined to be 79% association/21% HX elimination for X = F and 50% association/50% HX elimination when X = Cl. Reaction of Au(+)((3)D) with CH(3)F also results in HF elimination, which in this case is thought to produce (3)AuCH(2)(+). With CH(3)Cl, Au(+)((3)D) reacts to form AuCH(3)(+) and CH(3)Cl(+) in parallel steps. An additional product channel initiated by Au(+)((3)D) is also observed with both methyl halides, which yields CH(2)X(+) as a higher-order product. Kinetic measurements indicate that the reaction efficiency for both Au(+) states is significantly greater with CH(3)Cl than with CH(3)F. The observed two-body rate constant for depletion of Au(+)((1)S) by CH(3)F represents less than 5% of the limiting rate constant predicted by the average dipole orientation model (ADO) at room temperature and 226 K, whereas CH(3)Cl reacts with Au(+)((1)S) at the ADO limit at both room temperature and 218 K. Rate constants for depletion of Au(+)((3)D) by CH(3)F and CH(3)Cl were measured at 226 and 218 K respectively, and indicate that Au(+)((3)D) is consumed at approximately 2% of the ADO limit by CH(3)F and 69% of the ADO limit by CH(3)Cl. Product formation and overall efficiency for all four reactions are consistent with previous experimental results and available theoretical models.

Isomer-resolved dissociation of small carbon cluster cations, C7+–C10+

Isomer-resolved multiphoton dissociation (PD) with 3.5eV and collision-induced dissociation (CID) were applied to small carbon cluster cations, Cn+ (n=7–10), to investigate correlations between the isomer structures and dissociation reactivities. Linear and cyclic isomers, which coexist in this size range, were separated by ion mobility spectrometry using a home-made drift cell. The linear isomers mainly produced C3-loss fragments, while cyclic ones showed C2-loss as well as C3-loss products. The C2-loss products were predominantly observed by PD, whereas comparable amounts of C2- and C3-loss products were observed by CID. The range of the excitation energy of CID was overlapped with that of PD, whereas the former range was wider than the latter. Plausible origins of the difference between the fragment distributions from the cyclic and linear isomers are discussed in terms of the energetics of the fragment ions.

Temperature Dependence of Ion Mobility of Carbon Cluster Cations: Intermediate Region Connecting Low- and High-Field Conditions

Abstract Effective temperature (Teff) dependences of reduced mobilities and collision cross section of small carbon cluster cations, Cn+ (n = 6–25), were investigated by using a drift cell with temperatures of 170–300 K coupled with a reflectron time-of-flight mass spectrometer. We have measured ion mobility within E/N = 16–28 Td, values of which are slightly higher than the general experimental value at low-field conditions. Thus, our conditions should correspond to medium field strength between low- and high-field conditions. Under the present conditions, collision cross section was found to be proportional to Teff−0.1 − Teff−0.2. The power in the temperature dependence was slightly closer to zero than results obtained previously (A. A. Shvartsburg, G. C. Schatz, M. F. Jarrold, J. Chem. Phys., 1998, 108, 2416). This difference is probably due to E/N, resulting in the difference of relative kinetic energy between Cn+ and buffer gas atoms, in spite of the same Teff region. The result showed that the hard-sphere model is more applicable in the present conditions.

Collision Cross Sections of Proteins and Their Complexes: A Calibration Framework and Database for Gas-Phase Structural Biology

Collision cross sections in both helium and nitrogen gases were measured directly using a drift cell with RF ion confinement inserted within a quadrupole/ion mobility/time-of-flight hybrid mass spectrometer (Waters Synapt HDMS, Manchester, U.K.). Collision cross sections for a large set of denatured peptide, denatured protein, native-like protein, and native-like protein complex ions are reported here, forming a database of collision cross sections that spans over 2 orders of magnitude. The average effective density of the native-like ions is 0.6 g cm(-3), which is significantly lower than that for the solvent-excluded regions of proteins and suggests that these ions can retain significant memory of their solution-phase structures rather than collapse to globular structures. Because the measurements are acquired using an instrument that mimics the geometry of the commercial Synapt HDMS instrument, this database enables the determination of highly accurate collision cross sections from traveling-wave ion mobility data through the use of calibration standards with similar masses and mobilities. Errors in traveling-wave collision cross sections determined for native-like protein complexes calibrated using other native-like protein complexes are significantly less than those calibrated using denatured proteins. This database indicates that collision cross sections in both helium and nitrogen gases can be well-correlated for larger biomolecular ions, but non-correlated differences for smaller ions can be more significant. These results enable the generation of more accurate three-dimensional models of protein and other biomolecular complexes using gas-phase structural biology techniques.

Increased ion transmission in IMS: A high resolution, periodic-focusing DC ion guide ion mobility spectrometer

The resolution of ion mobility spectrometry (IMS) is of paramount importance for both post-ionization separations and structural characterization of ions that have similar ion-neutral collision cross sections; however, the instrumental features that lead to increased resolution also decrease ion transmission through the drift cell. The periodic-focusing DC ion guide (PDC IG) drift cell provides increased ion transmission with minimal loss in resolution. In earlier work we showed that the electrode geometry (inner diameter, thickness, and spacing) strongly affects ion focusing and ion transmission. Here, we critically evaluate the effect of the electrode geometry of a PDC IG drift cell on both ion transmission and resolution. In this study we examine two drift cells that differ in length (63 and 125 cm) and electrode configuration. We also examine the effects of applied voltage and pressure in an attempt to maximize both resolution and ion transmission. Experimental data obtained with fullerene and model peptide ions are compared with calculated ion trajectories using SIMION 8.0 simulations.

Artificial Photosynthesis over Crystalline TiO2-Based Catalysts: Fact or Fiction?

The mechanism of photocatalytic conversion of CO(2) and H(2)O over copper oxide promoted titania, Cu(I)/TiO(2), was investigated by means of in situ DRIFT spectroscopy in combination with isotopically labeled (13)CO(2). In addition to small amounts of (13)CO, (12)CO was demonstrated to be the primary product of the reaction by the 2115 cm(-1) Cu(I)-CO signature, indicating that carbon residues on the catalyst surface are involved in reactions with predominantly photocatalytically activated surface adsorbed water. This was confirmed by prolonged exposure of the catalyst to light and water vapor, which significantly reduced the amount of CO formed in a subsequent experiment in the DRIFT cell. In addition, formation of carboxylates and (bi)carbonates was observed by exposure of the Cu(I)/TiO(2) surface to CO(2) in the dark. These carboxylates and (bi)carbonates decompose upon light irradiation, yielding predominantly CO(2). At the same time a novel carbonate species is produced (having a main absorption at approximately 1395 cm(-1)) by adsorption of photocatalytically produced CO on the Cu(I)/TiO(2) surface, most likely through a reverse Boudouard reaction of photocatalytically activated CO(2) with carbon residues. The finding that carbon residues are involved in photocatalytic water activation and CO(2) reduction might have important implications for the rates of artificial photosynthesis reported in many studies in the literature, in particular those using photoactive materials synthesized with carbon containing precursors.

Calibration of the CMS drift tube chambers and measurement of the drift velocity with cosmic rays

This paper describes the calibration procedure for the drift tubes of the CMS barrel muon system and reports the main results obtained with data collected during a high statistics cosmic ray data-taking period. The main goal of the calibration is to determine, for each drift cell, the minimum time delay for signals relative to the trigger, accounting for the drift velocity within the cell. The accuracy of the calibration procedure is influenced by the random arrival time of the cosmic muons relative to the LHC clock cycle. A more refined analysis of the drift velocity was performed during the offline reconstruction phase, which takes into account this feature of cosmic ray events.

Structural Characterization of Phospholipids and Peptides Directly from Tissue Sections by MALDI Traveling-Wave Ion Mobility-Mass Spectrometry

Ion mobility-mass spectrometry (IM-MS) provides rapid two-dimensional separations based on analyte apparent surface area or collision cross section (CCS, A(2)) and mass-to-charge, respectively. Recently, traveling-wave (t-wave) IM-MS was developed which uses electrodynamic rather than electrostatic fields commonly used in drift cell IM-MS instruments. The underlying theory for obtaining CCS data is well developed for drift cell IM-MS, while strategies for obtaining CCS values from t-wave IM-MS data remains an active area of research. In this report, methods were developed and validated to obtain CCS values of phospholipids and peptides directly from thin tissue sections by MALDI t-wave IM-MS using CCS calibrants measured by MALDI drift cell IM-MS. Importantly, the average percent difference between t-wave and drift cell CCS measurements is minimized by calibrating with the same biomolecular class. Calibrating t-wave phospholipid CCS values with drift cell peptide CCS measurements results in an average percent difference of ca. 7% between the same lipids measured using t-wave and drift cell IM-MS, while this improves to <0.5% when drift cell phospholipid CCS values are used for calibrating t-wave data. A suite of CCS values are reported for lipids and peptides that were determined directly from tissue, i.e. without the need for tissue extraction and further purification steps.

Investigation of gaseous metabolites from moulds by Ion Mobility Spectrometry (IMS) and Gas Chromatography-Mass Spectrometry (GC-MS)

The metabolism of moulds results in the formation of various microbial volatile organic compounds (MVOCs). These substances can be used as an indicator for the presence of moulds in the indoor environment. Three different mould strains were cultivated on culture media and IMS spectra of gaseous mould metabolites were recorded using a portable mini system with a tritium source and a 5 cm drift cell. The headspace spectra are characteristic for mould species and their age. Typical gaseous components of the metabolites were identified and compared with results obtained from gas chromatography using a mass spectrometer detector. It was observed that the MVOCs formation depends on mould species and their growing stage with a maximum of MVOCs emission occurring during the first 10 days. These preliminary results show that IMS can be applied to detect MVOCs in indoor environment and indicate hidden mould growth.

Structural Analysis of Prion Proteins by Means of Drift Cell and Traveling Wave Ion Mobility Mass Spectrometry

The prion protein (PrP) is implicitly involved in the pathogenesis of transmissible spongiform encephalopathies (TSEs). The conversion of normal cellular PrP (PrP(C)), a protein that is predominantly alpha-helical, to a beta-sheet-rich isoform (PrP(Sc)), which has a propensity to aggregate, is the key molecular event in prion diseases. During its short life span, PrP can experience two different pH environments; a mildly acidic environment, whilst cycling within the cell, and a neutral pH when it is glycosyl phosphatidylinositol (GPI)-anchored to the cell membrane. Ion mobility (IM) combined with mass spectrometry has been employed to differentiate between two conformational isoforms of recombinant Syrian hamster prion protein (SHaPrP). The recombinant proteins studied were alpha-helical SHaPrP(90-231) and beta-sheet-rich SHaPrP(90-231) at pH 5.5 and pH 7.0. The recombinant proteins have the same nominal mass-to-charge ratio (m/z) but differ in their secondary and tertiary structures. A comparison of traveling-wave (T-Wave) ion mobility and drift cell ion mobility (DCIM) mass spectrometry estimated and absolute cross-sections showed an excellent agreement between the two techniques. The use of T-Wave ion mobility as a shape-selective separation technique enabled differentiation between the estimated cross-sections and arrival time distributions (ATDs) of alpha-helical SHaPrP(90-231) and beta-sheet-rich SHaPrP(90-231) at pH 5.5. No differences in cross-section or ATD profiles were observed between the protein isoforms at pH 7.0. The findings have potential implications for a new ante-mortem screening assay, in bodily fluids, for prion misfolding diseases such as TSEs.

The effect of drift gas on the separation of active pharmaceutical ingredients and impurities by ion mobility–mass spectrometry

The effect of drift gas and binary gas mixtures on the separation of active pharmaceutical ingredients and related compounds has been investigated for the first time using tri-wave ion mobility–mass spectrometry. The drift times for [M+H] + ions were measured in argon, carbon dioxide, nitrogen, helium and binary mixtures of these gases as a function of pressure and drift gas composition. At the same drift cell pressure the drift times of the protonated ions decreased in order; carbon dioxide > argon > nitrogen > helium. The drift times of compounds in the different drift gases were dependent on the polarizability of the drift gas, the size of the drift gas and the reduced mass of the analyte ion/drift gas pair. Resolution of the active pharmaceutical ingredients Lamotrigine and Rosiglitazone increased with drift cell pressure for argon and nitrogen, but showed no significant increase in carbon dioxide. Enhanced selectivity is demonstrated for the separation of pharmaceutical components using binary drift gas mixtures in tri-wave IM–MS.

Protonated Arginine and Protonated Lysine: Hydration and Its Effect on the Stability of Salt-Bridge Structures

Using a mass spectrometer equipped with a drift cell, water binding energies of protonated arginine (ArgH+) and protonated lysine (LysH+) were determined in equilibrium experiments and supplementary calculations at the B3LYP/6-311++G** level of theory. The binding energy of the first water molecule was measured to be 10.3 and 10.9 kcal/mol for ArgH+ and LysH+, respectively. Water binding energies decrease with increasing degree of hydration reaching values of 6-7 kcal/mol for the fourth and fifth water molecule. Theory reproduces this trend of decreasing binding energies correctly and theoretical water binding energies agree with experiment quantitatively within 2 kcal/mol. Lowest-energy theoretical structures of ArgH+ and LysH+ are characterized by protonated side chains and neutral alpha-amino and carboxyl groups which form intramolecular hydrogen bonds to the ionic group (charge solvation or CS structures). The salt bridge (SB) structures with two cationic groups (side chain and alpha-amine) and one anionic group (carboxyl) are 13.1 and 9.3 kcal/mol higher in energy for ArgH+ and LysH+, respectively. Theory indicated that the first water molecule binds to the ionic group of the CS structures of ArgH+ and LysH+. With increasing degree of hydration intramolecular interactions are replaced one by one with water bridges with water inserted into the intramolecular hydrogen bonds. Whereas the global minima of ArgH+.(H2O)n and LysH+.(H2O)n, n<7, were calculated to represent CS structures, 7-fold hydrated CS and SB structures, ArgH+.(H2O)7 and LysH+.(H2O)7, are nearly isoenergetic (within <1 kcal/mol).