Host–Guest Chemistry in the Gas Phase: Complex ... - ACS Publications

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Host Guest Chemistry in the Gas Phase: Complex Formation with 18-Crown-6 Enhances Helicity of Alanine-Based Peptides Jae Yoon Ko,† Sung Woo Heo,† Joon Ho Lee,† Han Bin Oh,‡ Hyungjun Kim,*,§ and Hugh I. Kim*,† †

Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea Department of Chemistry, Sogang University, Seoul, 121-742, Republic of Korea § Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea ‡

bS Supporting Information ABSTRACT: The gas-phase helix propensities of alanine-based polypeptides are studied with different locations of a Lys residue and host guest interactions with 18-Crown-6 (18C6). A series of model peptides Ac-Ala9 n-LysH+-Alan (n = 0, 1, 3, 5, 7, and 9) is examined alone and with 18C6 using traveling wave ion mobility mass spectrometry combined with molecular dynamics (MD) simulations. The gasphase helices are observed from the peptides whose Lys residue is located close to the C-terminus so that the Lys exerts its capping effect on the C-terminal carbonyl groups. The peptides, which interact with 18C6 in the gas phase, show enhanced helical propensities. The enhanced helicity of the peptide in the complex is attributed by isolation of the Lys butylammonium group from the helix backbone and the interaction of methylene groups of 18C6, which possess localized positive partial charges, with C-terminal carbonyl groups serving as a cap to stabilize the helix.

’ INTRODUCTION The helix is a ubiquitous secondary structural motif and a fundamental building block of proteins. This structural motif plays a key role in determining the structures and functions of membrane interacting proteins.1 Numerous research studies have endeavored to clarify the conditions for helical structure formation, which is strongly influenced by environmental factors such as the dielectric property of the solvent,2 4 pH,5 and molecular interactions.6 Helix propensities are also intensely affected by amino acid residues.7 9 Among 20 natural amino acids, alanine has the highest helix propensity,7,8,10 while glycine has the least propensity.7,11 Electrostatically, the helix contains a large macrodipole resulting from the accumulation of dipole moments from the hydrogen bonds of backbone amide groups.7,11 For a stable helical structure, this helix dipole needs to be stabilized by charged residues via capping the end of the helix.7,12 In order to understand the factors responsible for intrinsic helix propensities, numerous helix measurements have been carried out in gas phase where solvent interaction is absent.13 24 Gas-phase helices have been investigated thoroughly using polyalanine-based peptides, which include a Lys cap at the C-terminus and acetylated N-terminus.16,17 The butylammonium group of Lys functions to stabilize a peptide helix via the formation of hydrogen bonds with free carbonyl groups, and aligning of the helix dipole at the end of the C-terminal site.16,17 Studies using ion mobility mass spectrometry (IM-MS) have revealed that Ac-Alan-LysH+ peptides with n g 8 exhibit an extended helical structure in the gas phase.17 Recent spectroscopic study using Ac-Alan-LysH+ (n = 5, 10, and 15) supports r 2011 American Chemical Society

this, showing that longer molecules (n = 10, 15) yield one αhelical conformer, while Ac-Ala5-LysH+ is not a simple helix.13 Combined with theoretical calculations, IM-MS has been successfully applied for screening the structure of biomolecules in the gas phase.14 29 The experimentally measured ion mobility value provides a collision cross section (ΩD) of the ion,30 which depends on its structure.25 Recently developed traveling wave ion mobility mass spectrometry (TWIM-MS) has been applied to various chemistry fields,26,31 35 demonstrating its utility to estimate ΩD of the ion.34 37 In this study, we investigate the helical properties of alanine-based peptides using electrospray ionization (ESI) traveling wave ion mobility spectrometry (TWIMS) coupled with orthogonal acceleration time-of-flight (oa-TOF) mass spectrometry combined with molecular dynamics (MD) simulations.38 Of particular interest is the possible dependence of peptide helical propensities on the location of Lys and its interaction with 18-Crown-6 (18C6; Scheme 1) in the gas phase. 18C6 is a widely used host molecule with protein (or peptide) guest molecules,39 especially in the gas phase,40 43 with its specific recognition of Lys residue due to the strong ion dipole interactions.40,41

’ EXPERIMENTAL SECTION Chemicals and Reagents. All model peptides (Ac-A9 nKAn, n = 0, 1, 3, 5, 7, and 9) are purchased from Peptron (Daejeon, Received: August 21, 2011 Revised: October 25, 2011 Published: November 01, 2011 14215

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Table 1. Experimental and Theoretical ΩD of the Peptides Ac-Ala9 n-LysH+-Alan (n = 0, 1, 3, 5, 7, and 9) and Their 18C6 Complex Ionsa ΩD (Å2)

Korea) and used without further purification. 18-Crown-6 (18C6) is purchased from Sigma-Aldrich. All solvents (water and methanol) are HPLC grade and purchased from J. T. Baker (Phillipsburg, NJ). 18C6 stock solution is prepared by dissolving 18C6 in water. Then, sample solutions are prepared in a solvent consisting of 50/50 water and methanol by volume. The final concentrations of 18C6 and the guest peptides are adjusted to 40 and 20 μM, respectively, for electrospray ionization (ESI). Electrospray Ionization Traveling Wave Ion Mobility Mass Spectrometers. The experiments are performed on a Synapt G2 HDMS traveling wave ion mobility orthogonal acceleration time-of-flight (Waters, Manchester, U.K.) in positive ion mode. Source temperature of 80 °C, capillary voltage of 2.7 kV, desolvation temperature of 215 °C, and cone voltage of 24 V are set as parameters for ESI. Helium gas is introduced to the Helium Cell at a 180 mL/min flow rate. Nitrogen drift gas is introduced to the TWIMS stacked ring ion guide (SRIG) at a 50 mL/min flow rate, which corresponds to 4.75 Torr. The optimized traveling wave (T-wave) height and velocity are 21 V and 300 m/s, respectively. For each sample, 100 spectra are obtained and averaged for analysis. The drift times of analyte ions are extracted using MassLynx (version 4.1) software (Waters, Milford, MA). Collision Cross Sections. The collision cross sections (ΩD) of peptide and 18C6 complex ions are estimated using calibration methods adopted from Thalassinos et al.34 Polyalanine peptides and trypsin digests of cytochrome C, ubiquitin, bovine serum albumin, and hemoglobin are used to create a calibration curve with previously published ΩD values (Figure S1a and Table S1 in the Supporting Information).44 All calibrant ions are singlycharged species. The effective drift time of the calibrant is corrected for mass independent and mass dependent time. The effective drift time is then plotted against the corrected published collision cross section. The plot is used to fit following previously described methods.34 The equation from the fitting result is used to estimate ΩD of ions. Previous study of small peptides using TWIMS by Thalassinos et al.34 has reported that linear fit is appropriate for calibration. However, a slightly higher correlation is observed for power fitting in the present study. The power fit also shows good agreement with calibration curve obtained using the method from Ruotolo et al.,45 which is developed for large protein complexes (Figure S1b in the Supporting Information). The estimated ΩD values of peptides and peptide complexes of 18C6 using both methods by Thalssinos et al.34 and Ruotolo et al.45 are found in Table S2 in the Supporting Information. The experimental ΩD values used in the present study are from the power fit result using the method of Thalassinos et al.34 Computational Modeling. To sample the energetically favored conformations of peptides, we have performed molecular dynamics (MD) simulations using the large scale atomic/ molecular massively parallel simulator (LAMMPS) code.38 The interatom interactions are described by using the all-atom CHARMM PARAM27 force field (FF)46 combined with a FF specially designed for cyclic ethers.47 After the initial peptide

ΔE ΔΩD,exp‑theo (kcal/mol) (%)

experimentalb

theoretical

Ac-A9K

226.2 ( 4.2

214.2 ( 2.8

0

5.3 ( 2.2

Ac-A8KA

230.4 ( 3.9

214.9 ( 4.3

0

6.7 ( 2.5

225.1 ( 3.2

1.4

2.3 ( 2.2

218.5 ( 4.0

10.9

5.2 ( 2.4

214.9 ( 4.3

0

4.0 ( 2.4

215.7 ( 4.9 236.3 ( 6.8

8.7 14.6

3.6 ( 2.7 5.6 ( 3.4

201.0 ( 1.4

0

9.3 ( 2.1

210.7 ( 4.7

9.5

5.0 ( 2.9

210.0 ( 2.6

0

2.3 ( 1.8

215.3 ( 4.6

12.1

0.1 ( 2.5

205.9 ( 4.2

0

0.1 ( 2.5

207.2 ( 2.1

10.4

265.5 ( 3.1 266.5 ( 2.9

0 0

276.7 ( 4.2

1.2

263.6 ( 2.4

0

Ac-A6KA3

Ac-A4KA5 Ac-A2KA7 Ac-KA9

223.8 ( 3.4

221.7 ( 4.4 215.0 ( 2.8 205.7 ( 2.9

Ac-A9K+18C6 Ac-A8KA+18C6

277.4 ( 3.7 275.7 ( 2.7

Ac-A6KA3+18C6

270.5 ( 3.1

Ac-A4KA5+18C6

N/A

0.7 ( 1.7 4.3 ( 1.7 3.3 ( 1.4 0.4 ( 1.8 2.6 ( 1.4

269.4 ( 4.9

13.0

0.4 ( 2.1

252.6 ( 3.6

0

N/A

261.5 ( 5.2

0.5

267.2 ( 4.8

21.7

Ac-A2KA7+18C6

N/A

256.4 ( 4.6 262.1 ( 4.5

0 14.1

N/A

Ac-KA9+18C6

252.2 ( 2.4

250.7 ( 2.4

0

0.6 ( 1.3

a

The relative potential energy (ΔE) of each structure and the difference of ΩD (ΔΩD,exp‑theo) from theoretical ΩD to experimental ΩD are also shown. The structure corresponds to the theoretical ΩD is found in Figures S2 and S3. b Power fit result from the calibration method by Thalassinos et al.34

structure is minimized (with or without 18C6), we elevate the simulation temperature from 0 to 300 K for 25 ps to properly distribute the momentum to each atom. We then carry out 30 annealing cycles to efficiently search the entire conformational space; each annealing cycle consists of heating dynamics from 300 to 600 K for 100 ps and cooling dynamics from 600 and 300 K for another 100 ps. The helix stabilization enthalpy (ΔH) and entropy (ΔS) have been estimated as ∼ 1 kcal/mol48 53 and ∼ 2 to 5 cal/mol,48 50 respectively, per residue in solution phase. These lead the temperature required to denaturize the peptide with 10 amino acid residues as 200 500 K. Thus, we have chosen 600 K to anneal a gas phase peptide with 10 amino acid residues. We note that a Nose-Hoover thermostat was employed for the temperature control. After each 100 ps heating (or cooling) dynamics, we sample the structure and minimize it. Including the initially minimized structure, a total of 61 minimized structures are clustered based on the location of each amide backbone hydrogen bond. Then, clustered structures with their energies are sampled for the comparison with experimentally estimated collision cross sections (Table 1, Figures S2 and S3 in the Supporting Information). For the calculations of theoretical ΩD of ions, a projection approximation (PA) method, 14216

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Figure 2. (a) Ion mobility spectra of peptide series Ac-Ala9 n-LysH+Alan (n = 0, 1, 3, 5, 7, and 9). (b) MD simulated typical low energy structures and (c) secondary structure assignment of corresponding peptides whose calculated ΩD values agree well with experimental ΩD. To clarify secondary structure and backbone amide group interactions, Ala side chain atoms are omitted in the structures.

Figure 1. Electrospray ionization mass spectra of the peptides Ac-Ala9 nLysH+-Alan (n = 0, 1, 3, 5, 7, and 9) with 18-Crown-6 (18C6). Singlycharged peptide ions are found at m/z 828.5 as dominating species. Singly-charged peptide 18C6 complex ions are found at m/z 1092.7 from the peptides for n = 0, 1, 3, and 9. For n = 9, doubly-charged dimeric 18C6 complex of (Ac-LysH-Ala9)22+ is also observed (*).

which is based on a hard sphere description of the interaction potential with additional Lennard-Jones description, is used.54 We also calculate theoretical ΩD of ions using the trajectory (TJ) method,55 which adopts more realistic soft-core interactions, for the lowest energy structures whose calculated ΩD values using the PA method agree well with experimental ΩD. Then, we compare the ΩD of peptide and find good agreement between two methods with average deviations of 1.1% (Table S3 in the Supporting Information). The secondary structure of the peptide is assigned from atomic coordinates based on the combined hydrogen bond energy and statistically derived backbone torsional angle information using VMD (version 1.9).56,57 Electrostatic potential of typical low energy structures, whose theoretical ΩD was close to experimental ΩD, are evaluated using density functional theory (DFT) calculations. The DFT calculations are performed using Gaussian 09 (Gaussian Inc., Wallingford, CT)58 at M06-2X/6-31++G** level.59

’ RESULTS AND DISCUSSION Singly-Charged Peptides Ac-Ala9 n-LysH+-Alan. Singly-

charged peptides Ac-Ala9 n-LysH+-Alan (n = 0, 1, 3, 5, 7, and 9) are observed as dominating species in the TWIM-MS spectra (Figure 1). The 18C6 complex ions are also observed and with sufficiently high resolution with the peptides for n = 0, 1, 3, and 9.

First, we examine the conformations of peptide ions as a function of the Lys location from ion mobility (IM) spectra (Figure 2a). We find that the peaks of these isobaric peptides generally shift to smaller arrival times as the location of Lys moves from C-terminus to N-terminus. This implies that as the sizes of these peptides become smaller, the structures in turn change from extended conformations (i.e., helices) to globular conformations, as the Lys moves from C-terminus to N-terminus, which is supported by the low energy structures from our MD annealing dynamics trajectories. Typical low energy structures (Figure 2b), whose calculated ΩD values agree well with experimental ΩD, demonstrate that the helicities of Ac-Ala9 n-LysH+-Alan are preserved for n = 0 and 1, while other peptides prefer globular conformations (Figure 2c). For n = 0, the peptide favors a helical structure (Figure 2b), as reported from previous studies.15,17 Such a stable helical structure is preserved for the peptide n = 1 with a more extended form (Figure 2b), leading to a slightly longer arrival time (Figure 2a). This enhanced helicity is attributed from the increased flexibility of the Lys butylammonium group, which coordinates with C-terminal free carbonyl groups. Other peptides for n g 3 show generally globular structures with maximized hydrogen bonding interactions of backbone carbonyl groups with the Lys butylammonium group (Figure 2b). Singly-Charged 18C6 Complexes of Ac-Ala9 n-LysH+-Alan. We then examine the role of host guest chemistry in helix propensity. The general tendency of decreasing ΩD as the Lys moves from C-terminus to N-terminus is also shown from the IM spectra of 18C6 complex ions of the peptides for n = 0, 1, 3, and 9 (Figure 3a). However, the low energy structures with similar ΩD with experimental values from our simulations (Figure 3b) show that the peptide ions with 18C6 host have enhanced overall helicities compared to the peptide ions without 18C6 (Figures 3c). For instance, we find that the complex formation with 18C6 transforms the globular Ac-Ala6-LysH+Ala3 (n = 3) into an almost perfect helix. Even when n = 9, the 14217

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Figure 3. (a) Ion mobility spectra of the peptides Ac-Ala9 n-LysH+Alan (n = 0, 1, 3, and 9). For n = 9, doubly-charged dimeric 18C6 complex of (Ac-LysH-Ala9)22+ is observed at 6.06 ms (*). (b) MD simulated typical low energy structures and (c) secondary structure assignment of peptides whose calculated ΩD values agree well with experimental ΩD. To clarify secondary structure and backbone amide group interactions, Ala side chain atoms are omitted in the structures.

Figure 4. Plot of the relative ΩD against the location of the Lys residue in the peptides Ac-Ala9 n-LysH+-Alan (n = 0, 1, 3, 5, 7, and 9; black square) and the peptide 18C6 complexes (n = 0, 1, 3, and 9; red circle).

worst location of Lys residue for helix formation, 40% of partial helicity is induced by the complexation with 18C6. To evaluate the conformational effect on the ΩD, we calculate relative ΩD (ΩD,rel) adopting the method by Hudgins et al.17 ΩD,rel of the peptide with m Ala residues is described as ΩD,exp 14.9m Å2, where 14.9 Å2 is the average cross section per Ala residue calculated from ideal α-helical polyalanine peptides (of which backbone dihedral angles fixed at ϕ = 57° and ψ = 47°) with 5 20 Ala residues (Figure S4a and Table S4 in the Supporting Information). In the present study, we investigate isobaric peptides with 9 Ala residues, so ΩD,rel of peptide ion is described as ΩD,exp 134.1 Å2. As seen in Figure 4, in general, ΩD,rel becomes smaller as the location of the Lys moves from the C-terminal site to the N-terminal site. Ac-Ala9-LysH+ and Ac-Ala8-LysH+-Ala, of which helicities are preserved in the gas phase, show ΩD,rel larger than 90 Å2. For the ideal helix of a peptide in an 18C6 complex ion, average cross section per residue for an ideal polyalanine α-helix is calculated from 18C6 complexes of polyalalnine with 5 20 Ala

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Figure 5. MD simulated typical low energy structures of (a) Ac-Ala9LysH+, (b) Ac-Ala6-LysH+-Ala3, (c) Ac-LysH+-Ala9, and (d) Ac-Ala4LysH+-Ala5 peptide ions (left) and peptide 18C6 complex ions (right) whose calculated ΩD values agree well with experimental ΩD values (except peptide 18C6 complex ion of Ac-Ala4-LysH+-Ala5). To clarify secondary structures and backbone amide group interactions, Ala side chain atoms are omitted in the structures.

residues (Figure S4b in the Supporting Information). The structure of 18C6 is adopted from the typical low energy structure of 18C6 complex of Ac-Ala6-LysH+-Ala3. Average cross section per residue is calculated as 14.5 Å2 and ΩD,rel of a peptide 18C6 complex is described as ΩD,exp 130.5 Å2. Then, we subtract average cross section value of 18C6 in each complex from the low energy structures from our MD annealing dynamics trajectories (Table S5 in the Supporting Information). The peptide 18C6 complex ions for n = 0, 1, and 3, show ΩD,rel with larger than 90 Å2 indicating those peptides in the complexes have high helix propensities, with a clear decrease of ΩD,rel for n = 9. This supports the idea that the helix propensity of our peptide is stable for n = 0, 1, and 3 via host guest chemistry with 18C6 in gas phase. Origin of Enhanced Helicity of the Peptide in the 18C6 Complex. We can discern the origin of enhanced helicity by the host guest interaction from the detailed structural comparison between noncomplexed and complexed peptides (Figure 5). For the case of n = 0 (Ac-Ala9-LysH+) without complexation with 18C6, Ala7, and Ala8 (which are adjacent to the Lys) cannot participate in the helical alignment to provide more flexibility to the Lys butylammonium group to cap the end of the helix (Figure 5a). When the peptide forms the complex with 18C6, however, highly localized partial charges in oxygen atoms in 18C6 strongly interact with the butylammonium group of the Lys, letting it be directed outward from the helix backbone (we refer to this as a Lys isolation effect). Meanwhile, the methylene groups of 18C6, which possess localized positive partial charges, interact with C-terminal carbonyl groups serving as a cap to stabilize the helix (C-terminal capping effect; see Figure 6a). Thus, Ala7 and Ala8 in complex can participate in the helical alignment when the peptide forms a complex with 18C6, resulting in an enhanced helicity. For Ac-Ala6-LysH+-Ala3 (n = 3; Figure 5b) without 18C6, the Lys rather interacts with backbone amide carbonyl groups of N-terminal Ala residues (Ala2 to Ala4) than capping the C-terminus, which results in a globular conformation. When the peptide is complexed with 18C6, both the Lys isolation effect (which prevents direct interactions between Lys and Ala backbone groups) and the C-terminal capping effect occur by 18C6 (Figure 6b). These two 14218

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

Figure 6. Electrostatic potential maps for 18C6 in the complex with (a) Ac-Ala9-LysH+ and (b) Ac-Ala6-LysH+-Ala3.

aspects drive the peptide to transform from a globular conformation into a helix. In particular, the location of the Lys7 allows the peptide to form an almost perfect helix. The length of the extended butylammonium group is ∼5.2 Å, which matches with the measured last (partial) pitch of the helical turn from Lys7 to Ala10. As a result, this magic number location of the Lys and the host guest chemistry with 18C6 lead the peptide to have the maximum helicity (90%). The peptide Ac-LysH+-Ala9 (n = 9) also exhibits enhanced helicity when it is complexed with 18C6 (Figure 3), which can still be explained in terms of the Lys isolation effect and the C-terminal capping effect of 18C6. To cap the C-terminal in this case (i.e., to let 18C6 interact with both C-terminal amide carbonyl groups and N-terminal Lys simultaneously), the entire peptide should be bent, resulting in a loop structure (Figure 5c). This allows only Ala residues near the C-terminal to participate in a helical structure, resulting in a partial helicity of 40%. We find that peptides with n = 5 and 7 do not form the complex with 18C6 experimentally (Figure 1). In these peptides, 18C6 cannot interact with both Lys and C-terminus simultaneously. As seen in Figure 5d, the Lys of Ac-Ala4-LysH+-Ala5 interacts with backbone carbonyl groups via the hydrogen bonding interactions. The peptide backbone wraps around the Lys butylammonium group to maximize these interactions. A lack of complexation with 18C6 is observed, resulting in a globular conformation. Note that this observation supports the previous assertion that stabilizing the helical dipole by capping the C-terminus is essential for helix formation.

’ CONCLUSIONS The gas-phase helix propensities of alanine-based polypeptides Ac-Ala9 n-LysH+-Alan (n = 0, 1, 3, 5, 7, and 9) are investigated with different locations of a Lys and host guest interactions with 18C6 using traveling ion mobility mass spectrometry combined with MD simulations. Summarizing, the propensity of Ac-Ala9 n-LysH+-Alan to form a helix in the gas phase occurs when Lys residue is located close to the C-terminus so that the Lys interact with C-terminal carbonyl groups. Those peptides (n = 0, 1, 3, and 9), which interact with 18C6 in the gas phase, show improved helix propensity. The observed improved helicity of the peptide in the complex is well explained in terms of the Lys isolation effect and the C-terminal capping effect of 18C6. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures, tables, and a complete list of refs 46 and 58. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT This work was financially supported by Basic Science Research Program (Grant No. 2010-0021508) through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science, and Technology. H.K. acknowledges support from the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008000-10055-0). ’ REFERENCES (1) White, S. H.; Wimley, W. C. Annu. Rev. Biophys. Biomol. Struct. 1999, 28, 319–365. (2) Klibanov, A. M. Nature 2001, 409, 241–246. (3) Munishkina, L. A.; Phelan, C.; Uversky, V. N.; Fink, A. L. Biochemistry 2003, 42, 2720–2730. (4) Mattos, C.; Ringe, D. Curr. Opin. Struct. Biol. 2001, 11, 761–764. (5) Fulara, A.; Dzwolak, W. J. Phys. Chem. B 2010, 114, 8278–8283. (6) Fu, L.; Ma, G.; Yan, E. C. Y. J. Am. Chem. Soc. 2010, 132, 5405–5412. (7) Bryson, J. W.; Betz, S. F.; Lu, H. S.; Suich, D. J.; Zhou, H. X. X.; Oneil, K. T.; Degrado, W. F. Science 1995, 270, 935–941. (8) Marqusee, S.; Robbins, V. H.; Baldwin, R. L. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 5286–5290. (9) Albrieux, F.; Calvo, F.; Chirot, F.; Vorobyev, A.; Tsybin, Y. O.; Lepere, V. r.; Antoine, R.; Lemoine, J. r. m.; Dugourd, P. J. Phys. Chem. A 2010, 114, 6888–6896. (10) Scholtz, J. M.; York, E. J.; Stewart, J. M.; Baldwin, R. L. J. Am. Chem. Soc. 1991, 113, 5102–5104. (11) Blaber, M.; Zhang, X.; Matthews, B. Science 1993, 260, 1637– 1640. (12) Hirshberg, M.; Henrick, K.; Lloyd Haire, L.; Vasisht, N.; Brune, M.; Corrie, J. E. T.; Webb, M. R. Biochemistry 1998, 37, 10381–10385. (13) Rossi, M.; Blum, V.; Kupser, P.; von Helden, G.; Bierau, F.; Pagel, K.; Meijer, G.; Scheffler, M. J. Phys. Chem. Lett. 2010, 1, 3465– 3470. (14) Jarrold, M. F. Annu. Rev. Phys. Chem. 2000, 51, 179–207. (15) Kohtani, M.; Jarrold, M. F. J. Am. Chem. Soc. 2004, 126, 8454–8458. (16) Hudgins, R. R.; Jarrold, M. F. J. Am. Chem. Soc. 1999, 121, 3494–3501. (17) Hudgins, R. R.; Ratner, M. A.; Jarrold, M. F. J. Am. Chem. Soc. 1998, 120, 12974–12975. (18) Kohtani, M.; Schneider, J. E.; Jones, T. C.; Jarrold, M. F. J. Am. Chem. Soc. 2004, 126, 16981–16987. (19) McLean, J. R.; McLean, J. A.; Wu, Z. X.; Becker, C.; Perez, L. M.; Pace, C. N.; Scholtz, J. M.; Russell, D. H. J. Phys. Chem. B 2010, 114, 809–816. (20) Stearns, J. A.; Boyarkin, O. V.; Rizzo, T. R. J. Am. Chem. Soc. 2007, 129, 13820–13821. (21) Stearns, J. A.; Seaiby, C.; Boyarkin, O. V.; Rizzo, T. R. Phys. Chem. Chem. Phys. 2009, 11, 125–132. (22) Vaden, T. D.; de Boer, T.; Simons, J. P.; Snoek, L. C.; Suhai, S.; Paizs, B. J. Phys. Chem. A 2008, 112, 4608–4616. (23) Valentine, S. J.; Counterman, A. E.; Hoaglund-Hyzer, C. S.; Clemmer, D. E. J. Phys. Chem. B 1999, 103, 1203–1207. (24) Counterman, A. E.; Clemmer, D. E. J. Am. Chem. Soc. 2001, 123, 1490–1498. (25) Wyttenbach, T.; vonHelden, G.; Bowers, M. T. J. Am. Chem. Soc. 1996, 118, 8355–8364. 14219

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