Metal Cation Binding to Gas-Phase Pentaalanine: Divalent Ions

Article pubs.acs.org/JPCA

Metal Cation Binding to Gas-Phase Pentaalanine: Divalent Ions Restructure the Complex Robert C. Dunbar,*,†,# Jeffrey D. Steill,‡,⊥ Nicolas C. Polfer,§,# and Jos Oomens‡,∥,# †

Chemistry Department, Case Western Reserve University, Cleveland, Ohio 44106, United States FOM-Institute for Plasma Physics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands § Chemistry Department, University of Florida, Gainesville, Florida, United States ∥ University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands ‡

S Supporting Information *

ABSTRACT: Ion-neutral complexes of pentaalalanine with several singly- and doubly charged metal ions are examined using conformation analysis by infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) computations. The infrared spectroscopy in the 1500−1800 cm−1 region is found to be conformationally informative; in particular, the frequency of the CO stretching mode of the terminal carboxyl group is diagnostic for hydrogen bonding of the terminal hydroxyl. The doubly charged alkaline earth metal ions (Ca2+ and Ba2+) enforce a highly structured chelation shell around the metal ion, with six strongly bound Lewis-basic chelation sites, and no hydroxyl hydrogen bonding. With the more weakly binding alkali metal ions (Na+, K+, and Cs+), structures with intramolecular hydrogen bonds are more favorable, leading to dominance of conformations with lower degrees of metal ion chelation. The favored coordination mode correlates with ionic charge and binding strength but is not related to the ionic radius of the metal ion.

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INTRODUCTION Hydrogen bonds are central to the secondary structure of peptides, but when a metal ion is present, its noncovalent interactions with nucleophilic moieties in a peptide can play an equally key role in structure determination. When these two types of interactions come into competition, the resulting conformation of the metal-coordinated peptide around the metal ion center can depend in interesting ways on the specific nature of the metal ion. Observation of such effects can be particularly acute in the gas phase, where no solvent attenuates the strength of these electrostatic interactions, and where there is a strong driving force toward filling the coordination shell of the metal ion to occupy high-energy empty space around the bare metal charge. Complexes of metal ions with small oligopeptides present a useful domain for thinking in these terms, progressing upward in size from small peptide ligands toward large proteinmodeling systems.1−14 For small peptides with less than a halfdozen or so residues, the number of coordination sites available on the ligand for interaction with the metal ion is limited, and the system may face choices over whether to break strong hydrogen-bonding structures to free up coordinating groups for the metal. Using emerging spectroscopic tools, we have been able, as described here, to characterize in detail the playing out of such a competitive situation in metal-ion coordinated © 2012 American Chemical Society

polyalanine chains, demonstrating a switch from hydrogenbonding structure to metal-coordination restructuring as a function of the coordinating strength of the metal ion. Pentaalanine is an apt example for studying these types of interactions. It presents the possibility of six strong Lewis-basic binding sites to the metal ion (four amide carbonyls, one terminal carboxyl carbonyl oxygen, and one terminal amino nitrogen; the hydroxyl oxygen could also come into play, but probably only with the exclusion of the terminal carbonyl, which coordinates metal ions much more strongly). If the metal ion is not too small, and if steric considerations do not intervene, six coordinating atoms constitute a reasonable filled solvation shell for the ion, so that pentaalanine may be just large enough to have the possibility of full microsolvation of the metal ion. As will be shown below, pentaalanine fortuitously allows a folding state with maximal coordinative saturation, without imposing serious steric stresses. On the other hand, by freeing one or more Lewis-basic ligand sites from attachment to the metal, strong hydrogen bonding networks can be formed. Here, we characterize the competition between the fully chargeSpecial Issue: Peter B. Armentrout Festschrift Received: May 2, 2012 Revised: August 27, 2012 Published: August 28, 2012 1094

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Figure 1. Structures of Mn+-Ala5 complexes. Solvation of the metal ion by all five carbonyl oxygens is possible in structures CS-1 and CS-3, leaving the C-terminal hydroxyl proton free from hydrogen bonding interactions. Closing a peptide-chain macrocycle by H-bonding between the C-terminal OH and the N-terminal nitrogen necessitates disruption of the solvation shell around the metal ion (HB-A1), as does formation of other H-bonding networks (HB-B1). Relative enthalpies of the structures are given in kJ mol−1 for five metal ions (free energies at 298 K in parentheses). Note that the actual Ca2+(Ala5) ground state is calculated to be the similar diastereomerically related species CS-2.

seems to be best. In this case the amino terminus is freely flexible, conferring a substantial entropy advantage compared with the 6-fold coordinating structure. It is interesting to see in the following that this looser 5-fold CS binding pattern is also favored in the present sodium pentaalanine case, but that this is true only for the sodium ion among the metal ions examined.

solvated motif (CS) and the hydrogen-bonded motifs (HB) for several different metal ions. Figure 1 illustrates CS folding (CS1 and CS-3), and two low-energy hydrogen-bonded conformations (HB-A1 and HB-B1), out of a large pool of possible conformations. The combination of infrared spectroscopic information and quantum-chemical computations has been found to be a practical and incisive route to characterizing structures of gasphase ions.15−20 In particular, many successful applications of this combined approach to cationic peptide systems (both protonated and metal-ion-cationized) have been reported over the past decade.21−23 The present study continues the pursuit of such insights using the IR capability of the tunable free electron laser FELIX as described in numerous publications. Similarly directed work comes from the CLIO free electron laser facility. Most germane to the present study are recent studies of metal-ion complexes of polyalanines and polyglycines at both FELIX1,24 and CLIO.25,26 Recent studies of sodium octaglycine as well as a systematic survey of sodium ion complexes with a series of polyglycines by the Ohanessian group at CLIO26−28 present parallels and contrasts as background for the present work. They established the favorability of the macrocycle-forming hydrogen bond (as in HB-A1 in Figure 1), and they showed that octaglycine is large enough to permit simultaneously the full microsolvation of the metal ion by six ligand groups, at the same time as the formation of structure-forming hydrogen bonds. There is thus no competitive conflict between charge solvation and hydrogen bonding to resolve in this larger system, which nevertheless gives valuable insight into the thermochemical and spectroscopic behavior of metal ions interacting with a peptide chain in this general size range. They found a different situation for pentaglycine. A charge-solvated type of structure is favored, without the possibility of additional H-bonding interactions. The interaction with the sixth coordinating atom (the amine nitrogen) with Na+ is apparently not sufficiently strong to overcome steric crowding, and a 5-fold coordinating structure

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EXPERIMENTAL AND COMPUTATIONAL METHODS Infrared multiple-photon action spectra of the gas-phase complexes were obtained by infrared multiple photon dissociation (IRMPD) spectroscopy as described previously for experiments at the FELIX free electron laser light source.21 A Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer interfaced to the light source was used, as has been described in previous work.1 Electrospray ionization (ESI) using methanol/water as the solvent with peptide and salt (metal chloride or nitrate) at approximately 1 mM concentrations was applied as the ion production method. The complexes under study were accumulated for about 5 s in a hexapole linear trap, followed by mass isolation in the FTICR ion trap and irradiation by FELIX for typically 3 s. The action spectrum for the parent complex was derived by summing and plotting the yields of all major fragment ions as a function of the wavelength of the radiation. A linear power correction was applied as a function of wavelength. The reasonably wavelength-independent nature of the FEL output (dropping off only at the extremes of the tuning range by not more than a factor of 2) ensured that this approximation to the true intensity dependence of the yield did not result in severe distortions of the spectra. As always in multiple-photon action spectroscopy, the IRMPD spectrum is merely a surrogate for the true IR absorption spectrum. Peak wavelengths are found by much experience to be reasonably true to the linear IR spectrum, but the relative intensities are not quantitatively reliable indicators. 1095

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calculations of pentaglycine complexes were made for comparison with pentaalanine to make contact with ref 26. For Na+(Gly5), the CS-3-type structure was the ground state (as was reported in ref 26), being more stable than HB-B1 by 8 kJ mol−1. This latter comparison contrasts with the result found here for Na+(Ala5), according to which HB-B1 was more stable than CS-3 by 8 kJ mol−1. On a free-energy basis, this ordering for Na(Ala5) reversed, so that CS-3 was also the (free energy) ground state (by 5 kJ mol−1) for the pentaalanine case as in the pentaglycine case. Such small energy differences are not considered significant, because they are of the same order as the method-dependent variations in relative energies.

Computations were carried out on the Linux cluster LISA at the SARA supercomputer center in Amsterdam using the DFT/ B3LYP density functional method implemented in the Gaussian 0329 and Gaussian 0930 packages. Preliminary optimization was carried out at the B3LYP/3-21g level, followed by full optimization and IR frequency/intensity calculations at the B3LYP/6-31+g(d,p) level for all the structures chosen for IR modeling. For Cs and Ba complexes the sdd pseudopotential31,32 as parametrized in the Gaussian programs was used on the metal. Sodium, potassium, and calcium calculations were all-electron full optimizations. For Ba and Ca spectra, DFT frequencies were scaled by a scaling factor of 0.975 as in our previous work.24 For the alkali metals (Na, K, and Cs), the computational exploration by Ohanessian et al.27 for the sodium octaglycine system, using a slightly smaller factor (0.960), pointed to the desirability of a lower factor for alkali metals: we decided to use 0.965, which gave a good match to the strong amide II peaks near 1550 cm−1. An extensive exploration of the conformational possibilities for the potassium complex was carried out, although a complete survey of the entire conformation space was beyond our means. Starting with a number of chemically reasonable binding and hydrogen-bonding patterns, several thousand Monte Carlo conformations were generated by molecular mechanics in the HyperChem 7.5 program (Amber99 and Charmm27 force fields in different runs). Low energy structures (within about 80 kJ mol−1) were chosen for optimization by B3LYP/3-21g. Within each of the various families of structures several of the most stable were then fully optimized and vibrational frequencies/intensities calculated at the 6-31+g(d,p) level. Complete calculations of IR spectra at this latter level were ultimately done for about 30 candidate structures in this search. No imaginary vibrational frequencies were encountered in the vibrational calculations. Relative binding energies were estimated at the B3LYP/631+g(d,p) level. High accuracy is not expected, because (1) this level of computation is not considered very accurate for absolute binding energies, (2) no basis set superposition error corrections were applied (expected to be not greater than 10 kJ mol−1 from prior experience), and (3) no rigorous search was made for the most stable conformation of neutral pentaalanine (which gives an uncertainty in the absolute binding energies, but not the relative values, for different metals). A small conformational search of the pentaalanine conformation space was made (200 structures with the Amber99 force field) to locate a neutral-peptide conformation whose stability at least approximates the global ground state, and the conformation adopted for the binding energy calculations (geometryminimized at the B3LYP/6-31+g(d,p) level) is displayed in the Supporting Information. Free energies (298 K) were calculated in the rigid-rotor/ harmonic-oscillator approximation. Important changes in relative stability were frequently found (as much as 15 kJ mol−1) in going from enthalpies to free energies (Tables 1 and S-1, and figures). MP2 computations have been considered more reliable than DFT calculations for energy comparisons. Though this level of computation was not practicable for the present study, a few tests were made of some crucial comparative values versus the DFT results with matched basis sets (MP2 Full/6-31+g(d,p), with MP2 Full optimized geometries). The MP2 comparison of CS-1 and HB-1 for the Na+ and K+ complexes were found to be essentially the same as the B3LYP comparisons. Also, test DFT

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RESULTS In Figure 2, the IRMPD spectra (black traces) of the complexes of interest, three +1 alkali metal examples and two +2 alkaline

Figure 2. IRMPD spectra of the five metal ion complexes of pentaalanine (black). Superimposed (blue) is the calculated spectrum in each case of the charge-solvated (CS-1 or CS-2) conformation. Shown in red are HB conformations: For Ba2+ and Ca2+ HB-A1 is shown, whereas for the three alkali metals HB-B1 is shown because it gives a more satisfactory fit to the spectra than HB-A1, despite its higher calculated energy. CS conformations (blue) are CS-1 for Ba2+ and CS-2 for Ca2+. Note the excellent match of the CS conformation to the experimental spectrum for the two alkaline earth metal complexes, whereas for the complexes of K+ and Cs+ the H-bonded conformation (red) is much better.

earth metal examples, show the divergence of spectroscopic character that is highlighted in the present report. Much of the spectral range is conformationally uninformative, and it is the carbonyl stretching region between about 1630 and 1800 cm−1 that offers the clearest structural insight. It can be seen that the 1096

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Table 1. Calculated Relative Energies of M+ and M2+ Complexes with Ala5 of Interest in This Studya

two larger alkali metal complexes give somewhat similar spectra, as is also true of the two alkaline earth metal complexes, but the spectral patterns in this key region of the spectrum are strikingly different between these two sets of complexes. The two strong carbonyl stretching features at roughly 1660 and 1720 cm−1 in the alkaline earth metal spectra apparently collapse into a single feature near 1680 cm−1 in the alkali metal spectra. For these two alkali metal spectra there is only a very weak intensity at the 1750 cm−1 wavelength that is expected for a CO stretching mode of a metal-coordinated carboxyl group in the absence of hydrogen-bonding perturbations.1,5,24 This pattern suggests the conversion of the terminal COOH from a free−OH conformation (CS) in the alkaline earth metal complexes into a hydrogen-bonded OH conformation (HB) in the alkali metal complexes. The very small feature near 1750 cm−1 in these two alkali metal spectra is attributed to a minor extent of CS contamination in these two ion populations. Thus the set of spectra in Figure 2 graphically shows the transition from the hydrogen-bonding complexation motif to the charge-solvated motif as a consequence of the increased metal-ion charge and chelating strength of the divalent cations. This interpretation is supported by the computed spectra for CS and HB conformations shown in the figure. The CS-1 or CS-2 computation (blue traces) gives an excellent and convincing fit to the alkaline earth metal spectra. The spectra of the alkali metal complexes of K+ and Cs+ give clear evidence that the CS conformation is not a major constituent of the populations, as signaled by the near disappearance of the characteristic carboxyl carbonyl stretching peak around 1750 cm−1. In the Na+ case, the intensity at this wavelength is not negligible. It seems likely that the modest peak observed there corresponds to the presence of some CS ions mixed with a largely HB population, so that this complex constitutes a transition point between the two different binding modes. But the spectroscopic evidence cannot completely rule out the possibility that the population of the Na+ complex is predominantly CS. For K+ this feature is barely visible, and for Cs+ it is yet smaller. At this point, it is not certain which HB conformation, or mixtures of conformations, are dominant for the alkali metal complexes, because among the possibilities uncovered in the present search there are numerous candidates with reasonably good calculated stability and matching spectroscopic features in the carbonyl stretching region, and there are undoubtedly many more such candidates not located in the present searches. Further spectra and energy calculations shown in the Supporting Information give a survey of some of the candidate structures that were explored in our search. The Na+Ala5 complex is likely a transitional case and as such possibly represents a mixture of structures. Two features stand out that set it apart from either the alkaline earth metals or the larger alkali metals: One is the appearance of a significant peak around 1750 cm−1, which is considered characteristic of the CS conformations; the other is the calculated prediction that the five-coordinate CS-3 structure is the free-energy ground state, in contrast to the finding that the six-coordinate CS-1 or CS-2 conformations are the lowest-energy CS-type conformations for all the other metal ions (and are the global ground states for the alkaline earth metal complexes). Key results of the thermochemical calculations are shown in Tables 1 and 2. Table 1 surveys the conformational energy differences among the most interesting ligand conformations for the five metal ions. Table 2 summarizes the ionic radii of the

Cs CS-1 CS-2b CS-3 CS-4b HB-A1 HB-B1 HB-A2 HB-B3 HB-C1 SB-1

46 (35) 48 (37) 34 (22) 35 (24) 0 (0) 7 (7) 13 (17) 4 (6) 8 (2) 53 (68)

K

Na

Ba

Ca

(24) (24) (10) (13) 0 (0) 12 (12) 12 (16) 12 (12) 11 (5) 52 (64)

22 (21) 24 (23) 7 (0) 10 (3) 0 (5) 19 (20) 11 (20) 19 (21) 16 (9) 38 (62)

0 (0) 1 (2) 9 (2) 10 (1) 5 (11) 87 (89) 28

2 (3) 0 (0) 16 (8) 18 (10) 38 (43) 163 (159) 65 (69)

66 (84)

130 (144)

33 35 21 24

a

Computations at the B3LYP/6-31+g(d,p) level with zero-point energy corrections. Values in parentheses are free energies at 298 K. Values are comparable down the column for each metal ion (kJ mol−1). bStructure CS-2 is closely similar to CS-1, having inverted (diastereomeric) coordination around the metal. Similarly, CS-4 is diastereomerically related to CS-3. See Supporting Information for structures.

Table 2. Ionic Radii and Binding Strengths of Ions for the Ala5 Liganda rion (Å) rcb (Å) DA5 (kJ mol−1)c

Cs+

K+

Na+

Ba2+

Ca2+

1.67 2.43 185

1.37 2.03 246

1.02 1.68 324

1.35 2.04 762

0.99 1.72 973

a The rion values are standard ionic radii. The rc values are derived from the calculated metal−oxygen bonds in the Ala5 complexes. bDFT value of metal/amide carbonyl distance in Mn+Ala5 after subtraction of an oxygen radius of 0.68 Å. cBinding energy for lowest-energy complex conformation.

metal ions and the binding energies of the ground-state conformations for each metal ion. The free energy values in Table 1 signal entropy effects among the conformations for a given metal ion. If the free energy of conformation X is greater (relative to the ground conformation) than its energy (also relative to the ground conformation), then there is an unfavorable entropy change upon rearranging from the ground conformation to X. We can note a few generalizations evident in the table: (1) For metal ions having the CS motif for their ground conformations, rearrangement to the highly structured HB-A1 conformation is entropically unfavorable, but rearrangement to the looser CS-3/ CS-4 conformations is entropically favorable. (2) For metal ions having HB-A1 as their ground conformations, rearrangement to the CS conformations is entropically favorable. (3) Rearrangement to the zwitterionic SB-1 conformation is always entropically unfavorable, presumably because of the rigid structural constraints imposed by the salt-bridge structure.

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DISCUSSION Alkaline Earth Metal Complexes. The excellent agreement of the IRMPD spectra of Ca2+ and Ba2+ complexes with the predicted spectra for the CS conformation, bolstered by the fact that this conformation is most favored thermodynamically (Table 1), leaves little reason to look for other binding architectures for these two cases. We may ask whether we can distinguish among the four CS variants for these alkaline earth metal cases. The CS-1/CS-2 pair are diastereomers, in the sense that the arrangement of the six metal-coordinating sites is 1097

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Figure 3. Structures, spectra, and relative energies of a few representative K+ complexes, including additional hydrogen-bonded complexes, HB-A2 and HB-B3, as well as the free-OH complex HB-C1 and the zwitterionic complex SB-1. Calculated spectra of the complexes (red) are compared with experiment (black). See the Supporting Information for additional candidate structures.

type structures. Sodium is an interesting transitional case: This ion has sufficient coordinating power to bind all five carbonyl groups along with the amine group but releases the amine nitrogen atom (through the release of steric crowding and strain, aided by favorable entropy), giving 5-fold coordination (CS-3). The fact that the 1750 cm−1 peak is weaker in the experiment than theoretically predicted for a Na+ CS complex suggests that the Na+Ala5 population of structures is divided between CS and HB structures. The difficulty with a straightforward assignment of the lowest-energy known structures for the K+ and Cs+ complexes is evident in Figure S8 in the Supporting Information, which shows the predicted spectra for the structure (HB-A1) calculated to have the best calculated stability among those searched (Table 1, Table S1). (Both CS and HB structures are shown for the sodium ion complex, because these structures have nearly equal calculated energies and might well both be substantially present in the population.) As the figure shows, the alkali-complex spectroscopic fits to the HB-A1 structure are not very close. Other lowenergy HB conformations give a range of spectroscopic patterns in the carbonyl region, but none that we investigated combines high stability with a convincingly good spectroscopic matchthe suggested fits to HB-B1 shown in Figure 2, calculated to be higher in energy by 13 and 7 kJ mol−1 for K+ and Cs+, respectively, are as good as we were able to find. At this point, we conclude that non-H-bonded CS conformations are not important contributors to the ion populations, whereas several low-energy HB conformations have predicted spectra in rough but not perfect agreement with the experimental results. In a previous study of an analogous, though different, peptide

mirror-imaged. (They are not true enantiomers because of the optical asymmetry of the peptide chain itself). The diastereomers are very similar in energy (see Tables 1 and S-1) and calculated spectra. The pair CS-3/CS-4 is a similarly diastereomeric pair, and their predicted energies and spectra are also very similar. They differ from the CS-1/CS-2 pair in having the amine nitrogen detached from the metal ion, which gives them substantially different energies compared with CS1/CS-2. However, as illustrated in Figure S1 (Supporting Information), the four CS variants considered here are all spectroscopically similar, so that the specific assignments of CS1/CS-2 as predominant for the alkaline earth metals, and CS-3/ CS4 for the sodium ion (with possible admixtures of other CS as well as HB conformations) are based on less definitive considerations of their relative calculated energetics. For the Ba2+ complex, the spectroscopic fits to the CS pattern are excellent, but the free energies of the variant CS conformations are very similar, so there is little basis for making a choice, and a mixture of conformations seems likely. For the Ca2+ complex, the CS-1/CS-2 pattern seems to be significantly more likely than CS-3/CS4. CS-2 has a slight energetic preference over CS-1, but this has low significance, and a mixture of these two seems likely. What Are the Structures of the Alkali Metal Complexes? The spectroscopic argument advanced in the present report is that the nearly total absence of the 1750 cm−1 peak in the K+ and Cs+ complexes rules out their having dominant CS structures with a free (non-H-bonded) Cterminal OH group. Figure 2 illustrates that the spectra of these two complexes can be fitted with good success by HB1098

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complex, the K+ complex of the pentapeptide [Leu]-enkephalin was characterized by Polfer et al.6 and was also assigned as a hydrogen-bonded structure; it also gave rough but not perfect agreement with calculated candidate conformations of this type. For the K+ complex, calculations using MP2, as well as B3LYP calculations using the larger 6-311+g(d,p) basis, were performed, but they did not give major changes in the relative energetics of the conformations, which suggested that the computational method and level used in this work were adequate. Although there seems no particular reason to doubt the accuracy of this computational protocol for predicting the energetics of hydrogen bonds, it is possible that the strong O− H···N hydrogen bond occurring in HB-A1, similar to the shared-proton situation which is known to be difficult to calculate,33 might be overestimated, which would result in erroneously high relative stabilities for the HB-A class of conformations and could make the HB-B class of conformations more attractive. For the sodium complex, a major fraction of CS-3 can be included in the population to account for the 1750 cm−1 feature. Pursuing the alkali metal structure question a bit farther, Figure 3 displays a few additional types of low-energy conformations for the K+ complex along with their calculated spectra, supplementing those displayed in Figure 2. Among this group, HB-A2 and SB-1 have sufficient features of agreement with experiment to encourage further future exploration of their regions of conformation space. To complete the picture, we note the existence of some additional structure classes. The HB-C1 structure shown in Figure 3, exemplifies HB structures in which the OH proton is free rather than being H-bonded, and which therefore have a predicted carboxyl peak near 1750 cm−1. This structure, though higher in energy than the lowest HB structures, is considerably lower in energy than CS-1 for all the alkali metal complexes, and it is a reasonable possibility that a minor fraction of the ions in this HB conformation is giving rise to the small peaks near 1750 cm−1 in the spectra of the alkali metal complexes. Various zwitterionic salt-bridge (SB) structures have also been considered, in which the carboxyl proton is transferred to the amino group. Most interesting here is the SB-1 structure (Figure 3), which is lowest in energy out of the SB structures found, and which gives a quite attractive match to much of the experimental K+(Ala5) spectrum. However, SB structures have been found in general to be energetically unfavorable by a substantial margin in small alkali metal ion/peptide complexes (except those containing arginine).1,5,6,24,34−38 In the present case, structure SB-1 is more than 40 kJ/mol higher in energy than the low-energy HB structures (Table 1), so that it appears unlikely that zwitterionic conformations form a major constituent of the ion population. It might be thought that zwitterionic structures derived from structures such as HB-A1 by shifting the proton over toward the nitrogen side of the H-bond might lead to improved spectral matches. However, in the geometry optimization, the proton always migrates spontaneously back to the C-terminal oxygen, and our best estimate from constrained optimizations is that holding the proton on the N-terminus comes at a cost of at least 20 kJ mol−1. Relation between Structures and Metal Ion Properties. It is interesting to consider what it is about the metal ions that brings about the striking structural switch characterized in this study upon going from the larger alkali metal ions to the alkaline earth metals. Factors typically affecting organometallic

complexation include metal-ion size, strength of ion−ligand interaction, and electronic effects. Among these, electronic effects seem unlikely to be important here, because these ions are all closed-shell, main group species that interact with ligands largely through electrostatic interactions. Table 2 shows metalion characteristics relating to the other possible factors. The binding energies are straightforward energies of reaction estimated for the attachment process Mn + + Ala5(ground state) → Mn +· Ala5(ground state) (1)

Metal-ion radii are estimated in two ways. One, rion, is the standard ionic radius, as derived chiefly from packing observations of ionic crystals. Perhaps equally appropriate for the present purpose is a coordination radius, rc, which we have estimated as the average metal−oxygen distance in the most stable complex conformation in the present calculations, less 0.68 Å representing the oxygen share of the bond length. The absolute significance of any of these radius numbers is doubtful, but they are useful for comparative purposes. The utility of correlating ion sizes can be dismissed immediately. Two pairs of ions with practically the same radii can be picked out, namely the K+/Ba2+ pair, and the Na+/Ca2+ pair. In each case, the two ions of the pair, despite their similar radii, show different coordination preferences. Most clear-cut is K+/Ba2+, for which the first is clearly HB, and the second is clearly CS. The Na+/Ca2+ is less clear-cut, because the sodium complex is considered to be transitional between the two coordination modes, but still the sodium complex gives a clearly different spectral pattern than the calcium complex does. On the other hand, a correlation with binding strength is completely successful, with the weak-binding ions giving HB binding, and the strong-binding ions giving CS binding. The present interpretation thus puts the dividing point near sodium, at a binding energy of 324 kJ mol−1. In these electrostatically bound complexes, the charge on the ion is the dominant variable in determining the binding energy, so that the doubly charged ions are by far the stronger binders, and thus the preferred binding mode also correlates with the charge on the metal ion. Spectroscopic Considerations. Looking at the spectra in Figure 2, a number of features are common to all, not being strongly sensitive to either the identity of the metal ion or the geometry of the ligand binding. The peptide features amide I (collective amide CO stretch) and amide II (collective amide N−H bend mixed with C−N stretch) are universal, near 1650 and 1550 cm−1, respectively. The amide II position is weakly sensitive to metal-ion chelation of the amide carbonyl groups, showing a blue shift with increasing metal-ion binding energy (from Cs+ to Ca2+). The amide I peak position is more metalsensitive, showing a substantial red shift with increasing binding interaction with the metal, along with strong coupling and splitting of the four amide CO stretching modes. These two groups of modes, amide I and amide II, dominate the spectra of larger peptides (see, notably, ref 3) and are more dominant in the sodium octaglycine spectrum of Ohanessian et al.27 than they are in the present pentaalanine spectra. The terminal COOH group contributes a strong O−H bending peak near 1150 cm−1 when the OH is not H-bonded. This peak usually blue-shifts or weakens when the OH is H-bonded, but it is not a very useful marker of such H-bonding because it is often indistinguishable from C−H bending peaks in the same region. The terminal NH2 scissoring mode is quite consistently 1099

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calculated around 1600 cm−1, but it is typically weak, as well as being difficult to calculate accurately, probably because of the anharmonic nature of its potential.39,40 The diagnostic 1720−1750 cm−1 peak is always strong: Perhaps surprisingly, it is still calculated as one of the most intense peaks even in the present pentapeptide where one might have expected it to be somewhat overshadowed by the amide I cluster, made up of four individual amide CO bands. However, the calculations predict it to be a strong peak, not much weaker than the amide I cluster. Ohanessian et al. found it to be similarly intense in their MP2 calculations.25,26 The calculations appear to be accurate in predicting a high intensity for this peak, as the experimental Ca2+ and Ba2+ complex spectra display carboxyl CO stretching peaks of intensity almost equal to the Amide I cluster.

structures. Among conformations investigated here, the HB-B1 structure gave the most satisfactory match to the observed alkali-complex spectra. We conclude that the K+ and Cs+ complexes are predominantly in HB rather than CS conformations, and that this interpretation is in accord with the calculated high stability and IR-spectroscopic patterns found for such conformations in our calculations. The contrasting structures described here for the fully ligated alkaline earth metal complexes compared with the hydrogenbonding nature of the alkali metal complexes presents a graphic example of the superior structure-dominating properties of the doubly charged alkaline earth metal cations.

CONCLUSIONS IRMPD spectroscopy in combination with DFT calculations was used to study the competition between two structural motifs in complexes of M + /M 2+ ions with the Ala 5 pentapeptide: a charge-solvated (CS) structural motif, and one in which the ligand sacrifices one or more carbonyl/metal interactions to form intramolecular hydrogen bonds (HB). The IR band around 1720−1750 cm−1, belonging to the terminal COOH group without hydrogen-bonded hydroxyl, was considered to be a strong indicator of the relative abundance of hydrogen-bonded versus free terminal OH, which in turn correlates with the choice between full CS coordination of the five carbonyl groups versus structures having lower metal coordination and more extensive hydrogen-bonding networks. The spectroscopic results supported by the DFT calculations of both predicted spectra and thermochemistry indicate that the doubly charged alkaline earth metal cations have sufficiently strong chelating ability to structure the pentaalanine ligand into a fully ligated CS conformation with five Lewis-basic chelating carbonyls. The constraint of binding all of these Lewis-basic sites to the metal ion severely limits the possible structures for the CS motif. In contrast, the singly charged alkali metal ions K+ and Cs+ are not sufficiently strongly interacting to enforce such a tightly compacted structure, and instead allow at least one of the Lewis-basic sites to interact by hydrogen bonding, which also results in a much larger conformational phase-space. Na+ seems to be intermediatethe intensity of the characteristic 1740 cm−1 band is substantial, although smaller than calculated for the CS structures, and suggests at least a substantial fraction of CS conformation. This spectroscopic feature contrasts with K+ and Cs+, whose spectra have very little intensity around 1750 cm−1, suggesting HB-type conformations with at most a very small admixture of CS conformations. A competition between CS and H-bonded structures in the Na+ case is also suggested by the computed relative energies. Due to its combination of small size and weaker binding strength, the full 6-fold chelation of the metal ion which is seen with the alkaline earth metals (CS-1/CS-2) is slightly less favorable for Na+ than the 5-fold chelation having no amino/metal coordination (CS-3/CS-4) according to calculation, although preferences among the variant CS conformations could not be resolved from the spectroscopy. Although the structure shown here as HB-A1 was the most thermochemically favorable hydrogen-bonding motif found in the DFT computations, there are also a large number of other hydrogen-bonding alternatives of reasonable stability resulting from disruption of the complete solvation shell of the CS

Complete refs 29 and 30. Calculated energetics of the full set of K+(Ala5) isomers. Computed IR spectra of CS isomers of Ca2+(Ala5). Structures of all isomers. IRMPD spectra compared with computed spectra for HB-A1. Computed spectra of K+ and Cs+ complexes. Comparison of spectra of Na+ complexes of pentaalanine and pentaglycine. Structure of neutral pentaalanine. This material is available free of charge via the Internet at http://pubs.acs.org.

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ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*Tel. 216 368 3712. Fax: 216 368 3006. E-mail: [email protected]. edu. Present Address ⊥

Sandia National Laboratories, 7011 East Avenue, Livermore, CA 94551. E-mail: [email protected].

Notes

The authors declare no competing financial interest. # E-mail: N.C.P., [email protected]fl.edu; J.O., J.Oomens@ rijnhuizen.nl.

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ACKNOWLEDGMENTS This work is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek” (NWO). R.C.D. acknowledges support from the National Science Foundation, PIRE Grant OISE-0730072, and expresses gratitude for generous support by FOM during an extended visit. J.O. acknowledges support from the Stichting Physica. The FELIX staff, and particularly Drs. A. F. G. van der Meer, B. Redlich, and G. Berden, are gratefully acknowledged for their assistance.

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