Experimental and Theoretical Investigation of the Aromatic− Aromatic

J. Phys. Chem. A 2010, 114, 2973–2982

2973

Experimental and Theoretical Investigation of the Aromatic-Aromatic Interaction in Isolated Capped Dipeptides† Eric Gloaguen,*,‡ Haydee Valdes,§ Francesca Pagliarulo,‡ Rodolphe Pollet,‡ Benjamin Tardivel,‡ Pavel Hobza,⊥ Franc¸ois Piuzzi,‡ and Michel Mons‡ Laboratoire Francis Perrin, CEA/DSM/IRAMIS/SPAM - CNRS URA 2453, CEA/Saclay, 91191 Gif-sur-YVette, France, Dpto. Quı´mica Fı´sica y Analı´tica, UniVersidad de OViedo, C/ Julia´n ClaVerı´a, 8, 33006(OViedo) Asturias, Spain, and Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague 6, Czech Republic ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: September 1, 2009

Among the forces responsible for shaping proteins, interactions between side chains of aromatic residues play an important role as they are involved in the secondary and the tertiary structures of proteins contributing to the formation of hydrophobic domains. The purpose of this paper is to document this interaction in two capped dipeptides modeling a segment of a protein chain having two consecutive Phe residues, Ac-Phe-PheNH2 and Ac-Phe-D-Phe-NH2. These two molecules have been investigated in the gas phase by IR/UV double resonance spectroscopy, and the assignment of the observed conformers has been done by comparison with quantum chemistry calculations. Both peptides are found to adopt a β-turn type I conformation stabilized by an edge-to-face interaction between the two aromatic rings. Comparison with other dipeptides in the literature demonstrates the impact of this aromatic-aromatic interaction on the shape adopted by the peptide chain, and its role among the other shaping forces (H-bonds, NH-π interactions) is discussed. As an illustration, the H-bond strength is found to be significantly lower in the β-turn type I conformer, in which the two rings interact, as compared to the similar conformer where such an interaction does not exist. This structural feature due to the backbone distortion induced by the interaction between the aromatic rings makes this system a good test for evaluating the ability of computational methods to correctly account for the competition between these forces. MP2, SCS-MP2, DFT, and DFT-D methods have been assessed in this respect. Comparison between geometries, energies, and frequency calculations illustrate their respective limitations in describing conformations resulting from a subtle equilibrium between the several interactions at play. I. Introduction The folding preferences of a polymeric flexible chain, like a peptide or a protein, are partially defined by the topology of its conformational landscape. The presence of several significant accessible minima contributes to the flexibility of the molecule, which is a fundamental property related to its biological functionality. In this respect, the interactions that side chains can establish with their immediate environment provide a finetuning of the relative energetics of the competing conformations. Hydrophobic residues, in particular, like the three aromatic natural residues, phenylalanine, tyrosine, and tryptophan, are expected to favor backbone conformations that enable a closepacking of these side chains due to attractive interactions between the aromatic rings. Aromatic-aromatic (ar-ar) interactions are therefore considered as potential contributors to protein folding cores,1,2 as well as major actors in protein secondary structure stability,1-4 favoring the formation of hydrophobic domains5-7 which are involved, for instance, in amyloid fibril formation.8-10 In the past few years, gas-phase optical spectroscopy of short peptides chains has undergone a huge development, leading to detailed investigations of the interactions taking place in these †

Part of the “Benoît Soep Festschrift”. * To whom correspondence should be addressed. Tel: +33 1 69 08 35 82. Fax: +33 1 69 08 12 13. E-mail: [email protected]. ‡ CEA/DSM/IRAMIS/SPAM - CNRS URA 2453. § Universidad de Oviedo. ⊥ Academy of Sciences of the Czech Republic.

Figure 1. Intramolecular H-bonds along a peptide chain. They are labeled according to the number of atoms (n) involved in the ring (Cn) resulting from the intramolecular H-bond.

polymeric chains.11 Attention has been particularly focused onto the several types of H-bonds shaping the backbone, namely, the C5, C7, and C10 interactions (Figure 1). Besides, weaker interactions between the backbone and aromatic side chain12-20 exist and can be spectroscopically detected and characterized in the gas phase much more precisely than in the condensed phase, like NH-π interactions.12-19 These interactions have been found to be ubiquitous in peptide systems containing aromatic residues; they stabilize (i) βL conformations (C5), (ii) γL conformations (C7eq), as well as (iii) β-turns (C10). A previous gas-phase investigation19 on protected tripeptides has clearly demonstrated that these interactions can mediate the competition between several backbone forms, involving different secondary structures or local conformational preferences. In contrast, side-chain interactions between aromatic nonpolar residues are often more difficult to detect optically due to the absence of polar IR-active -X-H groups. The effect of these interactions is often detected indirectly, in particular, through

10.1021/jp904216f  2010 American Chemical Society Published on Web 10/09/2009

2974

J. Phys. Chem. A, Vol. 114, No. 9, 2010

their influence on the fluorescence lifetime of the aromatic chromophore, or through conformational competition. In the AcVal-Phe-NH2 protected dipeptide, the prevalence of the β-turn (C10) conformer in the gas phase, in contrast to other dipeptides with smaller side chains, was tentatively ascribed to an extra stabilization arising from dispersive London interactions taking place between the Val and Phe side chains.13 In this context, gas-phase studies of small peptide chains exhibiting several aromatic residues, like the bichromophoric Phe-Phe sequence, enable the investigation of the role of sidechain interactions (ar-ar interactions in this case) on the backbone conformational preferences. In particular, two important issues can be studied in detail, (i) the occurrence of an ar-ar interaction between the side chains and (ii) the possibility to form two consecutive NH-π interactions. In addition, these bichromophores studied in a supersonic expansion enable a selective ultraviolet (UV) photoexcitation of each of the Phe moieties, which should offer the possibility to investigate the influence of the originally excited site of the molecule21,22 (electronic excitation or photoionization) on the subsequent molecular dynamics (excited-state energy transfer, relaxation in the neutral, or fragmentation in the ion). The aim of the present work is to document the preferred conformations adopted by the homo- and heterochiral chemically protected diphenylalanine, Ac-Phe-Phe-NH2 and Ac-Phe-D-PheNH2, under the cold conditions of a supersonic expansion and to assign the relative role of the NH-π interactions and the ar-ar interactions in these preferences. This approach brings a new insight on the fundamental interactions at play in the hydrophobic core of proteins compared to the gas-phase study performed on unprotected Phe-Phe and Phe-D-Phe sequences by Abo-Riziq and co-workers.23 The strategy adopted stems from the assignment of the UV features in terms of local excitation of each Phe chromophore based on a comparison with a set of previously recorded data on similar molecules.13,14,16-18 Infrared (IR) spectroscopy in the NH stretch region recorded using the double resonance IR/UV technique reveals the H-bond network of the molecules. Several quantum chemistry methods have been selected for their potential ability to describe larger species. Density functional theory (DFT) methods are known to give harmonic frequencies reliable enough to perform a conformational assignment for this type of system24 despite their weakness in dealing with dispersion forces.25 In addition to DFT methods, DFT-D methods26 have also been used. Their main advantage in comparison to the standard DFT is that the London dispersion energy is empirically included along with an appropriately adjusted damping factor. This advantage is expected to be of primary importance in the description of such systems having an ar-ar interaction, even if it has been suggested that the contribution of dispersive interactions between aromatic rings is strongly dependent on both the distance and the relative orientation of the phenyl rings and may not be the main contributor to the stabilization energy.27 Finally, the secondorder Møller-Plesset perturbation theory (MP2) has been used for comparison purposes, even though it is known to overestimate dispersive interactions28 and is hampered by significant basis set superposition error. The spin-component-scaled modification (SCS-MP2), which improves the description of such systems by the MP2 theory,28 has also been tested. After detailing the experimental and theoretical methods (section II) and showing the experimental results (section III), this paper analyzes and discusses the results obtained on AcPhe-Phe-NH2 (section VI.A) and Ac-Phe-D-Phe-NH2 (IV.B). The observed conformer of Ac-Phe-Phe-NH2 will first be

Gloaguen et al.

Figure 2. Schematic of the laser spatial configuration used to record IR/UV double resonance spectra.

assigned by a simple interpretation of its IR signature, helped by the calculated relative energy of only two conformers (section IV.A.a). The geometry of the observed conformer is then described, and the calculated structures obtained by each method are simultaneously compared with each other (section IV.A.b). Methods are also evaluated through their ability to fit the measured IR frequencies with scaled harmonic frequency calculations (section IV.A.c). Consequences of the ar-ar interaction are specifically discussed regarding the IR (section IV.A.d) and the UV (section IV.A.e) signatures. Comparison with simpler dipeptides, finally, will provide insight on the role of the specific ar-ar interactions in the resulting conformational preference (section IV.A.f). Ac-Phe-D-Phe-NH2 (section IV.B) is finally presented as a typical case where comparison between experimental and theoretical results reaches its limits. II. Methods Section A. Experimental. The experimental apparatus has already been described elsewhere.13,14,29,30 Capped peptides were synthesized and purified (>95%) by GenScript Corporation. All residues have the natural chirality L, unless specified. Molecules are desorbed using a frequency-doubled Nd:YAG laser (Minilite Continuum, 10 Hz, 0.2-5 mJ/pulse at 532 nm) in a first vacuum chamber from a 6 mm diameter, 2 mm thick pellet containing a 4:1 molar mixture of graphite and peptide. The vaporization setup is coupled with a 0.3 mm diameter, 10 Hz pulsed valve, as previously described.29 A pulsed supersonic expansion of argon is used to cool down the molecules and carry them through a 2 mm diameter skimmer to the interaction region of a linear time-of-flight mass spectrometer in a second vacuum chamber. The UV source is an excimer 308 nm pumped (Lambda-Physik EMG 103) BBO doubled dye laser (LamdaPhysik FL 2002E). The output beam has a typical energy of 400 µJ/pulse and is mildly focused on the molecular beam, as described in Figure 2. A prism is used to send back the laser beam on the molecular beam in the interaction region of the mass spectrometer in order to get a second ionization region serving as a reference signal in the IR/UV experiments, as previously described by Page and co-workers.31 Two ion packets are thus created and extracted along the molecular beam axis. The voltages applied to the classical ion optics30 are chosen to be slightly off the Wiley-McLaren time-focusing conditions.32 In this case, the two ion packets formed with ions of the same m/z have times-of-flight that differ by a few hundreds of nanoseconds. Ions are detected by microchannel plates, and the ion signals are recorded and averaged by a 9350AM LeCroy digital oscilloscope. Resonant two-photon ionization (R2PI) UV spectra are obtained by scanning the wavelength of the dye laser while recording the signals of the two parent ion packets

Aromatic-Aromatic Interaction in Isolated Capped Dipeptides summed over 40 laser shots for each wavelength (0.35 cm-1 steps). IR spectra are obtained by using the IR/UV double resonance spectroscopy as previously described.14 The IR source is the idler of a Nd:YAG pumped LiNbO3 optical parametric oscillator (OPO) source (Euroscan, 3 mJ/pulse, 1 cm-1 resolution). The spectral region between 3460 and 3505 cm-1 is unfortunately not accessible due to a drop in efficiency of the OPO crystal. The IR laser beam is spatially overlapped with the UV laser beam in one of the ionization regions (Figure 2). The wavelength-scanned (3125-3460 and 3505-3565 cm-1 regions, 0.6 cm-1 steps) IR laser pulse is sent ∼50 ns before the UV laser pulse. Each time the IR wavelength is tuned to a vibrational transition of the species probed by the UV wavelength, the parent ion signal from the IR/UV-overlapped region drops. IR spectra are then deduced by plotting the ratio between the parent ion signal from the IR/UV-overlapped ionization region and the normalized parent ion signal from the UV-only ionization region. The normalization consists of applying a scaling factor in order to superimpose the time-integrated parent ion signals coming from both ionization regions when the IR beam is blocked. Finally, a broad bandwidth monochromator followed by a photomultiplier (Hamamatsu R15640U) is also available to detect fluorescence occurring in the first vacuum chamber of the setup.33 B. Theoretical. Conformers consistent with the experimental data were explored using the AMBER 99 force field included in the Hyperchem 7.52 package. The lowest-energy forms were kept for quantum chemistry calculations. Geometry optimizations and harmonic vibrational frequencies calculations were performed using the TURBOMOLE34 5.10 package. Several methods have been tested. As an example of DFT methods, B3LYP/6-31+G(d) was chosen as it had already been used in previous calculations on smaller molecules.15 The optimized structures were characterized as minima by carrying out harmonic frequency calculations at the same level of theory. This is the only method used without the resolution of the identity (RI) approximation, which significantly improves computational time. Two RI-DFT-D methods have been similarly tested. The augmented version of the TPSS35 functional by dispersion energy (TPSS-D) has been proven to perform reasonably well for the study of noncovalent complexes as well as isolated systems.26,36-38 The dispersion energy was calculated by a damped pair potential parametrized against CCSD(T)/CBS results for model complexes containing H-bonded, mainly dispersion-controlled, and mixed [H-bonds + dispersion] complexes. A reasonable performance of the TPSS-D/6311++G(3df,3pd) method for the study of peptides containing aromatic side chains is expected and has already been proven on similar systems.36,38 In order to test the importance of the dispersion energy in the stability of the systems investigated, TPSS/6-311++G(3df,3pd) optimizations have also been carried out. Another functional, B97-D, has also been tested through RI-B97-D/TZVP and RI-B97-D/TZVPP calculations. This functional has been specifically designed for dispersive systems39,40 by limiting the exchange-correlation contribution to short interelectronic distances in order to avoid double counting effects between dispersion already included in the functional and longrange dispersion corrections. Finally, MP2 methods have also been tested. In order to investigate a perturbation theory method that could also be applied to larger peptides, a small basis set has been chosen,41 making RI-MP2/SVP and RI-SCS-MP2/SVP the last two methods tested in this paper.

J. Phys. Chem. A, Vol. 114, No. 9, 2010 2975

Figure 3. Mass-selected R2PI near UV spectra of (a) Ac-Phe-PheNH2 and (b) Ac-Phe-D-Phe-NH2. The asterisk (*) marks the transitions where the IR spectra of Figure 4 were recorded. Dashed lines indicate the transitions belonging to the same conformer (see text).

Single-point calculations have also been carried out at the RI-JK-B2PLYP-D/TZVPP level.42,43 As already reported for a similar system,20 these calculations will serve as benchmark values. Calculated energies are given without taking into account the zero-point energy (ZPE) as this correction depends on the method used and may increase the uncertainty of the results obtained on the systems investigated. However, it has been verified at the RI-B97-D/TZVPP level that neither ZPE nor thermal effects can affect the assignment and the conclusions presented in this paper. The basis set superposition error (BSSE) may also modify the relative conformational energies44 but cannot be easily estimated for single molecules and is therefore not considered in the present paper. III. Experimental Results UV spectra of Ac-Phe-Phe-NH2 and Ac-Phe-D-Phe-NH2 are recorded in the region of the S1 r S0 transition of phenylalanine (Figure 3). Both spectra are made of discrete narrow bands as observed for Ac-D-Phe-NH2 (NAPA) and a much weaker spectrally broad absorption. As these two dipeptides have two phenylalanine residues, both UV chromophores are expected to contribute to these spectra. The IR spectrum of Ac-Phe-Phe-NH2 (Figure 4a) was recorded in the NH stretch region when fixing the UV laser wavelength on the most intense transition of the UV spectrum. The IR spectrum is composed of four bands, as expected for a molecule having four NH bonds. Assignment of these bands is presented in the discussion. IR absorptions at these four wavelengths are also observed when the UV laser wavelength is set on the five other major UV transitions (Figure 3a). Such an IR absorption has not been observed on the broad component of the UV spectrum. These results suggest that only one conformer mainly contributes to the sharp features of the UV spectrum, the broad contribution coming presumably from insufficiently cooled molecules. As a consequence, the set of multiple transitions observed on the UV spectrum reflect the vibrational Franck-Condon activity of at least one chromophore. A vibrational progression involving transitions 1, 2, and 4 can be observed. As transition 1 has the lowest energy and is the most intense, it can be assigned to the S1(V)0) r S0(V)0) transition, or the origin. The vibrational mode responsible for the observed progression has a vibrational constant of

2976

J. Phys. Chem. A, Vol. 114, No. 9, 2010

Gloaguen et al. TABLE 2: Comparison between C7 or C10 NH-Stretch Frequencies of Known Homochiral Dipeptides secondary structure C5C7a C7C7b C10c Ac-Phe-Phe-NH2

C7 or C10 wavenumber range (cm-1) 3337-3378 second residue: 3261-3340 first residue: 3360-3364 3385-3398 3391

a

Ac-Phe-Ala-NH2,16 Ac-Phe-Val-NH2 (2 conformers),16 Ac-PhePro-NH2 (2 conformers).16,18 b Ac-Ala-Phe-NH2,13 Ac-Val-Phe-NH2,13 Ac-Pro-Phe-NH2.18 c Ac-Phe-Val-NH2,16 Ac-Ala-Phe-NH2,13 Ac-ValPhe-NH2.13

Figure 4. IR/UV double resonance spectra of (a) Ac-Phe-Phe-NH2 and (b) Ac-Phe-D-Phe-NH2.

TABLE 1: Frequencies of the NH Stretches Obtained by IR/UV Double Resonance Spectroscopy (cm-1)a sym

NH2 NH (1) NH (2) NH2anti

Ac-Phe-Phe-NH2

Ac-Phe-D-Phe-NH2

3391 3437 3447 3524

3382 not observed (3460-3505)b 3440 3520

a NH amide stretches have been numbered according to their position along the peptide chain going from the N-terminal to the C-terminal. b Region not covered by the OPO IR laser.

54 ( 1 cm-1. Among the remaining bands of the UV spectrum, transition 3 has the lowest frequency and is the most intense. These arguments suggest that transition 3 is the S2(V)0) r S0(V)0) transition. A final assignment will be given further in the paper. The IR spectrum of Ac-Phe-D-Phe-NH2 (Figure 4b) was also recorded in the NH stretch region when tuning the UV laser wavelength to the most intense transition. As for the homochiral peptide, only one conformer is observed for this heterochiral peptide. Only three IR transitions can be observed. The missing IR transition is presumably in the gap not covered by the IR laser (3460-3505 cm-1). Although the UV spectrum of the heterochiral peptide (Figure 3b) contains several transitions, no clear Franck-Condon progression can be extracted, making the assignment of the origin transitions of both chromophores too speculative. No fluorescence has been detected within the limits of our apparatus sensitivity when exciting the main transitions of both molecules. IV. Analysis and Discussion A. Ac-Phe-Phe-NH2. a. Assignment of the Secondary Structure. The assignment procedure can be greatly simplified from general qualitative considerations about the IR spectrum. The IR bands observed for Ac-Phe-Phe-NH2 (Figure 4a and Table 1) are characteristic of a molecule having no free NH bonds (NH amide free stretch:45 3510 cm-1). Amide-NH2 stretches, symmetric and antisymmetric, are respectively redshifted and blue-shifted compared to the free NH stretch frequency. The vibration at 3524 cm-1 is then assigned to the

NH2anti stretch. This value allows us to estimate the coupling12,18 between both NH2 stretches, from which the NH2sym stretch can be assigned to the vibration observed at 3391 cm-1. The terminal NH2 is thus involved in one hydrogen bond that can only be either a C7 bond (γ-turn) or a C10 bond (β-turn). As the two other NH bonds of the molecule are also involved in intramolecular H-bonds (3437 and 3447 cm-1), only three types of structures have to be considered, (i) C5C7 with one NH-π interaction, (ii) C7C7 with one NH-π interaction, and (iii) C10 with two NH-π interactions. As it can be seen on the set of previously studied dipeptides (Table 2), the NH2sym stretch observed at 3391 cm-1 leads us to rule out the C5C7 and C7C7 structures and to assign the structure of Ac-Phe-Phe-NH2 to a C10 (β-turn) structure. Four major types of β-turns exist and are labeled I, I′, II, and II′ based on the values of the four Ramachandran dihedral angles defining the backbone torsions of the two residues making the β-turn.46 All C10 structures observed on previously studied homochiral protected dipeptides13,16,18 (Ac-Phe-Val-NH2, AcVal-Phe-NH2, and Ac-Ala-Phe-NH2) were of type I. However C10 type II′ have also been observed for two other dipeptides, Ac-Phe-Gly-NH2 and Ac-D-Ala-Phe-NH2. The molecular dynamics exploration has then been restricted to C10(I) and C10(II′) conformations. Two force field explorations were performed by fixing the geometry of the backbone to C10(I) and C10(II′) structures. Backbones were taken from the B3LYP/6-31+G* optimized geometry of Ac-Ala-Phe-NH2 for both types.17 All bond distances and angles of each side chain of Ac-Phe-PheNH2 were free to move during the exploration. Due to the geometrical constraints imposed by the C10(II′) backbone, no conformer having two NH-π interactions could be found. Only two C10(I) conformers are compatible with the two NH-π interactions experimentally observed. They differ by the orientation of their phenyl side chains, which are labeled according to the value of the NCCCaromatic dihedral angles, (g() if the angle lies in between 0 and (120° and (a) otherwise.18 The conformers were then labeled (g+)(g+) and (g-)(g+) according to the respective orientation of the Phe(1) and Phe(2) side chains of Ac-Phe(1)-Phe(2)-NH2 (Figure 5). These two conformers differ mainly by the interaction between the aromatic rings; the distance between ring centers is less than 0.6 nm for (g+)(g+), whereas it is around 0.8 nm for (g-)(g+). According to calculations on toluene dimers,47,48 the ar-ar stabilization is potentially as strong as 10 kJ mol-1 between the phenyl groups, whereas it is negligible in (g-)(g+). The conformer (g+)(g+) is then expected to be the lowest in energy mainly due to a larger contribution of the edge-to-face ar-ar interaction. Their structures have been optimized by quantum chemistry calculations using several methods (see Table 3). In all of the methods used, the (g+)(g+) is the lowest-energy conformer. On these grounds, the (g+)(g+) conformer is then assigned to the one observed.

Aromatic-Aromatic Interaction in Isolated Capped Dipeptides

Figure 5. RI-TPSS-D/6-311++G** calculated structures of Ac-PhePhe-NH2; (a) C10(I) (g+)(g+) and (b) C10(I) (g-)(g+). Distances are given in pm.

TABLE 3: Relative Energy and Inter-ring Distance Comparisons between the Calculated C10(I) Optimized Conformers of Ac-Phe-Phe-NH2, (g+)(g+), and (g-)(g+)

theoretical method B3LYP/6-31+G* RI-TPSS/6-311++G** RI-TPSS-D/6-311++G** RI-JK-B2PLYP-D/TZVPP RI-B97-D/TZVPP RI-B97-D/TZVP RI-SCS-MP2/SVP RI-MP2/SVP

E(g-g+) E(g+g+) drings(g+g+) drings(g-g+) (kJ mol-1) (pm) (pm) 0.4 1.4 13.7 14.0a 14.0 14.6 16.6 22.1

631 581 518

854 807 780

512 515 509 496

794 794 809 792

a Single-point calculations on RI-TPSS-D/6-311++G** structures (see text).

b. Description of the Ac-Phe-Phe-NH2 Conformer. The C10 H-bond, the two NH-π interactions, and the ar-ar interaction are the main stabilizing features shaping the (g+)(g+) conformer. However, geometrical constraints induce a competition between them. The structure will then result from a subtle balance between all of these stabilizing interactions, which makes very challenging a correct description of its shape by quantum chemistry. This is highlighted by the dependence of the distance between the aromatic rings and the quantum chemistry methods used (Table 3). DFT calculations like RITPSS/6-311++G** and B3LYP/6-3+G*, which cannot account properly for dispersive interactions as observed by Baquero and co-workers on a flexible bichromophore,22 provides the largest inter-ring distances, whereas RI-MP2/SVP, which overestimates these interactions, leads to the shortest distance. It is then believed that a more correct distance between the rings and, by extrapolation, a better structure are provided by methods giving intermediate results. This is confirmed by single-point RI-JKB2PLYP-D/TZVPP calculations of the optimized geometries of both conformers for each method. The lowest energies of these single-point calculations have been obtained for the RITPSS-D/6-311++G** geometries for both conformers, suggesting that this method gives the best structural description of this system. Single-point calculations carried out on RI-B97-D and RI-SCS-MP2/SVP structures also give very good results with absolute energies not higher than 3 kJ mol-1 above the RI-TPSS-D/6-311++G** structures. Other single-point calculations lead to energies higher than 4 kJ mol-1. The relative energy between both conformers reported in Table 3 also reflect the uncertainty of each method, RI-JK-B2PLYP-D/TZVPP serving as a reference. Thus, both DFT-D methods, which take dispersion into account, appear to be more trustworthy and give similar energies and inter-ring distances. Interestingly, the spin component scaling (SCS) that refines the MP2 method also leads to results similar to those obtained with DFT-D methods.

J. Phys. Chem. A, Vol. 114, No. 9, 2010 2977 The RI-TPSS-D/6-311++G** structure of the (g+)(g+) conformer is displayed in Figure 5. The phenyl rings of this conformer are closely interacting, making a kind of hydrophobic domain. This edge-to-face orientation of the phenyl rings is similar to the one calculated by Abo-Riziq and co-workers23 for a minor conformer of the natural Phe-Phe peptide. The distance between the two rings is 518 pm, which is close to the distance of 500 ( 10 pm calculated for the toluene T-shaped dimer.47,48 This indicates that the constraint brought by the backbone, which prevents both rings from reaching a shorter distance, is relatively weak. However, it does not allow the rings to adopt a stacked structure, predicted48,49 to be the most stable conformer of the toluene dimer and presumably observed50,51 simultaneously with the T-shaped dimer. As a consequence, the ar-ar stabilization can be estimated48 to be as strong as 10 kJ mol-1. If the side chain orientation is mainly governed by the ar-ar interaction, it is also interesting to note that the geometrical conditions in order to make two NH-π and one ar-ar stabilizing interactions are simultaneously fulfilled in this C10(I) (g+)(g+) conformer of Ac-Phe-Phe-NH2. This additional stability could also explain why no other competing conformer has been detected. Other studied single-chromophore dipeptides,13,16,18 Ac-Phe-Val-NH2, Ac-Val-Phe-NH2, and Ac-Ala-Phe-NH2, were indeed all detected with at least one other conformer competing with the C10(I). This illustrates the significant role played by two neighboring aromatic side chains in influencing the conformation adopted by the backbone. However, it cannot be concluded from these examples that either the additional NH-π or ar-ar interaction brought by the second chromophore is mainly responsible for the relative stabilization of the C10(I) observed for Ac-Phe-Phe-NH2. A way to test the global geometry proposed by the theoretical methods and not only the inter-ring distance is to compare the experimental and theoretical frequencies of the NH stretches. These calculated vibrations are indeed expected to be very sensitive to the structural differences induced by the theoretical treatment of dispersive effects. c. Calculated IR Frequencies of the C10(I) (g+)(g+). Harmonic frequency calculations have been carried out for each optimized structure. Several ways exist to account for anharmonicity using scaling factors for frequency comparison purpose. A first one consists of applying the same scaling factor for the four vibrations observed (two NH, one NH2sym, and one NH2anti stretches), assuming the anharmonic correction is very similar for each mode. This scaling factor can be determined ideally by fitting a set of calculated frequencies on the experimental frequencies of well-identified conformers. In the present case, the calculated frequencies of the conformer (g+)(g+) have been fitted on the experimentally observed ones for Ac-Phe-Phe-NH2, which would correspond to apply the best scaling factor possible to the calculated frequencies in order to fit the result of this experiment. The standard deviation and the maximum absolute shift have been reported in Table 4. It is clear that only a few computational methods are capable of giving results good enough to be used as predictive methods for assignment purposes (maximum deviation ∼ 10 cm-1). Surprisingly, B3LYP/6-31+G* and RI-MP2/SVP, which give the best fit, are those known to have trouble in describing correctly the dispersive interaction, the first one underestimating it and the second one overestimating it. Compensation errors between the poor description of the geometry and the electronics must be at play to give these fortuitous good results. A second way to compare calculated and experimental frequencies is to use specific scaling factors for each kind of

2978

J. Phys. Chem. A, Vol. 114, No. 9, 2010

Gloaguen et al.

TABLE 4: Comparison between Calculated and Measured Frequencies of Ac-Phe-Phe-NH2a theoretical method

mean deviation (cm-1)

maximum absolute shift (cm-1)

RI-SCS-MP2/SVP B3LYP/6-31+G* RI-MP2/SVP RI-B97-D/TZVP RI-B97-D/TZVPP RI-TPSS-D/6-311++G** RI-TPSS/6-311++G**

2.6 6.4 6.4 8.0 8.3 13 13

2.6 8.0 11 14 14 20 22

a The scaling factor is adjusted in order to fit the experimental data for each method. Calculated frequencies using a single scaling factor (fc,ssf) (see text) are compared with measured frequencies (f m) i∈[1,4] through the mean deviation [∑NHmodes ((fic,ssf - fim)2)/4]1/2 and the maximum shift Max|(fic,ssf - fim)|.

vibrational mode (NH, NH2sym, and NH2anti stretches) as a different conformation-averaged anharmonicity can be anticipated for each mode.52 One method consists of fitting the experimental data from a large set of well-characterized conformers in order to obtain these scaling factors.41,53 Another cheaper strategy consists of determining the scaling factor of each mode from the comparison between experience and theory on one single simple molecule serving as a reference.22 This strategy has been applied using acetamide (CH3-CO-NH2) and N-methylacetamide (CH3-CO-NH-CH3) as reference molecules for the NH2sym and NH2anti stretch modes and for the NH stretch mode, respectively.45 Scaled frequencies are presented in Table 5, and scaling factors are available in the Supporting Information. RI-MP2/SVP provides a good frequency prediction despite a presumably too compact structure. RI-B97-D gives a slightly worse frequency calculation mainly due to a poor estimation of the NH-π bands, which is a common feature to all of the methods except RI-MP2/SVP. The RI-SCS-MP2 leads to blue-shifted calculated frequencies from a structure similar to the DFT-D ones. The bad results of this improved version of the MP2 method suggest that the good results of RI-MP2/ SVP should be taken with great care. Both DFT methods overestimate the inter-ring distance, which leads to an inaccurate geometry revealed by the poor agreement between calculated and experimental frequencies, especially for the NH2sym mode. The case of TPSS-D, which predicts a too red shifted frequency for the NH2sym mode, is more surprising as the energetics and inter-ring distance were supposed to be as good as those obtained with RI-B97-D. In addition, harmonic frequency calculations of TPSS and TPSS-D give very similar results despite significant differences of the respective optimized geometries. This failure to predict satisfactory scaled frequencies using TPSS-D might be related to the specific strategy used to determine scaling factors from small molecules having free NH modes. This might indicate the nontransferability of TPSS-D from NH-free to NHbound species. This point is further supported by the analysis of the C10 H-bond distances presented in Table 6 and discussed in the following section. d. Signature of the ar-ar Interaction in the NH Stretch IR Spectrum. Harmonic frequency calculations have also been carried out for the (g-)(g+) conformer in order to test the assignment capabilities of the methods in this simple case. The (g-)(g+) and (g+)(g+) C10 H-bond calculated distances are compared in Table 6. The H-bond of (g-)(g+) is predicted to be shorter by several picometers. The calculated IR frequency of the NH2sym stretch is thus expected to be red-shifted. This trend is well predicted by all methods with, however, frequency differences ranging from 8 to 31 cm-1 when applying the same

mode-specific scaling factors as those used for the (g+)(g+) conformer. These differences in H-bond length and in IR frequency between the conformers were expected as there is no ar-ar interaction in the (g-)(g+) conformer compared to (g+)(g+). The structure of the (g-)(g+) β-turn is indeed less constrained, leading to a stronger H-bond. The analysis of the NH2sym stretch frequency clearly demonstrates how sensitive to the ar-ar interaction the C10 H-bond is and how difficult it is for theoretical methods to predict a molecular structure good enough to match this particular experimental frequency. The frequency difference of ∼30 cm-1 predicted by both RIDFT-D methods suggests a comparable treatment of the ar-ar interaction, whereas this is not reflected in the absolute frequencies of the NH2sym mode as stated above (Table 5). This difference between both methods comes from the H-bond distances presented in Table 6 that are predicted to be smaller for RI-TPSS-D relative to RI-B97-D. This might be the clue of an overlap between the density functional contribution and the long-range dispersion correction in TPSS-D, although further investigations are required to check this hypothesis. This effect of the dispersion term is also reflected by the C10 H-bond distances of the (g-)(g+) conformer that seem to be affected by the dispersion term (TPSS: 205.1 pm; TPSS-D: 200.3 pm), whereas this conformer has no ar-ar interaction. The mean deviation and the maximum shift compared to the experimental frequencies have been calculated for the (g-)(g+) conformer (not shown), as previously described for (g+)(g+). The comparison between both data sets confirms the assignment of the conformer experimentally observed to the (g+)(g+) conformer for most of the methods investigated. It can be noted that the method believed to calculate the most accurate frequencies, RI-B97-D/TZVPP, predicts the C10 H-bond of the (g+)(g-) conformer at 3354 cm-1 which is particularly discriminating for assignment purposes. However, B3LYP/631+G* and RI-SCS-MP2/SVP fail to assign the observed conformer as they would have fit the IR spectrum better with the (g-)(g+) calculated frequencies. This demonstrates the limitations of cheap strategies (functional not suitable for the system or too small basis set) in order to distinguish two conformers having the same kind of interactions involving NH groups. e. UV Spectroscopy of the Edge-to-Face (g+)(g+) Conformer. As the Ac-Phe-Phe-NH2 molecule has two UV chromophores which are not equivalent, one can hypothesize that the UV spectrum is the superposition of the two π* r π transitions localized on each chromophore. The comparison with the C10(I) structures13,16,18 of Ac-Phe(g+)-Val-NH2, Ac-ValPhe(g+)-NH2, and Ac-Ala-Phe(g+)-NH2 is helpful in order to assign the UV transitions observed in the bichromophore AcPhe(g+)-Phe(g+)-NH2 molecule. As UV spectra are sensitive to the chromophore environment,50 the comparison between the origin transitions of all of the C10(I) structures should be a good criterion to decipher the contributions of each chromophore to the UV spectrum of Ac-Phe-Phe-NH2. The origin transitions of the C10(I) of the single-chromophore peptides previously studied are, respectively, 37515, 37565, and 37560 cm-1. The origin bands, transitions 1 and 3 (Figure 3), of Ac-Phe(1)-Phe(2)NH2 are, respectively, measured at 37491 and 37567 cm-1. Transition 3 and the origin transitions of Ac-Val-Phe-NH2 and Ac-Ala-Phe-NH2 are all within 7 cm-1, supporting that S2 r S0 is localized on Phe(2). Transition 1 is red-shifted by 24 cm-1 compared with the origin transition of Ac-Phe-Val-NH2, making the S1 r S0 origin transition of Phe(1) its most probable assignment.

Aromatic-Aromatic Interaction in Isolated Capped Dipeptides

J. Phys. Chem. A, Vol. 114, No. 9, 2010 2979

TABLE 5: Comparison between Calculated and Measured Frequencies of Ac-Phe-Phe-NH2a method

NH2sym

NH (1)

NH (2)

NH2anti

mean deviation (cm-1)

maximum absolute shift (cm-1)

exp. RI-MP2/SVP RI-B97-D/TZVP RI-B97-D/TZVPP RI-TPSS-D/6-311++G** RI-TPSS/6-311++G** RI-SCS-MP2/SVP B3LYP/6-31+G*

3391 3390 3395 3385 3369 3368 3405 3398

3437 3436 3453 3450 3437 3443 3458 3462

3447 3445 3460 3463 3457 3455 3463 3475

3524 3512 3525 3525 3516 3514 3522 3525

6 10 11 13 13 14 19

13 16 17 23 23 20 28

a

Mode-specific scaling factors are determined for each method by fitting the experimental frequency of a reference molecule. Calculated frequencies using multiple scaling factors (fc,msf) (see text) are compared to measured frequencies (f m) through the mean deviation i∈[1,4] ((fic,msf - fim)2)/4]1/2 and the maximum shift Max|(fic,msf - fim)|. [∑NHmodes

TABLE 6: Calculated H-Bond Distances and NH2sym Stretch Frequency Differences between the (g+)(g+) and (g-)(g+) Conformers of Ac-Phe-Phe-NH2 Obtained with Several Theoretical Methods

theoretical method

(g+)(g+) C10 H-bond distance (pm)

(g-)(g+) C10 H-bond distance (pm)

C10 H-bond distance calculated stretch induced by the ar-ar interaction (pm)

RI-B97-D/TZVPP RI-B97-D/TZVP RI-MP2/SVP RI-SCS-MP2/SVP RI-TPSS-D/6-311++G** RI-TPSS/6-311++G** B3LYP/6-31+G*

220.3 220.5 213.3 217.7 210.9 213.8 213.9

204.3 204.5 200.2 204.3 200.3 205.1 206.2

16 16 13 13 11 9 8

A first remark on the excited-state lifetime can be developed by comparing this UV signature to the one previously observed for similar systems. Such sharp UV transitions, already observed on Phe-Phe,23 are compatible with excited-state lifetimes longer than hundreds of picoseconds. However, no fluorescence has been detected in our experiments, suggesting that the excited state lifetimes are in the sub-nanosecond time domain. In addition, the UV spectrum of the T-shaped toluene dimer presumably observed50,51 is broad compared to the distorted T-shaped Ac-Phe-Phe-NH2 (g+)(g+) conformer, possibly suggesting that the lifetimes of the excited ππ* states are very sensitive to the relative orientation of the aromatic rings, as already seen on NAPA.15 A second remark arises from the comparison with the Ac-Phe(g+)-Val-NH2, Ac-Val-Phe(g+)NH2, and Ac-Ala-Phe(g+)-NH2 C10(I) conformers. The origin of the π* r π transition of each chromophore is weakly dependent on the aromatic or alkyl nature of the side chain of the neighboring residue. This observation supports that π electrons interact very little in aromatic edge-to-face conformations.27 As mentioned above, a Franck-Condon progression related to transition 1 is observed. This mode has a vibrational constant of 54 ( 1 cm-1. It is assumed that this mode observed in the excited state is comparable to the modes calculated in the ground state. The list of the calculated low frequencies has been checked for every method. In every case, it is possible to find one mode within 12 cm-1 that changes the environment of the chromophore of Phe(1) significantly enough to be responsible for the observed Franck-Condon activity. However, these modes were too different from each others to propose an assignment for the observed progression. f. Influence of the ar-ar Interaction on the Conformational Landscape. The previously observed C10(I) structures in other dipeptides were found to be in competition with other conformers in every case.13,16 The secondary structures competing with the C10(I) have then been theoretically investigated for the AcPhe-Phe-NH2 system in order to estimate the influence of the

calculated NH2sym stretch frequency difference induced by the ar-ar interaction (cm-1) 31 30 10 8 28 13 9

TABLE 7: Energetics of the Conformational Landscape of Ac-Phe-Phe-NH2 at the RI-TPSS-D/6-311++G** Level of Theory conformers backbone

side chains

energy (kJ mol-1)

C10(I) C5C7(L) C5C7(D) C10(I) C7(D)C7(D) C7(D)C7(L) C7(L)C7(D) C7(L)C7(L)

(g+)(g+) (a)(g+) (a)(g-) (g-)(g+) (a)(g-) (a)(g-) (a)(g-) (a)(g-)

0 9.7 11.6 13.7 14.9 16.5 17.1 18.4

side chain interaction on the stabilization of this β-turn. A C5C7(L) structure was found to be the most stable for Ac-Phe-Val-NH2, and C7C7 structures were found to be either the most stable for Ac-Ala-Phe-NH2 or among the main conformers in Ac-ValPhe-NH2. A complete set of C5C7 and C7C7 backbone structures was considered. For each one, only the orientations of both side chains leading to the minimum energy are presented in Table 7. This table confirms the C10(I) (g+)(g+) structure as the most stable conformer. It appears that the (g+) orientation of Phe(2) in the calculated C10(I) structures of Ac-Phe-Phe-NH2 is the same as the one assigned to the Ac-Val-Phe-NH2 C10(I) structure.13 As the C7(L)C7(L) and the C10(I) conformers of Ac-Val-Phe-NH2 have roughly the same energy,13 it can then be concluded that the substitution of Val by Phe induces a stabilization of the C10(I) conformer of at least ∼18 kJ mol-1 relative to that of the C7(L)C7(L), as shown in Table 7. Part of this stabilization can be directly correlated to the additional ar-ar and NH-π interactions (respectively ∼10 and ∼3 kJ mol-1) brought by Phe(1) in the C10(I) structure and not existing in the C7(L)C7(L) structure. This is direct evidence of the role played by side chains in influencing the conformation adopted by a peptide.

2980

J. Phys. Chem. A, Vol. 114, No. 9, 2010

Gloaguen et al.

TABLE 8: IR Signature of Ac-Phe-D-Phe-NH2 Compared to Other Heterochiral Dipeptides

NH2sym NH(1) NH(2) NH2anti a

Ac-Phe-D-Phe-NH2 (present work)

Ac-D-Ala-Phe-NH217 C10(II′)

Ac-Phe-D-Ala-NH214 C5C7

3382 not observed (3460-3505)a 3440 3520

3383 not observed (3460-3505)a 3442 3517

3371 3457 3445 3525

Region not covered by the OPO IR laser.

A similar comparison can be made between the C10(I) and C5C7(L) structures of Ac-Phe-Val-NH2 and Ac-Phe-Phe-NH2 as the side chain orientation of Phe(1) is, respectively, (g+) and (a) in both molecules.16 The substitution of Val by Phe leads to a stabilization of ∼10 kJ mol-1 of C10(I) compared to C5C7(L), as suggested by Table 7. However, this stabilization cannot be attributed to the ar-ar interaction because the C5C7(L) conformer of Ac-Phe-Phe-NH2 allows the two phenyl rings to adopt an almost parallel V-shaped structure with an inter-ring distance of 410 pm, leading to an estimated ∼8 kJ mol-1 ar-ar stabilizing interaction48 on the same order of magnitude as that for the C10(I) conformer. The main apparent difference induced by the substitution of Val by Phe is the additional NH-π interaction existing in the C10(I) and not in the C5C7(L) conformer. However, this interaction can only account for ∼3 kJ mol-1 of stabilization of the C10(I) conformer. The remaining energy differences (∼5 kJ mol-1 in both cases) may reflect more subtle changes between the conformers or the uncertainty of the calculated energies which are not corrected for ZPE or BSSE. B. Ac-Phe-D-Phe-NH2. An interpretation of the IR spectrum (Figure 4b and Table 1) similar to the one previously presented for Ac-Phe-Phe-NH2 leads to consideration of three types of structures, C5C7, C7C7, and C10 with one NH-π interaction. Few IR data on heterochiral dipeptides with a ∼1 cm-1 resolution are available.14,17 A C5C7 structure has been observed on Ac-Phe-D-Ala-NH2, and a C10(II′) has been detected on AcD-Ala-Phe-NH2. It can be noted from the frequency analysis (Table 8) that the C10(II′) IR signature matches the one measured on Ac-Phe-D-Phe-NH2 (the largest frequency difference between both spectra is 3 cm-1). The C5C7 structure is less likely to correspond to the measured IR spectrum of Ac-Phe-D-Phe-NH2 (frequency differences ranging from 5 to 11 cm-1). Although not observed for heterochiral peptides, the C7C7 structure is expected to keep the characteristic signature of two red-shifted IR bands as already recorded for homochiral peptides.13 The C10 is then the most probable structure adopted by Ac-Phe-DPhe-NH2 according to IR data. As type II′ is observed for dipeptides having the chirality DL, like Ac-D-Ala-Phe-NH2, the enantiomer form, type II, has to be considered in the case of Ac-Phe-D-Phe-NH2 which has the chirality LD. A molecular dynamics exploration was then performed with a fixed backbone geometry taken as the enantiomer of the previously calculated17 Ac-D-Ala-Phe-NH2 backbone. The side chains were kept free to move during the exploration, as for the homochiral peptide. Several side chain orientations can lead to the IR signature of a C10 structure with only one NH-π interaction (Table 9). According to the geometrical constraints of the C10(II) structure, a NH-π interaction is possible in AcPhe-D-Phe-NH2 only when Phe(1) has the (g+) or (g-) orientation or when Phe(2) has the (g-) orientation. The (g+)(g+), (g+)(a), (g-)(g+) (g-)(a), and (a)(g-) orientations were then investigated at the RI-B97-D/TZVPP level. Two conformers matched the experimental frequencies (Table 1), (a)(g-) (3373; 3455; 3479; 3521 cm-1) and (g+)(a) (3388; 3441; 3465; 3524 cm-1). A definitive assignment based on the

TABLE 9: Energetics of C10(II) Conformers of Ac-Phe-D-Phe-NH2 at the RI-B97-D/TZVPP Level

conformers

number of NH-π interaction

RI-B97-D/TZVPP relative energy (kJ mol-1)

RI-B97-D/TZVPP distance between aromatic rings (pm)

(a)(g-) (g-)(g-) (a)(a) (a)(g+) (g-)(g+) (g+)(g-) (g+)(g+) (g-)(a) (g+)(a)

1 2 0 0 1 2 1 1 1

0

516

18

694

15 31 35

512 1073 864

calculated IR frequencies would demand an accuracy which cannot be reached by the theoretical method used. However, on an energetic basis, the most stable conformer compatible with the IR spectrum exhibits the (a)(g-) orientations. It is the best candidate that can be tentatively assigned to the conformer observed. This (a)(g-) C10(II) conformer found for Ac-Phe-D-Phe-NH2 is shown in Figure 6. Aromatic rings are in an edge-to-face interaction with an inter-ring distance of 516 pm. This structure can be compared with other similar structures. C10(II) and C10(II′) structures were found, respectively, in Ac-Phe-Gly-NH2 and AcGly-Phe-NH2 as minor conformers. The phenyl groups were in the respective orientations (a) and (g-) and were then conserved in the same orientation in Ac-Phe-D-Phe-NH2. The (a) orientation of Phe(1) which authorizes the ar-ar interaction in AcPhe-D-Phe-NH2 but prevents NH(1) from interacting with the π-system is thus not specific of the presence of the second aromatic residue. The (g-) orientation of Phe(2) is expected since it is the only one authorizing a NH-π interaction, besides being compatible with an ar-ar interaction. As a consequence, it cannot be said that the two phenyl groups adopt a conformation that favors the ar-ar interaction as their respective orientations were already observed in Ac-Phe-Gly-NH2 and AcGly-Phe-NH2. In addition, the C10(II′) (a) was already the unique conformer observed for Ac-Phe-D-Ala-NH2, which is evidence of the relative stabilization of the C10(II′) structure compared to the most stable one found for Ac-Phe-Gly-NH2. There is

Figure 6. RI-B97-D/TZVPP calculated C10(II) (a)(g-) structure of AcPhe-D-Phe-NH2. Distances are given in pm.

Aromatic-Aromatic Interaction in Isolated Capped Dipeptides therefore no experimental evidence of the additional stability brought by the ar-ar interaction in the case of Ac-Phe-D-PheNH2. However, UV spectroscopy (Figure 3b) reveals that few transitions are broader than others, suggesting that at least one chromophore might have a significantly shorter lifetime than the other, which has not been observed in other singlechromophore dipeptides. Unfortunately, this hypothesis is not supported by any measurement of the excited-state lifetime since no fluorescence could be detected for this system either. V. Conclusions This paper presents a combined experimental and theoretical study of two capped homo- and heterochiral dipeptides, AcPhe-Phe-NH2 and Ac-Phe-D-Phe-NH2. This study revealed that these systems simultaneously have H-bonds, NH-π and ar-ar interactions that act together to shape the molecules. It has been demonstrated how side chain interactions can influence the secondary structure adopted by dipeptides and that edge-to-face interactions between the two neighboring aromatic residues favor the formation of the C10(I) structure. As the C10 H-bond is very sensitive to the inter-ring distance, the frequency of the associated IR band can be used as an indirect probe of the aromatic pair conformation, which makes this system ideal to investigate the theoretical treatment of dispersive forces. These peptides are also good candidates to study solvation-related theoretical issues as their IR signature can potentially reveal quantitative hydrophobic effects, which will require further investigations. Several methods potentially suitable for calculations on larger systems have been tested on Ac-Phe-Phe-NH2, and optimized structures, energies, and harmonic frequencies have been compared. If the DFT-D and RI-SCS-MP2/SVP methods seem to describe correctly the Ac-Phe-Phe-NH2 structure and its conformational energetics, none of them reaches the accuracy needed for scaled harmonic frequency calculations to distinguish between the Ac-Phe-D-Phe-NH2 conformers either in the NH stretches or in the low-frequency mode domain. However, a combined analysis of energetics and IR calculated frequencies at the RI-B97-D/TZVPP level leads to a reliable assignment of the observed conformer. These two examples of dipeptides show that the theoretical methods investigated can be used to study systems which are subject to dispersion, including longer peptide sequences, provided a careful use. Acknowledgment. We are grateful to Dr Gilles Gre´goire for communicating useful preliminary results and for fruitful discussions. Part of the theoretical work was a part of the research project No. Z40550506 of the Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, and it was supported by Grant No. LC512 from the Ministry of Education, Youth and Sports of the Czech Republic. The support of Praemium Academiae, Academy of Sciences of the Czech Republic, awarded to P.H. in 2007 is also acknowledged. H.V. acknowledges the support of the government of Principado de Asturias under the program Plan de Ciencia, Tecnologı´a e Innovacio´n (PCTI) 2006-2009. Supporting Information Available: Scaling factors used to calculate the frequencies presented in Table 5. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Burley, S. K.; Petsko, G. A. Science 1985, 229, 23.

J. Phys. Chem. A, Vol. 114, No. 9, 2010 2981 (2) Thomas, A.; Meurisse, R.; Brasseur, R. Proteins: Struct., Funct., Genet. 2002, 48, 635. (3) Tatko, C. D.; Waters, M. L. J. Am. Chem. Soc. 2002, 124, 9372. (4) Butterfield, S. M.; Patel, P. R.; Waters, M. L. J. Am. Chem. Soc. 2002, 124, 9751. (5) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210. (6) Chelli, R.; Gervasio, F. L.; Procacci, P.; Schettino, V. J. Am. Chem. Soc. 2002, 124, 6133. (7) Marsili, S.; Chelli, R.; Schettino, V.; Procacci, P. Phys. Chem. Chem. Phys. 2008, 10, 2673. (8) Tjernberg, L.; Hosia, W.; Bark, N.; Thyberg, J.; Johansson, J. J. Biol. Chem. 2002, 277, 43243. (9) Gazit, E. FASEB J. 2002, 16, 77. (10) Azriel, R.; Gazit, E. J. Biol. Chem. 2001, 276, 34156. (11) Schermann, J.-P. Spectroscopy and modeling of biomolecular building blocks; Elsevier: New York, 2007. (12) Chin, W.; Piuzzi, F.; Dimicoli, I.; Mons, M. Phys. Chem. Chem. Phys. 2006, 8, 1033. (13) Chin, W.; Dognon, J.-P.; Canuel, C.; Piuzzi, F.; Dimicoli, I.; Mons, M.; Compagnon, I.; von Helden, G.; Meijer, G. J. Chem. Phys. 2005, 122, 054317. (14) Gloaguen, E.; Pagliarulo, F.; Brenner, V.; Chin, W.; Piuzzi, F.; Tardivel, B.; Mons, M. Phys. Chem. Chem. Phys. 2007, 9, 4491. (15) Chin, W.; Mons, M.; Dognon, J.-P.; Mirasol, R.; Chass, G.; Dimicoli, I.; Piuzzi, F.; Butz, P.; Tardivel, B.; Compagnon, I.; von Helden, G.; Meijer, G. J. Phys. Chem. A 2005, 109, 5281. (16) Chin, W.; Piuzzi, F.; Dognon, J.-P.; Dimicoli, I.; Mons, M. J. Chem. Phys. 2005, 123, 084301. (17) Brenner, V.; Piuzzi, F.; Dimicoli, I.; Tardivel, B.; Mons, M. Angew. Chem., Int. Ed. 2007, 46, 2463. (18) Chin, W.; Mons, M.; Dognon, J.-P.; Piuzzi, F.; Tardivel, B.; Dimicoli, I. Phys. Chem. Chem. Phys. 2004, 6, 2700. (19) Chin, W.; Piuzzi, F.; Dognon, J. P.; Dimicoli, I.; Tardivel, B.; Mons, M. J. Am. Chem. Soc. 2005, 127, 11900. (20) Haber, T.; Seefeld, K.; Engler, G.; Grimme, S.; Kleinermanns, K. Phys. Chem. Chem. Phys. 2008, 10, 2844. (21) Pillsbury, N. R.; Stearns, J. A.; Muller, C. W.; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2008, 129, 114301. (22) Baquero, E. E.; James, W. H., III; Choi, T. H.; Jordan, K. D.; Zwier, T. S. J. Phys. Chem. A 2008, 112, 11115. (23) Abo-Riziq, A. G.; Bushnell, J. E.; Crews, B.; Callahan, M. P.; Grace, L.; De Vries, M. S. Int. J. Quantum Chem. 2005, 105, 437. (24) Florio, G. M.; Christie, R. A.; Jordan, K. D.; Zwier, T. S. J. Am. Chem. Soc. 2002, 124, 10236. (25) Hobza, P.; Sponer, J.; Reschel, T. J. Comput. Chem. 1995, 16, 1315. (26) Jurecka, P.; Cerny, J.; Hobza, P.; Salahub, D. R. J. Comput. Chem. 2007, 28, 555. (27) Grimme, S. Angew. Chem., Int. Ed. 2008, 47, 3430. (28) Grimme, S. J. Chem. Phys. 2003, 118, 9095. (29) Piuzzi, F.; Dimicoli, I.; Mons, M.; Tardivel, B.; Zhao, Q. Chem. Phys. Lett. 2000, 320, 282. (30) Guillaume, C.; Le Calve´, J.; Dimicoli, I.; Mons, M. Z. Phys. D: At., Mol. Clusters 1994, 32, 157. (31) Page, R. H.; Shen, Y. R.; Lee, Y. T. J. Chem. Phys. 1988, 88, 4621. (32) Wiley, W. C.; McLaren, I. H. ReV. Sci. Instrum. 1955, 26, 1150. (33) Piuzzi, F.; Dimicoli, I.; Mons, M.; Millié, P.; Brenner, V.; Zhao, Q.; Soep, B.; Tramer, A. Chem. Phys. 2002, 275, 123. (34) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett. 1989, 162, 165. (35) Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. ReV. Lett. 2003, 91, 146401. (36) Valdes, H.; Spiwok, V.; Rezac, J.; Reha, D.; Abo-Riziq, A. G.; de Vries, M. S.; Hobza, P. Chem.sEur. J. 2008, 14, 4886. (37) Kabelac, M.; Valdes, H.; Sherer, E. C.; Cramer, C. J.; Hobza, P. Phys. Chem. Chem. Phys. 2007, 9, 5000. (38) Cerny, J.; Jurecka, P.; Hobza, P.; Valdes, H. J. Phys. Chem. A 2007, 111, 1146. (39) Grimme, S. J. Comput. Chem. 2004, 25, 1463. (40) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (41) Bouteiller, Y.; Poully, J. C.; Desfranc¸ois, C.; Gre´goire, G. J. Phys. Chem. A 2009, 113, 6301. (42) Grimme, S. J. Chem. Phys. 2006, 124, 034108. (43) Weigend, F. Phys. Chem. Chem. Phys. 2002, 4, 4285. (44) Gonzalez, C.; Lim, E. C. J. Phys. Chem. A 2000, 104, 2953. (45) Albrecht, M.; Rice, C. A.; Suhm, M. A. J. Phys. Chem. A 2008, 112, 7530. (46) Richardson, J. S. AdV. Protein Chem. 1981, 34, 167. (47) Chipot, C.; Jaffe, R.; Maigret, B.; Pearlman, D. A.; Kollman, P. A. J. Am. Chem. Soc. 1996, 118, 11217. (48) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M. J. Chem. Phys. 2005, 122, 144323.

2982

J. Phys. Chem. A, Vol. 114, No. 9, 2010

(49) Rogers, D. M.; Hirst, J. D.; Lee, E. P. F.; Wright, T. G. Chem. Phys. Lett. 2006, 427, 410. (50) Law, K. S.; Schauer, M.; Bernstein, E. R. J. Chem. Phys. 1984, 81, 4871. (51) Ishikawa, S.; Ebata, T.; Ishikawa, H.; Inoue, T.; Mikami, N. J. Phys. Chem. 1996, 100, 10531.

Gloaguen et al. (52) Nachtigallova, D.; Hobza, P.; Spirko, V. J. Phys. Chem. A 2008, 112, 1854. (53) Bouteiller, Y.; Gillet, J. C.; Gre´goire, G.; Schermann, J. P. J. Phys. Chem. A 2008, 112, 11656.

JP904216F