Photoionization-Induced Water Migration in the Amide Group of

Photoionization-Induced Water Migration in the Amide Group of...
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Photoionization-Induced Water Migration in the Amide Group of trans-Acetanilide-(H2O)1 in the Gas Phase Kenji Sakota, Satoshi Harada, Yuiga Shimazaki, and Hiroshi Sekiya* Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

bS Supporting Information ABSTRACT: IR-dip spectra of trans-acetanilide-water 1:1 cluster, AA-(H2O)1, have been measured for the S0 and D0 state in the gas phase. Two structural isomers, where a water molecule binds to the NH group or the CO group of AA, AA(NH)-(H2O)1 and AA(CO)-(H2O)1, are identified in the S0 state. One-color resonance-enhanced twophoton ionization, (1 þ 1) RE2PI, of AA(NH)-(H2O)1 via the S1-S0 origin generates [AA(NH)-(H2O)1]þ in the D0 state, however, photoionization of [AA(CO)-(H2O)1] does not produce [AA(CO)-(H2O)1]þ, leading to [AA(NH)-(H2O)1]þ. This observation explicitly indicates that the water molecule in [AA-(H2O)1]þ migrates from the CO group to the NH group in the D0 state. The reorganization of the charge distribution from the neutral to the D0 state of AA induces the repulsive force between the water molecule and the CO group of AAþ, which is the trigger of the water migration in [AA-(H2O)1]þ.

been reported by some groups.10-12 For example, Clarkson et al. reported that a water molecule migrates between the NH group and the CO group in trans-formanilide-water 1:1 cluster in the S0 state by using the stimulated emission pumping population transfer spectroscopy.10 In the cationic state, Gerhards et al. observed the water migration from the OH group to the NH2 group in [4-aminophenol-(H2O)1]þ by MATI (mass analyzed threshold ionization) and IR/UV double resonance spectroscopy.11 Kim et al. also observed a water migration from the CO group to the NH group in the biologically relevant [phenylglycine-(H2O)1]þ by ingenious analyses of the resonanceenhanced multiphoton ionization (REMPI) spectra measured by detecting the photofragment species.12 The water migrations observed in the cationic states of [4-aminophenol-(H2O)1]þ and [phenylglycine-(H2O)1]þ were initiated by photoionization. In addition, Ishiuchi et al. reported the ionization-induced Ar migration from the hydrophobic to the hydrophilic site in [phenol(Ar)2]þ by using the picosecond time-resolved IR spectroscopy.13 These studies provide information on the characteristic features of the rearrangement of the H-bond networks in the molecular clusters. However, to our best of knowledge, no spectroscopic study has been carried out on the water migration in the H-bonded clusters involving the amide group, although Tachikawa et al. theoretically investigated a water migration in the formanilide-water 1:1 cluster cation.14 The fluctuation and the rearrangement of H-bonded networks of hydration water on the surface of protein play an important role in the fluctuation of

1. INTRODUCTION Hydrogen-bonding (H-bonding) interactions play an important role in macromolecular systems such as proteins, which determine their stable structures.1 The rigidity and directionality of H-bonds are important factors to retain their structures properly. For example, secondary structures of proteins such as R-helix and β-sheet are stabilized by H-bonds formed between the peptide groups.2 In addition to the rigidity, moderate softness of H-bonds is another important feature to show the flexibility of biomolecular systems. For example, the concept of “induced-fit” in molecular recognition between an enzyme and a substrate is based on the flexibility of structures at the active site of the enzyme.3 Biomolecules usually show their functions in a water environment, thus, the local solvation structures and their fluctuations of H-bonds containing water molecules play a key role in determining their functions.4 H-bonded clusters in the gas-phase can be regarded as simple model systems of complicated H-bonded networks in the condensed phase. Sophisticated laser spectroscopy combined with supersonic expansion has been applied to the gas-phase H-bonded clusters to investigate the characteristic features of the H-bond at the molecular scale. In particular, IR spectroscopy at the 3 μm region combined with theoretical calculations is one of the most powerful techniques to determine structures of gas-phase H-bonded clusters because red-shifts of H-bonded groups such as OH and NH groups in the 3 μm region are sensitive to H-bonded structures. Thus, a lot of reports about structures of H-bonded clusters and the static characters of the H-bond have been published so far.5-9 On the other hand, the experimental studies on the fluctuation or rearrangement of H-bonded networks in the gas phase have r 2011 American Chemical Society

Received: December 10, 2010 Revised: December 21, 2010 Published: January 14, 2011 626

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the protein and its function.4 Amide groups constitute the backborn of the protein. The observation of the water migration in model clusters, where water is bonded to the amino group, provides insights into the mechanism of the rearrangement of H-bonded networks in biological systems. In this article, we report the IR-dip spectra of trans-acetanilidewater 1:1 cluster, AA-(H2O)1, in the S0 state and the cationic ground state (D0) in the gas phase. AA has an amide group which is one of the most important groups in proteins, because the amide groups consist of the backborn of proteins. The vibronic structure and the geometry in the D0 state of [AA-(H2O)1]þ were investigated by ZEKE spectroscopy,15 however, the dynamics of this cluster has not been reported. We report first observation of a migration of a water molecule from the CO group to the NH group in the amide group of [AA-(H2O)1]þ in the D0 state by using IR-dip spectroscopy. We will discuss the mechanism of the migration.

Figure 1. (a) (1 þ 1) RE2PI spectrum of AA. The S1-S0 origin band is observed at 35902 cm-1. The low frequency vibronic transitions in the vicinity of the origin band of AA were assigned to the internal rotations of the methyl group and the side-chain in-plane bending vibration. (b) (1 þ 1) RE2PI spectrum of AA-(H2O)1. The S1-S0 origin bands are observed at 35697 and 36050 cm-1 for AA(NH)-(H2O)1 and AA(CO)-(H2O)1, respectively. The wavenumbers in parentheses show the relative shifts of each origin band of AA-(H2O)1 from that of AA.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental setup was described in detail elsewhere.16 Briefly, AA was introduced in a stainless tube. The sample was heated to 373 K by a coiled heater, and the vaporized molecule was expanded into a vacuum chamber with Ne as a carrier gas, which passed through a reservoir containing water. The resonanceenhanced two-photon ionization (RE2PI) and IR-dip spectra were measured with a differentially pumped linear time-of-flight mass spectrometer. For the RE2PI experiment, a frequencydoubled dye-laser (Sirah Cobra stretch) pumped by a second harmonic of the Nd3þ:YAG laser (Spectra Physics INDI) was used as the UV source. For the IR-dip experiments, an optical parametric converter (LaserVision) pumped by an injectionseeded Nd3þ:YAG laser (Continuum Powerlite Precision II 8000) was used as the IR source. For the measurement of the IR-dip spectrum in the D0 state, the cluster ion was produced by the RE2PI process, followed by IR irradiation. When the IR photon energy is resonant with the vibrational transition of the cluster cation, the fragmentation occurs, which causes the reduction of the parent mass signal. Thus, we obtained the IR spectrum in D0 as the depletion of the parent mass signal. M06-2X/6-31þþG** calculations were performed to obtain the stable structures, the binding energies, the harmonic vibrational frequencies, and the IR intensities of AA-(H2O)1 and AA-(H2O)1þ. The calculated harmonic vibrational frequencies were scaled by 0.946. The natural bond orbital (NBO) charges of AAþ were calculated with the M06-2X/6-31þþG** level of theory. All calculations were performed by a GAUSSIAN 09 program package.17 The computations were carried out using the computer facilities at Research Institute for Information Technology, Kyushu University.

Figure 2. IR-dip spectra of (a) AA, (b) AA(NH)-(H2O)1, and (c) AA(CO)-(H2O)1 in the S0 states. The stick spectra of (b) and (c) correspond to the theoretical IR spectra of AA(NH)-(H2O)1 and AA(CO)-(H2O)1, respectively, obtained by the M06-2X/6-31þþG** level of theory.

3. RESULTS AND DISCUSSION Figure 1a shows the (1 þ 1) RE2PI spectrum of AA. In the previous study, the vibronic band at 35902 cm-1 was assigned to the S1-S0 origin band of AA.18-20 The weak vibronic bands observed in the vicinity of the origin band of AA were attributed to the internal rotations of the methyl group and the side chain in-plane bending vibration based on the ZEKE spectroscopy.19 Figure 1b shows the (1 þ 1) RE2PI spectrum of AA-(H2O)1. In the previous study, the vibronic band at 35697 cm-1 was assigned to the origin band of AA-(H2O)1 in which a water molecule binds to the NH group [AA(NH)-(H2O)1].15

In addition, the strong vibronic band was observed at 36050 cm-1. In the (1 þ 10 ) RE2PI spectrum reported previously, the weak vibronic bands were observed around 36050 cm-1, which were attributed to those of higher clusters.15 However, we assigned the vibronic band at 36050 cm-1 to the S1-S0 origin band of AA-(H2O)1 in which a water molecule binds to the CO group based on the IR-dip spectroscopy (see Figure 2c). Figure 2a shows the IR-dip spectrum of AA in the S0 state obtained by probing the origin band at 35902 cm-1, which is 627

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Figure 3. Calculated stable structures of (a) AA(NH)-(H2O)1 and (b) AA(CO)-(H2O)1 in the S0 states, and (c) [AA(NH)-(H2O)1]þ in the D0 state, respectively, obtained by the M06-2X/6-31þþG** level of theory. The zero-point energy and basis set superposition error corrected binding energies of each cluster are indicated. The H-bond distances are indicated in the units of Å.

essentially the same as that reported previously.20 The strong vibrational band was observed at 3472 cm-1. We can assign this vibrational band to the NH stretching vibration in the amide group, because the NH stretching vibration of AA is the only candidate for the strong infrared transition in this wavenumber region. The weak vibrational transitions observed in the region from ∼3040 to ∼3100 cm-1 are assigned to the CH stretching vibrations of the methyl group. Figure 2b shows the IR-dip spectrum of AA(NH)-(H2O)1 in the S0 state obtained by probing the origin band at 35697 cm-1. The stick spectrum in Figure 2b shows the theoretical IR spectrum of AA(NH)-(H2O)1 calculated at the M06-2X/631þþG** level of theory. The stable structure of AA(NH)-(H2O)1 obtained by M06-2X/6-31þþG** is shown in Figure 3a. The theoretical IR spectrum well reproduces the experimental one. Thus the observed species in Figure 2(b) corresponds to the AA(NH)-(H2O)1 isomer, which is consistent with the previous assignment based on the ZEKE spectroscopy.15 By comparing the theoretical IR spectrum with the experimental one, the vibrational transition observed at 3417, 3644, and 3743 cm-1 are assigned to the H-bonded NH stretching, ν1 and ν3 vibration of the water molecule, respectively. The H-bonded NH stretching vibration of AA(NH)-(H2O)1 is redshifted by 55 cm-1 from the free NH stretching vibration of AA. Figure 2c shows the IR-dip spectra obtained by probing the vibronic band at 36050 cm-1 together with the theoretical IR spectrum of the structural isomer shown in Figure 3b. The IR-dip spectrum is well reproduced by the theoretical one. Thus, the observed species in Figure 2c corresponds to the structural isomer in which a water molecule binds to the CO group, AA(CO)-(H2O)1, shown in Figure 3b, although the band at 36050 cm-1 was assigned to that of the higher cluster in the previous study.15 By comparing the theoretical IR spectrum with the experimental one, the vibrational bands observed at 3470, 3492, and 3718 cm-1 are assigned to the free NH stretching, H-bonded OH stretching, and free OH stretching vibration, respectively. The H-bonded OH stretching vibration of AA(CO)-(H2O)1 is red-shifted by 165 cm-1 from the ν1 vibration of the water molecule. Thus, the red-shift of the H-bonded OH stretching vibration of AA(CO)-(H2O)1 is greater than that of the H-bonded NH stretching vibration of AA(NH)-(H2O)1, which suggests that the H-bond strength of AA(CO)-(H2O)1 is

Figure 4. IR-dip spectra of (a) [AA-(Ar)2]þ, (b) [AA-(H2O)1]þ, produced via the S1-S 0 origin band of AA(NH)-(H 2O)1 , and (c) [AA-(H2 O)1]þ, produced via the S1-S 0 origin band of AA(CO)-(H2O)1, respectively. The stick spectrum shown in the bottom of the figure is the theoretical IR spectrum of [AA(NH)-(H2O)1]þ.

larger than that of AA(NH)-(H2O)1 in the S0 state. We compared the zero-point energies and the basis set superposition error corrected binding energies and the H-bond distances between two isomers shown in Figure 3a,b. The theoretical calculations indicate that the H-bond strength of AA(CO)(H2O)1 is larger than that of AA(NH)-(H2O)1, which is consistent with the experimental results. Figure 4a shows the IR-dip spectrum of [AA-(Ar)2]þ in the D0 state. The vibrational band at 3368 cm-1 can be assigned to the NH stretching vibration. In [AA-(Ar)1]þ, the small splitting of the NH stretching vibration was observed in the IR-dip spectrum (see Figure S1 in Supporting Information). We cannot well explain the reason for this splitting at present stage. In [phenol-(Ar)2]þ, the red-shifted OH stretching vibration was observed when Ar migrates from the aromatic π ring to the OH group.13 In [AA-(Ar)1]þ, however, the splitting of the NH stretching vibration is too small to assign the structural isomers where Ar binds to the NH group or the aromatic π ring. Figure 4b shows the IR-dip spectrum of [AA-(H2O)1]þ produced by the (1 þ 1) RE2PI process via the S1-S0 origin band of AA(NH)(H2O)1. The theoretical IR spectrum of [AA(NH)-(H2O)1]þ in which the water molecule binds to the NH group is shown in the bottom of Figure 4. The calculated stable structure of [AA(NH)-(H2O)1]þ is shown in Figure 3c. The theoretical IR spectrum well reproduces the experimental one shown in Figure 4b. Thus, we concluded that the observed species in Figure 4b corresponds to the structural isomer of [AA(NH)-(H2O)1]þ. As compared with the theoretical calculation, the vibrational bands observed at 3171 and 3212 cm -1 can be assigned to the H-bonded NH stretching vibration. The splitting of the H-bonded NH stretching vibration might be due to the Fermi resonance, but an unambiguous explanation of the observed splitting is not possible at the present stage. The vibrational 628

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Scheme 1. Water Migration from the CO Group to the NH Group of [AA-(H2O)1]þ in D0

Figure 5. NBO charges of AAþ calculated at the M06-2X/ 6-31þþG** level of theory.

transitions at 3622 and 3706 cm-1 can be assigned to the ν1 and ν3 vibrations of the water moiety, respectively. Figure 4c shows the IR-dip spectrum of [AA-(H2O)1]þ produced by the (1 þ 1) RE2PI process via the S1-S0 origin band of AA(CO)-(H2O)1. In Figure 4c, the vibrational transitions are observed at the same wavenumbers as compared with those in Figure 4b, although bandwidths of vibrational transitions at 3171 and 3212 cm-1 are somewhat broader than those in Figure 4b. Thus, the observed species in Figure 4c also corresponds to the structural isomer of [AA(NH)-(H2O)1]þ. When AA(CO)-(H2O)1 is ionized via the 0-0 band of AA(CO)(H2O)1, the water molecule should exist in the vicinity of the CO group, because the vertical transition occurs in the photoionization process. Thus, we expect that [AA(CO)-(H2O)1]þ, where the water molecule binds to the CO group, is observed when AA(CO)-(H2O)1 is ionized via the S1-S0 origin band of AA(CO)-(H2O)1. However, [AA(NH)-(H2O)1]þ is observed in Figure 4c. Thus, this observation explicitly indicates that the water molecule migrates from the CO group to the NH group in D0 (Scheme 1). If [AA(CO)-(H2O)1]þ coexists with [AA(NH)-(H2O)1]þ in D0, the sharp vibrational transition must be observed around 3368 cm-1 in Figure 4c, because [AA(CO)-(H2O)1]þ has a free NH stretching vibration which may be observed at the similar wavenumber of the NH stretching vibration in Figure 4a. In Figure 4c, however, there is no vibrational transition around 3368 cm-1. This indicates that the water migration of [AA-(H2O)1]þ observed in this study proceeds almost completely in D0. It should be noted that no stable structures corresponding to [AA(CO)-(H2O)1]þ have been obtained at the M06-2X/6-31þþG** level of theory. The adiabatic ionization energy of [AA-(H2O)1]þ via the S1-S0 origin band of AA(CO)-(H2O)1 has not been determined at the present stage, thus, the accurate excess internal energy of [AA-(H2O)1]þ in Figure 4c cannot be estimated. In ref. 15 however, the weak S1-S0 origin band of AA(CO)-(H2O)1 was observed at 36050 cm-1 by the (1 þ 10 ) RE2PI spectrum, where the wavenumber of the ionization laser was fixed at 28820 cm-1. Thus, the water migration of [AA-(H2O)1]þ may be decelerated by suppressing the internal energy of [AA-(H2O)1]þ by the (1 þ 10 ) RE2PI process. However, [AA(CO)-(H2O)1]þ might not be observed by suppressing the internal energy, because the internal energy distribution of [AA-(H2O)1]þ in D0 is governed by the Franck-Condon overlaps between the S1 state and the D0 state. Thus [AA(CO)-(H2O)1]þ, which is not produced by the (1 þ 1)

RE2PI process, might not be produced even by the (1 þ 10 ) RE2PI process. In [AA(CO)-(H2O)1]þ, the water molecule and the CO group of AAþ may act as the H-bond donor and acceptor, respectively, therefore, the H atom of the water molecule points to the CO group of AAþ. In the D0 state, however, the stable H-bond between the water molecule and the CO group of AAþ cannot be produced because there is the repulsive force between the partially positive-charged H atom of the water molecule and the positive-charged AAþ. The NBO charges of AAþ are shown in Figure 5. Although the O atom of the CO group in AAþ has the partially negative charge (-0.506), AAþ has the þ1 charge as a whole. Thus, the positive-charged AAþ repels the partially positive-charged H atom of the water molecule. Therefore, the repulsion of the positive charges between AAþ and the partially positive-charged H atom of the water molecule is regarded as the trigger of the water migration from the CO group to the NH group in the D0 state of [AA-(H2O)1]þ. We tried to calculate the intrinsic reaction coordinate of the water migration of [AA-(H2O)1]þ to obtain information of the reaction pathway. But this method could not be applied, probably because the potential energy surface may be quite flat. Tachikawa et al. reported the molecular dynamics simulations of [formanilide(H2O)1]þ after photoionization.14 Based on their calculations, the water molecule of [formanilide-(H2O)1]þ migrates from the CO group to the NH group within the plane of the formanilide cation. The water migration of [AA-(H2O)1]þ observed in this study may show the similar reaction pathway to that of [formanilide-(H2O)1]þ. In the S0 state, the water molecule and the CO group of AA form the stable H-bond which corresponds to AA(CO)-(H2O)1 observed in Figure 2c. Thus, the CO group of AA acts as the hydrophilic site in the S0 state. In the D0 state, however, the CO group of AAþ and the water molecule do not form the stable H-bond. In contrast, the CO group of AAþ acts as the hydrophobic site in the D0 state. Therefore, this study has demonstrated that the H-bond ability of the CO group in the amide group changes drastically from hydrophilicity to hydrophobicity by changing the charge distribution. The water migration in two H-bonding sites has been reported by some groups.10-12 Kim et al. observed a water migration of the CO group to the NH group in [phenylglycine-(H2O)1]þ by the REMPI spectra obtained by detecting the photofragment species.12 The phenomenon reported by Kim et al. is similar to 629

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The Journal of Physical Chemistry A the observation in this study. However, their observation of the water migration is rather indirect. It is worth noting that we have obtained the direct experimental evidence for the occurrence of the water migration by measuring the IR-dip spectra. Gerhards et al. also observed a water migration of the OH group to the amino group in [4-aminophenol-(H2O)1]þ by using IR/UV double resonance spectroscopy.11 Thus, they showed the direct experimental evidence of the water migration, which is similar to our results. We must emphasize that the water migration observed in this study is quite relevant to the biological system as compared with the study by Gerhards et al., because the backborn of protein consists of the amide groups; thus, the hydration dynamics of the amide group is the important issue in the biological systems. In biomolecular systems, such as proteins, H-bonds of CO groups in peptide groups play important roles in determining stable structures of proteins, because H-bonds of peptide groups control the stability of a secondary structure of protein. Thus, the drastic change of H-bond ability of the CO groups in the amide group reported in this study provides insights into the structural stability of real biomolecular systems. In the real biomolecular system, the charge redistribution induced by the oxidation or reduction of the biomolecule might change the H-bond ability of the CO group in the peptide groups, which may induce the drastic changes of the biomolecular structures. In addition, the water migration in the amide group observed in this study can be regarded as the rearrangement of the hydration structure of the amide group induced by the charge redistribution. The drastic change of the H-bond ability of the CO groups and the water migration in the amide group demonstrated in this study may shed new light on the detailed understanding of the dynamics of H-bonded networks and hydration structures of biomolecules.

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Functional Systems-Development of Advanced Methods for Exploring elementary Processes” (19056005) from the Japanese Ministry of Education, Sports, Science and Technology (MEXT).

’ REFERENCES (1) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological Structures; Springer-Verlag: New York, 1991. (2) Lodish, H.; Berk, A.; Matsudaira, P.; Kaiser, C. A.; Krieger, M.; Scott, M. P.; Zipurski, L.; Darnell, J. W. H. Mol. Cell. Biol.; Freeman & Co: New York, 2004. (3) Koshland, D. E. Proc. Natl. Acad. Sci. U.S.A. 1958, 44 (2), 98. (4) Nakagawa, H.; Kataoka, M. J. Phys. Soc. Jpn. 2010, 79, 08380. (5) Zwier, T. S. Annu. Rev. Phys. Chem. 1997, 47, 205. (6) Ebata, T.; Fujii, A.; Mikami, N. Int. Rev. Phys. Chem. 1998, 17, 331. (7) Brutschy, B. Chem. Rev. 2000, 100, 3999. (8) Sekiya, H.; Sakota, K. J. Photochem. Photobiol. C 2008, 9, 81. (9) Matsuda, Y.; Mikami, N.; Fujii, A. Phys. Chem. Chem. Phys. 2009, 11, 1279. (10) Clarkson, J. R.; Baquero, E.; Shubert, V. A.; Myshakin, E. M.; Jordan, K. D.; Zwier, T. S. Science 2005, 307, 1443. (11) Gerhards, M.; Jansen, A.; Unterberg, C.; Gerlach, A. J. Chem. Phys. 2005, 123, 074320. (12) Kim, H. M.; Han, K. Y.; Park, J.; Kim, G. S.; Kim, S. K. J. Chem. Phys. 2008, 128, 041104. (13) Ishiuchi, S.; Sakai, M.; Tsuchida, Y.; Takeda, A.; Kawashima, W.; Dopfer, O.; Muller-Dethlefs, K. Angew. Chem., Int. Ed. 2005, 44, 6149. (14) Tachikawa, H.; Igarashi, M.; Ishibashi, T. J. Phys. Chem. A 2003, 107, 7505. (15) Ullrich, S.; Muller-Dethlefs, K. J. Phys. Chem. A 2002, 106, 9188. (16) Kageura, Y.; Sakota, K.; Sekiya, H. J. Phys. Chem. A 2009, 113, 6880. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, € Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; S.; Daniels, A. D.; Farkas, O.; Fox, D. J. Gaussian 09, revision A.1, Gaussian, Inc.: Wallingford, CT, 2009. (18) Manea, V. P.; Wilson, K. J.; Cable, J. R. J. Am. Chem. Soc. 1997, 119, 2033. (19) Ullrich, S.; Muller-Dethlefs, K. J. Phys. Chem. A 2002, 106, 9181. (20) Miyazaki, M.; Saikawa, J.; Ishizaki, H.; Taira, T.; Fujii, M. Phys. Chem. Chem. Phys. 2009, 11, 6098.

4. CONCLUSIONS We have first to observe the water migration in the D0 state of [AA-(H2O)1]þ by using (1 þ 1) RE2PI and IR-dip spectroscopy. The experimental results explicitly indicate that the water molecule of [AA-(H2O)1]þ migrates from the CO group to the NH group. The water migration observed in this study is caused by the repulsion of charges between the positive-charged AAþ and the partially positive-charged H atom of the water molecule. The unstable H-bond between the CO group of AAþ and the water molecule indicates that H-bond nature of the CO group in the amide group changes from hydrophilicity to hydrophobicity in the S0 and the D0 state. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1 of IR-dip spectrum of [AA-(Ar)1]þ. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail:[email protected].

’ ACKNOWLEDGMENT This work was partly supported by the Grants-in Aid for Scientific Research B (20350011), the Grant-in-Aid for Scientific Research in Priority Area (477) “Molecular Science for Supra 630

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