Article pubs.acs.org/JPCA
Photoionization-Induced Water Migration in the Hydrated transFormanilide Cluster Cation Revealed by Gas-Phase Spectroscopy and Ab Initio Molecular Dynamics Simulation Takamasa Ikeda, Kenji Sakota, Yukio Kawashima, Yuiga Shimazaki, and Hiroshi Sekiya* Department of Chemistry, Faculty of Sciences, and Department of Molecular Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ABSTRACT: Photoionization-induced water migration in the trans-formanilide−water 1:1 cluster, FA-(H2O)1, has been investigated by using IR-dip spectroscopy, quantum chemical calculations, and ab initio molecular dynamics simulations. In the S0 state, FA-(H2O)1 has two structural isomers, FA(NH)-(H2O)1 and FA(CO)-(H2O)1, where a water molecule is hydrogen-bonded (H-bonded) to the NH group and the CO group, respectively. In addition, the S1−S0 origin transition of FA(CO)-(H2O)2, where a water dimer is H-bonded to the CO group, was observed only in the [FA-(H2O)1]+ mass channel, indicating that one of the water molecules evaporates completely in the D0 state. These results are consistent with a previous report [Robertson, E. G. Chem. Phys. Lett., 2000, 325, 299]. In the D0 state, however, [FA(H2O)1]+ produced by photoionization via the S1−S0 origin transitions of FA(NH)-(H2O)1 and FA(CO)-(H2O)1 shows essentially the same IR spectra. Compared with the theoretical calculations, [FA-(H2O)1]+ can be assigned to [FA(NH)-(H2O)1]+. This means that the water molecule in [FA-(H2O)1]+ migrates from the CO group to the NH group when [FA(H2O)1]+ is produced by photoionization of FA(CO)-(H2O)1. [FA-(H2O)1]+ produced by photoionization of FA(CO)-(H2O)2 also shows the IR spectrum corresponding to [FA(NH)-(H2O)1]+. In this case, the water migration from the CO group to the NH group occurs with the evaporation of a water molecule. Ab initio molecular dynamics simulations revealed the water migration pathway in [FA-(H2O)1]+. The calculations of classical electrostatic interactions show that charge-dipole interaction between FA+ and H2O induces an initial structural change in [FA-(H2O)1]+. An exchange repulsion between the lone pairs of the CO group and H2O in [FA-(H2O)1]+ also affects the initial direction of the water migration. These two factors play important roles in determining the initial water migration pathway.
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INTRODUCTION In aqueous solution, water molecules surrounding a solute have different hydrogen bond (H-bond) networks from bulk water. The functional expression of biomolecules such as proteins and neurotransmitters arises in water, thus water molecules surrounding biomolecules play important roles in determining the structures and functions of the biomolecules under physiological conditions.1−3 The H-bonding interaction is one of the dominant factors to determine hydration structures. In particular, the first hydration shell has a distinctive role, because water molecules comprising the first hydration shell directly form H-bonds with a biomolecule.4−6 In some cases, the Hbonds formed between water molecules and a biomolecule strongly restrict the structural preference of the biomolecule.7−11 A typical H-bond is, needless to say, a noncovalent interaction, thus its interaction energy is the same order of magnitude as the thermal energy under physiological conditions. Therefore, the H-bonds formed between a biomolecule and waters frequently break and reform, leading to a rearrangement of the hydration structure. Such a dynamic behavior of H-bonds must be taken into account for understanding the structures and functions of biomolecules in their native condition. Unfortunately, however, a molecularlevel inspection is often prevented under physiological © 2012 American Chemical Society
conditions because of a vast amount of water surrounding a biomolecule, hampering close assessments of such dynamic hydration. The combination of supersonic jet expansion with sophisticated laser spectroscopic techniques is one of the ideal means to investigate H-bonding interactions at the molecular level, which is free from disturbance of bulk water. It also enables us to characterize the state-specific dynamics of size-selected H-bonded clusters. There have been a number of studies concerning H-bonded clusters in the gas-phase.12−17 The H-bonded clusters are efficiently cooled with supersonic jet expansion, so that the individual structural isomers of the Hbonded cluster are isolated in each stable minimum on the potential energy surfaces due to the lack of thermal energy, enabling us to obtain detailed information on the H-bonds. Thus, the gas-phase studies have revealed the stable structures of H-bonded clusters and microsolvation effects. On the basis of these studies, we have achieved comprehensive understanding of the H-bond such as structural preferences of Hbonded networks and the microsolvation effect on conformaReceived: February 23, 2012 Revised: March 23, 2012 Published: March 23, 2012 3816
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tional preferences of flexible molecules. However, gas-phase spectroscopic study that sheds light on a dynamic rearrangement of the H-bond in H-bonded clusters is limited so far. For example, Clarkson et al. revealed that a water molecule shuttles at the amide group in the trans-formanilide−water 1:1 cluster in the S0 state.18 In the cationic state, Gerhards et al. reported a water migration event in [4-aminophenol-(H2O)1]+.19 Kim et al. revealed a water migration in the biological relevant molecule, [phenylglycine-(H2O)1]+.20 Recently, our group observed the water migration in [trans-acetanilide(AA)(H2O)1]+.21 However, such limited numbers of studies is yet insufficient for accomplishing extensive understanding of the rearrangement of the H-bond. The evident experimental result with a rigorous theoretical analysis is highly required to obtain the general picture of the H-bond rearrangement. In this article, we report the water migration dynamics in the trans-formanilide−water 1:1 cluster (FA-(H2O)1) cation by using IR spectroscopy, which is an extension of our previous study on the water migration of the trans-acetanilide−water 1:1 cluster.21 FA has an amide group, which is one of the important groups for proteins. The rearrangement of the water networks at the amide groups in a protein is likely a primary step for the folding dynamics. The water migration in [FA-(H2O)1]+ is one of the simple model systems that reveal the hydration dynamics associated with the amide groups. A pioneering theoretical work of the dynamics of the water migration in [FA-(H2O)1]+ was carried out by Tachikawa et al.22 However, the result was not directly compared with any experimental works. In this work, we discuss the pathway and mechanism of the water migration based on the ab initio molecular dynamics (MD) simulations and the calculation of classical electrostatic interactions.
laser lights was inverted as compared with that in the S0 state. In both measurements, the mass-selected ion signals were monitored while the wavelength of the IR laser was scanned. The amount of the ion signal decreases when the wavelength of the IR laser is resonant with the vibrational transitions of the monitored species. Thus the IR spectrum was obtained as the depletion of the ion signal. The repetition rate of the UV laser was 20 Hz, whereas that of the IR laser was 10 Hz. The ion signals with and without the IR pulse were stored separately so as to correct the artificial fluctuation of the spectral baseline. The time difference of ∼20 ns between the UV and IR lasers is enough to avoid the inappropriate overlap of the UV and IR laser pulses.
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COMPUTATIONAL METHODS
M06-2X24/6-311++G(2d,2p)25−27 calculations were performed to obtain stable structures, stabilization energies, harmonic vibrational frequencies, and IR intensities of FA-(H2O)1, FA(H2O)2, and [FA-(H2O)1]+. The calculated harmonic vibrational frequencies in the S0 and D0 states were scaled by 0.939 and 0.942, respectively. Twelve constant energy (NVE) ab initio MD simulations based on ωB97XD/6-31+G(d,p)27−29 force calculation were executed on the D0 state for [FA-(H2O)1]+ to investigate the dynamics of the water migration after ionization from the S1 state of FA(CO)-(H2O)1. The initial velocity and displacement of each atom are obtained by randomly sampling the initial normal mode coordinates and velocities of all normal modes, so that the harmonic oscillator energy is equal to the zero-point vibrational energy for each normal mode obtained from harmonic vibrational frequency analysis of the S1 state of FA(H2O)1. The harmonic vibrational frequency analysis was computed using time-dependent density functional theory (DFT)30−35 based on ωB97XD/6-31+G(d,p) calculation. Each simulation consisted of 10 000 steps with a 0.5 fs time step. The Leapfrog Verlet algorithm was used to integrate the equation of motion. We chose hybrid DFT functionals so that the HF exchange is included to treat the intermolecular interaction. For MD simulation, we chose ωB97XD to include long-range correction based on range separation and additional dispersion to treat the weak intermolecular interaction in the dynamics. The calculation results using these functionals and basis sets agreed well with our experimental data. The UTCHEM software package36 was used to carry out the MD simulation, and all quantum mechanical calculations were performed by the GAUSSIAN 09 program package.37
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EXPERIMENTAL METHODS The experimental setup used in this study has been described in detail elsewhere.23 FA was purchased commercially (Wako) and used without further purification. The sample introduced in a stainless steel tube was heated to 363 K by a coiled heater. Ne was used as a carrier gas, which passed through a reservoir containing water. The water reservoir was cooled down to 268 K. A typical stagnation pressure was 2 atm. A mixture of sample, water, and Ne was expanded into a vacuum chamber by using a commercially available pulsed valve (General Valve, series 9, 500 μm as an orifice diameter) at 20 Hz. The pulsed supersonic expansion was skimmed into an ion source chamber. An UV laser was irradiated into the skimmed molecular beam, then ions were analyzed with a linear time-of-flight mass spectrometer. For the one-color resonance-enhanced two photon ionization ((1 + 1) R2PI) experiment, a frequency doubled dye-laser (Sirah Cobra stretch) pumped by the second harmonic of a Nd3+:YAG laser (Spectra Physics INDI, 20 Hz) was used as a UV source. Pyrromethene 580 was used as a laser dye. The mass-selected ion signals were measured as a function of the UV wavelength in order to obtain the (1 + 1) R2PI spectrum. For IR-dip experiments, an optical parametric converter (LaserVision) pumped by an injection seeded Nd3+:YAG laser (Continuum Powerlite Precision II 8000, 10 Hz) was used as an IR source. The IR and UV lights were focused by cylindrical lenses (300 mm focal length), and then spatially overlapped with each other. For IR-dip spectroscopy in the S0 state, IR irradiation preceded UV irradiation by ∼20 ns. For the measurements in the D0 state, the irradiation sequence of two
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RESULTS AND DISCUSSIONS (1+1) R2PI spectra and IR-dip spectra of FA-(H2O)1,2 are presented in Figures 1, 2, and 4. The spectral features and their assignments are essentially the same as those reported previously.38−40 We briefly explain the essential features of these spectra below, because structural information on FA(H2O)1,2 in the S0 state is important for understanding of the water migration dynamics in [FA-(H2O)1]+. (1+1) R2PI spectra monitoring the FA+, [FA-(H2O)1]+ and [FA-(H2O)2]+ mass channels are shown in Figure 1. The S1−S0 origin band of FA is observed at 36005 cm−1 in Figure 1a. In Figure 1b, the vibronic bands are observed at 35787 and 36118 cm−1, which were assigned to the S1−S0 origin bands of FA(NH)-(H2 O) 1 and FA(CO)-(H2 O) 1, where a water 3817
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Figure 1. (a) (1+1) R2PI spectrum of FA. The S1−S0 origin band is observed at 36005 cm−1. (b) (1+1) R2PI spectrum obtained by monitoring the [FA-(H2O)1]+ mass channel. The S1−S0 origin bands of FA(NH)-(H2O)1 and FA(CO)-(H2O)1 are observed at 35787 and 36118 cm−1, respectively. The S1−S0 origin band of FA(CO)-(H2O)2 is also observed at 36101 cm−1 in the mass channel of [FA-(H2O)1]+. (c) (1 + 1) R2PI spectrum obtained by monitoring the [FA-(H2O)2]+ mass channel. The S1−S0 origin band of FA(NH)-(H2O)1 is observed at 35521 cm−1. The band corresponding to the origin transition of FA(CO)-(H2O)2 is not observed in this mass channel due to a complete evaporation of one water in the D0 state. Figure 2. IR-dip spectra of (a) FA(NH)-(H2O)1 and (b) FA(CO)(H2O)1 in the S0 state. The theoretical IR spectra are also shown as the stick spectra. The calculations were carried out at the M06-2X/6-311+ +G(2d,2p) level of theory. Harmonic frequencies are scaled by 0.939.
molecule is H-bonded to the NH and CO groups, respectively.38−40 In addition, the vibronic band at 36101 cm−1 in Figure 1b was assigned to the origin band of FA(CO)(H2O)2 where a water dimer is H-bonded to the CO group of FA. In Figure 1c, a vibronic band observed at 35521 cm−1 was assigned to the S1−S0 origin band of FA(NH)-(H2O)2, where a water dimer binds to the NH group of FA. It should be noted that the origin band of FA(CO)-(H2O)2 (36101 cm−1) is not observed in Figure 1c. This indicates that one of the water molecules evaporates completely from FA(CO)-(H2O)2 after (1+1) photoionization. IR-dip spectra of FA(NH)-(H2O)1 and FA(CO)-(H2O)1 in the S0 states in the 3000−3800 cm−1 region are shown in Figure 2a,b. The lower parts in Figure 2a,b are the theoretical IR spectra of FA(NH)-(H2O)1 and FA(CO)-(H2O)1. The stable structures of FA(NH)-(H2O)1 and FA(CO)-(H2O)1 obtained by DFT calculations are shown in Figure 3a,b. By comparing theoretical IR spectra, the experimental IR spectra in Figure 2a,b were assigned to those of FA(NH)-(H2O)1 and FA(CO)-(H2O)1, respectively. Figure 4a displays the IR-dip spectrum of FA(CO)-(H2O)2 obtained by probing the vibronic band at 36101 cm−1 in the R2PI spectrum (Figure 1b). Figure 4b,c shows theoretical IR spectra of FA(CO)-(H2O)2 and FA(NH)-(H2O)2, respectively. The stable structures obtained by DFT calculations are indicated in Figure 3c,d. The IR-dip spectrum in Figure 4a is well reproduced by the theoretical IR spectrum of FA(CO)(H2O)2, confirming that the vibronic band at 36101 cm−1 can be attributed to that of FA(CO)-(H2O)2. The IR-dip spectrum of [FA-(Ar)1]+ in the D0 state is shown in Figure 5a in order to obtain information on the NH stretching vibration of FA+. The IR spectrum of the bare FA cation cannot be measured by using our experimental scheme, thus we used the Ar-tagging technique. [FA-(Ar)1]+ was produced from the neutral precursors of FA-(Ar)n by
photoionization, thus the fragmentation of Ar atoms occurs in the photoionization process. In the case of [phenol-(Ar)2]+, an Ar atom is attached to the OH group of phenol, so that the OH stretching vibration of [phenol-(Ar)2]+ is red-shifted by ∼200 cm−1 as compared with that of phenol in the S0 state due to the formation of the H-bond between the OH group and the argon atom.41 In the case of [FA-(Ar)1]+, however, a prominent vibrational transition is observed at 3386 cm−1 in Figure 5a, which is red-shifted by 77 cm−1 from the free NH stretching vibration of FA in the S0 state. This small red shift indicates that the vibrational transition at 3386 cm−1 in Figure 5a can be assigned to the free NH stretching vibration of the FA+ moiety in [FA-(Ar)1]+, where the Ar atom is likely attached to the aromatic ring of FA+. The attached Ar atom in the aromatic ring of FA+ hardly affects the NH stretching vibration, therefore the free NH stretching vibration of FA+ might have almost the same vibrational frequency as that of [FA-(Ar)1]+. The IR-dip spectra of [FA-(H2O)1]+ are indicated in Figure 5b,c. The observed vibrational structures in Figure 5b,c are quite similar, although [FA-(H2O)1]+ was produced by the (1+1) R2PI processes via the S1−S0 origin bands of FA(NH)(H2O)1 at 35787 cm−1 and FA(CO)-(H2O)1 at 36118 cm−1, respectively, i.e., [FA-(H2O)1]+ was produced from the different structural isomers in Figure 5b,c. The theoretical IR spectrum of [FA(NH)-(H2O)1]+ is shown in Figure 5e. The stable structure of [FA(NH)-(H2O)1]+ obtained by the DFT calculation is indicated in Figure 3e. In [FA(NH)-(H2O)1]+, a water molecule is H-bonded to the NH group of FA+. The theoretical IR spectrum in Figure 5e well reproduces the IR-dip spectra of [FA-(H2O)1]+ in Figure 5b,c. As compared with Figure 5e, the broad vibrational transitions at ∼3200 cm−1 in 3818
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Figure 4. (a) IR-dip spectrum of FA(CO)-(H2O)2 in the S0 state. The calculated IR spectra of (b) FA(CO)-(H2O)2 and (c) FA(NH)(H2O)2 are shown as the stick spectra. The calculations were carried out at the M06-2X/6-311++G(2d,2p) level of theory. Harmonic frequencies are scaled by 0.939.
Figure 3. Stable structures of (a) FA(NH)-(H2O)1, (b) FA(CO)(H2O)1, (c) FA(CO)-(H2O)2, (d) FA(NH)-(H2O)2, and (e) [FA(NH)-(H2O)1]+, respectively, obtained at the M06-2X/6-311+ +G(2d,2p) level of theory. The basis set superposition error (BSSE) and zero-point vibrational energy corrected relative stabilities are indicated in structures a−d. The H-bond distances are also shown in units of Å.
corresponding to [FA(CO)-(H2O)1]+ by DFT calculations, being consistent with the mechanism of the water migration discussed below. In Figure 5b,c, [FA-(H2O)1]+ is produced by the (1+1) R2PI process. Thus, the internal energy distribution of [FA-(H2O)1]+ is governed by Franck−Condon overlaps between the S1 and D0 states. From the structural similarity between FA(NH)(H2O)1 and [FA(NH)-(H2O)1]+, [FA-(H2O)1]+ produced from FA(NH)-(H2O)1 is expected to have lower internal energy than that produced from FA(CO)-(H2O)1. On close inspection, two vibrational bands at ∼3050 and 2950 cm−1 in Figure 5b turn into a broad vibrational band in Figure 5c. In addition, the bandwidths of the ν1 and ν3 vibrations of water in Figure 5c are slightly broader than those in Figure 5b. These observations suggest that [FA-(H2O)1]+ in Figure 5b has lower internal energy than that in Figure 5c. The IR-dip spectrum of [FA-(H2O)1]+ produced by photoionization via the vibronic transition of FA(CO)(H2O)2 at 36101 cm−1 is shown in Figure 5d. The vibrational structures in Figure 5d are almost the same as those in Figure 5b,c, indicating that [FA-(H2O)1]+ in Figure 5d is also attributed to [FA(NH)-(H2O)1]+, although the neutral precursor is FA(CO)-(H2O)2. As is the case of [FA-(H2O)1]+ in Figure 5c, the water dimer must be found in the vicinity of the CO group of FA+ just after photoionization of FA(CO)(H2O)2. In Figure 5d, however, we observed the IR-dip spectrum of [FA(NH)-(H2O)1]+, indicating that a water molecule migrates from the CO group to the NH group in the D0 state when FA(CO)-(H2O)2 is photoionized. As discussed above, one of the water molecules evaporates completely in the D0 state when FA(CO)-(H2O)2 is ionized. This implies that the water molecule evaporates on the way to
Figure 5b,c are assigned to the H-bonded NH stretching vibration of [FA(NH)-(H2O)1]+. In addition, fairly sharp vibrational transitions at 3630 and 3714 cm−1 are assigned to the ν1 and ν3 vibrations of the water moiety in [FA(NH)(H2O)1]+, respectively. On the basis of these assignments, we conclude that the water molecule is H-bonded to the NH group in Figure 5b,c, i.e., the structure is [FA(NH)-(H2O)1]+. It is plausible that [FA(NH)-(H2O)1]+ is produced by photoionization via the S1−S0 origin band of FA(NH)-(H2O)1, because the vertical transition occurs preferentially in the photoionization process. In the case of FA(CO)-(H2O)1, however, the scenario is totally different from that of FA(NH)(H2O)1. In the S0 state, the water molecule is H-bonded to the CO group in FA(CO)-(H2O)1. When FA(CO)-(H2O)1 is ionized via the S1−S0 origin transition, the water molecule must be found in the vicinity of the CO group in FA+ just after photoionization due to vertical transition. In Figure 5c, however, the IR-dip spectrum of [FA(NH)-(H2O)1]+ is observed although [FA-(H2O)1]+ is produced by photoionization via the S1−S0 transition of FA(CO)-(H2O)1. Therefore, this observation explicitly indicates that the water molecule migrates from the CO group to the NH group in FA+. In Figure 5c, no vibrational transition is observed around 3386 cm−1 that corresponds to the free NH stretching vibration of FA+, indicating that [FA(CO)-(H2O)1]+ is not observed in Figure 5c. Thus, we can presume that the migration of the water observed in [FA-(H2O)1]+ mostly proceeds. It should be noted that we could not obtain the stable structure 3819
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Figure 6. Snapshots of structures of [FA-(H2O)1]+ after photoionization of FA(CO)-(H2O)1 obtained by the ab initio MD simulation. The ωB97XD/6-31+G(d,p) level of theory was used for calculating the forces acting on each atom.
trajectories obtained from our MD simulation. In Figure 6, the water molecule is kicked out toward the NH group from the initial (0 fs) position, and the water molecule migrates within the molecular plane of FA+. The oxygen atom of the water molecule points to FA+ in the migration pathway, while the direction of the water molecule largely rocks. These features are observed in all trajectories. In all trajectories, the migration pathways of [FA-(H2O)1]+ for the first 500 fs, which is the initial approach to the NH group, are very similar. However, the migration pathways are different after the first approaches to the NH group in each trajectory. Figure 7 shows the potential energy profiles of representative trajectories having different behaviors. In Figure 7a−c, the water molecule approaches the NH group at ∼500 fs. In Figure 7a, the water molecule is trapped in the potential well of the NH group after the first approach to the NH group, so that the water molecule fluctuates in the vicinity of the NH group. The potential energy profile after 1 ps, where the water molecule is trapped in the NH group, shows a fairly large fluctuation, since the intermolecular distance and angle between the water molecule and the NH group vary widely. This is because the excess energy released by the water migration is stored in the intermolecular motions between the water molecule and the NH group. It should be noted that the average value of the potential energy after 1 ps is lower than the initial potential energy. Figure 7b shows that the water molecule is trapped in the potential well of the NH group for the first 2.5 ps, and the water molecule approaches the NH group three times with large intermolecular motions in this period. However, the water molecule escapes from the potential well of the NH group after 2.5 ps, and the water molecule rotates around the out of plane of FA+, then the water molecule returns to the NH group at ∼4.2 ps. In Figure 7c, the water molecule passes through the potential well of the NH group after the initial approach, and continues to linger around FA+ on its molecular plane. As the water molecule approaches the CO group, the water molecule is immediately kicked out toward the out of plane of FA+, and the water molecule finally returns to the NH group at ∼5 ps. It should be noted that these fairly complicated pathways in Figure 7b,c were not reported in the previous work.22 Among the 12 trajectories, three trajectories show that the water molecule is trapped in the potential well of the NH group just after the first approach to the NH group. In the other trajectories, the water molecule escapes from the potential well,
Figure 5. (a) IR-dip spectrum of [FA-(Ar)1]+. The free NH stretching vibration is observed at 3386 cm−1. (b) IR-dip spectrum of [FA(H2O)1]+ produced by photoionization via the origin transition of FA(NH)-(H2O)1. (c) IR-dip spectrum of [FA-(H2O)1]+ produced by photoionization via the origin transition of FA(CO)-(H2O)1. (d) IRdip spectrum of [FA-(H2O)1]+ produced by photoionization via the origin transition of FA(CO)-(H2O)2. (e) The theoretical IR spectrum of [FA-(H2O)1]+ calculated at the M06-2X/6-311++G(2d,2p) level of theory. Harmonic frequencies are scaled by 0.942.
the water migration. Thus, the water migration is entangled with the evaporation dynamics after photoionization of FA(CO)-(H2O)2. We carried out ab initio MD simulations to obtain information on the migration dynamics in [FA-(H2O)1]+ produced by photoionization of FA(CO)-(H2O)1. In our experiment, [FA-(H2O)1]+ was produced via the S1−S0 origin transition of FA(CO)-(H2O)1. Thus, just after photoionization, each atom of [FA-(H2O)1]+ has a velocity corresponding to the zero-point level vibration in the S1 state, which must affect the migration dynamics. As mentioned in the Computational Methods section, we mimic this situation by randomly sampling the initial normal mode coordinates and velocities of all normal modes in the S1 state, so that we obtained the 12 trajectories, which have different initial velocities. Tachikawa et al. reported the ab initio MD simulations of [FA-(H2O)1]+.22 In the previous work, however, the MD simulation was carried out without initial velocities, which does not consider the influence of the ionization from the S1 state on the water migration dynamics, and the difference in each trajectory having different initial velocities could not be assessed. Moreover, their force calculation was based on the lower ab initio molecular orbital methodology and much smaller basis sets (HF/3-21G*) than ours (ωB97XD/6-31+G(d,p)). Figure 6 shows snapshots of structures of [FA-(H2O)1]+ after photoionization of FA(CO)-(H2O)1 taken from one of our 3820
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Figure 7. Potential energy profiles of [FA-(H2O)1]+ produced by photoionization of FA(CO)-(H2O)1 obtained in the ab initio MD simulation. Initial conditions are different among a−c: In a, the water molecule is trapped in the potential well of the NH group. In b and c, the water molecule escapes from the potential well of the NH group, then returns to the NH group. (d) The MD run with no initial velocities for all atoms. All potential energy profiles are indicated as values relative to the 0 fs value.
where +ZFA+, μFA(+), and μw are the charge of FA+, the dipole moment of FA+, and that of H2O, respectively. The angles, θ, θ′, and χ are defined in Figure 9. The distance, r, was taken as the distance between their center of mass. The charge and dipole moments, +ZFA(+), μFA(+), and μw, were calculated by using electrostatic potential (ESP) charges in each atom.43 In Figure 8, the potential energy profile (UMD) as a function of the simulation time is divided into three regions: (i) the potential energy immediately decreases at ∼50 fs, (ii) the reduction of the potential energy is gradual between ∼50 fs and ∼400 fs, and (iii) the potential energy is stabilized steeply from ∼400 fs to ∼500 fs. In region (i), the direction of the water molecule changes such that the oxygen atom rotates and points toward FA+ (Figure 6). In Figure 8, the charge−dipole interaction (Ucharge−dipole) stabilizes [FA-(H2O)1]+ in the initial 50 fs. Thus, this initial dynamics is induced so as to make the direction of the dipole moment of the water molecule to be suitable for the charge−dipole interaction, due to the immediate response triggered by the change of the electronic structure from the S1 state to the D0 state. The dipole−dipole interaction (Udipole−dipole) has a quite small contribution to the initial dynamics. In region (ii), the water molecule moves around the CH group in the amide group of FA+, but the electrostatic interactions (Ucharge‑dipole + Udipole−dipole) do not change so much because the distance between the charge center of FA+ and the dipole of the water molecule is fairly large in region (ii). In region (iii), the water molecule approaches the NH group to form the H-bond, so that the distance between the charge center of FA+ and the dipole of the water molecule becomes small. This causes strong electrostatic interactions, which stabilize [FA(NH)-(H2O)1]+. In particular, the for-
but in seven trajectories the water molecule returns to the NH group. These simulations clearly show that the water migration in [FA-(H2O)1]+ is not straightforward; the water migration event cannot be characterized only by the initial approach to the NH group. The simulations also suggest that the NH group is the most preferential H-bonding site along the migration path. We also executed a MD run with no initial velocities for all atoms. The potential energy profile of this trajectory is shown in Figure 7d. This allows us to investigate the potential energy profile along the migration pathway more easily than the other 12 MD runs. In this trajectory, the potential energy is steeply stabilized at ∼500 fs, which corresponds to the first approach to the NH group. The water molecule escapes from the potential well of the NH group, and does not return to the NH group due to the limited simulation time. On the basis of the potential energy profile in Figure 7d, it is obvious that the NH group is the most preferential H-bonding site, which is consistent with the result obtained from the other 12 MD runs with initial velocities. In order to obtain an intuitive picture of the water migration, we calculated the electrostatic charge−dipole and dipole− dipole interactions between FA+ and H2O along the migration coordinate of the MD run with no initial velocities for all atoms. The interactions were treated classically by using the formulas42 Ucharge − dipole =
1 +Z FA(+)μw cos θ 4πε0 r2
Udipole − dipole = −
1 μ FA(+)μw (2 cos θ cos θ′ 4πε0 r3
− sin θ sin θ′ cos χ) 3821
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Figure 8. Potential energy of [FA-(H2O)1]+ produced by photoionization of FA(CO)-(H2O)1. The black curve is the potential energy profile obtained from the ab initio MD simulation with no initial velocities for all atoms (UMD). The circles connected with the green and blue lines indicate the relative potential energies of the charge− dipole and dipole−dipole interactions of FA+ and the water molecule as a function of the simulation time, respectively (Ucharge‑dipole and Udipole−dipole). The circles connected with the red line is the sum of the energies of the electrostatic interactions (Ucharge‑dipole+Udipole−dipole). All potential energy profiles are indicated as values relative to the 0 fs value.
Figure 9. Definitions of angles, θ, θ′, χ, in the charge−dipole and dipole−dipole interactions. The distance between two center of mass (FA+ and H2O) is defined as r.
mation of the H-bond results in larger dipole−dipole interaction in region (iii) as compared to the other two regions, due to the realignment of the water orientation to form a H-bond with the NH group of FA+. It is worth noting that the water molecule is always kicked out toward the NH group in all trajectories, in spite of the randomness of the initial velocities for the 12 MD runs. The water molecule never moves toward the opposite direction in the initial structural change. This remarkable tendency is explained by taking the exchange repulsion as well as the electrostatic interaction into account. As mentioned above, the initial structural change is induced so as to stabilize the charge− dipole interaction, thus, the oxygen atom of the water molecule points to FA+. In this circumstance, the water molecule cannot move to the opposite direction, because the exchange repulsion between the lone pairs of the CO group in FA+ and the water molecule pushes away the water molecule toward the NH group, even if the water molecule has the initial velocity toward the opposite direction. Thus, the exchange repulsion plays an important role in determining the pathway of the water migration. We reported the observation of the water migration in [AA(H2O)1]+ previously.21 The experimental results on [AA(H2O)1]+ are very similar to those on [FA-(H2O)1]+, suggesting that the water migration dynamics is similar between the two molecular systems. However, the effect of the presence of the methyl group on the water migration pathway is unclear. Theoretical investigations may clarify the difference in the dynamics between [FA(H2O)1]+ and [AA-(H2O)1]+. In both cases of [FA(H2O)1]+ and [AA-(H2O)1]+, the CO group of the amide group shows hydrophobicity, i.e., the water molecule does not bind to the CO group in the D0 state, while in the S0 state the CO group of the amide group shows hydrophilicity. In physiological conditions, biomolecules are
fully hydrated, so that the detailed rearrangement dynamics of the H-bond around the amide group in [FA-(H2O)1]+ and [AA-(H2O)1]+ may be different from those in real biomolecular systems. However, the switching between hydrophobicity and hydrophilicity of the CO group, whose mechanism at the molecular level is revealed by our studies for the first time, may regulate the initial step of the rearrangement of the H-bonded networks in real biomolecular phenomena. Thus, molecular level information on the H-bond rearrangement obtained by simple model systems such as [FA(H2O)1]+ and [AA-(H2O)1]+ may shed light on the detailed understanding of the rearrangement dynamics of the H-bond networks in real biomolecular systems.
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CONCLUSION This study explored the migration dynamics of the water molecule in the [FA-(H2O)1]+ cation in the gas-phase by using (1+1) R2PI and IR-dip spectroscopy. [FA-(H2O)1]+ produced by the R2PI process via the vibronic transition of FA(CO)(H2O)1 has a H-bonded structure where a water molecule is Hbonded to the NH group of FA+. This experimental result indicates that the water molecule in [FA-(H2O)1]+ migrates from the CO group to the NH group to form the stable intermolecular H-bond. Thus, the rearrangement of the Hbonded structure in the amide group of FA+ occurs in the D0 state. This means that the CO group in the amide group of FA+ is no longer a preferential H-bonding site. When [FA-(H2O)1]+ is produced by photoionization of FA(CO)-(H2O)2, a water molecule migrates from the CO group to the NH group, but the other water molecule evaporates from FA+. We predict that the water migration is likely entangled with the evaporation dynamics. 3822
dx.doi.org/10.1021/jp301804w | J. Phys. Chem. A 2012, 116, 3816−3823
The Journal of Physical Chemistry A
Article
The ab initio MD simulation of [FA-(H2O)1]+ revealed that the water molecule migrates from the CO group to the NH group within the molecular plane of FA+, and the oxygen atom of the water molecule points toward FA+ during the migration. The analyses of 12 trajectories showed that the NH group is the most preferential H-bonding site along the migration path, although the water molecule in [FA-(H2O)1]+ may take a fairly complicated migration pathway. On the basis of the potential energy profile along the migration coordinate in this simulation, the migration dynamics is divided into three regions for the first 500 fs: (i) the initial dynamics responding to the change in the electronic structure takes place, (ii) the water molecule moves around the CH group of the amide group in FA+, and (iii) the water molecule forms a stable H-bond with the NH group. The calculations of the electrostatic interactions show that the charge−dipole interaction induces the initial dynamics. In addition, the exchange repulsion between the lone pairs of the CO group in the amide group and the water molecule controls the initial direction of the water migration toward the NH group.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by the Grants-in Aid for Young Scientists A (23685005) and the Grant-in-Aid for Scientific Research in Priority Area (461) “Molecular Theory for Real Systems” (No.19029034) and (477) “Molecular Science for Supra Functional Systems − Development of Advanced Methods for Exploring Elementary Processes” (No. 19056005) from the Japanese Ministry of Education, Sports, Science and Technology (MEXT).
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