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Solvation of the Amphiphilic Diol Molecule in Aliphatic Alcohol-Water and Fluorinated Alcohol-Water Solutions Toshiyuki Takamuku,*,† Miho Tanaka,† Takunosuke Sako,† Takuya Shimomura,† Kenta Fujii,†,‡ Ryo Kanzaki,§ and Munetaka Takeuchi| Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga 840-8502, Japan, Graduate School of Science and Engineering, Kagoshima UniVersity, Korimoto, Kagoshima 890-0065, Japan, and Department of Chemistry, Faculty of Science, Kyushu UniVersity, Hakozaki, Higasi-ku, Fukuoka 812-8581, Japan ReceiVed: October 11, 2009; ReVised Manuscript ReceiVed: February 16, 2010
We investigated the solvation properties of aqueous solutions of aliphatic alcohols and fluorinated alcohols. These included ethanol (EtOH), 2-propanol (2-PrOH), 2,2,2-trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro2-propanol (HFIP). The amphiphilic diol, 1,4-pentanediol (1,4-PD), was used as the solute to probe solvation properties at the molecular level. Small-angle neutron scattering (SANS) experiments revealed that the inherent microheterogeneity of HFIP-water binary solutions was significantly enhanced by addition of 1,4-PD. In contrast, the addition of 1,4-PD to EtOH-, 2-PrOH-, and TFE-water solutions hardly changed the mixing state. Molecular dynamics simulations were used to obtain the spatial distribution functions for the oxygen atom of water molecules and the carbon and fluorine atoms of alcohol molecules around 1,4-PD. Of the alcohols studied, these spatial distributions illustrated that HFIP molecules formed the strongest hydrophobic solvation shell around the hydrocarbons of 1,4-PD. This preferential solvation of 1,4-PD by HFIP leads to enhancement of HFIP clusters in the solutions. 13C NMR and infrared spectroscopic measurements on 1,4PD in the different alcohol-water solutions suggested that the number of water molecules around the hydrocarbons of 1,4-PD decreased in aliphatic alcohol-water solutions. Additionally, HFIP molecules are thought to strongly interact with the hydrocarbons of 1,4-PD in HFIP-water solutions. Introduction Aqueous binary solutions of aliphatic alcohols and fluorinated alcohols are often used as solvents for chemical reactions and extractions. However, until recently, the mixing states of alcohol-water binary solutions have only been discussed in terms of thermodynamic parameters, such as enthalpies of mixing and partial molar volumes. Recent development of smallangle X-ray and neutron scattering techniques has enabled direct observation of the mixing states on a mesoscopic scale. Smallangle scattering experiments have shown that alcohol and water molecules are often inhomogeneously mixed, with alcohol clusters and water clusters coexisting in the solutions. Smallangle neutron scattering (SANS) has revealed that fluorinated alcohol molecules, such as 2,2,2-trifluoroethanol (TFE)1 and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP),2 form stronger alcohol clusters in aqueous solutions than aliphatic alcohol molecules like ethanol (EtOH) and 2-propanol (2-PrOH). The formation of alcohol clusters and water clusters in alcohol-water binary solutions should affect chemical reactions and equilibria in the solutions. Alcohol-water solutions have often been used as eluents for high performance liquid chromatography (HPLC). In particular, fluoroalcohol-water binary solutions can be utilized to extract polyamides and * Author to whom all correspondence should be addressed. E-mail:
[email protected]. † Saga University. ‡ Present address: Neutron Science Laboratory, Institute for Solid State Physics, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa 277-8581, Japan. § Kagoshima University. | Kyushu University.
nylon, which cannot be extracted with conventional eluents such as acetonitrile-water.3,4 The distribution of the organic compounds between fluoroalcohol clusters and water clusters is probably key to the underlying mechanism of the extraction. The alcohol in these binary solutions can also induce conformational changes in proteins and peptides, such as folding or denaturation. TFE5 and HFIP6 do this to a greater level than EtOH and 2-PrOH. To elucidate the mechanism for these conformational changes, researchers have focused on the number of hydroxyl groups within the alcohol molecules.7 In particular, the relationship between the hydrophobic contact energy for the alcohol and the helix energy of the protein has been elucidated.8 The energy of transfer of the protein from water to alcohol has also been investigated.9 Goto et al. studied how both aliphatic alcohols and fluoroalcohols changed the secondary structure of the bee venom peptide melittin and bovine β-lactoglobulin.10-14 They found a good correlation between the ellipticities of peptide molecules in alcohol-water solutions and the smallangle X-ray scattering intensities for alcohol-water solutions, particularly for TFE-water and HFIP-water solutions. They concluded that the formation of alcohol and water clusters in alcohol-water solutions is the main driver of protein conformational changes. However, the interactions of protein and peptide molecules with alcohol and water clusters were not elaborated. Several reports have studied the solvation of peptide molecules in alcohol-water solutions using nuclear Overhauser effect (NOE) NMR spectroscopy techniques. Gerig et al. investigated interactions of peptide molecules with TFE and HFIP by heteronuclear 1H{19F} NOE.15-17 Berger and his co-
10.1021/jp9097414 2010 American Chemical Society Published on Web 03/10/2010
Solvation of Diol in Alcohol-Water Solutions
J. Phys. Chem. B, Vol. 114, No. 12, 2010 4253 TABLE 1: Compositions of the Four 1,4-PD-Alcohol-Water Ternary Systems Examined by MD Simulationsa
Figure 1. Structure of 1,4-PD with notation of carbon atoms.
workers utilized both homonuclear 1H{1H} and heteronuclear 1 H{19F} NOEs to elucidate intermolecular interactions between a tetrapeptide molecule and ethanol or TFE in EtOH- and TFE-water solutions.18-20 The trifluoromethyl group of TFE was found to strongly interact with the hydrophobic moieties of the peptide, whereas the interactions between EtOH and the hydrophobic moieties of the peptide were weak. Fioroni et al. used molecular dynamics (MD) simulations to clarify solvation of peptide molecules in fluoroalcohol-water solutions.21,22 Even after these studies, the effects of alcohol clusters on solvation for simple organic molecules in alcohol-water solutions are not well understood at the molecular level. An understanding of the solvation behavior of organic solute molecules in alcohol-water solutions is essential for determining the underlying mechanisms for extraction of organic solutes and protein conformational changes. The addition of an organic solute to an alcohol-water solution should enhance the inherent inhomogeneous mixing of the solution if the organic solute is preferentially solvated by either an alcohol cluster or a water cluster. However, this requires confirmation. In this study, we used 1,4-pentanediol (1,4-PD) (Figure 1) to clarify at the molecular level the solvation of both hydrophobic and hydrophilic moieties of an amphiphilic molecule in aqueous solutions of EtOH, 2-PrOH, TFE, and HFIP. Although 1,4-PD is a much simpler molecule than a protein, the length of its hydrocarbon chain and the separation of its two hydroxyl groups are sufficient to allow both hydrophobic and hydrophilic interactions with solvent molecules in alcohol-water solutions. The asymmetrical structure of 1,4-PD enables us to probe the electron densities of the carbon atoms in the molecule by 13C NMR spectroscopy. The 1,4-PD-alcohol-water solutions were first studied by SANS to examine enhancement of the microheterogeneity caused by addition of 1,4-PD. MD simulations were then performed on 1,4-PD in water, EtOH-water, TFE-water, and HFIP-water. The MD results were used to compare the solvation structures of 1,4-PD in the different solutions. Changes in electron density and the C-H stretching vibration for the hydrocarbons of 1,4-PD in the ternary solutions were observed as a function of alcohol content using 13C NMR and infrared (IR) spectroscopy, respectively. Experimental Section Sample Preparation. 1,4-PD (Aldrich, 99%), EtOH (Wako Pure Chemicals, HPLC grade), 2-PrOH (Wako Pure Chemicals, HPLC grade), TFE (Tokyo Chemical Industry, high purity grade), and HFIP (Tokyo Chemical Industry, high purity grade) were used for all experiments without further purification. Doubly distilled water was used for NMR experiments. Deuterium oxide (D2O) (Cambridge Isotope Laboratories, D content ) 99.9%) was used in SANS to contrast water molecules with alcohol molecules and in IR to avoid the wide and strong absorption bands of H2O. To produce the binary solutions, alcohol and H2O or D2O were mixed at the required molar ratio. Then, 1,4-PD was added at a concentration of 0.100 or 0.200 mol dm-3. Densities for
system
n1,4-PD
nA
nW
xA
dMD
dexp
1,4-PD-water 1,4-PD-EtOH-water 1,4-PD-TFE-water 1,4-PD-HFIP-water
1 1 1 1
0 50 50 50
499 449 449 449
0 0.10 0.10 0.10
0.9796 0.9671 1.121 1.250
0.9970 0.9643 1.136 1.252
a n1,4-PD, nA, and nW give the number of 1,4-PD, alcohol, and water molecules in a MD cell, respectively, xA represents the mole fraction of alcohol, and dMD and dexp are the densities (g cm-3) derived from the simulations and determined experimentally, respectively.
the sample solutions were measured at 298 K using an electronic densimeter (ANTON Paar K.G., DMA60 and DMA602), and the densities were used during analysis of SANS data. SANS Measurements. SANS measurements were conducted at 298 K on D2O, EtOH-D2O, 2-PrOH-D2O, TFE-D2O, and HFIP-D2O binary solutions and their equivalent 1,4-PD solutions using the SANS-U spectrometer installed at reactor JRR-3M in the Japan Atomic Energy Agency (JAEA), Tokai, Japan. The sample solutions were held in a quartz cell that was 10 mm in width, 40 mm in height, and 2 mm in sample thickness. The temperature of the solutions was controlled at 298.2 ( 0.1 K. A camera length of 2 m between the sample and detector position was employed to cover the momentum transfer Q ()4πλ-1 sin θ) range of 0.05-0.15 Å-1.23,24 The scattering intensities for the sample solutions were collected for 4 h. The wavelength of the neutron beam was λ ) 7 Å, and its diameter at the sample position was 5 mm. The transmission from a sample and a cell was measured with a 3He detector placed at the beam-stopper position. The observed intensities were background corrected by subtracting the intensities of an empty cell, and then normalized by dividing the intensities for each sample solution by those for Lupolen, a standard polyethylene sample.25 Incoherent scatter was subtracted from the normalized intensities. All parameter values required for the above corrections were taken from the literature.26 MD Simulations. MD simulations were performed with an NTP ensemble in a cubic cell under a periodic boundary condition at 298 K and 1 atm, using the Material Explorer 3.0 program package (Fujitsu Co.). The compositions of the four systems tested are outlined in Table 1. In the MD simulations, bending and torsion terms for the intramolecular interactions and Lennard-Jones and Coulomb terms for the intermolecular interactions were taken into account by the potential function:
E)
∑
Vn {1 + cos(nφ 2 torsion σij 12 σij 6 1 qiqj + Rij Rij 4πε0 Rij
Kθ(θ - θ0)2 +
angle
φ0)} +
∑ i TFE > EtOH. The EtOH solvation shell around 1,4-PD is much thinner than that of HFIP. The TFE shell is not as strong as that of HFIP, as shown by a break in the main blue cloud and dispersion of small fragments. Interestingly, fluoroalcohol molecules mainly interact with 1,4-PD from the side of the most hydrophobic C5 group, which projects from the skeleton of the 1,4-PD molecule. In comparison, EtOH molecules approach 1,4PD from the reverse side of the C5 group. This finding shows that the fluoroalkyl groups of fluoroalcohol molecules favorably interact with the hydrophobic moiety of 1,4-PD. The MD simulations in this study suggest that the microheterogeneity enhancement for HFIP-water solutions on addition of 1,4-PD is due to preferential solvation of the 1,4-PD hydrocarbon chain by the fluoroalkyl groups of HFIP. Thus, HFIP clusters can form around a 1,4-PD core. In contrast, TFE does not provide sufficient solvation of the 1,4-PD hydrocarbon chain for TFE cluster formation. This is due to the smaller fluoroalkyl group of TFE compared to HFIP. Despite the presence of an EtOH solvation shell around the hydrophobic moiety of 1,4-PD, SANS experiments indicate that microheterogeneity is not enhanced. The magnitude of EtOH alkyl group solvation for the 1,4-PD hydrocarbon chain may be comparable to the water hydration for the hydroxyl groups. 13 C NMR Measurements. The 13C NMR chemical shifts for 1,4-PD (0.100 mol dm-3) in the different alcohol-water solutions were obtained and plotted over the entire range of alcohol mole fraction (Figure 8). Variations in the chemical shifts for the carbon atoms of 1,4-PD with increasing alcohol content are obviously different among the alcohol-water solutions. The hydroxyl group and the terminal methyl C5 group of 1,4-PD are bound to the C4 atom. In EtOH- and 2-PrOH-water solutions, this atom is gradually shielded with increasing alcohol content, while, in TFE- and HFIP-water solutions, it is deshielded. The deshielding of C4 in HFIP-water is more remarkable than that in TFE-water. The same patterns are observed for the C1 atom, to which the hydroxyl group is bound, but the shielding/deshielding is less significant than that for C4. In contrast, the C2, C3, and C5 atoms of the 1,4-PD hydrophobic hydrocarbon chain show the opposite pattern. In the aliphatic alcohol-water solutions, these atoms are gradually deshielded when the alcohol content increases, whereas those in the fluoroalcohol-water solutions are shielded. The exception is C3, which shows almost constant values in the TFE-water solutions. These findings should reflect the solvation structure of 1,4PD in the alcohol-water solutions, such as its hydrogen bonds with water and hydrophobic interactions with alcohol alkyl or fluoroalkyl groups. The changes in the 13C NMR chemical shifts for C1 and C4 of 1,4-PD in the aliphatic alcohol-water solutions can be explained by the weakening of the hydrogen bonds between the adjacent hydroxyl group and water molecules, as seen in the MD simulations. The hydrogen bonds between the hydroxyl groups of 1,4-PD and water are gradually weakened with increasing alcohol content due to hydrogen bond interactions between the alcohol and water. Thus, the electron densities for C1 and C4 increase in the aliphatic alcohol-water solutions with increasing alcohol content because the electrons of the
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Figure 8. 13C NMR chemical shifts for each carbon atom within 1,4PD in various alcohol-water solutions as a function of the alcohol mole fraction. The chemical shifts for EtOH-, 2-PrOH-, TFE-, and HFIP-water solutions are indicated by red, white, blue, and green circles, respectively.
hydroxyl oxygen atom flow to the hydroxyl hydrogen and carbon atoms.39 In comparison, the hydrogen bonds between the hydroxyl groups and water do not easily weaken in the fluoroalcohol-water solutions due to weaker interactions between the fluoroalcohols and water. This is illustrated by the heterogeneous mixing of both molecules observed in the SANS measurements. Thus, the deshielding for C1 and C4 in the HFIP-water solution cannot be interpreted in terms of the hydrogen bonds between the hydroxyl groups and water. The hydrophobic interactions between 1,4-PD and the fluoroalcohol may influence the deshielding of C1 and C4 as discussed below. To clarify hydration for water-miscible organic solutes, such as dimethyl sulfoxide (DMSO), in aqueous solutions, Mizuno et al. observed the C-H stretching vibrations and 1H and 13C NMR chemical shifts of the alkyl groups within solute molecules as a function of water content.44 The C-H vibration of DMSO shifted to a high frequency, and the carbon atoms of the methyl groups were gradually shielded with increasing water content. They interpreted both of these changes with increasing water content in terms of hydrophobic hydration of the DMSO methyl groups. Due to a dispersion interaction, the electronegative water oxygen atom pushes the hydrogen atoms of the C-H bonds toward the carbon atoms.44 Thus, the C-H bond lengths in DMSO are gradually shortened with increasing water content, resulting in the strengthening of the C-H stretching vibration. Furthermore, this increases the electron density of carbon atoms in the C-H bonds as electron flow occurs from the hydrogen atom to the carbon atom. On the basis of this, we can explain the changes in the 13C NMR shifts for the hydrophobic C2, C3, and C5 atoms of 1,4-PD in the alcohol-water solutions with increasing alcohol content as follows. In the aqueous 1,4-PD solution, water molecules surround the 1,4-PD hydrocarbon chain and form a hydrophobic hydration shell, as shown in the
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SDF (Figure 6b). Thus, water molecules in the aqueous solution affect the C-H bonds within 1,4-PD. However, when the alcohol content increases in the aliphatic alcohol-water solutions, water molecules around the 1,4-PD hydrocarbon chain may be gradually replaced by alcohol alkyl groups. This may occur due to hydrogen bonding between alcohol and water molecules and the hydrophobic interactions between 1,4-PD and the alcohol. Nevertheless, C2, C3, and C5 are deshielded with increasing alcohol content. This may be because the aliphatic alcohol alkyl groups have a lower electron density and therefore have less influence on the C2, C3, and C5 C-H bonds than the water oxygen atoms. Additionally, in the SDF for the 1,4PD-EtOH-water system, we observed only a thin EtOH solvation shell around the 1,4-PD hydrocarbon chain. This exclusion of water molecules from around the 1,4-PD hydrocarbon chain is a main cause of deshielding for C2, C3, and C5. In contrast, the SDFs show that fluoroalcohols, and in particular HFIP, form stronger solvation shells around the 1,4PD hydrocarbon chain. Due to the three electronegative fluorine atoms, the 1,4-PD hydrocarbons may interact more strongly with the trifluoromethyl groups than with the water oxygen atoms. This results in shielding of C2, C3, and C5 in the fluoroalcoholwater solution. The carbon atoms in HFIP-water are more significantly shielded than in TFE-water due to the two trifluoromethyl groups of HFIP. The hydrophobic interaction between C2, C3, and C5 and the alcohol fluoroalkyl groups may reduce the electron densities of C1 and C4. In particular, the alcohol fluoroalkyl groups significantly affect the terminal methyl C5 because it projects from the 1,4-PD skeleton into the solvation field of the fluoroalcohol clusters. The stronger interaction between the methyl C5 and the alcohol fluoroalkyl groups more significantly induces the electron flow from C4 to C5 atom than the interaction between C2 and the fluoroalkyl groups. Thus, C4 is more deshielded than C1. For the HFIP-water solutions, the chemical shift for C5 remarkably decreases when increasing the mole fraction to xHFIP ≈ 0.1. After this (xHFIP ≈ 0.1-1), the decrease in chemical shift becomes moderate, resulting in a break point at xA ≈ 0.1. This finding is consistent with the previous SANS results that HFIP clusters gradually form in HFIP-water binary solution with increasing HFIP mole fraction, with the greatest enhancement at xHFIP ) 0.06.2 Thus, C5 may be stably surrounded by HFIP clusters in HFIP-water solutions above xHFIP ) 0.06. It is probable that 1,4-PD is accommodated into the HFIP structure mainly by interactions with the HFIP fluoroalkyl groups in the HFIP solution (xHFIP ) 1). IR Spectroscopy. Figure 9 shows a representative IR spectrum of the C-H stretching vibration bands of 1,4-PD, in this case in HFIP-D2O. To deconvolute the absorption bands into each component, they were fitted by a pseudo-Voigt function consisting of Lorentzian and Gaussian components. Five peaks were extracted from 2825 to 3025 cm-1. DFT calculations at the B3LYP/6-311G(d,p) level45,46 were performed using the Gaussian 98 program package47 to assign the peaks observed. The C-H stretching vibrations of 1,4-PD are complex. However, generally, the C-H stretching vibration for the methyl group appears at higher wavenumber than those for the methylene and methine groups.48 1,4-PD has only one methyl group. Thus, the peak at 2975 cm-1 can be confidently assigned to the C5-H stretching vibration. In Figure 10, the wavenumbers for the C5-H vibration of 1,4-PD in the different alcohol-water solutions are depicted as a function of alcohol content. Peaks for the vibrations of
Takamuku et al.
Figure 9. Observed IR bands (circles) for 1,4-PD in HFIP-D2O. Each component (dashed line) was deconvoluted using a pseudo-Voigt function. The sum of components (solid line) and theoretical IR bands (bars) for the C-H stretching vibrations of 1,4-PD were obtained from a DFT calculation.
Figure 10. Wavenumbers for the C5-H vibration of 1,4-PD in various alcohol-water solutions as a function of the alcohol mole fraction. The colors of the circles are the same as those used in Figure 8.
1,4-PD in the alcohol-water solutions above the xA range depicted could not be extracted from the original spectra due to the contribution of vibrations for the alcohol. As in the 13C NMR data, this figure clearly illustrates a difference between the aliphatic alcohol-water and fluoroalcohol-water solutions. For the aliphatic alcohol-water solutions, the C5-H vibrations gradually shift to lower frequencies with increasing alcohol content. In contrast, in the fluoroalcohol-water solutions, they shift to higher frequencies when the alcohol content increases. These findings are consistent with our discussion on the 13C NMR data. The effect of water molecules on the C-H bonds within 1,4-PD in the aliphatic alcohol-water solutions becomes weaker with increasing alcohol content because water molecules around the 1,4-PD hydrocarbons are gradually excluded. This leads to the low frequency shift for the C5-H vibration. In comparison, the C-H bonds of 1,4-PD in the fluoroalcohol-water solutions are affected by the high electron density of the fluoroalkyl groups due to the strong fluoroalcohol solvation shell around the 1,4-PD hydrocarbons. In particular, the two trifluoromethyl groups of the HFIP molecule strongly influence the 1,4-PD hydrocarbons. Solvation for the 1,4-PD Molecule. The SANS results obtained in this study show that the inherent microheterogeneity of HFIP-water solutions is significantly enhanced by addition
Solvation of Diol in Alcohol-Water Solutions of 1,4-PD. However, when 1,4-PD is added to the TFE-water solution at xTFE ) 0.097, heterogeneous mixing hardly increases. The mixing states of EtOH-water and 2-PrOH-water are not influenced by 1,4-PD. These results suggest that HFIP can aggregate around 1,4-PD due to HFIP’s preferential solvation of the 1,4-PD hydrophobic moieties. In contrast, EtOH and 2-PrOH do not strongly interact with 1,4-PD. The MD simulations support this. In all of the 1,4-PD-alcohol-water systems studied, water molecules hydrogen bond to and hydrate the 1,4PD hydroxyl groups. However, alcohol solvation of the 1,4PD hydrocarbon chain is significantly different between the aliphatic alcohol-water and fluoroalcohol-water systems. The SDFs indicate that fluoroalcohols, particularly HFIP, form strong solvation shells around the hydrophobic moiety of the 1,4-PD molecule. In contrast, the EtOH solvation shell around 1,4-PD is thin. This suggests that the hydrophobic interaction between the 1,4-PD hydrocarbon chain and the TFE and HFIP fluoroalkyl groups is strong. This is corroborated by the results that TFE and HFIP approach 1,4-PD from the side of the projecting methyl C5 group, while EtOH mainly surrounds 1,4-PD from the opposite side. The 13C NMR and IR spectroscopic measurements also suggest strong solvation of the 1,4-PD hydrophobic moiety by HFIP. The C2, C3, and C5 hydrocarbons of 1,4-PD are shielded with increasing HFIP content in the HFIP-water solutions. When the HFIP content increases, the C5-H vibration shifts to a higher frequency. Thus, the strong interaction between the 1,4-PD hydrocarbons and the HFIP fluoroalkyl groups increases the electron densities of the carbon atoms and strengthens the C-H bonds of 1,4-PD. In contrast, the patterns observed for aliphatic alcohol-water solutions are opposite to those for HFIP. The C2, C3, and C5 hydrocarbons of 1,4-PD in EtOH- and 2-PrOH-water solutions are deshielded with increasing alcohol content. The C5-H vibration of 1,4-PD in the aliphatic alcohol-water solutions shifts to a lower frequency when the alcohol content increases. These results mainly arise from the exclusion of water molecules around the 1,4-PD hydrocarbons due to increasing alcohol content in the aliphatic alcohol-water solutions. The influence of water molecules on the 1,4-PD C-H bonds gradually weakens with increasing alcohol content. This decreases the electron densities of the 1,4-PD hydrocarbons and weakens the C-H bonds. Of the alcohols studied, this investigation shows that HFIP forms the strongest hydrophobic solvation field around the amphiphilic solute 1,4-PD. This arises from not only the hydrophobicity of individual HFIP molecules but also their aggregation into clusters. Simultaneously, water clusters formed in the HFIP-water solutions provide a hydrophilic solvation field around the solute. This means HFIP-water solutions can provide both hydrophilic and hydrophobic solvation fields for an amphiphilic solute. The balance between both solvation fields may govern the underlying mechanisms for HPLC extraction processes and conformational changes in peptide and protein molecules in HFIP-water solutions. Acknowledgment. This work was supported partly by Grants-in-Aid (nos. 15550016 and 19550022) from the Japan Society for the Promotion of Science. The SANS experiments were carried out in joint research with the Institute for Solid State Physics, the University of Tokyo (Proposal No. 5565). The density, NMR, and IR measurements for the sample solutions were conducted at the Analytical Research Center for Experimental Sciences of Saga University.
J. Phys. Chem. B, Vol. 114, No. 12, 2010 4259 Supporting Information Available: Table showing the atom-atom bond lengths, the potential parameters, and the atomic point charges for the model of each molecule used in the MD simulations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Takamuku, T.; Kumai, T.; Yoshida, K.; Yamaguchi, T.; Otomo, T. J. Phys. Chem. A 2005, 109, 7667. (2) Yoshida, K.; Yamaguchi, T.; Adachi, T.; Otomo, T.; Matsuo, D.; Takamuku, T.; Nishi, N. J. Chem. Phys. 2003, 119, 6132. (3) Mengerinka, Y.; van der Wala, S.; Claessensb, H. A.; Cramersb, C. A. J. Chromatogr., A 2000, 871, 259. (4) Du, C. M.; Valko, K.; Bevan, C.; Reynolds, D.; Abraham, M. H. J. Chromatogr. Sci. 2000, 38, 503. (5) For example: Nelson, J. W.; Kallenbach, N. R. Proteins: Struct., Funct., Genet. 1986, 1, 211. (6) For example: Barrow, C. J.; Yasuda, A.; Kenny, P. T.; Zagorski, M. G. J. Mol. Biol. 1992, 225, 1075. (7) Gerlsma, S. Y.; Stuur, E. R. Int. J. Peptide Protein Res. 1972, 4, 377. (8) Thomas, P. D.; Dill, K. A. Protein Sci. 1993, 2, 2050–2065. (9) Liu, Y.; Bolen, D. W. Biochemistry 1995, 34, 12884–12891. (10) Shiraki, K.; Nishikawa, K.; Goto, Y. J. Mol. Biol. 1995, 235, 180. (11) Hirota, N.; Mizuno, K.; Goto, Y. Protein Sci. 1997, 6, 416. (12) Hirota, N.; Mizuno, K.; Goto, Y. J. Mol. Biol. 1998, 275, 365. (13) Hirota-Nakaoka, N.; Goto, Y. Bioorg. Med. Chem. 1999, 7, 67. (14) Hong, D.-P.; Hoshino, M.; Kuboi, R.; Goto, Y. J. Am. Chem. Soc. 1999, 121, 8427. (15) Martinez, D.; Gerig, J. T. J. Magn. Reson. 2001, 152, 269. (16) Strickler, M. A.; Gerig, J. T. Biopolymers 2002, 64, 227. (17) Chatterjee, C.; Gerig, J. T. Biochemistry 2006, 45, 14665–14674. (18) Dı´az, M. D.; Berger, S. Magn. Reson. Chem. 2001, 39, 369. (19) Fioroni, M.; Dı´az, M. D; Burger, K.; Berger, S. J. Am. Chem. Soc. 2002, 124, 7737. (20) Angulo, M.; Berger, S. Anal. Bioanal. Chem. 2004, 378, 1555. (21) Fioroni, M.; Burger, K.; Mark, A. E.; Roccatano, D. J. Phys. Chem. B 2001, 105, 10967. (22) Fioroni, M.; Burger, K.; Mark, A. E.; Roccatano, D. J. Phys. Chem. B 2003, 107, 4855. (23) Okabe, S.; Nagao, M.; Karino, T.; Watanabe, S.; Adachi, T.; Shimizu, H.; Shibayama, M. J. Appl. Crystallogr. 2005, 38, 1035. (24) Okabe, S.; Karino, T.; Nagao, M.; Watanabe, S.; Shibayama, M. Nucl. Instrum. Methods Phys. Res., Sect. A 2007, 572, 853. (25) Wignall, G. D.; Bates, F. S. J. Appl. Crystallogr. 1987, 20, 28. (26) Sears, V. F. Thermal-Neutron Scattering Lengths and Cross Sections for Condensed-Matter Research; Chalk River Lab: Ontario, 1984. (27) Rychaert, J. P. J. Comput. Phys. 1977, 23, 327. (28) Weiner, S. J.; Kollman, P. A.; Case, D. A.; Singh, U. C.; Ghio, C.; Alagona, G.; Profeta, S., Jr.; Weiner, P. J. Am. Chem. Soc. 1984, 106, 765. (29) Watkins, E. K.; Jorgensen, W. L. J. Phys. Chem. A 2001, 105, 4118. (30) Jorgensen, W. L.; Madura, J. D.; Swenson, C. J. J. Am. Chem. Soc. 1984, 106, 6638. (31) Jorgensen, W. L. J. Phys. Chem. 1986, 90, 1276. (32) Fioroni, M.; Burger, K.; Mark, A. E.; Roccatano, D. J. Phys. Chem. B 2000, 104, 12347. (33) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179. (34) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (35) Mahoney, M. W.; Jorgensen, W. L. J. Chem. Phys. 2000, 112, 8910. (36) Berendsen, H. J. C.; van Gunsteren, W. F. In Molecular dynamics simulation of statistical mechanical system, Proceeding of the Enrico Fermi Summer School, Varenna, 1985. (37) (a) Nose, S. Mol. Phys. 1984, 52, 255. (b) Nose, S. J. Chem. Phys. 1984, 81, 511. (38) (a) Parrinello, M.; Rahman, A. Phys. ReV. Lett. 1980, 45, 1196. (b) Parrinello, M.; Rahman, A. J. Appl. Phys. 1981, 52, 7182. (39) Mizuno, K.; Tamiya, Y.; Mekata, M. Pure Appl. Chem. 2004, 76, 105. (40) Momoki, K.; Fukazawa, Y. Anal. Chem. 1990, 62, 1665. (41) Momoki, K.; Fukazawa, Y. Anal. Sci. 1994, 10, 53. (42) Nishikawa, K.; Iijima, T. J. Phys. Chem. 1993, 97, 10824. (43) Hayashi, H.; Nishikawa, K.; Iijima, T. J. Phys. Chem. 1990, 94, 8334. (44) Mizuno, K.; Imafuji, S.; Ochi, T.; Ohta, T.; Maeda, S. J. Phys. Chem. B 2000, 104, 11001. (45) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
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(46) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
Takamuku et al. J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.1; Gaussian, Inc.: Pittsburgh, PA, 1998. (48) Lu, R.; Gan, W.; Wu, B.; Zhang, Z.; Guo, Y.; Wang, H. J. Phys. Chem. B 2005, 109, 14118.
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