Effect of Pressure on Conformational Equilibria of 1-Chloropropane

Jun 1, 2009 - Sciences, Ritsumeikan UniVersity, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan. ReceiVed: January 5, 2009; ReVised Manuscript ...
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J. Phys. Chem. B 2009, 113, 8607–8612

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Effect of Pressure on Conformational Equilibria of 1-Chloropropane and 1-Bromopropane in Water and Organic Solvents: A Raman Spectroscopic Study Kunihiro Kasezawa† and Minoru Kato*,†,‡ Graduate School of Science and Engineering, and Department of Pharmacy, College of Pharmaceutical Sciences, Ritsumeikan UniVersity, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan ReceiVed: January 5, 2009; ReVised Manuscript ReceiVed: March 27, 2009

We have investigated the effect of pressure on the conformational equilibria of 1-chloropropane and 1-bromopropane in water and organic solvents using Raman spectroscopy. In particular, we focus on 1-chloropropane and 1-bromopropane in water as a model for characterizing the hydrophobic effect on the molecular conformation. From the pressure dependence of the Raman intensities of the carbon-halogen stretching bands, the volume differences (∆Vtfg) between trans and gauche conformers of 1-chloropropane and 1-bromopropane in water and organic solvents are determined. All the values are found to be negative. The values of ∆V for aqueous solutions deviate significantly from the expected results obtained for organic solvents. The ∆V is divided into several contributions and interpretation of these contributions indicates that the significantly large negative ∆V for aqueous solutions is explained by the considerably low packing density of solvent water compared to the density of the organic solvents. The results for the aqueous solutions are compared with the pressure effect on the dimerization of methane in water calculated by Hummer et al. (Hummer, G.; et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 1552.). Our results strongly support the validity of the increasing desolvation barrier with increasing pressure predicted by Hummer et al. Finally, we discuss the biological relevance of the results for aqueous solutions in terms of the pressure denaturation of proteins. 1. Introduction Molecular conformation is a central concept in the structural chemistry of chain molecules. The conformational equilibrium is a critical factor for chemical and biological phenomena, and is frequently influenced by changes in temperature, pressure, and solvent (i.e., medium effects). Such structural flexibility of molecules plays an important role in chemical and biological processes in the liquid phase. The medium effects on the conformational equilibrium of simple molecules in organic solvents have been studied extensively,1-10 and understood8 on the basis of the dielectric continuum model and/or packing effects. Some studies9,10 have focused on conformational equilibria of 1-halopropanes to clarify the nature of the medium effects in organic solvents. Kato and Taniguchi9 showed that the differences in volumes between trans and gauche conformers of neat liquid 1-chloropropane and 1-bromopropane are primarily due to packing effect contributions. Ben-Amotz et al.10 analyzed enthalpy and volume differences between trans and gauche conformers of 1-chloropropane and 1,2-dichloroethane in diethyl ether using thermodynamic perturbation theory. However, such treatments and interpretations are not applicable to conformational thermodynamics in water. A previous Raman study11 demonstrated that the dielectric continuum model is not applicable to the conformational thermodynamics in an aqueous system using the trans-gauche equilibrium of 1,2-dichloroethane in water. Moreover, this study also suggested that the hydration (hydrophobic) effect would dominate the conformational thermodynamics. Since most biological molecules (e.g., proteins, lipids) are chain molecules, * Corresponding author. E-mail: [email protected]. Tel: +8177-561-2761. Fax: +81-77-561-2659. † Graduate School of Science and Engineering. ‡ Department of Pharmacy.

the medium effects on conformational equilibria of model molecules in water is a research subject of vital importance. In particular, the conformational equilibria of hydrophobic molecules are important model systems for characterizing the hydrophobic effect on biomolecular structures. The hydrophobic interaction has been thought an important factor governing the structural stability of proteins. This commonly held view derives from the correlation between the thermodynamics for the transfer of hydrophobic molecules from a hydrophobic liquid to water and for the thermal denaturation of proteins.12 That is, the dominant factor for the thermodynamic stability of proteins is due to the assembly of the hydrophobic side chains in the interior of proteins. However, this model is not consistent with pressure denaturation.13 Using information theory, Hummer et al.14 showed that pressure stabilizes the solvent-separated minimum in the potential of mean force between two methane molecules in water. The authors suggested that pressure denaturation was to the result of the penetration of water into the hydrophobic core of the protein. However, this methane dimerization model is experimentally unsuitable because of the difficulty in studying such a system experimentally. The trans-gauche equilibria of simple molecules are analogous model systems. This is because the approaching terminal atom/group is associated with the change from the trans to the gauche conformation and is similar to the dimerization of methane. Although there have been several theoretical studies on the conformational equilibria of hydrophobic molecules (1,2-dichloroethane,15,16 butane17-22) in aqueous solutions, most of these studies have focused on solvent or temperature effects.15,18-20 The only related study involving pressure effects has been done by Imai et al.22 who calculated the volume difference between trans and gauche conformers of butane in aqueous solution using the reference interaction site model theory coupled with the Kirkwood-Buff solution theory (RISM-

10.1021/jp900073p CCC: $40.75  2009 American Chemical Society Published on Web 06/01/2009

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Figure 1. Raman spectra of the C-Cl stretching mode of 1-chloropropane and the C-Br stretching mode of 1-bromopropane in water at various pressures and 298.2 K. Solid, dashed, and broken lines represent the observed spectra, the curve-fitted spectra, and the residual, respectively.

KB). Although it was shown that the partial molar volume of the gauche conformer was found to be smaller than the trans conformer, the volume difference appears to be too large and the contribution from hydration is poorly defined. In the present study, we have investigated the pressure effect on conformational equilibria of 1-chloropropane and 1-bromopropane in water and organic solvents by Raman spectroscopy. The volume differences between the trans and gauche conformers (∆V) of 1-chloropropane and 1-bromopropane in several solvents were determined. The solubility in water of 1-chloropropane and 1-bromopropane is extremely low23-25 and the dipole moment difference between the trans and gauche conformers of 1-chloropropane and 1-bromopropane were found to be considerably smaller than 1,2-dichloroethane, which was previously used as a model molecule.11 Thus, 1-chloropropane and 1-bromopropane are suitable model molecules to understand the hydrophobic effect on molecular conformation. We compare ∆Vs in water with ∆Vs in organic solvents and discuss the differences by dividing the ∆V into several contributions. Finally, the biological relevance: the pressure denaturation of proteins based on the present ∆V in water is presented. 2. Materials and Methods 2.1. Experimental Section. Methanol (99.7%) (Wako Co.), acetonitrile (99%), ethanol (99.5%), cyclopentane (98%), and diethyl ether (99.5%) purchased from Nacalai Tesque Co. were used as solvents, and 1-chloropropane (99%) and 1-bromopropane (99%) purchased from Tokyo Kasei Co. were used as solutes without further purification. Ion-exchanged water was distilled just before sample preparation. The concentrations of the solutes (mole fraction of solute, x) in various organic solvents were prepared to be x ) 0.02. In the aqueous solutions, the solutes were dissolved to their maximum solubilities at room temperature: x ) 5.74 × 10-4 (298 K) for 1-chloropropane23 and x ) 3.34 × 10-4 (298 K) for 1-bromopropane.24,25 Raman spectra were measured using an NR-1800 Raman spectrophotometer (JASCO, Tokyo, Japan) equipped with an F-single polychromator and a liquid nitrogen cooled CCD detector (Princeton Instruments Inc.). The spectral resolution was 4.5 cm-1 and the exposure time was between 120 and 600 s. Raman scattered light was collected in the 90° direction from the incident laser beam. The 514.5 nm line from an Ar+ laser beam (Spectra Physics Co.) was used as an exciting source with

Kasezawa and Kato

Figure 2. Raman spectra of the C-Cl stretching mode of 1-chloropropane and the C-Br stretching mode of 1-bromopropane in various organic solvents at 0.1 MPa and 298.2 K. Solid, dashed, and broken lines represent the observed spectra, the curve-fitted spectra, and the residual, respectively.

Figure 3. Temperature dependence of IS/IF for the C-Br stretching modes of the trans (b) and the gauche (O) conformers of 1-bromopropane (neat liquid). Here, IS and IF are the Raman intensities of the shoulder and fundamental bands, respectively.

a power of 500 mW at the sample position. A hydrostatic optical cell with three sapphire windows was used for pressure-tuning the Raman measurements. The temperature of the samples was controlled within an error of (0.1 °C by circulating thermostatic water around the cell. The pressure was determined using a Heise Bourdon-tube type gauge with an error of 1 MPa. For the spectral analyses, spectral lines were fitted with GaussianLorentzian mixing functions using GRAMS/386 software (Galactic Ind., Co.). 2.2. Quantum Chemical Calculations and Molecular Volume Calculations. All density functional theory (DFT) calculations were carried out using the Gaussian 03 program.26 The DFT method used the Becke (B) exchange function and Becke’s three-parameter (B3) exchange function27,28 combined with the Lee-Yang-Parr correlation function (B3LYP).29 Calculation of the geometric optimization was performed for the trans and gauche conformers of 1-chloropropane and 1-bromopropane at the B3LYP/6-31G(d,p) level. We have employed the polarizable continuum model (PCM) method30 for the estimation of the free energy difference between the trans and gauche conformers of 1-chloropropane and 1-bromopropane in the dielectric continuum medium. The alpha-shapes program31 was used to calculate the molecular volumes. Calculations were carried out for the optimized geometry of molecules using atomic radii by Bondi.32

Conformation of C3H7Cl and C3H7Br in Water

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TABLE 1: Experimental Volume Differences (∆Vtfg) between the Trans and Gauche Conformers of 1-Chloropropane and 1-Bromopropane in Various Solvents at 298.2 K and a Breakdown of the Various Contributions solute

solvent

ra (Å)

∆Vtfg (cm3 mol-1)

∆VVtfg (cm3 mol-1)

tfg ∆VM (cm3 mol-1)

tfg ∆Velec (cm3 mol-1)

tfg ∆Vlocal (cm3 mol-1)

1-chloropropane

water methanol acetonitrile ethanol neat liquid cyclopentane water methanol acetonitrile ethanol neat liquid diethyl ether

1.40 2.07 2.22 2.34 2.65 2.74 1.40 2.07 2.22 2.34 2.65 2.75

-1.65 ( 0.20 -1.16 ( 0.06 -1.26 ( 0.16 -1.29 ( 0.13 -1.1 ( 0.2b -1.41 ( 0.23 -1.73 ( 0.16 (-1.86 ( 0.12c) -1.13 ( 0.13 (-1.20 ( 0.09c) -1.21 ( 0.10 (-1.21 ( 0.14c) -1.26 ( 0.07 (-1.25 ( 0.07c) -1.2 ( 0.2b -1.35 ( 0.13 (-1.35 ( 0.11c)

0.05 0.06 0.06 0.06 0.07 0.07 0.01 0.02 0.03 0.03 0.03 0.03

0.08 0.09 0.09 0.09 0.10 0.10 0.01 0.00 -0.01 -0.01 -0.01 -0.01

0.00 0.00 0.00 0.00 -0.01 0.10 0.00 0.00 0.00 0.00 0.00 0.07

-1.73 ( 0.20 -1.25 ( 0.06 -1.35 ( 0.16 -1.38 ( 0.13 -1.2 ( 0.2b -1.61 ( 0.23 -1.74 ( 0.16 -1.14 ( 0.13 -1.21 ( 0.10 -1.25 ( 0.07 -1.2 ( 0.2b -1.41 ( 0.13

1-bromopropane

a The molecular radii estimated from VW calculated by the alpha-shapes program assuming the solvents behaved as spheres. b Data from ref 9. c The values were obtained from analyses without the hot band intensities.

In the following section, we perform quantitative analyses of Raman intensities to obtain the volume properties of the conformers. All the spectra in Figures 1 and 2 show wellresolved bands and therefore enable accurate analyses of the conformational equilibria. The two compounds in the aqueous solution (Figure 1) show relatively low S/N ratios because of the low solubility in water. However, while the S/N is low, the error was determined to be only 3% of the intensities. 3.2. Volume Difference between Trans and Gauche Conformers. The volume change, ∆Vtfg, for the transformation from the trans to the gauche form was determined from the pressure dependence of the Raman band intensities of the C-X (X ) Cl or Br) stretching mode. Assuming that the ratio of the scattering cross section for the conformers is independent of pressure,34,35 ∆Vtfg is given by

{

∆V ) -RT

Figure 4. Pressure dependence of the integrated intensities ratios of the trans-gauche equilibria of 1-chloropropane and 1-bromopropane (298.2 K) in various solvents: water (O), methanol (9), acetonitrile (2), ethanol ([), cyclopentane (3), and diethyl ether (1).

3. Results and Discussion 3.1. Raman Spectra. Figure 1 shows Raman spectra of 1-chloropropane and 1-bromopropane in water at various pressures, whereas Figure 2 shows Raman spectra of 1-chloropropane and 1-bromopropane in different organic solvents at 0.1 MPa. The bands at 643 and 718 cm-1 for 1-chloropropane in water are assigned to the C-Cl stretching modes of the gauche and trans conformers, respectively.33 The bands at 559 and 626 cm-1 for 1-bromopropane in water are assigned to the C-Br stretching modes of the gauche and trans conformers, respectively.33 Each band of the C-Br stretching mode has a shoulder on the lower frequency side. If this shoulder band is a hot band, the ratio of the intensity of the shoulder band to the corresponding fundamental intensity of the C-Br stretching band should increase with rising temperature. In Figure 3, the ratios of the intensity of the shoulder band to the intensity of the fundamental band of each conformer of 1-bromopropane are plotted against temperature. This figure indicates an increase in the relative intensity of the shoulder band for each conformer with increasing temperature. This result suggests that the shoulder bands are the hot bands of the C-Br stretching mode.

∂ln(Ig /It) ∂p

}

T

(1)

where Ig and It indicate the Raman intensities, respectively, for the gauche and trans conformers, and the other symbols have standard definitions. The ratios of the integrated intensities of the gauche stretching mode to the trans stretching mode for 1-chloropropane and 1-bromopropane are plotted against pressure in Figure 4. The ∆Vtfg values were determined from the slopes of the regression lines. The values of ∆Vtfg for 1-chloropropane and 1-bromopropane in water and organic solvents are summarized in Table 1. In the case of 1-bromopropane, two approaches to estimate the intensities of the conformers were employed: We analyzed the intensities including or excluding the hot bands. Within experimental errors, both analyses gave identical values of volume differences. To characterize the hydrophobic hydration in terms of the partial molar volume, we divided ∆Vtfg into various contributions: tfg tfg ∆Vtfg ) ∆VM + ∆Vsolv

(2)

tfg where ∆VM is due to tfg and ∆Vsolv is due to

the geometrical (molecular) contribution the solvation contribution. The former contribution is composed of the van der Waals volume (VW) and the structural void volume within the solvent-inaccessible core of solute molecule (VV): tfg tfg ∆VM ) ∆Vtfg W + ∆VV

(3)

For approximate estimation of VV, we estimated the probe radii (r) of the solvents assuming the solvents were spheres. The

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TABLE 2: Solvation Gibbs Free Energy of the Trans and the Gauche Conformers of 1-Chloropropane and 1-Bromopropane in Various Solvents Calculated Using the PCM Method and the B3LYP/6-31G(d,p) Basis Set 1-chloropropane solvent

εa

cyclohexane diethyl ether tetrahydrofuran ethanol methanol acetonitrile water

2.02 4.24 7.58 24.85 32.61 35.68 78.34

a

1-bromopropane

trans gauche trans gauche Gelec Gelec Gelec Gelec (kJ mol-1) (kJ mol-1) (kJ mol-1) (kJ mol-1)

-5.17 -8.23 -9.47 -10.66 -10.80 -10.83 -11.06

-4.99 -7.99 -9.22 -10.42 -10.57 -10.60 -10.85

-5.02 -7.96 -9.14 -10.28 -10.41 -10.45 -10.66

-4.79 -7.66 -8.83 -9.97 -10.12 -10.14 -10.38

Dielectric constants at atmospheric pressure (data from ref 36).

values of ∆VVtfg for various solvents are summarized in Table 1. The rough approximations do not influence the following conclusions since the values of ∆Vtfg V do not significantly depend tfg for various solvents on the solvent radii. The values of ∆VM are summarized in Table 1. These values are negligibly small, compared with the experimental values, ∆Vtfg. Although the above estimations are approximations, it is concluded that the tfg is the dominant contribution to the ∆Vtfg of 1-chloro∆Vsolv propane and 1-bromopropane. The solvation contribution arises from the difference between the solvation structures around the conformers. The solvation structure is associated with the solute-solvent interactions. The interactions are primarily classified into repulsive and attractive interactions. The former interaction is the key factor to the packing effect that restricts the orientation and configuration of molecules. Previous studies8,9 have concluded that the packing effect is dominant for nonpolar solutions and the electrostatic effect is an additional significant factor for situations where the dipole moments of the conformers are significantly different to each other in organic solutions. In the aqueous solution; however, another significant factor resulting from hydrophobic hydration is possible. To discuss ∆Vtfg solv in detail, the contribution tfg was separated into the electrostatic effect (∆Vtfg of ∆Vsolv elec ) and tfg the local solute-solvent interaction effect (∆Vlocal ): tfg tfg tfg ∆Vsolv ) ∆Velec + ∆Vlocal

Figure 5. Gelec of the trans and gauche equilibrium of 1-chloropropane plotted as a function of the dielectric constant ε of the solvent.

(4)

∆Vtfg local

Here, the contribution of includes the packing effect due to repulsive interactions and/or the effect of hydrogen bonding. We calculated ∆Vtfg elec from the pressure dependence of the electrostatic free energy difference (∆Gtfg elec ). For the calculation of the electrostatic free energy Gelec of each conformer, we used the PCM method by Tomasi and co-workers.30 This method is a dielectric continuum model. On the basis of the dielectric continuum model, the electrostatic free energy, which is the free energy change accompanied by the medium change from the gas to the continuum medium, is a function of the dielectric constant (ε). Thus, ∆Velec is given by the following equation: tfg ∆Velec )

(

) (

)(

tfg gauche trans ∂∆Gelec ∂(Gelec - Gelec ) ∂ε ) ∂p ∂ε ∂p

)

(5)

For the determination of ∂Gelec/∂ε, we calculated the values of Gelec of the trans and gauche conformers of 1-chloropropane and 1-bromopropane at various dielectric constants and then estimated the values ∂Gelec/∂ε of the target solvents from the dependence of Gelec on the dielectric constant. The values of Gelec calculated using the PCM method30 are summarized in Table 2. The values for 1-chloropropane and 1-bromopropane are plotted against the dielectric constant of solvents in Figures

Figure 6. Gelec of the trans and gauche equilibrium of 1-bromopropane plotted as a function the dielectric constant ε of the solvent.

5 and 6, respectively. According to the Kirkwood expression,37 Gelec is given by the sum of the dipole moment and the quadruple moment terms, which are rational functions of ε - 1/2ε + 1 and ε - 1/3ε + 2, respectively. Thus, the following rational function was used for the regression analysis to estimate the values of ∂Gelec/∂ε at atmospheric pressure.

Gelec )

ε-1 aε + b

(6)

Here, a and b are parameters for the regression curve analysis. The goodness of the fit for Figures 5 and 6 is satisfactory to obtain the differential coefficients with respect to ε. All the data necessary for the calculation of ∆Velec and the obtained ∆Velec for 1-chloropropane and 1-bromopropane in several solvents are summarized in Table 3. Thus, the obtained ∆Vtfg elec values

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TABLE 3: Parameters for the Calculation of ∆Velec of 1-Chloropropane and 1-Bromopropane in Several Solvents 1-chloropropane a

solvent

ε

cyclopentane diethyl ether 1-bromopropane 1-chloropropane ethanol methanol acetonitrile water

1.96 4.24 8.05 8.35 24.85 32.61 35.68 78.34

-1

2

10 (∂ε/∂p) (MPa ) 0.16b 1.18c 0.99d 0.99d 2.37b 4.06b 4.02e 4.78c

trans ∂Gelec /∂εf

-1

(kJ mol )

-2.721 -0.184 -0.022 -0.012 -0.010 -0.002

gauche ∂Gelec /∂εf

1-bromopropane -1

(kJ mol )

trans ∂Gelec /∂εf

-2.659 -0.185 -0.022 -0.012 -0.010 -0.002

(kJ mol-1)

-0.653 -0.188 -0.020 -0.012 -0.010 -0.002

gauche ∂Gelec /∂εf (kJ mol-1)

-0.653 -0.188 -0.020 -0.012 -0.010 -0.002

a Dielectric constants at atmospheric pressure (data from ref 36). b Values at 303 K (data from ref 38). c Values at 293 K (data from ref 39). Values at 293 K (1-chlorobutane data from ref 40 was used to estimate the corresponding values for 1-chloropropane and 1-bromopropane). e Values at 298 K (data from ref 41). f Values obtained from the fitted curves of Figures 5 and 6. d

TABLE 4: Distances between Methanes in the Methane Dimer and Distances between the CH3 Group and Halogen Atoms of the Trans and Gauche Conformers of 1-Chloropropane and 1-Bromopropane Methane-Methane Dimerization: C-C Distance (Å) configuration

contact configuration

desolvation

solvent-separated configuration

CH4-CH4

3.9a

5.5a

7.4a

Trans-Gauche Equilibrium of Halopropanes: C-X Distance (Å) conformation 1-chloropropane (X ) Cl) 1-bromopropane (X ) Br) a

gauche

trans

3.33 3.41

4.17 4.32

Data from ref 44.

are summarized in Table 1. Most of the ∆Vtfg elec values are close to zero. However, the ∆Vtfg elec values for cyclopentane and diethyl ether, whose dielectric constants are very low, were found to be small and negative. Thus, ∆Vtfg elec is negligible compared with tfg tfg in all cases. Hence, ∆Vlocal is the dominant contribution ∆Vsolv to the volume difference between the trans and gauche conformers of 1-chloropropane and 1-bromopropane in various solvents. In a previous study,9 Kato and Taniguchi showed that the contribution of the packing effect dominates ∆Vtfg of neat liquid 1-chloropropane and 1-bromopropane. Pratt et al.6 used a statistical mechanics calculation to show that the volume difference between conformers of n-butane increased as the radius of the solvent molecule increased. Here, we investigated tfg and the sizes of the solvent the correlation between ∆Vlocal tfg values against the molecules. Figure 7 shows a plot of ∆Vlocal radii of solvent molecules. Here, we employed the radius of a sphere having the same volume of the solvent as the radius of solvent. The |∆Vtfg local| values were found to increase as the organic solvent radii increased for both 1-chloropropane and 1-brotfg for mopropane. This result implies that values of ∆Vlocal 1-chloropropane and 1-bromopropane in organic solvents are dominated by the packing effect contribution. However, the tfg | for 1-chloropropane and 1-bromopropane in values of |∆Vlocal water are significantly larger than the values expected from the lines obtained by the least-squares method using values for organic solvents. The above discussion is not rigorous since the partial molar volume depends not only on the solvent size but also on the packing density of the solvent. Thus, the deviation arises from the significantly lower packing density of solvent water compared to the density of the organic solvents. This interpretation is in agreement with recent theoretical studies42,43 on partial molar volumes of hydrophobic molecules.

Figure 7. Relationship between solvent radii, r, and the ∆Vlocal of 1-chloropropane and 1-bromopropane in water (O) and organic solvents (b) at 298.2 K.

Hence, the present result demonstrated that the low packing density around the solute influences the high pressure behavior of conformational equilibria of hydrophobic molecules in an aqueous solution. 4. Concluding Remarks In the present study we investigated the effect of pressure on the conformational equilibria of 1-chloropropane and 1-bromopropane in various water and organic solvents by Raman spectroscopy. We determined the volume change, ∆Vtfg, for the transformation from the trans to gauche form of 1-chloropropane and 1-bromopropane in water and various organic solvents. The values of ∆Vtfg for 1-chloropropane and 1-bromopropane in water and organic solvents were negative. The ∆Vtfg values were dominated by the solvation contribution ∆Vtfg solv since the geometrical contribution was negligible. Of the solvation contributions, the electrostatic contribution, which is due to long-range interactions, was also found to be negligible tfg values for the organic for all the solvents. Finally, ∆Vsolv solutions were dominated by the packing effect. The values of tfg ∆Vsolv for the aqueous solutions deviate significantly from the values for organic solutions. This deviation is due to the low

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packing density of solvent water, which is consistent with other theoretical studies characterizing hydrophobic effects. As mentioned in the Introduction, the present model system, which can be studied experimentally, is analogous to the methane dimerization model presented by Hummer et al.14 They examined the pressure dependence of the potential minima (contact and solvent-separated configurations) and a desolvation barrier, which separates the two minima, in the methane-methane potential of mean forces. It showed that pressure induces a relative favoring of the solvent-separated minimum and increases the desolvation potential barrier. The former leads to a prediction that the solvent-separated hydrophobic interaction is a driving force for the pressure denaturation of proteins. The latter leads to another prediction that pressure induces a decrease in the folding and unfolding rates of proteins. This agrees with the experimental observations presented herein. In this study, we elucidated the pressure effect on the conformational behavior of hydrophobic molecules in water and determined the volume differences between the conformers of 1-halopropanes in water: ∆Vtfg were approximately -1.7 cm3/ mol. The gauche conformers correspond to the contact configuration of methane in the methane dimerization model. The shorter distances of the gauche conformers are due to the difference in the sizes of the terminal group/atoms radii (i.e., methyl group, 1.90 Å; Cl, 1.75 Å; Br, 1.80 Å as shown in Table 4).32 In the same way, the trans conformers correspond to the desolvation configuration of methane in terms of geometry. Thus, the present volume change, ∆Vgft, corresponds to the volume change between the contact configuration and the desolvation configuration. The presented ∆Vgft ) 1.7 cm3/mol are surprisingly close to the value of 1.6 mL/mol for methane in water.14 This agreement between the present experimental and theoretical results strongly supports the validity of one of Hummer’s major predictions,14,44,45 although a rigorous comparison of these values is challenging because of the difference in molecular size between the present model and the methane dimer. As a model analogous to the methane dimerization one, the present conformational change corresponds to folding/unfolding activation processes as described above. From a different point of view, however, the present system can be a model for local conformational changes around internal rotational bonds of side chains and the main chain of a protein. According to this model, the present result suggests that there is another factor playing a role in pressure denaturation. That is, local geometrical (conformational) changes not involved with the solvent-separated hydrophobic interactions are induced by applying pressure. Hence, pressure denaturation induces collective structural changes with water molecules penetrating the hydrophobic core14 and local conformational changes around internal rotational bonds. Acknowledgment. We gratefully acknowledge financial support from a Grant-in-Aid for Scientific Research and “Academic Frontier” Project from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). K.K. is supported by a Nishio Memorial Scholarship (Ritsumeikan University). References and Notes (1) Kirkwood, J. G. J. Chem. Phys. 1934, 2, 351. (2) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1586.

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