J. Phys. Chem. B 2010, 114, 14947–14954
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Interfacial Behavior of Surfactin at the Decane/Water Interface: A Molecular Dynamics Simulation Hong-Ze Gang, Jin-Feng Liu, and Bo-Zhong Mu* State Key Laboratory of Bioreactor Engineering and Institute of Applied Chemistry, East China UniVersity of Science and Technology, Shanghai, P. R. China 200237 ReceiVed: June 21, 2010; ReVised Manuscript ReceiVed: September 27, 2010
The structural and dynamical properties of protonated surfactin molecules at the decane/water interface have been studied by molecular dynamics simulation. The rigidity of the surfactin hydration layer and the dynamics of surfactin-water and water-water hydrogen bonds have been evaluated. The simulation shows that the peptide rings slightly tilt at the interface and the aliphatic chains exhibit more extended conformation protruding into decane phase, and thus a smaller interfacial molecular area is obtained. Dynamical motions of surfactin at the interface are largely restricted by the strong polar interaction between surfactin and water molecule. Motion activities of the water molecules are decreased in the vicinity of surfactin and lead to longer lifetimes of water-water hydrogen bonds and a higher rigidity of the hydration layer. The lifetimes and the structural relaxation behaviors of surfactin-water hydrogen bonds are consistent with those of the corresponding water-water hydrogen bonds as well as the dynamics of the hydration layer water. 1. Introduction Surfactin (ST) is the general term for a class of cyclic lipopeptides produced by several strains of Bacillus subtilis. ST consists of a heptapeptide chain linked to a β-hydroxyl fatty acid, and the carboxyl terminal of the peptide chain connects with the hydroxyl group of the fatty acid by a lactone bond. The typical composition and chiral sequence of the heptapeptide moiety is L-Glu1-L-Leu2-D-Leu3-L-Val4-L-Asp5-D-Leu6-L-Leu7, and the carbon number of the β-hydroxyl fatty acid moiety varies from 12 to 17.1-3 It has been demonstrated that ST exhibited various biological activities, such as antibacterial,4 antiviral,4,5 antifungal,6 and hemolytic properties.5 In addition, ST is one of the most powerful biosurfactants for its prominent interfacial activity. Earlier studies showed that ST could lower the surface tension of aqueous solution from 72 to ∼30 mN m-1 at a concentration of the order 10-5 M.7-9 ST also has a tendency to interact with membranes by penetrating into the membrane10,11 and disturbing its integrity.12 These remarkable surface and membrane activities of ST remind that more attention should be paid to the interfacial behavior of ST at the hydrophobic/hydrophilic interface. In the studies of Bonmatin et al.,13 the peptide ring of ST was characterized to adopt a “horse-saddle” topology in DMSO solution by means of 1H NMR combined with molecular modeling. The two acidic amino acid residues and the peptide ring backbone formed the hydrophilic domain of the amphiphilic molecule, and the ST molecules stably absorbed to the interface through these multiple anchoring points, accounting for its significant interfacial properties. Further investigations showed that the conformations and the dynamics of ST molecule at the interface depended on the surrounding conditions, such as the nature of the hydrophobic/hydrophilic interface and the local interfacial concentration. A part of the ST conformations * To whom correspondence should be addressed. Tel: +86-21-64252063. Fax: +86-21-64252458. E-mail:
[email protected]. Postal address: East China University of Science and Technology, Mailbox 424, 130 Meilong Road, Shanghai, P. R. China 200237.
obtained in the sodium dodecyl sulfate micellar solution14 were different from those in DMSO solution in intramolecular hydrogen bond though the backbones also exhibited a horsesaddle topology. At a lower interfacial concentration, the peptide rings as well as the aliphatic chains were assumed to lie flat at the interface.7 It was concluded in the previous simulation15 that ST presented at the interface in the form of dimer and the ST dimer exhibited a tumbling-over motion. Neutron reflectivity data implied that the aliphatic chains folded back to interact with the hydrophobic residues in an aggregated monolayer,16 and the ST conformation that with its aliphatic chain fold back to interact with Leu2 and Val4 was the most stable structure representing ST behavior at a hydrophobic/hydrophilic interface in weak compression.17 At a moderate interfacial concentration, the peptide rings tilted at the interface and the molecular motion activities decreased.15 While under a compression condition, the aliphatic chains were suggested to orient to the normal to the interface,7 and in some cases the peptide rings were proposed to stand vertically at the interface because a ultralow interfacial molecular area was obtained.18 The interfacial behavior of ST is governed by the conformational state and the dynamics of molecules at the hydrophobic/hydrophilic interface, and partly mediated through the nearby water molecules. Though the previous simulations15,17 gave the conformational and the dynamical properties of ST molecules performed at the hydrophobic/hydrophilic interface at a molecular level, the understanding of the interfacial behavior of ST molecules is still limited, especially the lack of the dynamics of water molecules that surrounding the ST molecules as well as the strong polar interaction between ST and water molecule as hydrogen bonds. In this simulation, interfacial behavior of protonated surfactin at the decane/water interface was studied by molecular dynamics simulation. Conformational and dynamical properties of ST molecules at the interface were evaluated by the analysis of the molecular orientation, the interfacial molecular area, and the translational and rotational motion. Dynamics of the surrounding water molecules and the rigidity of the hydration layer of ST molecules were determined. In addition, the dynamics of
10.1021/jp1057379 2010 American Chemical Society Published on Web 11/02/2010
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Figure 1. (a) Chemical composition and (b) secondary structure of n-C15 surfactin. The secondary structure of surfactin is displayed with the VMD software.19 The coloring scheme is Glu, blue; Leu, orange; Val, purple; Asp, red; and the fatty acid moiety, white. For visual clarity, all the nonpolar hydrogen atoms are not displayed.
surfactin-water (SW) and water-water (WW) hydrogen bonds (H-bonds) were also investigated. 2. Simulation Methods ST used here is the n-C15 surfactin, where 15 denotes the carbon number in the β-hydroxyl fatty acid moiety, and the peptide ring backbone of ST adopts a “horse-saddle” conformation proposed by Bonmatin.13 Its chemical composition and secondary structure are presented in Figure 1, a and b. The two hydrophilic amino acid residues Glu1 and Asp5 orient to one side of the peptide ring, and the aliphatic chain as well as some of the hydrophobic amino acids extend to the opposite direction. The peptide ring here is considered as the headgroup of ST, and the remainder of the aliphatic chain from the γ-C atom is defined as ST tail. In the simulation, an all-atom model was employed to described the molecules and the adopted force field was MMFF94.20-24 The setup steps of the simulation were similar to the previous simulation.25 First, a box that contains water molecules was performed to achieve equilibrium and then the box was set in the center of a larger cube, which had the same scales in both x and y directions (38 Å × 38 Å), and thus two water/vacuum interfaces were formed. Second, protonated ST molecules were added at the water/vacuum interfaces, with their peptide ring planes laid flat at the interface and the hydrophilic residues inserted into water, and the ST concentration was 7 molecules at each interface. After that, a short run of 10 ps with a small step was performed to eliminate the inconsequential overlap between atoms as well as to randomize the conformations of the ST tails. Third, n-decane molecules were added into the residual part of the cube and a short equilibrium process
Gang et al.
Figure 2. (a) Density profiles of water, ST, n-decane, ST tail, the leucine fragment (the four leucines), and the hydrophile (the remainder of the headgroup) along the direction normal to the decane/water interface. (b) Normalized probability distribution of RH, the angle of the head vector respect to the normal to the interface (from bulk water to bulk decane), and RT, the angle between the tail vector and the normal to the interface. The head vector is defined as the vector connecting the β-C atom and the R-C atom in Val4 residue, and the tail vector is defined as the vector connecting the β-C atom and the 15th carbon atom of the aliphatic chain.
was applied beginning with a small time step for the removing of the bad van der Waals contact, and the time step then gradually increased to a value of 1.0 fs. Periodic boundary condition was applied in all the three directions. Ewald summation26 was used to calculate the electrostatic potential energy, and the cutoff radius of the van der Waals term and the real part of electrostatic interaction was 9 Å. The simulation was carried out in the NVT ensemble. The motion equations were solved by Verlet velocity algorithm,27 and the Berendsen thermostat28 was applied to maintain the system temperature at 293 K. The composition of the final cube was 245 n-decane molecules, 14 ST molecules, and 1928 water molecules. The cube dimension in the z direction was ∼113 Å. The simulation process to achieve equilibrium was continued for 400 ps, and the equilibration of the simulation was checked by monitoring the potential energy as well as the root-mean-square deviation of the peptide ring backbone. After equilibrium was achieved, MD trajectories were collected every 0.4 ps during a 2.0 ns run. To investigate the ultrafast properties as the dynamics of H-bonds, a shorter simulation of 600 ps was stored with a time resolution of 10 fs. 3. Results and Discussion 3.1. Density Profiles. Density profiles of n-decane, water, and ST along the z direction are shown in Figure 2a. It is implied in the figure that the head groups of ST reside at the decane/water interface region and the aliphatic chains protrude into oil phase. To support this assumption, density profiles of ST tail, the leucine fragment (the four leucines), and the hydrophile (the remainder of the headgroup) are also displayed in Figure 2a. It clearly appears that the ST tail is steep in the decane phase. The orientation of ST tails at the oil/water interface is more different from that at the
Behavior of Surfactin at the Decane/Water Interface surface of aqueous solution, with its aliphatic chain laid flat in the air/water interface.7 Density profiles of these three fragments are respectively fitted by Gaussian formula29 and a separation of 7.13 ( 0.07 Å (the error estimation was obtained from the average value of the two interfaces) is obtained between the tail and the hydrophile, and the separation for leucines-hydrophile is 0.52 ( 0.06 Å. In a recent neutron reflectivity experiment by Shen et al.,16 separations of about 6 and 5.5 Å for tail-hydrophile and leucines-hydrophile were found within the surfactin monolayer at the air/water interface at pH 7.5. The larger separation for the tail-hydrophile obtained in this study suggests that the ST tails adopt a more extended conformation at the oil/water interface even at low surface coverage. Further analysis showed that the dihedrals in the ST tails had 72.38% trans character. Similar extended conformation of the hydrocarbon chains in conventional chemical surfactant was also found in the presence of oil phase.29,30 3.2. Surfactin Orientation and Dynamics. The molecular orientation of ST at the decane/water interface is analyzed by the evaluation of the angular distribution of RH, the angle of the head vector, H, with respect to the normal to the oil/water interface, n (from bulk water to bulk decane), as well as the angular distribution of RT, the angle between the tail vector, T, and n. Here H is defined as the vector connecting the β-C atom and the R-C atom in Val4 residue (as shown in Figure 1b), and T is the end-to-end vector that connects the β-C atom and the 15th carbon atom of the aliphatic chain. As can be seen in Figure 2b, the angular distribution of RH centers at about 110°, while the distribution of RT centers at 0° but exhibits a broad distribution. It can be easily concluded that the head groups slightly tilt at the oil/water interface with its β-C atom raised from the interface, and the main angle between the peptide ring plane and the interface is about 20°. The main contribution 90° suggests that the terminal part of the tails sometimes approach the oil/water interface, and this benefits the appearance of hydrophobic contact between the tail and the hydrophobic amino acid residue, which is also proposed in experimental study.16 By means of screening the ST conformations, nearly 68.2% of the tails preferentially interact with Leu3 and Val4 according to a
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Figure 3. Order parameter, Sn, of C3Cn vector in surfactin tail, n is the carbon atom number along the hydrophobic tail. The error bars were obtained from the average value of the two interfaces.
hydrophobic contact criterion,34 since these two residues are both on the hydrophobic side of the peptide ring. The molecular projection area of ST molecules on the decane/ water interface is calculated and the value is 109.6 ( 10.1 Å2. However, the projection area cannot give clear indication of the actual interfacial area that ST molecules controlled according to the previous projection areas of surfactin derivatives with different ionization state.25 Hence a two-dimensional radial distribution function of the centers of mass of the ST head groups has been evaluated (Figure S1 in the Supporting Information), and a distance of ∼13 Å between ST molecule and its neighboring layer is obtained resulting in an interfacial occupied area of approximate 133 Å2. Previous studies demonstrated that the interfacial molecular area of ST molecules was affected by the interfacial concentration,15 the temperature,18 the pH value,7,18 and the ionic strength18 of the subphase. Actually, the interfacial molecular area mostly depended on the molecular orientation and the ionization state of ST molecules under different surrounding conditions. A computer simulation showed that the interfacial molecular area of ST molecules with their aliphatic chains in extended conformation is smaller than that in folded conformation.17 Good agreement of areas per ST molecule at the air/water interface and at a hydrophobic solid/ water interface were attributed to the same conformation of ST molecules at the two interfaces.16 Interfacial molecular area of ST molecules increased with the pH value for the increase of the percent of ionized ST as well as the consequent stronger electrostatic repulsion between ionized ST molecules.7,18 The interfacial occupied area in the present simulation is lower than the interfacial molecular area of 145-283 Å2 that was determined on ionized ST molecules16,18,35-37 and approaches the minimum value of 126-220 Å2 that was evaluated on ST molecules in the protonated state,7,17,18 and this difference can be attributed to the tilted orientation of the ST head groups and the extended conformation of ST tails at the decane/water interface as well as their protonated state. Diffusion coefficients of ST are obtained by the well-known Einstein relationship38 for the mean square displacement (MSD), and the MSDs of the center of mass of ST in the tangential and normal direction of the xy plane are separately calculated since the oil/water interface region that ST molecules inhabit in it is an inhomogeneous region. The tangential (Dxy) and the normal (Dz) diffusion coefficients derived from the slope of MSDs are (3.1 ( 0.4) × 10-6 cm2 s-1 and (3.4 ( 1.4) × 10-7 cm2 s-1, respectively. The mobility of ST molecules exhibits a highly anisotropic at the oil/water interface region, and the translational activity in the normal direction of the xy plane is extremely slow, indicating a restricted translational motion in this direction. The tangential diffusion coefficient of ST obtained here agrees
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Figure 4. Reorientational dynamics of surfactin head groups and tails.
fairly well with that in previous simulation, in which study the diffusion coefficient of ST was (2.6 ( 1.4) × 10-6 cm2 s-1 at the hexane/water interface.15 Besides the translational activity, the rotational motion of ST head groups and tails are also calculated by the reorientational time correlation functions (RTCF),30 which is in the form of
Figure 5. Survival time correlation functions of water molecules in region I, region II, and bulk region. Region I is the hydration layer of the peptide rings that is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen); region II is the hydration layer of the Glu1 and Asp5 that within 5 Å from the carboxyl groups; and the bulk region is located in a range of -7 to 7 Å.
TABLE 1: Multiexponential Fitting Parameters for the Survival Time Correlation Function, CΩ(t), of Water Molecules in Region I, Region II and Bulk Region region
〈b(t + τ) · b(τ)〉 CR(t) ) 〈b(τ) · b(τ)〉
(1)
region I
a
region IIb
where 〈 · · · 〉 denotes an average over all molecules and different time origins, b is the vector in ST. In the ST tail, the end-toend vector, T, is monitored, while in ST headgroup, eight vectors are tagged to characterize the reorientational dynamics of the whole head group because of the complex composition and the huge structure of the peptide moiety. Figure 4 displays the relaxation of these RTCFs, and we notice from the figure that the relaxation process of ST head groups is even slower in a 1.5 ns time interval compared with that of ST tails. RTCFs of ST tails relax more quickly for the rotational motion is more flexible due to its simple composition and streamline structure of the chains. There are many hydrophilic segments in the ST head groups, such as the acidic amino acid residues and the amido bonds as well as the lactone bond, and ST molecules stably adsorb at the oil/water interface as a result of the strong polar interactions between the hydrophilic segment and water molecule. Compared with conventional nonionic surfactant (monododecyldiethylene glycol) that form a monolayer at decane/water interface,30 the biosurfactant in the present study, ST, exhibits a much slower mobility in both tangential and normal direction of the interface, as well as a delayed relaxation of the reorientational dynamics of the head group even at low surface coverage. This comparison demonstrates that the translational and the reorientational dynamics of ST at the decane/water interface are more restricted and leads to a consequent stable adsorption of ST molecules at the interface which is proposed to be responsible for its prominent interfacial activity. 3.3. Dynamics Analysis of Water Molecules. The dynamics of water molecules are investigated especially for those in the vicinity of ST molecules, and three regions are defined for water molecules. The hydration layer of the peptide rings which is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen) is considered as region I. Region II is defined as the hydration layer of Glu1 and Asp5 that is within 5 Å from the carboxyl groups. The region located in a range of -7 to 7 Å that far away from the interface region is considered as bulk region. Dynamical properties of water molecules in these three regions are determined as follows.
bulk regionc
amplitude (%)
ti (ps)
τΩ (ps)
0.39 0.30 0.24 0.44 0.38 0.09 0.10 0.15 0.74
0.84 9.90 59.71 1.11 10.08 72.56 0.91 7.13 60.29
17.63 10.85 45.78
a Region I is the hydration layer of the peptide rings that is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen). b Region II is the hydration layer of the Glu1 and Asp5 that is within 5 Å from the carboxyl groups. c Bulk region is located in a range of -7 to 7 Å.
First of all, the residence times of water molecules in the three regions are calculated for their essential role in obtaining a better statistic dynamics of water molecules in each region. In the present simulation, the survival time correlation function (STCF), CΩ(t), is employed to evaluate the residence time of water molecules in each region, which is defined as39
CΩ(t) )
〈PΩ(0) · PΩ(t)〉 〈PΩ(0) · PΩ(0)〉
(2)
where PΩ(t) is a binary function which is equal to 1 if a monitored water molecule resides in a region, Ω, during a t time interval without leaving and is equal to zero otherwise. The STCFs of water molecules in the three regions are shown in Figure 5. Since the STCFs measure the probability that a monitored water molecule resides in a certain region for a time interval t, the average residence times can be deduced by the fitting of the decay of STCFs by a sum of three exponentials, 3 Aie-t/ti. The fitting parameters are listed in Table CΩ(t) ) ∑i)1 1, and the average residence times of water molecule, τΩ, in region I, region II, and bulk region are about 18, 11, and 46 ps, respectively. In this section, the translational and rotational properties of water molecules are analyzed to obtain the useful insight of the influence of ST molecules on the surrounding water. The tangential and normal components of MSD for center of mass
Behavior of Surfactin at the Decane/Water Interface
J. Phys. Chem. B, Vol. 114, No. 46, 2010 14951 TABLE 2: Multiexponential Fitting Parameters for the Reorientational Time Correlation Functions, CR(t), of Water Dipoles in Region I, Region II, and Bulk Region region region I
a
region IIb bulk regionc Figure 6. Reorientational dynamics of water dipoles in region I, region II, and bulk region. Region I is the hydration layer of the peptide rings that is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen); region II is the hydration layer of the Glu1 and Asp5 that is within 5 Å from the carboxyl groups; and the bulk region is located in a range of -7 to 7 Å.
of water molecules are separately calculated using the timedisplacement correlation functions:40
Dxy ) lim tf∞
1 〈[x(t + τ) - x(τ)]2 + [y(t + τ) - y(τ)]2〉 4tN(t) (3) Dz ) lim tf∞
1 〈[z(t + τ) - z(τ)]2〉 2tN(t)
(4)
where N(t) is the number of data points that contribute to the sum at time t. N(t) takes into account the fact that, when a molecule crosses the boundary into a new region, its time variables t and τ are set to zero. According to the averaged residence time of water molecules in the three regions, the MSDs are fitted by linear equation within 3-20, 3-15, and 5-50 ps for region I, region II, and bulk region, and the diffusion coefficients are (4.53 ( 0.45) × 10-6, (5.89 ( 0.26) × 10-6, and (2.57 ( 0.10) × 10-5 cm2 s-1, respectively. It appears that the translational motion of the hydration layer water is more restricted, especially for those in region I. Additionally, the rotational motion of water molecules is evaluated and the RTCFs of water dipoles are displayed in Figure 6. It shows that the decays of RTCFs of water molecules are substantially slowed down when they are entering the hydration layer of ST molecules. Further information of the reorientational time constant of water molecules can be obtained by the three exponentials fitting of RTCFs up to 30, 15, and 60 ps for region I, region II, and bulk region, respectively, and the fitting parameters are given in Table 2. It is distinctly shown in the table that the reorientational time constant, τR, of the hydration layer water is about 43-56 times longer than those in the bulk region, and the time constant in region II is 24% times shorter than those in region I. The strong electrostatic interaction between ST and water molecule as well as the hydrogen bonds formed between them are considered as the main factors that account for the decrease of motion activities of the hydration layer water. The higher density of hydrophilic segments in region I results in an intense interaction on water molecules, and consequently a slower motion in region I is observed comparing with that of water molecules in region II. The slower translational and rotational motions of water molecules in vicinity of the ST head groups demonstrate that the structure of the hydration layer water is rigid than the structure of bulk water. Such structural and dynamical behavior had been
amplitude (%)
ti (ps)
τR (ps)
0.12 0.26 0.60 0.12 0.29 0.58 0.09 0.24 0.67
0.19 4.55 442.54 0.13 3.10 350.51 0.08 1.94 6.34
266.73 204.21 4.72
a Region I is the hydration layer of the peptide rings that is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen). b Region II is the hydration layer of the Glu1 and Asp5 that is within 5 Å from the carboxyl groups. c Bulk region is located in a range of -7 to 7 Å.
observed earlier for the hydration layer water of a polypeptide41 as well as a cationic micelle.42 3.4. Hydrogen Bond Dynamics. A purely geometry criterion is employed in this work to define the SW and the WW H-bonds. The criterion are a maximum distance of 3.3 Å between the D (hydrogen donor) and A (hydrogen acceptor), and a minimum angle of 135° for DsH · · · A. The dynamics of H-bonds are evaluated by two time correlation functions,43-45 namely, the continuous hydrogen bond time correlation function, SHB(t), and the intermittent hydrogen bond time correlation function, CHB(t), which are in the forms of
SHB(t) )
〈h(0) · H(t)〉 〈h〉
(5)
CHB(t) )
〈h(0) · h(t)〉 〈h〉
(6)
and
where 〈 · · · 〉 means the calculation are carried out by averaging the H-bonds that formed at different time origins, h(t) is unity when a pair of sites is hydrogen bonded at time t according to the definition and zero otherwise, and H(t) is unity when the monitored pair of sites continuously hydrogen bonded in a interval of 0-t and zero otherwise. Therefore, SHB(t) describes the probability that a hydrogen bond formed between two sites at time zero and continually bonded up to time t, and gives the direct information of the lifetime of H-bonds. The correlation function CHB(t) describes the probability that a monitored hydrogen bond is formed at time zero as well as at time t, and it permits the breaking of H-bonds at intermediate times caused by the change of distance or orientation between the two sites. Thus, the relaxation of CHB(t) provides the information about the structural relaxation of a monitored hydrogen bond. The intermittent time correlation function for the SW HSW (t), are separately calculated for the oxygen and the bonds, CHB nitrogen of the peptide ring (SW-I) and the carboxyl groups in Glu1 and Asp5 (SW-II), and the curves are shown in Figure 7. The inset of the figure shows the time correlation function WW (t), in the three regions. It reveals for the WW H-bonds, CHB SW (t) for SW-I H-bonds is much slower that the relaxation of CHB than that for SW-II H-bonds, and the inset shows that the structural relaxations of the H-bonds formed between the hydration layer water molecules are slower than that in bulk
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Figure 7. Intermittent hydrogen bond time correlation function, CSW HB (t), for SW H-bonds. The inset shows the corresponding correlation WW function, CHB (t), for WW H-bonds formed in the three regions.
TABLE 3: Average Relaxation Time Constant, τC, the Average Lifetime, τS, and the Average Interaction Energy of the Surfactin-Water and Water-Water Hydrogen Bondsa hydrogen bonds
τCb (ps)
τSc (ps)
energy (kcal mol-1)
SW-I SW-II region Id region IIe bulk regionf
135.32 13.80 34.70 13.57 3.83
0.61 0.28 0.56 0.44 0.40
-8.72 -4.93 -5.24 -5.20 -5.15
a The multiexponential fitting parameters of the continuous hydrogen bond time correlation function, SHB(t), and the intermittent hydrogen bond time correlation function, CHB(t), are shown in Table S1 of the Supporting Information. b The amplitude-weighted average time constants, τC, of hydrogen bonds were obtained by fitting CHB(t) curves by a sum of three exponentials. c The average lifetime, τS, of hydrogen bonds were obtained by fitting SHB(t) curves by a sum of two exponentials. d Region I is the hydration layer of the peptide rings that is not farther than 5 Å away from the heavy atoms (oxygen and nitrogen). e Region II is the hydration layer of the Glu1 and Asp5 that is within 5 Å from the carboxyl groups. f Bulk region is located in a range of -7 to 7 Å.
SW region. We have used a sum of three exponentials to fit CHB (t) (t) curves due to the slow long-time decay components and CWW HB in all curves. The amplitude-weighted average time constants, τC, obtained from the fits are listed in Table 3. We can conclude that the τCSW value of SW H-bonds is about 3.6-35.3 times longer than that for bulk water. It is also shown that the τCWW value for the hydration layer water is 3.5-9.1 times longer than those for bulk water. As have been discussed previously, the translational coefficients of the hydration layer water are about 77-82% lower than those for bulk water, and the reorientational time constants for the hydration layer water are 43.3-56.5 times longer than those for bulk water. Consequently, the relaxation behaviors of the intermittent hydrogen bond time correlation function for water, CWW HB (t), are well consistent with the dynamics of water molecules, and the relaxation behaviors of CSW HB (t) agree WW with those corresponding to CHB (t) as well as the dynamics of water molecules. These relaxation behaviors had been observed earlier for the H-bonds formed between the hydration layer water and the surfactant monolayer,46 the surfactant micelle47 as well as the protein.45 The longest structural relaxation of SW-I H-bonds indicates the existence of bound water molecules around the peptide rings of ST, and the analyses of the survival time correlation function of water suggest that water molecules can reside in the hydration layer of the peptide rings for more than 200 ps. This can mainly be attributed to the abundance of the hydrophilic segments on the peptide ring that can form H-bonds with water. As can be seen in Figure 8, hydrogenbonding bridges are formed between the residues via the bound water molecules and these water molecules have a preference
Figure 8. Snapshots of representative configuration of the strongly bound water molecules that (a) hydrogen binding to one ST molecule with two hydrogen bonds and (b) hydrogen binding to two ST molecules with three hydrogen bonds. The atom coloring scheme is C, green; N, blue; O, red; and H, white. For visual clarity, all the nonpolar hydrogen atoms are not displayed.
Figure 9. Continuous hydrogen bond time correlation function, SSW HB (t), for SW H-bonds. The inset shows the corresponding correlation WW function, SHB (t), for WW H-bonds formed in the three regions.
to reform H-bonds with ST. The conformation that one water molecule forms one or two hydrogen bonds with ST molecule was also observed in previously simulation.15 Figure 9 displays the relaxation of the continuous time correlation function, SHB(t), for SW H-bonds, and the inset shows the continuous time correlation function for WW H-bonds. It can be clearly observed that all the correlation functions decayed rapidly. Thus, a sum of two exponentials is employed to fit the
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SW WW SHB (t) and SHB (t) decay curves, and the estimated amplitudeweighted average hydrogen bond lifetimes, τS, obtained from the fits are listed in Table 3. The lifetime of H-bonds not only depends on the translational and the rotational motion of the hydrogen-bonded sites but also is influenced by the energies of the H-bonds. Therefore, the average interaction energies of H-bonds are also calculated and given in Table 3. The longer lifetimes for the WW H-bonds are obtained in the hydration layer comparing with those for bulk water. Similar trend of the lifetimes of WW H-bonds was obtained in the vicinity of a polypeptide, and the reason was suggested to be partly due to their slightly higher interaction energy.41 These lifetimes of the WW H-bonds are consistent with the corresponding τC values, as well as agree with the dynamics of the hydration layer water. Much lower average interaction energies for SW-I H-bonds result in stronger hydrogen bonds, and hence longer lifetime of the SW-I H-bonds is obtained, which is about 50% longer than those for bulk water. The relaxation behavior of the functions, SW (t), for SW-II H-bonds should be noted for their higher SHB average interaction energy and the shorter lifetime. It is proposed that the intramolecular hydrogen bonds formed between the carboxyl groups and the hydrophilic segments on the peptide ring have an effect on the formation of hydrogen bonds between carboxyl group and water molecule. Further investigation confirms that more stable H-bonds could be formed between the carboxyl group and the peptide ring, with even a long lifetime of 1.58 ps, and the consequent result is that the lifetime for SW-II H-bonds is even shorter than those for bulk water. In the present simulation, a pure geometry criterion is applied to judge whether a hydrogen bond is formed, and therefore the change of the distance or the orientation caused by the dynamical motion of the hydrogen-bonded sites results in the rupture of H-bonds. The motion of water molecules is faster compared with the dynamics of ST molecules, and thus the main factors affecting the lifetime for SW H-bonds are the fast translational and rotational motions of the hydrogen-bonded water. In order to exclude the effect of the fast motion of water molecules on the relaxation of SW H-bonds, we calculate the time correlation function48
NHB(t) )
〈h(0)(1 - h(t))H′(t)〉 〈h〉
(7)
where H′(t) is unity if the monitored pair of sites is within the cutoff distance at time t and zero otherwise. Therefore, a nonzero value for NHB(t) indicates that the monitored pair of sites reside in the vicinity of each other but are no longer hydrogen-bonded, and the relaxation of NHB(t) provides the information of the rigidity of the hydration layer. Figure 10 shows the time correlation function, NHB(t), for the H-bonds that formed between ST and water molecule. It was clearly displayed in the figure that the relaxation of the function, NHB(t), for SW-I H-bonds is more slower than those for SW-II H-bonds. Since the relaxation of NHB(t) occurs due to the reformation of the broken hydrogen bond as well as the separation of the two sites that is mainly caused by the translational motion, the slower relaxation of NHB(t) for SW-I H-bonds indicates a more rigid hydration layer of the ST peptide rings. The rigidity of the hydration layer is also consistent well with the dynamics of water molecules discussed before. It was expected that the rapid motion of the hydration layer water facilitated the binding process of the active-site residues.45 The fast relaxation of the SW-II H-bonds and the lower rigidity of the hydration layer are likely to help the chelation of carboxyl groups with cations,
Figure 10. Time-dependent probability that a SW hydrogen bond is broken but the water molecule remains in the vicinity of surfactin, NHB(t).
and the cation chelation process plays important role in the biological activities of ST molecule, such as the deeper penetration of ST into membrane with the present of univalent or divalent cations,32 and the inhibition of some enzymes which need divalent ions for their activity.49,50 4. Conclusions In the present simulation, a molecular dynamics simulation has been carried out to study the molecular orientation, the interfacial molecular area, and the dynamics of protonated ST molecules at the decane/water interface. The rigidity of the hydration layer of ST molecules has been determined by the calculation of the motion activities of water molecules in the vicinity of ST molecules. The dynamics of hydrogen bonds formed between ST and water as well as among water molecules have been explored in detail. The results showed that the ST peptide rings slightly tilted at the interface with its β-C atom rising from the interface, and the ST tails exhibited more extended conformation protruding into the decane phase. A smaller interfacial molecular area of a value about 133 Å2 was obtained comparing with that at the air/water interface due to the tilted molecular orientation of ST molecules at the decane/water interface. This molecular orientation was suggested to benefit the insertion of ST molecules into membrane. Both the translational and the rotational motion of ST molecules at the decane/water interface were largely restricted by the strong polar interaction between ST headgroup and water molecule, and led to a stable adsorption of ST molecules at the interface which was proposed to be responsible for its prominent interfacial activity. Compared with bulk water, the motion activities of hydration layer water decreased and the lifetime of WW H-bonds was longer, and resulted in a higher rigidity of the hydration layer water. The lifetimes and the structural relaxation behaviors of SW H-bonds were consistent with those of corresponding WW H-bonds as well as the dynamics of the hydration layer water. The average interaction energy of the SW-I H-bonds was the strongest and the corresponding hydration layer of the peptide rings was more rigid. Bound water molecules were detected in the vicinity of the ST peptide rings, and hydrogen-bonding bridges were formed between the residues via these bound water molecules. In contrast, the hydration layer of the carboxyl groups in Glu1 and Asp5 was less rigid, and the structural relaxation of the SW-II H-bonds was faster, and these were suggested to help the cation chelation of carboxyl groups. The studies of the interfacial behavior of surfactin molecules and the dynamics of surfactin-water hydrogen bonds are expected to help us gain a better understanding of the interfacial activities of surfactin at the hydrophobic/hydrophilic interface.
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Acknowledgment. This work has been supported by the National High Technology Research and Development Program of China (Grant No. 2009AA063503) and the Department of Science and Technology Shanghai (Grant No. 071607014). Supporting Information Available: Multiexponential fitting parameters for the intermittent hydrogen bond time correlation function, CHB(t), and the continuous hydrogen bond time correlation function, SHB(t), of the surfactin-water and water-water hydrogen bonds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, X. Y.; Yang, S. Z.; Mu, B. Z. J. Pept. Sci. 2008, 14, 864. (2) Li, Y. M.; Haddad, N. A.; Yang, S. Z.; Mu, B. Z. Int. J. Pept. Res. Ther. 2008, 14, 229. (3) Hue, N.; Serani, L.; Laprevote, O. Rapid Commun. Mass Spectrom. 2001, 15, 203. (4) Vollenbroich, D.; Ozel, M.; Vater, J.; Kamp, R. M.; Pauli, G. Biologicals 1997, 25, 289. (5) Kracht, M.; Rokos, H.; Ozel, M.; Kowall, M.; Pauli, G.; Vater, J. J. Antibiot. 1999, 52, 613. (6) Tendulkar, S. R.; Saikumari, Y. K.; Patel, V.; Raghotama, S.; Munshi, T. K.; Balaram, P.; Chattoo, B. B. J. Appl. Microbiol. 2007, 103, 2331. (7) Ishigami, Y.; Osman, M.; Nakahara, H.; Sano, Y.; Ishiguro, R.; Matsumoto, M. Colloids Surf. B 1995, 4, 341. (8) Peypoux, F.; Bonmatin, J. M.; Wallach, J. Appl. Microbiol. Biot. 1999, 51, 553. (9) Zou, A. H.; Liu, J.; Garamus, V. M.; Yang, Y.; Willumeit, R.; Mu, B. Z. J. Phys. Chem. B 2010, 114, 2712. (10) Carrillo, C.; Teruel, J. A.; Aranda, F. J.; Ortiz, A. Biochim. Biophys. Acta 2003, 1611, 91. (11) Heerklotz, H.; Wieprecht, T.; Seelig, J. J. Phys. Chem. B 2004, 108, 4909. (12) Buchoux, S.; Lai-Kee-Him, J.; Garnier, M.; Tsan, P.; Besson, F.; Brisson, A.; Dufourc, E. J. Biophys. J. 2008, 95, 3840. (13) Bonmatin, J. M.; Genest, M.; Labbe, H.; Ptak, M. Biopolymers 1994, 34, 975. (14) Tsan, P.; Volpon, L.; Besson, F.; Lancelin, J. M. J. Am. Chem. Soc. 2007, 129, 1968. (15) Nicolas, J. P. Biophys. J. 2003, 85, 1377. (16) Shen, H. H.; Thomas, R. K.; Chen, C. Y.; Darton, R. C.; Baker, S. C.; Penfold, J. Langmuir 2009, 25, 4211.
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