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Molecular Dynamics Simulations of Surfactant‚Poly(r,L-glutamate) Complexes in Chloroform Solution: Influence of the Chemical Constitution of the Surfactant in the Molecular Organization David Zanuy*,† and Carlos Alema´n*,‡ Laboratory of Experimental and Computational Biology, NCI-Frederick, Building 469, Room 151, Frederick, Maryland 21702, and Departament d’Enginyeria Quimica, ETSEIB, Universitat Politecnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain Received September 13, 2002. In Final Form: January 17, 2003 Results of molecular dynamics simulations of different stoichiometric complexes constituted by poly(R,L-glutamate) and oppositely charged amphiphilic surfactants (n-hexylammonium, n-dodecylammonium, n-hexyltrimethylammonium and n-dodecyltrimethylammonium) are presented. Simulations were performed in dilute chloroform solution using explicit solvent molecules. It is shown that the conformation of the polypeptide chain is strongly influenced by the constitution of the surfactant polar headgroup. Thus, the polypeptide adopts a canonical R-helix conformation in complexes containing n-alkyltrimethylammonium surfactants, while a nonregular but also elongated conformation was found in complexes formed by alkylammonium surfactants. The latter structure is due to the interaction of alkylammonium cations with the amide group of the polypeptide chain. On the other hand, we show the intrinsic tendency of the molecular cations to form multiple interactions with the polypeptide chain, independently of their constitution.
Introduction Over the past few years, theoretical methods based on both quantum mechanical and force-field calculations have been increasingly applied to investigate fundamental aspects of macromolecular systems. The advantage of these methods lies in their ability to provide atomistic information that rarely can be obtained through experimental techniques. Thus, theoretical calculations are used to obtain a complete understanding, in terms of microscopic constitution, of chemical and physical properties, which are of fundamental and technological interest. Complex materials formed by synthetic polyelectrolytes and oppositely charged low-molecular weight amphiphilic surfactants, which are bonded through ion-pair interactions, have motivated a great interest during the past decade.1-8 Stoichiometric complexes are in general insoluble in water but dissolve in organic solvents of low polarity forming in many cases well-defined supramolecular structures. Thus, complexes with different supramolecular structures have been prepared according to the chemical properties of the components.3-8 Surfactant‚polyelectrolyte complexes based on polypeptides are especially interesting because the latter can adopt a * To whom correspondence should be addressed. † Laboratory of Experimental and Computational Biology. E-mail:
[email protected]. ‡ Departament d’Enginyeria Quimica. E-mail: carlos.aleman@ upc.es. (1) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (2) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (3) Antonietti, M.; Kaul, A.; Thu¨nemann, A. Langmuir 1995, 11, 2633. (4) Higashi, N.; Massaaki, S.; Niwa, M. Langmuir 1995, 11, 1864. (5) Macknight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781. (6) Ciferri, A. Macromol. Chem. Phys. 1994, 195, 457. (7) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (8) Ponomarenko, E. A.; Waddon, A. J.; Tirrell, D. A.; MacKnight, W. J. Langmuir 1996, 12, 2169.
variety of highly ordered secondary structures.5 Furthermore, the polypeptide conformation can be modified by both the temperature and the solvent, which should help to control the properties of the complexes. Work in this area involves complexes with different chemical constitutions but we will concentrate on compounds formed by poly(R,L-glutamate), abbreviated PALG, and amphiphilic molecular cations. Surfactant‚PALG complexes have been extensively investigated by Tirrell’s group.5,8,9 Stoichiometric complexes consisting of PALG and alkyltrimethylammonium cations with alkyl lengths of 12-18 carbon atoms were prepared by mixing equimolar quantities of surfactants and sodium PALG at room temperature. In the solid state the resulting complexes behaved as comblike poly(γ-alkylR,L-glutamate)s,10 even though the alkyl side groups are covalently attached to the backbone in the latter polymers. As a result, surfactant‚PALG complexes adopt a biphasic structure, which consists of a layered arrangement of polypeptide helical rods immersed in a paraffinic pool. On the other hand, in dilute chloroform solution the polypeptide chain adopts a well-defined R-helical conformation, but no information has been reported about the organization of the surfactants with respect to the polyanion. The amount of experimental data available on surfactant‚PALG complexes, although insufficient to attain a detailed atomistic description of the structure, is very useful for supporting the theoretical analysis. In this sense, we have recently reported some studies to gain more insight into the microscopic organization of these systems.11-13 More specifically, the ion-pair interaction (9) Ponomarenko, E. A.; Waddon, A. J.; Bakeev, K. N.; Tirell, D. A.; MacKnight, W. J. Macromolecules 1996, 29, 4340. (10) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141. (11) Alema´n C.; Zanuy D. Chem. Phys. Lett. 2000, 319, 318. (12) Alema´n C.; Zanuy D. Chem. Phys. Lett. 2001, 343, 390.
10.1021/la026549o CCC: $25.00 © 2003 American Chemical Society Published on Web 03/21/2003
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Table 1. Summary of the Chemical Constitution, Average rmsd, and Average RG for the Four Complexes Investigateda surfactant complex I II III IV
head polar group +NH +NH
3
3 +N(CH +N(CH
3)3 3)3
alkyl chain
complex
rmsd (Å)
RG (Å)
-(CH2)5-CH3 -(CH2)11-CH3 -(CH2)5-CH3 -(CH2)11-CH3
6-AA‚PALG 12-AA‚PALG 6-ATMA‚PALG 12-ATMA‚PALG
0.49 ( 0.08 0.49 ( 0.10 1.45 ( 0.19 1.06 ( 0.16
7.11 ( 0.07 7.17 ( 0.07 7.73 ( 0.12 7.37 ( 0.13
a
The averages are calculated considering the 2000 snapshots stored through 2 ns trajectories. Standard deviations of the averages are also included.
involved in surfactant‚PALG complexes was exhaustively investigated using sophisticated ab initio quantum mechanical calculations.11,12 Thus, the more relevant features of this interaction (intermolecular geometry and both binding and cooperative energies) were investigated using small model systems and considering different surrounding media. On the other hand, the structural properties of the stoichiometric complex formed by PALG and octyltrimethylammonium, henceforth abbreviated 8-ATMA‚PALG, were investigated in dilute chloroform solution using molecular dynamics (MD) simulations.13 It was found that the surfactant ions predominantly adopt an extended conformation that is stabilized by favorable interactions with the organic solvent, while the polypeptide chain is organized in an R-helix. Furthermore, results indicated that 8-ATMA cations form multiple electrostatic interactions with the polypeptide chain, the simultaneous interaction of every surfactant molecule with three carboxylate groups being the most frequent situation. In this article we examine the influence of the chemical constitution of the surfactant molecules in the structural organization of the surfactant‚PALG complexes. Four different surfactant molecules have been considered, which differ among them in the polar headgroup and/or the length of the alkyl group: (i) n-hexylammonium (6-AA), (ii) n-dodecylammonium (12-AA), (iii) n-hexyltrimethylammonium (6-ATMA), and (iv) n-dodecyltrimethylammonium (12-ATMA). MD simulations in dilute chloroform solution reveal that the chemical nature of the polar headgroup plays a crucial role in both the conformation of the polypeptide chain and the supramolecular organization of the complex, whereas the length of the alkyl group has an almost negligible influence. These results enable us to gain new insights into the molecular organization of surfactant‚polypeptide complexes in solution.
Methods Molecular Model. MD simulations were run on stoichiometric complexes formed by PALG and oppositely charged surfactants (n-AA and n-ATMA with n ) 6 and 12). Complexes, which were modeled by explicitly considering all the atoms, were generated in the following way. A polypeptide chain of 15 negatively charged residues was generated and blocked at the N-terminus with an (13) Zanuy D.; Alema´n C.; Munoz-Guerra, S. Biopolymers 2002, 63, 151
acetyl group and at the C-terminus with a N-methylamide group. Then, the polypeptide chain was surrounded 15 surfactant cations. It should be emphasized that only a type of surfactant was considered for each complex. Details about the chemical constitution of the four complexes considered in this work, which have been denoted I (6-AA‚PALG), II (12-AA‚PALG), III (6ATMA‚PALG), and IV (12-ATMA‚PALG), are provided in Table 1. The initial set of atomic coordinates was generated for each complex according to our previous studies on related systems. The R-helical conformation recently modeled for the poly(R,Lglutamic acid)14 was assumed for the PALG chain in the complex. This is consistent with the experimental evidences reported for self-assembled n-ATMA‚PALG complexes with n) 12, 16, and 18, which allowed us to propose a helical conformation in dilute chloroform solution.5,8 The dihedral angles used to generate such structure were φ) -64.7°, ψ) -36.9°, ω) 177.1°, and χ1) -67.8°. On the other hand, the surfactant molecules were arranged in the following way: (i) a fully extended conformation was considered for the alkyl side groups and (ii) the polar headgroups were oriented facing a carboxylate moiety of the helical polypeptide. This orientation was not identified as the most favorable for the polar headgroups.12,13 However, we used such a specific disposition as the starting point for checking that the arrangement of the molecular cations with respect to the polypeptide chain does not depend on their chemical constitution. Computational Details.In our previous study we demonstrated that, if atomic charges are accurately described, forcefield calculations are able to reproduce quantitatively the ionpair binding energies provided by ab initio quantum mechanical methods.13 Following this established procedure atomic point charges were derived by fitting the rigorously defined quantum mechanical molecular electrostatic potential, which was computed at the ab initio HF/6-31G(d)15 level, to the Coulombic electrostatic potential. The atomic charges obtained for the four surfactants considered in this work are displayed in Figure 1a. The rest of the force-field parameters were taken from the libraries of the Amber force field.16 Simulations were performed with the Amber 4.1 package of programs.17 Bond lengths were constrained to their standard values using the SHAKE algorithm.18 Residue-based cutoffs were applied at 14 Å, i.e., if two residues or a residue and a chloroform molecule have any atom within 14 Å, the interaction between the entire pair is evaluated. Periodic boundary conditions were applied using the nearest image convention and the nonbonded pair list was updated every 25 MD steps. A numerical integration time step of 2 fs was used for all the calculations, while MD trajectories were saved every 500 steps (1 ps interval) for subsequent analysis. A cubic solvent box (cell-axis ) 66.56 Å) of 2200 chloroform molecules (F ) 1.46 g/cm3) was created and equilibrated, the OPLS model being used to describe the solvent molecules.19 (14) Zanuy, D.; Alema´n, C. Biopolymers 1999, 49, 497. (15) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (16) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M., Jr.; Ferguson, D. M.; Spellmeyer, T.; Fox, J.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179. (17) Pearlman, D. A.; Case, D. A.; Caldwell, J. W.; Ross, W. S.; Cheatham, T. E., III; Ferguson, D. M.; Seibel, G. L.; Singh, C.; Weiner, P. K.; Kollman, P. A. Amber 4.1; University of California: San Francisco, 1995. (18) Ryckaert, J.-P.; Ciccotti, G.; Berendsen, H. J. C. J. Comput. Phys. 1977, 23, 327. (19) Jorgensen, W. L.; Briggs, J. M.; Contreras, M. L. J. Phys. Chem. 1990, 94, 1683.
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Figure 1. (a) Atomistic representation of the four surfactant molecules investigated in this work. The atomic partial charges derived from quantum mechanical calculations are indicated. (b) Schematic representation of the simulation box. A ribbon representation is used to depict the polypeptide main chain. The surfactant molecules and the polypeptide side chains are represented with sticks, while the chloroform molecules are indicated with solid balls. Molecular systems were constructed by centering each complex in the solvent box and deleting the overlapped chloroform molecules. A schematic representation of a starting simulation box is depicted in Figure 1b. The temperature of each system was brought to 300 K by 56 ps of NVT (constant volume and temperature)-MD with the solvent molecules fixed. After this, the energy was equilibrated by performing 60 ps of NVT-MD, in which both the solvent molecules and the complex were allowed to move freely. The resulting structure was used as starting point of 2 ns NVT-MD at 300 K.
Results and Discussion Conformation of the Polypeptide Chain. Among the results previously reported for the 8-ATMA‚PALG complex,13 the large stability of the polypeptide R-helical conformation deserved special attention. To check if this preference is retained in other surfactant‚PALG complexes, we monitored the temporal progress of several geometrical parameters related with the main chain conformation. Specifically, we followed (i) the root-meansquare deviation (rmsd) with respect to the initial R-helical conformation, (ii) the radius of gyration (RG), (iii) the number and position of intramolecular hydrogen bonds between amide groups, and (iv) the conformational distribution of the main chain dihedral angles φ and ψ. The average rmsd value, which was computed considering only the backbone atoms of the polypeptide chain, is listed in Table 1 for all the complexes. This parameter strongly depends on the chemical constitution of the polar headgroup of the molecular cation. For complexes III and IV, which are formed by 6-ATMA and 12-ATMA surfactants, respectively, the rmsd remains relatively low and
fluctuates little over the whole trajectory (data no shown). Similar results were previously obtained for the 8-ATMA‚ PALG complex.13 Accordingly, ionically attached n-ATMA surfactants do not alter the conformational stability of the R-helix. These results are supported by 1H NMR data obtained for different n-ATMA‚PALG complexes in dilute chloroform solution.5,8,9 In all cases, the R-CH protons of the polypeptide backbone were observed at δ ) 4.0 ppm, suggesting the presence of hydrogen bonded secondary structure. Thus, no proton resonances characteristic of main chain units free of hydrogen bonding were detected. Furthermore, dynamical light-scattering measures indicate the absence of intermolecular aggregates, which are characteristic of β-sheet polypeptides. On the basis of these experimental observations, a R-helical conformation was proposed for the n-ATMA‚PALG complexes. Complexes containing n-AA surfactants show a significant different behavior: the rmsd rapidly increases to higher values (about 1.0-1.5 Å), even though the fluctuations are small once a certain value is reached. Indeed, a visual inspection to the recorded structures indicates that in complexes I and II the polypeptide chain evolves toward a distinct helical conformation (details will be provided below), which remains stable until the end of the simulation. However, although the conformation reached differs from the canonical R-helix, no unfolding event was detected in the temporal evolution of the rmsd. Another interesting feature is manifested by the RG, which is displayed in Table 1 for all the investigated complexes. The differences of the average RG values with respect to that of a canonical R-helix (7.25 Å) do not allow
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Figure 2. Spatio - temporal evolution of the CdO(i) r H-N(i+4) type hydrogen bonds, R-helix (in black dots), and CdO(i) r H-N(i+3) type hydrogen bonds, 310 helix (in gray dots), for (a) complex I, (b) complex II, (c) complex III, and (d) complex IV. Note that residue number i denotes the ith residue with the oxygen acceptor.
us to conclude that the organization reached by the polyanion in n-AA‚PALG complexes implies a loss of helical order. Indeed, results displayed in Table 1 suggest that the conformations associated to the polypeptide chain in n-AA‚PALG and n-ATMA‚PALG complexes apparently do not differ in terms of global shape. This information leads us to reach some interesting conclusions. The stability and topology of the PALG conformation strongly depend on the chemical composition of the polar headgroup of the surfactant chains. Conversely, the polypeptide conformation does not depend on the length of the surfactant alkyl chain. On the other hand, the apparent behavior of the bulk system seems to be analogous to that experimentally observed in solid state.5,8 Thus, as was already mentioned, analyses of solid films revealed that the surfactant alkyl chains and the polar groups (the polypeptide chain and the surfactant polar heads) are organized in different phases. The temporal-positional distribution of the 13 initial CdO(i) r H-N(i+4) hydrogen bonds, which correspond to a R-helix of 15 residues, is monitored in Figure 2. The criterion used to define a hydrogen bond was that the O‚‚‚H distance must be shorter than 2.5 Å and the ∠NH‚‚‚O angle must be larger than 135°. As can be seen, the patterns depicted for complexes I and II are completely different from those obtained for III and IV. For complexes III and IV the polypeptide chain clearly retains the majority of the initial hydrogen bonds, with a high efficiency at the center of the helix. Indeed, only some occasional disruption is observed at both extremes of the helices, as a consequence of the finite size of our model systems. The general tendency of the PALG backbone to maintain its original hydrogen bonding scheme is consistent with the low rmsd previously
discussed. On the other hand, the patterns for the two n-AA‚PALG complexes are completely different. Thus, the disruption of the hydrogen bonds occurs not only in the extremes but also in the central part of the helix, this trend being especially notable for the complex I (Figure 2a). To help in the interpretation of this disruption, new possible intramolecular hydrogen bonding schemes were investigated. It has been longer reported that the extremes of an R-helix can temporally adopt the hydrogen bonding scheme characteristic of a 310-helical conformation,20-22 i.e., CdO(i) r H-N(i+3). Furthermore, in peptides and proteins it has been established that this kind of conformational transition plays a key role on the kinetics of the folding process.23,24 Nevertheless, the data derived from the analysis of this kind of hydrogen bond did not provide us any clues about the polypeptide conformation in nAA‚ complexes since the number of CdO(i) r H-N(i+3) hydrogen bonds was very scarce (Figure 2). Complexes III and IV showed the presence of 310-helix arrangements for very short periods of time, not as a general restructuring but as a consequence of the extremes frying. Figure 3 represents the accumulated Ramachandran plots for the eight central residues of the polypeptide chains. It is worth noting that the distribution of the backbone dihedral angles {φ,ψ} strongly depends on the chemical constitution of the assembled surfactants. Com(20) Bollin, K. A.; Millhauser, G. L. Acc. Chem. Res. 1999, 32, 1027. (21) Barlow, D. J.; Thornton, J. M. J. Mol. Biol. 1988, 201, 601. (22) (a) Karpen, M. E.; de Haseth, P. L.; Neet, K. E. Protein Sci. 1992, 1, 1333. (b) Aurora, R.; Rose, G. D. Protein Sci. 1998, 7, 21. (23) Gerstein, M.; Chothia, C. J. Mol. Biol. 1991, 220, 133. (24) McPalen, C. A.; Vincent, M. G.; Picot, D.; Jansonius, J. N.; Lesk, A. M.; Chothia, C. J. Mol. Biol. 1992, 227, 197.
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Figure 3. Ramachandran plot containing the eight central residues of the polypeptide chain for (a) complex I, (b) complex II, (c) complex III, and (d) complex IV. Dihedral angles were defined as is usual in polypeptides: φ ) C-N-CR-C and ψ ) N-CR-C-N. Circles point the different conformational regions that have been characterized.
plexes III and IV present a tight and uniform distribution around the region associated to the canonical R-helix (φ ∼ -60°; ψ ∼ -50°), which is usually denoted R. Conversely, complexes I and II exhibit a multiple conformational distribution. Thus, three different conformational regions are notably populated for the n-AA‚PALG complexes, two of them (denoted A and B in Figure 3) involving an important alteration in the spatial disposition of the amide groups with respect to that of the R region. In the conformations associated to A and B the carbonyl groups are perpendicular to the helical axis, while in the R region the carbonyl groups and the helical axis are parallel. Accordingly, a drastic conformational change is induced by the presence of residues with {φ,ψ} belonging to regions A and B. These features are illustrated in Figure 4, which shows the PALG chain of complexes II and IV after 2 ns of simulation. On the other hand, it should be noted that, as far as we know, no experimental data about the polypeptide conformation of n-AA‚PALG complexes in organic solvents is available in the literature. The organization of the negatively charged carboxylate groups along the polypeptide chain was investigated using the same procedure that in our previous study.13 Accordingly, we carried out a population analysis for the three dihedral angles of the PALG side chain χi, where i ranges from 1 to 3, considering all the stored structures. Each conformer was classified into one of six established categories: trans, gauche+, gauche-, skew+, skew-, and cis. The results, which are displayed in Table 2, indicate that in all cases the trans is the preferred conformation for χ1 and χ2. However, the frequencies are remarkable
smaller for I and II than for III and IV. On the other hand, the interactions between the carboxyl groups and the surfactant molecules induce a wide conformational distribution for χ3, i.e., this angle is closely related with the position of the side chain oxygen atoms in the space. Conformational Properties of the Surfactant Molecules. Figure 5 shows the evolution of the average endto-end distance for the surfactant chains, which was measured as the distance between the nitrogen atom of the headgroup and the last carbon atom of the alkyl chain. The influence of the polar headgroup in the conformational behavior of the alkyl chains is almost negligible. Thus, the surfactant molecules tend to preserve the initial extended arrangement. The average end-to-end distance is 6.88 and 6.55 Å for 6-AA and 6-ATMA, respectively, while the 12-AA and 12-ATMA surfactants achieve values of 12.47 and 12.48 Å, respectively. These values are very close to those expected for a canonical fully extended alkyl chain containing 6 and 12 carbon atoms, which are 1.225 Å × 6 ) 7.350 Å and 1.225 Å × 12 ) 14.700 Å, respectively. The strong tendency of the molecular surfactants to preserve the extended arrangement can be explained by considering that the interaction with the solvent is the conformational driving force of the alkyl chains. Thus, the molecular surface exposed to the solvent increases with the number of dihedral angles in trans conformation. Supramolecular Organization. Surfactant‚Polypeptide Structure. The distinctive organization of the surfactant molecules with respect to the carboxylate groups of the polypeptide chain was a striking finding of our previous theoretical studies.12,13 Thus, calculations
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Figure 5. Evolution of the average end-to-end distance for the cation alkyl chains with respect to the simulation time. Averages correspond to the 15 surfactant molecules. Complexes I (solid) and II (dashed) are represented by black lines, while gray lines correspond to complexes III (solid) and IV (dashed).
Figure 4. Representative snapshots of the main chain conformation for complexes (left) IV and (right) II. Amide groups are depicted explicitly by spheres, thick dashed lines represent the intramolecular hydrogen bonds and the small solid arrows point the amide groups that are not parallel to the helical axis (long dashed arrow). Table 2. Dihedral Angle Distributions for the Polypeptide Side Chainsa torsion no,
trans
gauche+
c1 c2 c3
55.86 60.62 9.38
13.61 11.59 7.83
c1 c2 c3
66.41 34.64 6.16
c1 c2 c3 c1 c2 c3
gauche-
skew+
skew-
cis
Complex I 26.91 7.8 32.16
0.09 5.84 18.39
2.84 14.08 27.00
0.68 0.07 5.24
4.93 23.13 24.85
Complex II 21.19 18.86 28.00
0.29 4.04 15.69
7.05 18.58 18.61
0.13 0.75 6.69
85.11 82.87 2.53
3.92 8.86 20.79
Complex III 3.73 3.92 26.93
0.7 4.17 16.83
4.45 0.13 27.63
2.09 0.05 5.28
77.63 79.46 6.55
0.01 15.48 18.81
Complex IV 16.18 0.29 22.62
0.02 3.49 14.2
6.12 1.27 31.59
0.04 0.01 6.23
a The populations (in %) for the different complexes have been averaged considering the 2000 snapshots stored during the 2 ns trajectories.
on stoichiometric complexes indicated that the molecular cations always interact with the maximum number of accessible anionic groups rather than follow a one-to-one pattern. The influence of the chemical constitution on the supramolecular organization was investigated by examining the geometries associated to the electrostatic interactions between the molecular cations and the polyanion. An electrostatic interaction was identified as such when the distance d between the nitrogen atom of the molecular cation and the carbon atom of the carboxylate group was smaller than 5.50 Å. This value was found to provide an acceptable representation for the electrostatic energy within the molecular mechanics approximation.13
Figure 6 displays the distribution of the number of carboxylate groups that interact with each surfactant molecule during the 2 ns trajectory. The four complexes show, in general, similar trends: every surfactant molecule, independently of the polar headgroup and the length of the alkyl chain, interacts simultaneously with several carboxylate groups. Accordingly, during the equilibration period, the molecular cations left their original positions, facing only one carboxylate group, and migrated to new locations in order to achieve a multiple interaction pattern. A more detailed examination of the distributions displayed in Figure 6 shows important differences between complexes constituted by n-AA and n-ATMA surfactants. The surfactant molecules of complexes I and II interact, in average, with less carboxylate groups than those of III and IV. As can be seen, with the exception of those associated to the N-terminus segment, the n-AA cations interact with two carboxylate groups, the interaction with three carboxylate groups at the same time being very infrequent. Conversely, the simultaneous interaction between a surfactant molecule and three carboxylate groups is very usual for complexes III and IV. Moreover, in both cases the surfactant chains sporadically present interactions with four carboxylate groups at the same time. These results are very surprising, since we expected that the smaller size of the polar heads groups would allow a higher degree of multiplicity in the interaction pattern, due to a lower steric hindrance. Another interesting trend of the investigated compounds is the asymmetric distribution of the electrostatic interactions. The interaction between a molecular cation and a given number, n, of anions was characterized by a set of distances {di; i ) 1, ..., n}, where di refers to the distance between the nitrogen atom of the surfactant and the carbon atom of the carboxylate group. The results (data not shown) indicate that such distances can differ by even ∼0.5 Å. On the other hand, these intermolecular parameters are almost 1 Å shorter for complexes containing n-AA than for complexes formed by n-ATMA. The smaller size of the ammonium group allows to the n-AA surfactants a greater proximity to the carboxylate anions. Nevertheless, this result provides a contradiction since the same argumentation should lead to a higher number of electrostatic interactions. We find out an explanation to this feature after visual examination of the recorded structures: complexes I and II present a new and nonexpected interaction between the surfactant cations and the amide groups of the polypeptide chain. Furthermore, as we will show in the next section, this interaction justifies the unusual conformational properties exhibited by the polypeptide chain.
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Figure 6. Distribution of the number of carboxylate groups that interact with every of the 15 molecular cations at the same time: (a) complex I, (b) complex II, (c) complex III, and (d) complex IV. From top to button, with one (black), two (dark gray), three (medium gray), and four (light gray) carboxylate groups.
Surfactant-Amide Interaction. Interestingly, we detected a frequent interaction between the polypeptide amide groups and the molecular cations in n-AA‚PALG complexes. This kind of interaction was mentioned in our previous work about 8-ATMA‚PALG, although in that case was attributed to the finite size of the system since it was only detected at the C-terminus.13 The surfactant-amide interaction was characterized using the distributions of the distances between the nitrogen atom of the surfactants and the amide oxygen atoms of the polypeptide chain. The main trends of such distributions, which are displayed in Figure 7, can be summarized as follows: (a) a sharp peak appears for complexes I and II and (b) the four complexes present a broad region at distances larger than ∼4.5 Å. The later region involves to all possible distances between any molecular cation and the amide groups of the polypeptide. However, the sharp peaks obtained for I and II exhibit distances ranging from 2.5 and 3.1 Å and, therefore, must be related with a specific interaction. Accordingly, the surfactant-amide interactions can be identified as such when the N‚‚‚O distance is smaller than 3.1 Å. Figure 8a shows the distribution of the number of amide groups that interact with each surfactant cation during the trajectory for complexes I and II. As can be seen, the presence of the surfactant-amide interaction strongly depends on the length of the alkyl group. For complex I the interaction was never detected in 7 of the 15 molecular cations, while it is absent in only two molecular cations of complex II. Furthermore, each molecular cation tends to interact with only a carbonyl group indicating it is not
Figure 7. Distribution of the distances between the nitrogen of the molecular cations and the amide oxygen atoms of the polypeptide backbone for complexes I (black solid line), II (black dashed line), III (grey solid line), and IV (grey dashed line).
a multiple interaction. However, the combination of the surfactant-carboxylate and surfactant-amide distributions provides similar level of multiplicity in the interacting pattern for the four complexes. Figure 9 displays the multiple interaction patterns for a representative microstructure of complexes II and IV. Figure 8b represents the space-temporal evolution of the CdO(i) r H-N(i+4) and CdO(i) r H-N(i+3) hydrogen bonds together with the surfactant-amide interactions for complexes I and II. It is worth noting that many of the gaps previously showed in Figure 2a,b
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Figure 8. (a) Distribution of the number of amide oxygen atoms that simultaneously interact with every molecular cation for complexes (left) I and (right) II. From top to button, with none (white), one (black), two (dark gray), and three (medium gray) amide oxygen atoms. (b) Spatiotemporal evolution of the CdO(i) r H-N(i+4) type hydrogen bonds, R-helix (in black dots), CdO(i) r H-N(i+3) type hydrogen bonds, 310 helix (in medium gray dots), and surfactant-amide interactions (light gray dots). Left and right graphics correspond to complexes I and II, respectively.
Figure 9. (a) Detailed view of the interaction between a 12-ATMA surfactant molecule and four different carboxylate groups. The parameters di indicate a degree of multiplicity of four (see ref 13). (b) Atomistic view of a representative interaction between a 12-AA surfactant and the polypeptide chain. The amide group that interacts with the cation is explicitly depicted while for the rest of the polypeptide we only displayed the backbone.
disappeared when action of the surfactant-amide interaction is taken in account. Accordingly, it can be stated that the surfactant-amide interaction is clearly the cause of the conformational change previously described for the polypeptide chain of I and II. This interaction is strong enough to break the intramolecular amide-amide hydrogen bonds but maintaining the elongated shape of the polypeptide chain, i.e., the surfactants keep the solvent
away from the polypeptide chain allowing the backbone to remain in a stationary-like state. Conclusions In this work we present a molecular dynamics study of different self-assembled complexes based on PALG. We investigated not only the conformational preferences of the molecular ions but also the structural organization of
Surfactant‚Poly(R,L-glutamate) Complexes
the whole complexes in chloroform solution. The results have allowed us to gain important new insights about the microscopic organization of surfactant‚polypeptide systems. The polypeptide main chain adopts a stable conformation in all the investigated complexes. This particular feature might be understood in terms of the polypeptide global shape. Thus, molecular cations enhance the conformational stability of the polypeptide backbone by providing protection toward the solvent accessibility and stabilizing the enormous electrostatic repulsions associated to the spatial proximity of the negatively charged peptide side chains. On the other hand, independently of the surfactant polar headgroup, the interaction pattern of surfactant‚PALG complexes always implies multiplicity and asymmetry. The structural differences found among complexes that differ in the polar headgroup of the surfactant ions have been explained according to the specific interactions with
Langmuir, Vol. 19, No. 9, 2003 3995
the polypeptide chain. Thus, when the size of the polar headgroup is small enough, i.e., ammonium, the cations tend to interact not only with the carboxylate groups but also with other electronegative atoms, i.e. the oxygen atom of the amide groups. The surfactant-amide interactions induce an elongated conformation in the polypeptide chain, which remains stable along the simulated time even although it is not regular. However, when no surfactantamide interaction can be achieved due the volume of the polar headgroup, the interaction of the surfactants with the polypeptide enhances the stability of the canonical R-helix conformation. Acknowledgment. This work was supported by DGICYT with Grant Nο. 3QU20000990 and the Centre de Supercomputacio´ de Catalunya (CESCA). LA026549O
