Quantifying Surfactant Alkyl Chain Orientation and Conformational

Oct 13, 2014 - Tatsuhiko Ohto , Ellen H. G. Backus , Wataru Mizukami , Johannes Hunger , Mischa Bonn , and Yuki Nagata. The Journal of Physical Chemis...
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Quantifying Surfactant Alkyl Chain Orientation and Conformational Order from Sum Frequency Generation Spectra of CH Modes at the Surfactant−Water Interface Michael Schleeger,* Yuki Nagata, and Mischa Bonn* Department of Molecular Spectroscopy, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: We combine second-order nonlinear vibrational spectroscopy and quantumchemical calculations to quantify the molecular tilt angle and the structural variation of a decanoic acid surfactant monolayer on water. We demonstrate that there is a remarkable degree of delocalization of the vibrational modes along the backbone of the amphiphilic molecule. A simulation-based on modeled sum frequency generation (SFG) spectra offers quantitative insights into the disorder of surfactant monolayers at the water−air interface. It is shown that an average of one gauche defect in the alkyl chain suffices to give rise to the methylene stretch intensity similar in magnitude to the methyl stretch.

SECTION: Biomaterials, Surfactants, and Membranes

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the vibrational responses, it is challenging to disentangle the SFG feature into individual contributions. A second complication in SFG is that although gauche defects clearly manifest themselves in the SFG spectrum through the appearance of methylene (CH2) stretch intensity, it has been difficult to quantify the defect density using the SFG spectra. Here, we combine experimental SFG spectroscopy of a surfactant on water and inexpensive theoretical calculations based on density functional theory (DFT) in vacuo. With this approach, we assess the relationship between the vibrational response and the structure of the monolayer and obtain insights into the involved normal modes in the C−H stretching region. From combined calculated and experimentally measured SFG spectra, we uniquely determine the tilt angle of the principal axis of surfactant alkyl chains by mapping the simulated molecular structures onto the SFG spectra, rather than assuming molecular orientation from the orientation of individual vibrational modes, and quantify the gauche defect density from the spectra. We focused on the C−H stretching SFG features of decanoate at the water−air interface. The resulting experimental SFG responses of the surfactant at SSP (with Spolarized SFG, S-polarized visible lights, and P-polarized infrared lights) and SPS polarization conditions are shown in Figure 1 with (a) H2O and (b) D2O as a subphase. The spectra show that the SFG peaks of the C−H stretching mode are affected by a broad tail of water stretch bands for both the H2O and D2O cases. The O−H and O−D stretch bands are centered

urfactant monolayers at the water−air interface constitute controllable and well-defined models for the behavior of lipids in biological membranes. The self-organization of surfactants occurring spontaneously at water−air interfaces leads to well-ordered systems due to the amphiphilic character of hydrophilic head groups and hydrophobic surfactant alkyl chains. The structures of such self-assembled surfactant and closely related lipid monolayers have been widely investigated in the last decades.1−8 Most of the applied techniques such as NMR, X-ray reflectivity, and fluorescence microscopy either lack surface specificity or provide only limited information about molecular structure. A particular fruitful method to gain molecular level insights into the structure of interfaces is vibrational sum frequency generation (SFG) spectroscopy because of its intrinsic surface specificity.2,9−14 Because vibrational frequencies are highly sensitive to structure and structural variations, the complex local environment at the interface can be uniquely addressed. To advance the understanding of the molecular structure at interfaces, different polarization conditions have been applied to identify the orientation of molecules at interfaces.13−17 The SFG signals from different polarization conditions can reveal the orientation of a vibrational mode, as demonstrated by Hore et al., who determined the orientation of the organosulfate (SO4−) headgroup of a sodium dodecyl sulfate monolayer.16 However, because the vibrational mode is localized at specific segments of molecules, vibrational techniques provide information on the local molecular structure. Therefore, it is often challenging to determine the principal axis of whole molecules. This drawback of the SFG technique typically emerges when the alkyl chain of the lipid/surfactant has gauche defects; because multiple microscopic structures with a gauche defect can contribute to © 2014 American Chemical Society

Received: September 17, 2014 Accepted: October 13, 2014 Published: October 13, 2014 3737

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Figure 2. (a) Structure of a specific decanoate rotameric conformer at the water interface. The molecular orientation is defined by the tilt angle θ toward the surface normal, the angle of rotation ψ around the molecular long axis, and the angle of rotation φ of the molecule around the surface normal. (b−d) Experimental (filled curves) and calculated SFG spectra (lines) in SSP (black) and SPS polarization (red). Spectra are calculated for the tilt angles θ = 80° (b), 65° (c), and 50° (d).

Figure 1. Experimentally measured SFG spectra of vibrational C−H stretching modes of a decanoate monolayer at (a) the H2O-air interface and (b) the D2O-air interface. (c) SFG intensity from only the C−H stretching modes reconstructed by fitting the experimental data. (d) Calculated SFG spectra for a molecular tilt angle of 65°. The SPS SFG spectra were all enhanced by the factor ×5.

calculated SSP and SPS spectra is observed for a molecular tilt angle of θ = 65° as presented in Figure 1c and d. As discussed below, this value represents an upper limit for the molecular tilt angle. The selection of θ from a comparison of simulated spectra with variable tilt angles is pointed out in Figure 2b−d (see below), demonstrating the sensitivity of the strongest bands in the calculated spectra to θ. The relative ratio of the main three bands in the SSP spectrum in Figure 1c and d and the two main bands in the SPS spectrum as well as the intensity ratio between the main bands in the SSP and SPS spectra are reproduced. The main discrepancies between experimental and calculated spectra are the lower predicted SSP band intensities corresponding to the experimental bands at 2855 and 2940 cm−1. Both can be attributed to the inability of our approach to include Fermi resonance (FR).13,19−21 The arrows in Figure 1d demonstrate the effects a FR would have on the calculated SSP spectrum: A new high-frequency band would appear at the expense of the main band (symmetric CH3, see assignment below) leading to a compensation of the main discrepancies between experiment and calculations. Because the Fermi resonance arises from the frequency overlap between the CH2 bending overtone and CH3 stretching modes, the normal mode calculation where the coupling between different modes are neglected cannot properly describe this effect.22 Additionally, the frequencies of several calculated bands are shifted with respect to the experimental ones. Our calculations are based on a harmonic approximation to calculate the normal-mode frequencies and a sigmoidal correction function is applied to compensate for the anharmonicity of vibrational modes. The observed frequency shifts hint on a more complex influence of anharmonicity on the contributing vibrational modes than assumed in our procedure. Given the overall reasonable agreement between the modeled and measured spectra, we can now attempt a more rigorous assignment of the experimental bands to vibrational normal modes. To aid the discussion on the nature of the SFG normal modes, we employ the vibrational amplitude correlation function (VACF).23 This function describes the average spatial correlation of the vibrational amplitudes between two oscillators for a given frequency, as a function of distance between those oscillators.24 The high frequency features above 2940 cm−1, labeled “*” in Figure 3b, correlate to the major band in the calculated SPS spectrum (transparent red line). It is

at ∼3400 and ∼2500 cm−1 and influence the higher and lower frequency region of the C−H stretching modes, respectively. To extract the C−H stretch peaks and remove the water stretch contributions, we fit the SFG spectra at the H2O and D2O/ decanoate interfaces using Lorentzians. The results of the fitting procedure are given in the Supporting Information. By using the parameters associated with the C−H vibrational mode, we reconstructed the SFG intensity contributed only by the C−H stretch modes, which is plotted in Figure 1c. To extract the information on the tilt angle of the decanoate at the water−air interface, we calculated the SFG responses decanoate in vacuum, whose headgroup was coordinated by four water molecules and determined the tilt angles through the direct comparison between the simulated and experimental data. In the SFG spectra calculation, we consider decanoate molecules containing maximally one single gauche defect. This is motivated by both the high surface pressure in the experiment, which will suppress gauche defects, and the high energetic cost, implying that the amount of decanoate molecules with two gauche defects is expected to be negligible at room temperature. The details of the calculation protocols can be found in the Supporting Information and elsewhere.18 The left panel of Figure 2 depicts a rotameric conformer of decanoate, exhibiting a kink at the end of the hydrocarbon tail. In our calculations, the average number of gauche defects per molecule was set to 0.83, see Supporting Information. The SFG spectra are calculated by taking the average of the nonlinear responses from six rotameric conformers and the individual SFG responses were numerically averaged for the rotations along the Euler angles ψ and φ. The calculated SSP and SPS spectra are scaled by the same factor, which sets the highest peak in the SSP spectrum equal to the experimental one at 2880 cm−1. We employ the main bands of the experimental SSP and SPS C−H stretch spectra and search for a delta-distribution of molecular tilt angles θ for which the calculated SSP and SPS spectra reproduce the experimentally observed relative intensities of these peaks (for angular distributions, see below). The best agreement between experimental and 3738

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Figure 3. Panels a and b depict the vibrational amplitude correlation function (VACF) vs distances of the vibrational sites, which was weighted with the calculated SSP and SPS amplitudes of the corresponding normal mode, respectively. On the right a representation of normal modes is illustrated, which mainly contributes to the symmetric and antisymmetric CH3 bands. The coloring indicates sites of vibration. In-phase and out-of-phase displacements of the C−H groups are displayed in red and blue, respectively, with their brightness indicating the relative vibrational amplitude. (c) Order ratio as a function of the fraction of all-trans decanoate as a measure of conformational order. Three molecular tilt angles of θ = 0° (black spheres), 50° (red squares), and 80° (blue diamonds) were investigated.

agreement between predicted and experimental SFG spectra. Notably, the tilt angle of decanoate at a surface pressure at 30 mN·m−1 of 65° is larger than determined for fatty acids with longer alkyl chains.2,13 This value of 65° constitutes an upper limit on the angle and should be considered with some care, as the calculations overestimate the symmetric CH3 vibrational band by not accounting for the effect of FR. The tilt angle is not only an essential parameter for the molecular structure but also is crucial to discuss the order/ disorder of alkyl chains within an amphiphilic monolayer. This order is regularly correlated with the ratio of two bands in the SSP SFG spectrum, commonly assigned to symmetric stretching vibrations of the CH3 and CH2 groups.2,9 To explore the relation between this disorder ratio, the conformational order and the tilt angle, we performed a demonstrative calculation of the ratio in dependence of the fraction of the alltrans conformer and by varying the tilt angles θ.25 The resulting data is plotted in Figure 3c, indicating that the order ratio is, as expected, sensitive to the fraction of kinked rotational conformers. Yet, this dependence is in part overcompensated by the sensitivity of the order ratio on the molecular tilt angle θ, especially at higher fractions of the all-trans conformer. An analogous approach as presented in Figure 3c was applied to reveal the effect of orientational disorder on the order ratio. Allowing for a broad and uniform distribution of the tilt angle θ has only a small effect on the order ratio, as shown in the Supporting Information. In conclusion, we proposed a pathway to consider gauche defect contributions for the prediction of nonlinear SFG spectra of a surfactant based on quantum chemical calculations. In combination with experimental data at the water−air interface, it was possible to rigorously address the contributions

built up by two types of localized asymmetric CH3 vibrational modes. The strongest VACF values are found at short distances of vibrational sites, predominantly corresponding to the distance of two C−H sites in a methyl group. To the right of Figure 3b the displacements within two normal modes, which significantly contribute to the discussed SFG band are illustrated. The all-trans form of decanoate as well as a rotameric conformer with a kink at carbon atom no. 4 show, as expected, the localized nature of the antisymmetric CH3 vibrational mode. The prominent features around 2880 cm−1 in Figure 3a, marked with “**”, reflect vibrational modes constituting the main band in the SSP SFG spectrum (transparent black line). They arise primarily from symmetric CH3 vibrational modes. These are mostly strongly localized, as indicated by the normal mode representation to the right of Figure 3a for the all-trans form of decanoate. Thus, in specific rotameric conformers, a gauche defect leads to delocalized normal modes, which involves symmetric CH2 stretching vibrations throughout the molecule. This is illustrated for the rotamer structure with a kink at C atom no. 6 to the right of Figure 3a. In average, additional pronounced VACF values are present in Figure 3a, at distances of 3, 7, and 10 Å. To further illustrate the sensitivity of the calculated spectra to the molecular tilt angle θ, the calculated SFG responses are plotted by varying θ. As panels b−d of Figure 2 indicate, the best agreement between experimental and calculated SSP and SPS C−H stretch spectra is observed for a molecular tilt angle of θ = 65°. Furthermore, this figure clearly illustrates that the 2965 cm−1 band in the SPS signal and 2880 cm−1 band in the SSP signal constitute the key features to uniquely determine the tilt angle. Assuming a distribution of molecular tilt angles toward the surface normal does not lead to an improved 3739

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(2) Roke, S.; Schins, J.; Muller, M.; Bonn, M. Vibrational Spectroscopic Investigation of the Phase Diagram of a Biomimetic Lipid Monolayer. Phys. Rev. Lett. 2003, 90, 128101−1−128101−4. (3) Daillant, J.; Benattar, J. J.; Bosio, L. X-Ray Reflectivity Study of Monolayers of Amphiphilics at the Air−Water-Interface. J. Phys. Condens. Mater. 1990, 2, Sa405−Sa410. (4) Daillant, J.; Bosio, L.; Benattar, J. J. X-Ray Reflectivity Study of the Liquid-Expanded Liquid-Condensed Phase-Transition. Europhys. Lett. 1990, 12, 715−720. (5) Perly, B.; Smith, I. C. P.; Jarrell, H. C. Effects of the Replacement of a Double-Bond by a Cyclopropane Ring in Phosphatidylethanolamines: a 2H NMR Study of Phase Transitions and Molecular Organization. Biochemistry 1985, 24, 1055−1063. (6) Lafleur, M.; Cullis, P. R.; Fine, B.; Bloom, M. Comparison of the Orientational Order of Lipid Chains in the Lα and HII Phases. Biochemistry 1990, 29, 8325−8333. (7) Thurmond, R. L.; Lindblom, G.; Brown, M. F. Influences of Membrane Curvature in Lipid Hexagonal Phases Studied by Deuterium NMR-Spectroscopy. Biochem. Biophys. Res. Commun. 1990, 173, 1231−1238. (8) Thurmond, R. L.; Lindblom, G.; Brown, M. F. Curvature, Order, and Dynamics of Lipid Hexagonal Phases Studied by Deuterium NMR Spectroscopy. Biochemistry 1993, 32, 5394−5410. (9) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. Observation of Molecular Ordering at the Liquid−Liquid Interface by Resonant SumFrequency Generation. J. Am. Chem. Soc. 1995, 117, 8039−8040. (10) Walker, R. A.; Gruetzmacher, J. A.; Richmond, G. L. Phosphatidylcholine Monolayer Structure at a Liquid−Liquid Interface. J. Am. Chem. Soc. 1998, 120, 6991−7003. (11) Watry, M. R.; Tarbuck, T. L.; Richmond, G. I. Vibrational SumFrequency Studies of a Series of Phospholipid Monolayers and the Associated Water Structure at the Vapor/Water Interface. J. Phys. Chem. B 2003, 107, 512−518. (12) Bonn, M.; Bakker, H. J.; Ghosh, A.; Yamamoto, S.; Sovago, M.; Campen, R. K. Structural Inhomogeneity of Interfacial Water at Lipid Monolayers Revealed by Surface-Specific Vibrational Pump−Probe Spectroscopy. J. Am. Chem. Soc. 2010, 132, 14971−14978. (13) Guyotsionnest, P.; Hunt, J. H.; Shen, Y. R. Sum-Frequency Vibrational Spectroscopy of a Langmuir FilmStudy of MolecularOrientation of a Two-Dimensional System. Phys. Rev. Lett. 1987, 59, 1597−1600. (14) Beattie, D. A.; Fraenkel, R.; Winget, S. A.; Petersen, A.; Bain, C. D. Sum−Frequency Spectroscopy of a Monolayer of Zinc Arachidate at the Solid−Solid Interface. J. Phys. Chem. B 2006, 110, 2278−2292. (15) Chen, X.; Wang, J.; Boughton, A. P.; Kristalyn, C. B.; Chen, Z. Multiple Orientation of Melittin Inside a Single Lipid Bilayer Determined by Combined Vibrational Spectroscopic Studies. J. Am. Chem. Soc. 2007, 129, 1420−1427. (16) Hore, D. K.; Beaman, D. K.; Parks, D. H.; Richmond, G. L. Whole-Molecule Approach for Determining Orientation at Isotropic Surfaces by Nonlinear Vibrational Spectroscopy. J. Phys. Chem. B 2005, 109, 16846−16851. (17) Rivera, C. A.; Fourkas, J. T. Reexamining the Interpretation of Vibrational Sum−Frequency Generation Spectra. Int. Rev. Phys. Chem. 2011, 30, 409−443. (18) Volkov, V.; Bonn, M. Structural Properties of gp41 Fusion Peptide at a Model Membrane Interface. J. Phys. Chem. B 2013, 117, 15527−15535. (19) Sibert, E. L., 3rd; Kidwell, N. M.; Zwier, T. S. A First-Principles Model of Fermi Resonance in the Alkyl CH Stretch Region: Application to Hydronaphthalenes, Indanes, and Cyclohexane. J. Phys. Chem. B 2014, 118, 8236−8245. (20) Schachtschneider, J. H.; Snyder, R. G. Vibrational Analysis of the N-Paraffins II. Normal Co-ordinate Calculations. Spectrochim. Acta 1963, 19, 117−168. (21) Lu, R.; Gan, W.; Wu, B. H.; Chen, H.; Wang, H. F. Vibrational Polarization Spectroscopy of CH Stretching Modes of the Methylene Group at the Vapor/Liquid Interfaces with Sum Frequency Generation. J. Phys. Chem. B 2004, 108, 7297−7306.

of various vibrational modes involved in the measured spectra with particular focus on molecular structure and sample composition. Our method offers an alternative to extract molecular tilt angles at interfaces using the main bands of SSP and SPS spectra as demonstrated here for a short-chain fatty acid. An analysis of the vibrational normal modes demonstrates further that the symmetric CH3 stretch band in the SSP spectrum is mainly build up by localized modes for all-trans and several kinked rotameric conformers of decanoate. In addition, we could show a pronounced delocalization in contributing normal modes for distinct gauche defects. Such rigorous insight into the normal modes involved in SFG spectra of a monolayer of amphiphilic molecules was developed for the first time.

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EXPERIMENTAL METHODS We studied decanoic acid at the water−air interface. For SFG experiments, a 0.2 M solution of decanoate (decanoic acid from Sigma-Aldrich, purity ≥98.0%) in water/NaOH was prepared above the critical micelle concentration (cmc =102 mM at pH = 11.8, 25 °C) at pH = 12.26 The decanoate solution was filtered using a 0.2 μm syringe filter, transferred into a Teflon trough, and equilibrated for 3 h to allow the formation of a stable monolayer. For the preparation of a decanoate sample in D2O, the pD was adjusted by 30 wt % NaOD in D2O (SigmaAldrich, 99 atom % D) and determined from the pH-meter reading as pD = pHread − 0.4.27 The surface pressure was determined to be 30 mN·m−1 using a tensiometer (KBN 315, Kibron Inc., Helsinki, Finland) with a needle as a probe and a laser displacement sensor (Hl-6103-S-J, Panasonic) to ensure the same height conditions for water calibration and sample solutions. The employed SFG setup is similar to that described by Engel at al.28 Incident angles of the visible and IR laser pulse were 41° and 46°, respectively, the spectral bandwidth of the visible laser pulse was restricted to 15 cm−1 full width at halfmaximum (fwhm).

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ASSOCIATED CONTENT

S Supporting Information *

Further details to the computational methods, the individual calculated spectra of all regarded rotameric conformers of decanoate as well as a figure investigating the order ratio for orientational disordered model monolayers of decanoate are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected]. *[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NanoNextNL, a micro- and nanotechnology consortium of the Government of The Netherlands and 130 partners. M.S. thanks Dr. Victor Volkov for helping with calculations of nonlinear spectra and analysis of delocalization within vibrational modes.

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REFERENCES

(1) McConlogue, C. W.; Vanderlick, T. K. A Close Look at Domain Formation in DPPC Monolayers. Langmuir 1997, 13, 7158−7164. 3740

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(22) Martin, J. M. L.; Lee, T. J.; Taylor, P. R.; Francois, J. P. The Anharmonic-Force Field of Ethylene, C2H4, by Means of Accurate AbInitio Calculations. J. Chem. Phys. 1995, 103, 2589−2602. (23) Torii, H. Extent of Delocalization of Vibrational Modes in Liquids as a Result of Competition between Diagonal Disorder and Off-Diagonal Coupling. J. Phys. Chem. A 2004, 108, 2103−2107. (24) We define a vibrational C−H site within the equilibrium structure of the respective molecule as the center of mass of a carbon and hydrogen atom. The distance between two vibrational sites is received from the equilibrium structure. (25) Here, the order ratio is defined by the SFG transition strengths χn as follows: χ(ν(CH3,sym.))/χ(ν(CH2,sym.)), which are accessed by fitting the simulated spectra to the absolute square of the sum of three complex Lorentzian curves. The molecular tilt angle θ is the angle between the molecular long axis, and the surface normal. The molecular long axis coincides with the long axis of inertia. (26) Namani, T.; Walde, P. From Decanoate Micelles to Decanoic Acid/Dodecylbenzenesulfonate Vesicles. Langmuir 2005, 21, 6210− 6219. (27) Glasoe, P. K.; Long, F. A. Use of Glass Electrodes to Measure Acidities in Deuterium Oxide. J. Phys. Chem. 1960, 64, 188−190. (28) Engel, M. F.; Vandenakker, C. C.; Schleeger, M.; Velikov, K. P.; Koenderink, G. H.; Bonn, M. The Polyphenol EGCG Inhibits Amyloid Formation Less Efficiently at Phospholipid Interfaces than in Bulk Solution. J. Am. Chem. Soc. 2012, 134, 14781−14788.

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