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Interaction of Sodium Dodecylsulfate with Lipid Monolayer Studied by Sum-Frequency Generation Spectroscopy at Air-Water Interface Ankur Saha, Sumana SenGupta, Awadhesh Kumar, and Prakash D. Naik J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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The Journal of Physical Chemistry

Interaction of Sodium Dodecylsulfate with Lipid Monolayer Studied by Sum-Frequency Generation Spectroscopy at Air-Water Interface Ankur Saha, Sumana SenGupta, Awadhesh Kumar*, Prakash D. Naik Radiation & Photochemistry Division Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085 Also affiliated to: Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai–400 094

*

Corresponding author

Tel.: 91-22-25590302 Fax: 91-22-25505151, 25505331

1 ACS Paragon Plus Environment


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ABSTRACT We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular level understanding of interaction of anionic surfactant sodium dodecylsulfate (SDS), in low bulk concentration at the µM level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) at the air-water interface. These results are different from that reported at higher bulk concentration of SDS at mM level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase. But, with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPPC are responsible for an increase in the conformational order of the alkyl chain of DPPC with a decrease in the gauche defect and increase in trans conformer. Similarly, the orientation and concentration of interfacial water molecules of DPPC monolayer at SDS subphase are controlled by concentration of both SDS and DPPC. The VSFG results are complemented by the surface pressure measurements.


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1. INTRODUCTION A phospholipid monolayer at the air−water interface is subjected to several studies for decades, mainly because of its biological importance. This system is an excellent model for cell membrane, lung surfactant, and 2D material at asymmetric interface1-3. Among all the phospholipids, phosphatidylcholines (PC) have been investigated extensively as a simple model system for a cell membrane. Basic understanding on structure of phospholipid monolayer, employing sum frequency generation spectroscopy, has been reported4,5. The molecular interaction of the lipid monolayer with various biologically active molecules has a significant effect on biophysical and structural properties of lipid monolayers. In this regard, surfactants 6-9, amino acids (particularly, phenylalanine

10,11

), various drugs

12,13

, carbon-based particles

14-16

and ions 17,18 are being investigated by many researchers. Importance of such studies particularly between charged surfactants and lipids cannot be over emphasized, because of their relevance to biological models19, packaging material for DNA/RNA based drug delivery20-22 and atmospheric aerosols23. Studying the interaction between lipids, models for cell membrane, and charged surfactants, models for soluble amphiphilic biomolecules, can provide information on interaction between biological surfactants and cell membranes. Such understanding is important for development of gene delivery agents. In addition, fundamental understanding of interactions of surfactants is needed at an air-water interface, because atmospheric aerosols have a variety of organic surfactants. Similarly, interactions of phospholipids are applicable to marine aerosol surfaces as well24. Since these molecular interactions are weak with long-range characters, their determination is challenging, and hence these studies are pursued employing different experimental techniques, in addition to theoretical modeling9.



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Among various surface techniques, vibrational sum-frequency generation (VSFG) spectroscopy provides surface specific molecular level understanding of interfacial molecules, and, therefore, VSFG is extensively employed for interfacial studies. Recent reviews provide details on theory and applications of VSFG in general25, and on Langmuir monolayers in particular26. We have employed this spectroscopy to investigate interaction of anionic surfactant sodium dodecylsulfate (SDS) with the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3phosphocholine (DPPC) monolayer. Chemical structures of DPPC and SDS are given in Scheme 1. The interaction leads to a change in the structure and other surface properties of the monolayer, and it is strongly dependent on the concentration27. Since most of the studies are reported with Scheme 1. Chemical Structures of DPPC and SDS

relatively high concentration (mM level), we investigated interaction of SDS with DPPC monolayer in lower concentration regime of SDS to examine if there is any difference. Surfactant SDS was selected because it is soluble and has less surface activity (in comparison to cationic cetyltrimethylammonium bromide, CTAB)

28

. Thus, a low bulk concentration (at µM

level) of SDS will not show any VSFG feature in the CH stretching region, and effect of its interaction with DPPC resulting into a change in the VSFG spectra can be easily discerned. Some of our results are in contrast to similar VSFG studies using higher bulk concentration (at


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mM level) of SDS6, mainly because of different SDS concentrations used in these experiments. At higher bulk concentration, VSFG intensity of lipid monolayer in the CH region due to acyl chain is reported to be lower in the SDS subphase than the water subphase6. Similarly, the VSFG intensity of SDS at the air-water interface is found to be lower in the presence of the lipid monolayer than in its absence, due to decreased surface number density of SDS molecules in the former 6. We have combined the VSFG studies with surface pressure measurements to get insights into the nature of interaction. In earlier reports on such interaction using surface pressure-area (π-A) isotherm, the number of molecules in the monolayer is kept constant, but the surface area is changed by compression to alter the number of molecules per unit surface area. In the present work, the surface area is kept constant, but the number of molecules is changed to alter number of molecules per unit surface area. THEORETICAL BACKGROUND Theoretical background on SFG is described in detail in the literature29-32. Briefly, the SFG intensity, ISFG, is proportional to the absolute square of the second-order nonlinear susceptibility, χ (2 ) , which consists of a non-resonant ( χ nr(2 ) ) and resonant terms ( χ r(2 ) ), and given by eqn. 1 , I SFG (ω SFG ) ∝ χ (2 ) (ω IR ) ∝ χ nr(2 ) + χ r(2 ) (ω IR ) 2

2

(1)

The resonant part is given by eqn. 2, 

   (ω IR − ω q ) + iΓq 

χ r(2 ) (ω IR ) = ∑  q

Aq

(2)



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in which Aq, ωq and Γq represent the magnitude, resonant frequency and damping constant, respectively, for the qth vibrational mode. The measured VSFG spectra can be fitted to obtain these parameters for a particular polarization combination, which is denoted based on the polarization states of the SFG, visible and IR beams in the sequence. We have employed the relative values of Assp (in ssp polarization combination) for the CH3 symmetric and asymmetric stretch to estimate the value of the orientation angle (θ) of the terminal methyl group, which is the angle between the symmetric axis of the methyl group and the surface normal, employing a standard equation (eqn. 3). The equation is given as follows for an isotropic surface and a delta function of the orientation distribution, 6 ( 2) χ ssp (CH 3 symmetric) β ccc [(1 + r ) cosθ − (1 − r ) cos3 θ ] = , (2) χ ssp (CH 3 asymmetric ) 2 β caa (cosθ − cos3 θ )

(3)

where depolarization ratio (r) is given by βaac/βccc with a range of possible values between 1.66 and 4.2.29,33

A similar equation (eqn. 4) was also employed to estimate the orientation angle of

the terminal methyl group using the relative values of fitting parameter Appp to Assp 29. ( 2) χ ppp (CH 3 symmetric) 2[ r cosθ + (1 − r ) cos3 θ ] = ( 2) χ ssp (CH 3 symmetric) [(1 + r ) cosθ − (1 − r ) cos3 θ )]

(4)

2. EXPERIMENTAL 2.1. Vibrational Sum-Frequency Generation. VSFG spectroscopy involves a nonlinear process with interaction of two input beams of tunable infrared (IR beam, 2.3 to 10.0 µm) and fixed visible (generally 532 nm or 800 nm) wavelengths to produce the third beam (signal) at the frequency of sum of the input frequencies.


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These two input laser beams are partially focused and overlapped spatially and temporally at the interface. The angles of incidence for the IR and visible laser beams were kept at 55° and 60°, respectively, with respect to the surface normal. The IR wavelength is scanned across the vibrational resonances (2800 to 3000 cm-1 for the CH region and 3000 to 3800 cm-1 for the OH region) of the interfacial molecules to obtain the IR absorption spectra of interfacial molecules. We performed the experiments in two different polarization schemes of ssp and ppp, and these are denoted based on the polarization states of the SFG, visible and IR beams in the sequence. For example, the ssp polarization implies that the SFG and visible beams are s-polarized, and the IR beam is p-polarized. The VSFG setup has been described in our earlier publications34,35. Briefly, the VSFG spectrometer is a commercial Ekspla laser system having the repetition rate of 10 Hz and the pulse width of 30 ps. This SFG spectrometer has the spectral resolution of about 2 cm-1 in the C-H stretching range. The fundamental output of a Nd:YAG laser (PL2241B, Ekspla, Lithuania) is frequency doubled to generate visible beam at 532 nm. The idler output of an optical parametric generator (OPG) is mixed with the fundamental output of the Nd:YAG laser (1064 nm) in a difference frequency generator (DFG) with silver thiogallate (AgGaS2) crystal to generate the tunable IR beam. The third harmonic beam of the Nd:YAG laser (355 nm) was used to pump the OPG (PG401, Ekspla), which has lithium triborate (LiB3O5) as a nonlinear crystal. In general, about 30 min were allowed for the DPPC monolayer to reach equilibrium before polarized SFG spectra were measured. Each spectral data point in the VSFG spectra was averaged for 60 shots, and normalized by the energies of the input IR and visible beams. We have measured VSFG spectra in both the CH and OH stretching regions for a fixed DPPC surface concentration (1.2x1014 molecules/cm2) and varying SDS bulk concentration (0 to 120 µM). Similar measurements were carried out for a fixed SDS concentration (60 µM) and



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varying DPPC concentration (6.0x1013, 1.2x1014 and 2.4x1014 molecules/cm2). We fitted the measured spectra employing a standard procedure29,36,37 and using equations 1 and 2 to quantitatively analyze the dependence of the intensity of VSFG peaks on bulk or surface concentration.

2.2. Surface Pressure-Time (π− −t) Adsorption Kinetics. To measure the surface pressure of a Langmuir monolayer of DPPC at the water subphase and SDS solution subphase, the π−t adsorption kinetics was recorded at room temperature of ∼297 K, using a platinum Wilhelmy plate microbalance with an accuracy of ±0.02 mN/m. A known volume of DPPC in CHCl3 solution was spread at either the water or the SDS solution subphase in a Teflon trough using a Hamilton syringe. The surface pressure was recorded on equilibration of the system at various concentrations of SDS and DPPC.

2.3. Materials and Sample Preparations. All chemicals used were commercial products, which were used without any further purification. DPPC and SDS were purchased from Sigma-Aldrich (purity ≥ 99%), whereas chloroform was procured from Thomas Baker (Mumbai, purity ≥ 99%). DPPC stock solution (~100 µM) in chloroform was stored in a refrigerator even between uses, and was used for a maximum of 2 days. SDS solutions were prepared in ultrapure Millipore water (pH=5.56, resistivity 18.2 MΩ-cm, surface tension of 71.9 mN/m), which was devoid of any hydrocarbon impurity as checked with VSFG measurement. These samples were stored in glass containers with stoppers. For SFG measurements a known volume of either aqueous subphase or the SDS subphase was taken in a Teflon trough, and required volume of DPPC stock solution was added carefully to the subphase, employing a Hamilton syringe. Measurements were initiated after 30 minutes to allow sufficient time for evaporation of chloroform. The SDS commercial sample can 8 ACS Paragon Plus Environment

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have dodecanol impurity to the extent of 1%, which can have some contribution to our measured VSFG spectra.

3. RESULTS AND DISCUSSION We applied VSFG spectroscopy to understand the structures and orientation of the monolayer of DPPC, and how these are affected in the presence of SDS. The CH stretching vibrations of both CH2 and CH3 groups in the hydrophobic alkyl chain of DPPC were measured in the wavenumber range of 2800 to 3000 cm-1. VSFG spectra of the OH stretching vibration of interfacial water molecules were also measured in the wavenumber range of 3000 to 3800 cm-1, to know how structure and orientation of these water molecules in DPPC monolayer are influenced by SDS molecules in the subphase.

3.1. VSFG Spectra of DPPC Monolayer with SDS Subphase VSFG spectra of DPPC monolayer were measured in the CH (with ppp and ssp polarizations) and OH (with ssp polarization) stretching regions in both the water and the SDS subphases.

3.1.1. CH Vibrational Stretch Region: Effect of SDS Concentration We measured the surface density dependence of DPPC monolayer on its VSFG intensity in the CH stretch region, and observed mainly three vibrational bands with ssp polarization at 2850, 2879 and 2941 cm-1, which are assigned to the CH2 symmetric stretch, νs(CH2), CH3 symmetric stretch, νs(CH3) and CH3 Fermi resonance, νFR(CH3), due to interaction of the symmetric stretching mode and the bending overtone of the methyl group, respectively (shown in Figure 1)

4-6

. The figure shows that the ppp spectra are dominated by the band at 2964 cm-1,

which is assigned to an asymmetric stretch, νas(CH3). The observation of 2850 cm-1 band suggests presence of gauche defect in the alkyl chains of DPPC. At increased DPPC



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concentration, one additional weak band is observed at ~2902 cm-1. This band is assigned to the νFR(CH2) band of the acyl group with some contribution from the νs(CH2) band of the phosphocholine headgroups4-6, and observed because of enhanced surface concentration of DPPC. This band can have some contribution from the CH2 symmetric stretch of the phospholipid glycerol moiety, and labeled as ν(CH2) in Table S1 of the Supplementary Material. The dependence of VSFG intensities of DPPC monolayer at the air-water interface in the CH and OH vibrational frequency regions was investigated in presence of SDS between 0 to 120 µM aqueous solutions. The VSFG intensity of DPPC monolayer in the CH stretching region is changed in the presence of SDS solution, which is depicted in Figure 1. This figure depicts the VSFG spectra with ssp [marked as (a)] and ppp [marked as (b)] polarizations, having different SDS concentrations (0 to 120 µM). With an increasing SDS bulk concentration, the intensities of the bands due to the CH3 group of the acyl chain increase. At this low concentration of SDS, the VSFG signal for the CH3 stretch from SDS is either absent or negligibly small (shown as the lowest trace in both the panels of Figure 1 for 120 µM bulk concentration). The νas(CH3) signal due to the DPPC monolayer (1.2x1014 molecules/cm2) in the water subphase is weak (marked with “DPPC only”). However in the SDS subphase, the signal goes on increasing with increased SDS bulk concentration (30 µM to 120 µM). In general, the VSFG intensity depends on both the surface number density and orientation of interfacial molecules. Since the average surface number density of DPPC molecules in the monolayer is not expected to increase with increasing SDS molecules in the subphase, a change in structure and orientation of DPPC molecules is responsible for enhanced VSFG intensity. It suggests that the SDS molecules intercalate in the DPPC monolayer, thereby facilitate stronger hydrophobic interaction between long hydrocarbon chains. Similar enhancement of the VSFG intensity of the CH3 group has been observed by 10 ACS Paragon Plus Environment

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several researchers in different systems4,37, including us in the mixed surfactants of SDS and CTAB34. Increased trans structure of the hydrocarbon chains and reduced gauche defects with increasing SDS concentration are supported by our VSFG measurements in the ssp polarization. The νs(CH2) band at 2850 cm-1 (ssp polarization, Figure 1) decreases in intensity with respect to the νs(CH3) band at 2879 cm-1. The results imply highly ordered conformation of the methylene chains with increasing trans structure in the presence of SDS 8, because of a strong hydrophobic interaction of alkyl chains. This interaction is shown schematically in Scheme 2, wherein the two tails of a DPPC molecule are shown as one for better clarity.

Scheme 2. Schematic representation of interaction between SDS and DPPC monolayer at air-water interface

The interactions of similar nature are observed by other researchers6, who found an increase in the order of the acyl chain on incorporation of SDS molecules in the DPPC monolayer. However, in contrast to present work they found that the VSFG intensity of the lipid layer decreased at the SDS subphase in comparison to the water subphase. The authors6 explained the effect due to decrease in the monolayer number density of the lipid molecules because of their aggregation in the presence of SDS with bulk concentration in the mM range. The aggregates can have an inversion symmetry precluding their detection by VSFG, and thus



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apparently reduced number density. However, it appears that the authors have compared only the ssp spectra, which many times do not represent correct concentration because of complications due to Fermi interaction. In general, the νas(CH3) band intensity in the ppp spectra of the CH stretch region is relatively more accurate measure of surface concentration than the νs(CH3) and νFR(CH3) band intensities in the ssp spectra because of Fermi interaction33 and interference from interfacial water OH stretch in the ssp spectra38. Their ppp spectra reveal no significant change in VSFG intensity of the lipid monolayer at the water and SDS subphases. In the present work, the SDS bulk concentration is low at the µM level, and hence the aggregation of DPPC molecules induced by SDS molecules does not play a role, and we do not observe a decrease in DPPC number density in the presence of SDS molecules. This observation is in agreement with earlier findings that incorporation of SDS molecules in the lipid monolayer does not affect the surface number density of lipid molecules 8.

3.1.2. CH Vibrational Stretch Region: Effect of DPPC Concentration We observed enhanced VSFG intensity of the terminal CH3 group of DPPC monolayer in the presence of SDS because of incorporation of the SDS molecules in the monolayer of DPPC. Similarly, the VSFG intensity should be modified with a change in the DPPC surface concentration at a fixed concentration of SDS. The VSFG spectra were measured in both ssp and ppp polarizations with varying surface concentrations of DPPC for a fixed SDS bulk concentration of 60 µΜ. These spectra for DPPC concentration of 6.0x1013, 1.2x1014, and 2.4x1014 molecules/cm2 are depicted in Figures 2(a) and 2(b) for the ppp and ssp polarizations, respectively. The concentration of DPPC is marked in the figures without its unit, which is molecules/cm2. SDS alone does not produce any feature due to its low concentration. Even DPPC at the lowest concentration of 6.0x1013 molecules/cm2 does not produce any signal at the


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water subphase. But, DPPC at the SDS subphase shows usual VSFG features in both the polarizations, implying a strong interaction between these molecules. We observed mainly two effects on the VSFG spectra with increasing DPPC concentration. The features due to DPPC at the water subphase can be seen at higher concentrations, and their intensities increase with its increasing concentration due to increased surface number density. The intensity increases with increasing DPPC concentration at the SDS subphase (with a fixed SDS concentration) also, since at higher concentration more DPPC molecules are available which can get oriented on induced by SDS molecules. One can expect the increase in the intensity with respect to the air-water interface (that is, with respect to DPPC alone) to be higher at higher DPPC concentration. On the contrary the increment in the VSFG intensity at the SDS subphase reduces with increasing DPPC concentration. This reduction in increment at higher DPPC concentration can be explained based on decreased incorporation probability of SDS molecules into the monolayer because of greater steric hindrance imposed by increased number of DPPC molecules for SDS molecules.

3.1.3. Orientation angle of the terminal methyl group We have estimated the average orientation angle of the terminal methyl group of DPPC molecule with respect to the normal to the surface, employing methods used by several researchers6,29,30,39, to understand how it changes from the water subphase to the SDS subphase at the air-water interface. We have employed equation 3 and the relative values of Assp (given in Table S1 of the supplementary material) for the CH3 symmetric and asymmetric stretch to estimate the value of the orientation angle (θ) of the terminal methyl group. We have used values of r = βaac/βccc = 4.0 33,40 and βcaa/βaac = 4.241 in this equation. The average orientation angle was estimated to be 17°±1° for the CH3 group of DPPC at the water subphase as well as at SDS subphase for the CH3 group of DPPC. Thus, there is a weak or no dependence of the θ value on



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the SDS concentration. The measured orientation angle of 17° for the terminal CH3 group is in good agreement with the reported values of 16.6±0.6° 6,27, but much lower than that of 35° to 40° reported recently33. The difference lies in the symmetry used for the CH3 group in calculations, which is C3v for the former and Cs for the latter. The difference is also associated with inaccuracy in estimating the relative intensity of the vas(CH3)/vs(CH3) in the ssp polarization because of complications due to Fermi interaction33 and overlapping OH stretching band of interfacial water molecules 42. The relative intensity of the vs(CH3) in the ppp and ssp polarizations is suggested to be a better method for estimation of the orientation angle of the terminal CH3 group, because the effects of the Fermi interaction get cancelled33. Moreover, the OH stretching bands of water molecules do not overlap in this CH stretch region. Using the relative values of Appp to Assp (given in Table S1 of the supplementary material) for the CH3 symmetric stretch in equation 4 and taking the value of r =4.0 40, we estimated the average orientation angle to be 22°±2° for the CH3 group of DPPC at the water subphase and 26°±4° at the SDS subphase. These calculated values of the orientation angle are in better agreement with the reported values 33 of 35° to 40° than that (17°±1°) calculated based on the relative values of Assp for symmetric and asymmetric CH3 stretch.

3.1.4. OH Vibrational Stretch Region: Effect of SDS Concentration In the OH vibrational stretch region (3000 to 3800 cm-1), VSFG features are observed due to polar oriented water molecules at the air-water interface. Two broad features are detected at ~3200 and 3400 cm-1 due to hydrogen bonded water molecules, with a sharp feature at ~3720 cm-1 due to free OH with the ssp polarization. In the presence of DPPC at the water subphase


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only both broad features (but enhanced in intensity) are observed, implying the absence of free OH at the interface. DPPC molecules are zwitterionic neutral species, and the charge separation in the headgroup produces a small net electric field, which is responsible for enhancement of VSFG intensity. But, SDS molecules being charged species, produce relatively strong electric field, which orient interfacial water molecules more efficiently to generate enhanced VSFG intensity in the OH stretch region. A detailed dynamics of interaction between SDS and water molecules are now being revealed using two-dimensional sum frequency generation spectroscopy43. SDS molecules orient water molecules even in the presence of DPPC monolayer, and the VSFG intensity increases with increasing SDS bulk concentration from 30 to 120 µM (spectra shown as dotted curves in Figure 3) because of increasing surface concentration of SDS molecules. The VSFG intensity in the OH stretching region of the DPPC monolayer at the SDS subphase is found to be greater (shown as solid curves in Figure 3) than that at the water subphase and also greater than only SDS at the air-water interface (dotted curves). This enhanced VSFG intensity at the SDS subphase can be due to increased alignment of interfacial water molecules normal to the interface and/or an increase in the sampling depth. The result implies greater number of SDS molecules getting incorporated in the DPPC monolayer. However, at higher SDS concentrations of 90 and 120 µM the increased VSFG intensity in the OH stretch region remains almost the same. It implies that the SDS uptake in the DPPC monolayer is almost saturated. These results are supported by an initial increase in the surface pressure of the monolayer and finally getting saturated with increasing SDS concentration (vide infra). An increase in the VSFG intensity with increasing SDS concentration with a fixed DPPC surface number density can result from incorporation of SDS molecules in the DPPC monolayer leading to a packed structure of DPPC and SDS molecules at the interface. The packed structure will



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lead to a rigid and narrow distribution of their polar heads, which can be responsible for more effective polar orientation of interfacial water molecules. The interfacial SDS molecules interact with DPPC monolayer involving mainly hydrophobic interactions between their hydrocarbon chains with probably weak electrostatic interactions between their polar headgroups. If the headgroups interact, the SDS molecules with negatively charged headgroup can bind to the positively charged region of DPPC. On this binding the charge on SDS molecules should reduce. Since the SDS molecules alone produce greater VSFG intensity than DPPC monolayer, SDS binding can lead to reduced polar orientation of the interfacial water molecules, and hence reduced VSFG intensity in the OH stretching region. However in the present studies the VSFG intensity of SDS in the OH stretching region is either enhanced (at lower SDS concentration) or remains almost unaltered (at higher SDS concentration) in the presence of the DPPC monolayer, this electrostatic interaction may not occur to an observable level between SDS and DPPC molecules. But, electrostatic interactions with water molecules play an important role in orientation of interfacial water molecules. However, in case of the interactions between the negatively charged SDS headgroup with the positively charged choline group in DPPC at the interfaces, as mentioned earlier, the negatively charged phosphate groups can get exposed and still make the overall surface negatively charged. Therefore, with our current spectral observations in the OH stretching regions we cannot comment unambiguously about the role of interactions between the charged headgroups.

3.1.5. OH Vibrational Stretch Region: Effect of DPPC Surface Density In the previous section we observed an increase in the VSFG intensity in the OH stretch region of the DPPC monolayer with increasing SDS concentration at the subphase in comparison


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to the water subphase. Similarly, at a fixed SDS concentration (~28 µM), the intensity is enhanced significantly with increasing DPPC surface density, which is shown in Figure 4(a). However, with high SDS concentration of 85 µM and at high DPPC concentration (2.3x1014 molecules/cm2), the enhanced VSFG intensity is slightly greater or almost the same as only due to SDS at the air-water interface [depicted in Figure 4(b)]. Effects of DPPC on VSFG intensity with a fixed concentration of SDS in the OH region can have similar explanation as that of SDS. Increasing surface density of DPPC leads to a more packed structure of DPPC and adsorbed SDS molecules at the interface. The packed structure will generate a rigid and narrow distribution of their polar heads, which can be responsible for more effective polar orientation of interfacial water molecules. Thus, the highly packed structure of DPPC and SDS molecules at the interface is responsible for enhanced VSFG intensity due to more oriented water molecules. However, at higher concentration of SDS and DPPC, further incorporation of SDS molecules into the DPPC monolayer is discouraged due to steric hindrance, as explained earlier. Surface pressure measurements support these results on VSFG spectra.

3.2. Surface Pressure of DPPC Monolayer Surface pressure of a Langmuir monolayer of DPPC at the water subphase and SDS subphase was measured to get a better insight into the VSFG results. A known volume of DPPC (50 to 125 µL) in CHCl3 solution was spread at the water or SDS subphase in a Teflon trough using a Hamilton syringe. We measured the surface pressure dependence of DPPC on its amount at the air-water interface, which shows an expected increasing trend. We observed an increase in surface pressure of monolayer at the SDS subphase in comparison to the water subphase. The increase in the surface pressure (∆π) with respect to the water subphase is greater for higher concentration of SDS. Figure 5 depicts dependence of ∆π on the SDS bulk concentration (5 to



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110 µM) for a fixed DPPC concentration. An increase in surface pressure, implying decrease in surface tension, suggests replacement of interfacial water molecules by SDS molecules. Thus, SDS molecules are adsorbed from the subphase, and get incorporated in the Langmuir DPPC layer. At lower surface spread of DPPC (9.2x1013 molecules/cm2), the surface pressure increases with increasing SDS bulk concentration. But at higher surface spread (1.5x1014, 1.8x1014 and 2.3x1014 molecules/cm2), the surface pressure rapidly increases and then gets almost saturated with SDS concentration. Thus, at the high DPPC surface concentration with high SDS subphase concentration further incorporation of SDS molecules into the monolayer is restricted. These results on the surface pressure measurements are in good agreement with that on VSFG findings. Similar to the present measurements, the surface pressure of phospholipid DOPC(1,2-dioleoylsn-glycero-3-phosphocholine) monolayer at the air-water interface is also reported to increase with increasing SDS concentration (up to ~1 mM) due to penetration of SDS molecules into the monolayer 7. However, the maximum surface pressure is observed at greater SDS concentration (~1 mM) than in the present work, probably because of contribution from both the hydrophilic and hydrophobic interactions between SDS and DOPC molecules44, and mainly hydrophobic interaction between SDS and DPPC molecules.

4. CONCLUSIONS We have investigated the interaction of DPPC molecules with SDS molecules at low bulk concentration in µM range, employing VSFG spectroscopy and surface pressure measurement. At some low concentration neither DPPC nor SDS produces VSFG signal in the CH stretch region in the water subphase at the air-water interface. However, the same concentration of DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction at the air-water interface originates from mainly hydrophobic 18 ACS Paragon Plus Environment

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interaction of alkyl chains. However, the role of electrostatic interaction can not be ruled out based upon the present work. DPPC molecules get oriented with enhanced trans structure with no observable effect on molecular charges in the presence of anionic SDS molecules. Relatively strong hydrophobic interaction, because of long hydrocarbon chains of both SDS and DPPC molecules, is responsible for enhanced polar ordering of these chains. The evaluated orientation angle, θ, of the terminal CH3 group of DPPC suggests its value remains almost unaffected in the presence of SDS. This interaction leads to significantly increased VSFG intensity of the terminal CH3 relative to CH2 stretch of DPPC molecules with low gauche defect and increase in trans conformers. The VSFG intensity in the SDS subphase increases with increasing concentration of both DPPC and SDS, but the increment in intensity is reduced with increasing concentration. Thus, the extent of polar ordering of the acyl and alkyl chains diminishes with increasing SDS bulk concentration, because the bulk SDS molecules encounter increasing steric hindrance to get incorporated into the lipid monolayer. Similarly, the interfacial water molecules are also more polar ordered in the SDS subphase, and the extent depends on concentration of both DPPC and SDS. The VSFG results are supported by surface pressure measurements. With increasing either the bulk concentration of SDS or the surface density of DPPC to certain extent the surface pressure increased due to interaction of SDS molecules into the lipid monolayer. At higher concentration, the surface pressure tends towards saturation because of increasing steric hindrance felt by SDS molecules for adsorption from the DPPC molecules in the monolayer. In contrast to our measurements at low bulk concentration of SDS (at µM level), at higher bulk concentration (at mM level) the VSFG intensity of lipid monolayer in the CH region due to acyl chain is reported to be lower in the SDS subphase than the water subphase6. Similarly, the authors found that the VSFG intensity of SDS at the air-water interface is lower in



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the presence of the lipid monolayer than in its absence, and explained due to decreased surface number density of SDS molecules in the presence of lipid monolayer. Thus, these results suggest that at lower bulk concentration of SDS, the predominant effect of hydrophobic interaction between acyl and alky chains is to orient these chains along the normal to the surface, and thus increasing the VSFG intensity of the terminal CH3 stretch. Since we observed that even small concentration of SDS can influence the structure and orientation of DPPC monolayer, the interaction can have observable effect on the surface physical and chemical properties of marine and atmospheric aerosols, and other environmental activities. Further work on VSFG experiments with deuterated DPPC molecules under similar low bulk concentration of SDS can provide still better understanding of interaction of DPPC monolayer with SDS molecules.

AUTHOR INFORMATION *Corresponding Author Email: [email protected] Tel.: 91-22-25590302 Fax: 91-22-25505151, 25505331

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors acknowledge assistance from Dr Sipra Choudhury in surface pressure measurements.



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Figure Captions

Figure 1: The figure depicts a comparison between the VSFG spectra of DPPC in the water (trace marked with “DPPC only”) and that in the SDS subphase (with varying SDS bulk concentration of 30, 90, and 120 µM) at the air-water interface in the ssp (a) and ppp (b) polarization combinations. The surface concentration of DPPC is fixed at 1.2x1014 molecules/cm2. The traces are shifted vertically for clarity.

Figure 2: Dependence of VSFG intensity (in the CH stretch region) of DPPC monolayer in the SDS subphase at the air-water interface on its surface concentration (6.0x1013, 1.2x1014 and 2.4x1014 molecules/cm2), which is marked in the figure without its unit. SDS bulk concentration is fixed at 60 µM, and the polarization combination is ppp (a) and ssp (b). The traces are shifted vertically for clarity.

Figure 3: Dependence of VSFG intensity in the ssp polarization (in the OH stretch region) of the DPPC monolayer (1.2x1014 molecules/cm2) on concentration of SDS aqueous solutions (30, 90, and 120 µM) at the air-water interface. DPPC, SDS and DPPC+SDS spectra are denoted by dashed, dotted and solid curves, respectively.

Figure 4: Enhancement in the VSFG intensity in the ssp polarization (in the OH stretch region) of DPPC monolayer (1.4x1014 and 2.3x1014 molecules/cm2) in presence of SDS (fixed bulk concentration) at the air-water interface. (a) At low SDS concentration (28 µM), enhancement is greater for higher concentration of DPPC (2.3x1014 molecules/cm2). (b) At higher SDS (85 µM) and DPPC concentrations, the enhancement is negligible; intensity is almost the same as that of SDS alone.


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Figure 5: The figure depicts dependence of increase in the surface pressure (∆π) of the DPPC monolayer on the SDS bulk concentration (5 to 110 µM) for a fixed DPPC concentration of 9.2x1013, 1.5x1014, 1.8x1014, and 2.3x1014 molecules/cm2.



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Figure 1 213x290mm (600 x 600 DPI)

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Figure 2a 190x231mm (600 x 600 DPI)


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Figure 2b 187x225mm (600 x 600 DPI)



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Figure 3 192x165mm (150 x 150 DPI)


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Figure 5 157x154mm (150 x 150 DPI)


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