1H NMR Spectroscopic Investigations on the Conformation of

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H NMR Spectroscopic Investigations on the Conformation of Amphiphilic Aromatic Amino Acid Derivatives in Solution: Effect of Chemical Architecture of Amphiphiles and Polarity of Solvent Medium R. Vijay,† A. B. Mandal,‡ and Geetha Baskar*,† Industrial Chemistry Laboratory and Chemical Laboratory, Central Leather Research Institute (CLRI), Chennai-600020, India ReceiVed: May 8, 2010; ReVised Manuscript ReceiVed: September 26, 2010

In this study, the conformation of the amphiphilic lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) in water and dimethyl sulfoxide is established. The alkyl chain protons of LEP in D2O appear at δ 1.010-1.398 and show an upfield shift and large line width, suggesting the proximity of the phenyl ring to the alkyl chain in contrast to that of LET. Quite interestingly, in DMSO-d6, the 1H NMR spectra of LET and LEP show a strong similarity that is suggestive of an orientation that positions the aromatic ring and aliphatic chain away from each other. These results are substantiated with two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOSEY). Theoretical molecular models of the conformation at the interface corroborate the experimental findings. Investigations of the solvent polarity and chemical structure-dependent conformation are discussed. 1. Introduction The self-organized assembly of amphiphiles in the presence of a solvent has immense potential in reaction media,1-3 the design of advanced materials,4-6 and the solubilization of functional materials such as drugs and cosmetics.7-11 The phenomenon of self-assembly and the conformation of the assembled structures at the interface continue to draw significant attention. This area of research has immediate relevance to the generation of advanced ecofriendly materials and technologies.12-14 Toward this goal, we aim to design amphiphiles from biodegradable sources such as amino acids and elucidate the structure-property correlation related to interfacial organization. We have chosen to synthesize the lauryl ester of two structurally related aromatic amino acids, viz., phenylalanine and tyrosine. The choice of ester derivative, in view of many reports on amido derivatives of amino acid amphiphiles,15-18 is meant to retain the amino functional group in the amphiphile. The advantage of the amino functionality is in the broad scope of various counterions available for the protonated amine, which could lead to novel structures,19-22 and in the ease of chemical modification to design species with new functional characteristics.23,24 The amino acids phenylalanine and tyrosine are important especially in view of their UV-visible absorbing properties and well-known biological relevance.25 The lauryl ester derivatives of these amino acids adsorbed at different interfaces and formed micellar assemblies in aqueous medium.26 In recent reports from Marx et al. the amphiphilic decyl derivatives of D- and L-tyrosine in micellar solutions were described to promote a controlled enzymatic polymerization reaction to form soluble polymers in contrast to that of nonamphiphilic phenolic derivatives which form insoluble crosslinked polymers.27,28 The packed structures of C18 esters of various amino acids including tyrosine and phenylalanine occurring at the air/water interface have been shown to favor polycondensation under alkaline conditions.29 * Corresponding author. E-mail: [email protected]. † Industrial Chemistry Laboratory. ‡ Chemical Laboratory.

Figure 1. Chemical structural representation of (a) L-LET and (b) L-LEP, as projected at the oil/water interface based on the theoretical model from Cerius2.

It is understandable that the chemical structure of the surfactant and nature of the interface dictate the type and the conformation of the assembled structures. The conformation of surfactants in micelles influences important parameters of regioselectivity,30 reaction kinetics,30,31 and enantioselectivity.32 In recent reports, we showed that the amphiphilic lauryl ester derivatives of L-tyrosine (LET) and L-phenylalanine (LEP) exhibit monolayer packing at different interfaces, viz., air/ solution and oil/water, that is significantly dependent on the chemical architecture.26 LET showed more efficient packing than LEP. The molecular models of conformation of LET and LEP at the liquid/liquid interface are presented in Figure 1. The OH group in LET prefers contact with water whereas, in LEP, the phenyl ring, lacking the hydrophilic hydroxyl group, was folded down into the oil. In our very recent report,33 the core shell model of micellar assemblies of these surfactants was hypoth-

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esized to be similar to that at the oil/water interface.26 The difference in conformation was shown to confer ionic character to LEP in contrast to nonionic character to LET. We recently observed the implication of the conformational difference between LET and LEP in the template-assisted design of hollow silica spheres. LET was shown to stabilize the air bubbles that acted as templates, in sharp contrast to that of LEP.34 It is important to understand how the polarity of the solvent affects the conformation of LET and LEP. Toward this aim, we have chosen to investigate the microstructures of LET and LEP in aqueous solvent mixtures that exhibit polarity over a wide range. We have selected various dimethyl sulfoxide-water solvent mixtures. Note that dimethyl sulfoxide (DMSO) at high concentrations influences the unfolding mechanism and perturbs the secondary structures of proteins.35,36 This study of LET and LEP, consisting of an amino acid skeleton similar to that of peptides or protein residues, in DMSO-d6-deuterium oxide (D2O) solvent mixtures might provide insight into the folding of proteins. From the literature, some novel characteristics of DMSO-watersolventmixturescouldberecognized.DMSO-water mixtures are shown to provide an interface with tunable polarity characteristics depending on the volume % of DMSO.37,38 In fact, the merits of DMSO-water mixtures among various reaction media are well-documented.39-44 In this study, we have performed surface tension measurements for DMSO-water solvent mixtures of identified composition to illustrate the effects of change in polarity, and the results are in accordance with literature information. Surface tension estimations were further performed for LET and LEP in the solvent mixtures to understand the changes in the interfacial adsorption behavior of LET and LEP as a function of the polarity of the solvent. We have employed 1H NMR methods to obtain information on the conformation of surfactant molecules in the aggregated structures.45,46 This method is, in fact, sensitive to changes in the microenvironment of aggregated structures.47-58 As a first approach, we address the issue of influence of the polarity of the solvent on the conformation of LET and LEP, from changes in chemical shift values (δ) of various protons. Therefore, 1H NMR spectral measurements were performed at different concentrations of LET and LEP in DMSO-d6-D2O solvent mixtures consisting of D2O in DMSO-d6 over the range of 0-100% by volume. The ring current effect on the alkyl chain and the changes in line positions, line features, and the half height peak width of aromatic and aliphatic alkyl chain protons for LET and LEP solutions are discussed. Two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOSEY) can be employed to obtain information on the conformation of aggregated structures as shown in proteins or surfactant solutions.59,60 These measurements were performed on LET and LEP in D2O and DMSO-d6. The validation of results from NMR studies with theoretical molecular models has been well reported in the literature.61 In this study, the experimental findings on the conformation from 1H NMR measurements are corroborated with the theoretical molecular models. 2. Experimental Section 2.1. Materials. The lauryl ester of tyrosine (LET) and phenylalanine (LEP) were synthesized according to the method reported elsewhere.26,62 D2O, DMSO-d6, and 3-(trimethylsilyl)1-propanesulfonic acid sodium salt (DSS) were from Aldrich Chemicals and used as received. DMSO AR grade was purchased from sd fine chem. LTD. Milli-Q water was used throughout the experiment. 2.2. Methods. NMR Experiments. 1H NMR experiments were performed at a frequency of 500 MHz on a JEOL ECA

Vijay et al. 500 spectrometer, at 28 °C. All chemical shifts were measured relative to sodium 4,4-dimethyl- 4-silapentane-1-sulfonate (DSS), which acted as an internal standard. The peak assignments were made based on literature report and theoretical values. A 200 mM solution of the surfactants was prepared in DMSO-d6 and D2O. An appropriate volume of surfactant in D2O and DMSOd6 was mixed to change the composition of D2O in DMSO-d6 to 5-90% (v/v), without any change in the concentration of surfactant. In the case of LEP, the solutions were further diluted to make 50, 100, and 150 mM solutions by serial dilution method. All solutions were equilibrated for 24 h before proceeding with the experiments. 2D NOESY Experiments. 2D NOESY experiments were performed on 200 mM solutions of LET and LEP in D2O. Mixing times of 700 ms and 32 accumulations were employed in all measurements. For LEP in D2O, the experiments were performed with a lower mixing time of 50, 150, and 400 ms. Tensiometry Experiments. Surface tension measurements were performed on a GBX 3S tensiometer from France, employing a platinum du Nuoy ring probe with an accuracy of 0.1 mN/m and standardized with milli-Q water. The reported values are the average of at least three measurements and represent the equilibrium values. All measurements were performed at 28 ( 0.1 °C. These measurements were carried out on LET and LEP in neat DMSO and in mixtures with 25, 50, and 75% water (v/v). Molecular Modeling Studies. A molecular model that was developed to understand the organization of LET and LEP at the liquid/liquid interface was considered for substantiating the experimental findings. As detailed in our report,26 in the molecular modeling studies, we have used the Cerius2 package and minimized in different arrangements using the Discover module by employing condensed phase optimized molecular potentials atomistic simulation studies (COMPASS). The entire model was minimized in the steepest descent algorithm followed by the conjugate gradient method until the rms derivate reached 0.001 kcal/mol/Å2. The minimized energy models were developed using Chem Draw Ultra 8.0 software by fixing the rms gradient at 0.001 kcal/mol/Å2. 3. Results and Discussion 3.1. 1H NMR Spectra of LET and LEP in D2O. The 1H NMR spectra of LET and LEP in D2O at a concentration of 200 mM presented in Figure 2(vii) and Figure 5A(vii) show the basic difference in conformation of the respective micellar structures. In view of the cmc of LEP and LET at 1.26 × 10-4 M and 4 × 10-5 M,26 a 200 mM solution consists predominantly of micellar structures. We mainly focus on the features of chemical shift (δ) values of the protons of the alkyl chain that include the end methyl group, nine middle methylene groups, β methylene, and the phenyl ring, to illustrate the orientation of these groups in the micelles. The variations in line features of protons of methine, R CH2, and benzyl are also discussed. In LET, β protons at δ 1.513, a methyl group at δ 0.794, and middle methylene protons at δ 1.176, ppm, accounting for nine methylene groups, appear as separate peaks in sharp contrast to that of LEP (Table 1). In LEP, the alkyl chain protons are characterized by larger line width and coalescence of peaks and, as a consequence, two distinct signals are obtained. On the basis of δ and integral values, it could be inferred that the peak of the β methylene protons (δ 1.57, theoretical) merges with the peak of middle methylene protons (δ 1.398) of the alkyl chain that account for nine methylene groups. The peak at δ 1.010 suggests the coalescence of a methylene and methyl protons. These results are indicative of distinct shielding (upfield shift)

Conformation of Amphiphilic Aromatic Amino Acid Derivatives

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Figure 2. 1H NMR spectra of LET at 200 mM in DMSO-d6-D2O solvent mixtures, i-vii: 0, 20, 40, 50, 70, 80, and 100% (by volume) D2O, 500 MHz, temp 28 °C.

TABLE 1: 1H NMR Peak Assignments, δ (ppm) for LET and LEP at 200 mM in D2O and DMSO-d6, 500 MHz, Temperature 28 °C LET in DMSO-d6 LET in D2O LEP in DMSO-d6 LEP in D2O

Ar-OH

NH3+

p-ArH

o-ArH

m-ArH

CH

R CH2

benzyl-H

β CH2

alkyl CH2

CH3

9.547 -

8.651 8.816 -

7.262 7.128

7.046 6.970 7.301 7.212

6.755 6.775 7.301 7.242

4.213 4.213 4.193 4.373

4.083 4.109, 4.010 4.002 4.046, 3.942

4.109 3.080, 2.998 3.265, 3.069 3.347, 3.142

1.513 1.513 1.412 1.398

1.280 1.176 1.244 1.398

0.893 0.794 0.858 1.018

of methylene protons of the alkyl chain in LEP. The shielding of these protons in this case could arise only from a possible ring current effect, which suggests the proximity of the aromatic ring to the alkyl chain in LEP. From a general understanding of the packing of alkyl chains in the core or nonpolar region of a micelle, it could be then deduced that the aromatic ring and the alkyl chain are both located in the core region of the micelle in LEP. Note that a similar inference on the shielding of alkyl chain protons due to ring current effect was reported in single and double chain alkyl surfactants in the presence of aromatic additives.51,63 The methine, benzyl, and R methylene groups could be considered as linker groups, and their positions in the micelle could vary among different regions of the micelle, viz., core, palisade, and micelle-water interface. The methine and benzyl protons of LET (Table 1) show an upfield shift in comparison to that of LEP. On the other hand, R protons in LET show a large downfield shift (Table 1).52,54,26 All these results substantiate the packing of the aromatic ring and aliphatic chain of LEP in close proximity to each other in the micellar structure in D2O. 3.2. LET and LEP in DMSO-d6. The 1H NMR spectral features in DMSO-d6 of LET and LEP are analyzed to understand the conformation in DMSO, which is less polar than water. The spectra of LET and LEP at a concentration of 200 mM in DMSO-d6 are presented in Figure 2(i). and Figure 5A(i), respectively. The signals due to NH3+ protons at δ 8.651 in LET and δ 8.816 in LEP could be observed. In addition, in LET, the signal at δ 9.547 due to the phenolic group could be seen. These results suggest that these protons do not undergo

exchange with DMSO-d6 as observed with D2O, and this could be anticipated. The resonance lines of protons of almost all groups in LET and LEP in DMSO-d6 are much narrower and are similar between spectra in contrast to those observed in D2O. For example, with respect to the alkyl chain, the signals due to various protons, viz., methyl, nine middle methylenes, and β proton, appear in LET and LEP as detailed in Table 1. However, methylene chain protons in LEP appearing at δ 1.125-1.244 are broader than those in LET and show splitting of lines. These features suggest that alkyl chain protons in LEP and LET exhibit different relaxation time scales. An almost negligible or very small change in δ, especially of methyl and the β CH2 group, suggests a similar microenvironment for these parts of the alkyl chain of LET and LEP in DMSO-d6, in contrast to that observed in D2O. LET and LEP could both be present predominantly as a monomer at this concentration in DMSO as inferred from surface tension measurements. This is also supported from the half height widths of various protons of LEP and LET that are much narrower in DMSO in comparison to that in water. Therefore, it is reasonable to deduce that the conformations of LET and LEP monomers in DMSO are similar. The remarkable difference in the conformation of LEP in DMSO and D2O suggests a significant role played by the polarity of the solvent. Previous reports have shown64 that DMSO-water solvent mixtures exhibit Dielectric Constant (DEC) (48-80) (Table 5) that indicate polarity characteristics between that of water and DMSO. We have therefore chosen this solvent mixture for detailed investigations to understand the influence of polarity on the solution structures. Thus, 1H

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TABLE 2: 1H NMR Peak Assignments, δ (ppm), for LET at 200 mM in D2O, DMSO-d6, and Their Mixtures, 500 MHz, Temperature 28 °C % D2O in DMSO-d6

m-ArH

CH

β CH2

alkyl chain

CH3

0 20 40 50 60 70 80 100

7.046 7.045 7.007 7.001 6.994 6.983 6.975 6.970

4.213 4.186 4.204 4.215 4.227 4.235 4.213

1.513 1.513 1.513 1.514 1.513 1.513 1.513 1.513

1.280 1.260 1.218 1.213 1.206 1.195 1.184 1.176

0.893 0.873 0.833 0.829 0.820 0.811 0.800 0.794

NMR investigations were carried out in solvent mixtures of different compositions. 3.3. LET in DMSO-d6-D2O Solvent Mixtures. The 1H NMR spectra of a 200 mM solution of LET in solvent mixtures wherein the volume ratio of D2O to DMSO-d6 was varied from 0.05:0.95 to 0.90:0.10 are presented in Figure 2. On the basis of cmc values estimated from surface tension measurements (Table 6), it could be considered that a 200 mM solution in the solvent mixture consists of micellar structures. The peak assignments of various protons in the solvent mixture are summarized in Table 2. Interestingly, the signals due to protons of various segments (as shown in the inset of Figure 2) appear in the solvent mixture irrespective of the solvent composition. The plot of δ as a function of D2O content in the solvent mixture for β CH2 (a), alkyl chain (b) methyl (c), methine (d), and aromatic (meta) protons (e) is presented in Figure 3A and 3B. The significant inferences are an almost negligible change in δ with respect to β CH2 (a), the methine (d), and aromatic (meta) protons (e), and the absence of spectral line coalescence as the solvent composition changed. These results suggest an absence of ring current effect and the location of the alkyl chain and aromatic ring away from each other irrespective of solvent mixture composition, similar to that observed with neat D2O or DMSO solvent. The half height width changes for β (a) and alkyl (b) protons as a function of D2O % is given in Figure 4A. For β CH2 (a) protons, half height width increases from 5.24 to 30.80 Hz upon increasing D2O from 0 to 100%. A similar trend of line broadening is observed with alkyl chain (b) protons from 5.19 to 17.57 Hz. The line broadening of protons of various groups in LET with change in concentration of D2O in the solvent mixture supports the presence of aggregated structure52,54 in accordance with tensiometry results. From these results, it could be understood that the conformation of LET as a monomer in DMSO-d6 or micellar assemblies in D2O and the solvent mixtures remains almost the same, i.e., the aromatic ring and aliphatic chain are located far apart. 3.4. LEP in DMSO-d6-D2O Solvent Mixtures. 1H NMR spectral measurements were performed on LEP at selected concentrations of 50, 100, 150, and 200 mM in the solvent mixtures of D2O and DMSO-d6 similar to those used for LET. These concentrations of LEP are above the cmc in all the solvent mixtures as seen from surface tension measurements (Table 5). The 1H NMR spectra of LEP at 50 and 200 mM in a solvent mixture of selected composition in comparison to those in simple D2O or DMSO-d6 are presented in Figure 5a and 5b. (For the other two concentrations, 100 and 150 mM, spectra are provided in Supporting Information SI A and SI B). The peak assignments for different protons are detailed in the inset of Figure 5a and 5b. The summary of peak assignments for LEP at 200 mM in the solvent mixture and the neat solvents are presented in Table

Figure 3. Plot of δ vs concentration of D2O in DMSO-d6 for LET at 200 mM. (A) a-c: βCH2, alkyl chain, and methyl; (B) d and e: m-ArH and methine.

Figure 4. Plot of half height width vs concentration of D2O in DMSOd6 for LET at 200 mM. a and b: β CH2 and alkyl chain.

3. Alkyl, aromatic, and the linker protons in LEP exhibit considerable changes in chemical shift and line broadening with D2O in the solvent mixture. The changes in the alkyl chain portion are described as follows. The methylene protons from



Figure 5. 1H NMR spectra of LEP at (A) 200 mM and (B) 50 mM in DMSO-d6-D2O solvent mixtures, i-vii: 0, 20, 40, 50, 70, 90, and 100% (by volume) D2O, 500 MHz, temp 28 °C.

a part of alkyl chain and those from the end methyl show progressive coalescence. The splitting of resonance lines for a part of the alkyl chain protons is also observed. This enables the assignment of peaks individually to different protons, viz., b2, b3, and b4. The inference on the coalescence of proton peaks is based on the integral values. The plot of change in δ of different protons, viz., β CH2 (a), alkyl protons (b), γ CH2 (c), methyl (d), protons of aliphatic chain, methine protons (f), and aromatic (para) protons (e) with the increase in D2O in solvent mixture for different concentrations of LEP is shown in Figure 6A and 6B. From Figure 6A(iv) (200 mM), significant changes in δ start to appear in the solvent mixture consisting of J25% D2O with respect to protons of all segments of LEP. For example, the methyl group shows δ at 0.886 ( 0.008 ppm, the

γ CH2 proton at 1.135 ( 0.016 ppm, and the β CH2 protons at 1.437 ( 0.007 over the concentration range of 5-20% (v/v) D2O in the mixture. Above these concentrations, progressive changes could be observed. The γ CH2 protons show continuous upfield shifts from δ 1.146 to 1.029 upon increasing the concentration of D2O from 5 to 70% after which the peak merges with the methyl proton peak (at δ 0.992). This indicates greater shielding of the methylene protons. The β CH2 protons show an upfield shift up to δ 1.399 after which the peak merges with alkyl chain proton peak (at δ 1.331) upon increasing the concentration of D2O to more than 30%. This is inferred from the integral of the alkyl chain protons, which account for 14 protons, in the solvent mixture with D2O > 30%. The β CH2 and γ CH2 protons (marked c and b4 in the inset, Figure 5)

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TABLE 3: 1H NMR Peak Assignments, δ (ppm), for LEP in D2O, DMSO-d6, and Their Mixtures, 500 MHz, Temperature 28 °C % D2O in DMSO-d6

p-ArH

CH

β CH2

alkyl chain

γ CH2

CH3

0 5 10 20 30 40 50 60 70 80 90 100

7.262 7.285 7.283 7.288 7.237 7.201 7.184 7.173 7.167 7.158 7.146 7.128

4.193 4.240 4.243 4.252 4.280 4.298 4.316 4.333 4.348 4.362 4.374 4.373

1.412 1.441 1.440 1.431 1.399 -

1.244 1.270 1.269 1.281 1.314 1.331 1.345 1.357 1.368 1.380 1.396 1.398

1.125 1.146 1.139 1.119 1.082 1.055 1.041 1.035 1.029 -

0.858 0.883 0.882 0.894 0.925 0.943 0.958 0.969 0.980 0.992 1.007 1.018

show an upfield shift that results in coalescence with the alkyl chain and methyl protons, respectively. This is accompanied by a downfield shift of protons from part of the alkyl chain and from the methyl. The upfield shift, which indicates shielding of the β CH2 and γ CH2 protons, in this case must arise due to ring current effect. The opposite trends of shielding and deshielding in the alkyl chain protons demonstrate the difference in the orientation of the alkyl chain with respect to the phenyl ring. These shielding effects occur because of the progressive folding of the ring more toward the nonpolar region and the existence of the alkyl chain predominantly in the coiled form in the micellar or aggregated structure that is promoted by D2O in the solvent mixture. In the case of aromatic ring protons, a continuous upfield shift is seen with D2O in the solvent mixture, and the extent of upfield shift is more with respect to the para protons. Here also, a similar trend of small changes up to a concentration of 20% D2O in the solvent mixture is observed (Figure 6). It could be inferred that the progressive addition of D2O to solvent mixtures promotes formation of aggregated structures. The enhanced shielding of aromatic protons under these conditions once again suggests the favorable packing of aromatic chains inside the core of aggregated/micellar structures. The methine proton that could be considered as a linker group shows a similar trend of negligible change up to 20% D2O in the solvent mixture but at higher concentration shows progressive downfield shift. These results demonstrate that the different shielding effects on the protons arise mainly from different conformations. The increase in polarity of the solvent by way of increasing water content in the solvent mixture brings about progressive folding of the aromatic chain into more nonpolar regions of the micelle. It is significant to note that at all concentrations of LEP, negligible changes in δ occur for all the protons up to a concentration of 20% D2O. The concentrations of D2O in the solvent mixture at which coalescence of γ CH2 and β CH2 protons occur vary with the concentration of LEP. For example, coalescence of the β CH2 protons with alkyl chain protons occurs in the solvent mixture consisting of 60, 50, 40, 40% D2O for 50, 100, 150, and 200 mM LEP. A similar trend showing a decrease in concentration of D2O in the solvent mixture with an increase in concentration of LEP is observed for γ CH2 protons. The plot of half height width of β CH2 (a) and γ CH2 (b) protons of LEP vs D2O in the solvent mixture is presented in Figure 7A and 7B. In the case of 200 mM LEP solution (Figure 7B), the half height width of β CH2 (a) protons increases from 19.80 to 22.52 Hz with an increase in D2O from 5 to 30%. Similarly, the half height width of γ CH2 (b) protons increases from 19.76 to 23.15 Hz with an increase in D2O from 30 to 60%. These results indicate the promotion of aggregated

structure formation with the addition of water in the solvent mixture. The changes in the δ, resonance line features, and the half height width at and above ∼25% D2O in the solvent mixture are important. This composition corresponds approximately to a mole ratio of DMSO:water of 0.33:0.67. This composition of the solvent mixture was established to allow significant changes in surface tension and interaction characteristics.38 The characteristics of the solvent mixture seem to play a significant role in influencing the conformation and aggregation of LEP. Essentially, the progressive changes related to the coalescence of the β CH2 and γ CH2 protons that eventually undergo more shielding, the deshielding effect on protons of a part of the alkyl chain and methyl protons, and the systematic increase in shielding of aromatic protons with an increase in D2O volume in the solvent mixture all indicate the ring current effect on the related protons at different levels. It is known that the ring current effect is governed by eq 165

σ ) iB(1 - 3 cos2 θ)/r3

(1)

where i is a ring-current factor, B is the constant of proportionality, θ is the angle, and r is the distance. From this equation, the ring current effect is directly proportional to angle θ and inversely proportional to the distance between the aromatic ring and the neighboring group. The decrease in distance between the aromatic ring and the aliphatic chain and in the angle would depend upon the orientation of the aromatic ring and the packing density of surfactants in the micellar assembly. The enhanced packing density that could be anticipated with an increase in polarity of the solvent mixture or, in other words, with addition of water to the solvent mixture is expected to promote aggregation.38 This process could enable folding of the aromatic ring that eventually would decrease the distance between aromatic and aliphatic groups in the micelle. On the contrary, in DMSO, the ring current effect on alkyl chain protons is negligible for LEP, similar to the situation for LET in DMSOd6, D2O, or the solvent mixtures. It is thus established that the polarity of the solvent serves as a useful handle in tuning the aggregation process and the conformation of the monomer in the micellar aggregate. The theoretical molecular model (Figure 1) at the liquid/liquid interface of LET and LEP that was developed using the Cerius2 package was inferred to agree very well with the minimized energy models from the ChemDraw Ultra 8 software. The molecular model (Figure 8) from ChemDraw Ultra 8 has been speculated for LET and LEP monomers in D2O, DMSO-d6, and the solvent mixture. The results on polarity and chemical structure-dependent conformation are highly significant especially in the context of employing these surfactants in applications related to polarity-dependent functions, such as chemical reactions and solubilization. 3.5. 2D NOSEY Spectra of LET and LEP in D2O and DMSO-d6. The NOESY spectra of LET and LEP in D2O obtained at a mixing time of 700 ms are presented in Figure 9a,b. Note that for LET in D2O (Figure 9a), cross-peaks could be seen mainly for the neighboring protons, e.g., m-ArH, o-ArH, β CH2 alkyl chain protons. Significantly, no cross-peaks between aromatic protons and alkyl chain protons are observed. The absence of cross-peaks in the aromatic-tail proton region demonstrates that, on an average, the aromatic protons and the tail protons are far apart. For LEP in D2O, the appearance of cross-peaks between the tail, R CH2 protons (at δ ) 0.98-1.40, 3.99), and the aromatic protons (δ ) 6.97-7.20 ppm) is the most interesting and significant result. Besides cross-peaks



Figure 6. Plot of δ vs concentration of D2O in DMSO-d6 for LEP. A(i)-A(iv), 50, 100, 150, and 200 mM. a-d: β CH2, alkyl chain, γ CH2, and methyl. I represents the number of protons, indicative of the coalescence of protons. B(i)-B(iv), 50, 100, 150, and 200 mM. e and f: p-ArH and methine.

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Vijay et al. and aliphatic chain protons that are unique with respect to LEP in D2O, 2D NOESY measurements were performed on 200 mM LEP at mixing times of 50, 150, and 400 ms (Figure 10). This was performed especially to obtain information on the interproton distance (rnm) between the aromatic ring and aliphatic chain in the aggregated structure. A representative plot of the ratio of the intensity of the diagonal peak of the p-ArH proton to that of the cross-peak between p-ArH and the alkyl chain protons vs mixing time is presented in Figure 11. The intensity ratio increased in the region up to 150 ms after which it remained almost constant up to 700 ms. Equation 2 was used to calculate rnm.

rnm ) rab ×

Figure 7. Plot of half height width vs concentration of D2O in DMSOd6 for LEP at (A) 50 and (B) 200 mM. a and b: β CH2 and γ CH2.

between adjacent protons, e.g., m-ArH and o-ArH, γ and alkyl chain protons could also be observed. These results demonstrate the proximity of the alkyl to the aromatic protons with a distance parameter governed by the NOE dependence on the interproton distance r-6 .49 In view of the cross-peaks between the aromatic

6

Iab Inm

(2)

where Iab and Inm are the respective intensities of the crosspeaks for the Ar(pH-oH) and the required aliphatic aromatic proton. rab, the distance of Ar(pH-oH), is reported as 2.40 Å in the literature.66 In eq 2, Iab and Inm, measured at a mixing time of 150 ms is fitted in order to ensure that the distance specificity is not destroyed by spin diffusion and also to get sufficiently good intensity for all peaks.67 The results on the interproton distance of the most important groups, viz., aliphatic and aromatic, are presented in Table 4. The interproton distance between different aromatic and aliphatic chain protons are calculated as 3.78 and 3.60 Å. The higher interproton distance between aromatic ring and aliphatic chain establishes the spatial interaction between these segments. The distance of 2.45 Å estimated by applying eq 2 for the aromatic protons (Ar(mH-pH)) agrees within limits of experimental error with that reported for related structures.49 We attempted to calculate the interproton distance of the selected groups mentioned above in the model developed using the Cerius2 package.26 The results are presented in Table 4. The close agreement in the values substantiates our understanding of the conformation of LEP in aggregated structures and validates the model developed. The contour plot of LET and LEP in DMSO-d6 is presented in Figure 12. The cross-peaks between neighboring aromatic protons similar to those observed with LET in D2O could be seen both

Figure 8. Theoretical molecular model of (a) LET in DMSO and water, and LEP in (b) DMSO and (c) D2O.



Figure 9. 2D NOESY spectrum of (a) LET and (b) LEP at 200 mM in D2O for a mixing time of 700 ms.

Figure 10. 2D NOESY spectrum of 200 mM LEP in D2O at different mixing times: (a) 400, (b) 150, and (c) 50 ms.

with respect to LET and LEP in DMSO-d6. The absence of cross-peaks in the aromatic-tail proton region clearly demonstrates that the aromatic protons and the tail protons are far apart in both LET and LEP solutions. These results further substantiate our inferences on the conformation of LEP and LET in DMSO. 3.6. Tensiometry. Surface tension measurements were performed on solvent mixtures in the absence and presence of LET and LEP. The results on the surface tension of neat water,

DMSO, and the solvent mixtures consisting of 25, 50, and 75 (v/v) weight% water are presented in Table 5. The values agree with those reported in the literature within limits of experimental error. The lower surface tension of DMSO (42.84 mN/m) in comparison to water (72 mN/m) is indicative of the more nonpolar character of DMSO. The lower surface tension mainly arises from adsorption of DMSO at the interface due to the preference of the methyl groups for a nonpolar microenviron-

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Vijay et al. TABLE 5: Surface Tension (γ) and DEC of Water, DMSO, and Their Mixtures

Figure 11. Plot of the intensity ratio with respect to p-ArH protons vs mixing time of 200 mM LEP in D2O: (9) m-ArH-alkyl chain H, (2) m-ArH-γCH2.

TABLE 4: Comparative Interproton Distance from the Cerius2 Model and 2D NOESY Spectra of a 200 mM Solution of LEP in D2O, at 500 MHz, Temperature 28 °C interproton distance (Å) groups

Cerius2 model (theoretical model)

NOESY

m-ArH-γ CH2 m-ArH-alkyl CH2 p-ArH-m-ArH

3.47 4.80 2.40

3.78 3.60 2.45

ment. The addition of DMSO to water brings about a progressive decrease in the surface tension of water. For example, in a 1:1 (volume ratio) DMSO-water mixture the surface tension was 50.8 mN/m (Table 5). The decrease in the surface tension of water in the presence of DMSO could be attributed to a change in the solvent structure of water and replacement of water molecules with DMSO at the interface. The intermolecular association promoted among DMSO molecules, and between water and DMSO in the mixtures, as established from theoretical methods37,38,64 is well indicated by the significant changes in surface tension. The plot of surface tension vs concentration of LET and LEP in neat solvents and in the mixtures is presented in Figure 13a and 13b. In the presence of LET and LEP at

system

γ

DEC

water 75% water in DMSO (v/v) 50% water in DMSO (v/v) 25% water in DMSO (v/v) DMSO

72.00 57.01 50.83 45.40 42.84

80 68 64 57 48

different concentrations, the surface tension of DMSO shows negligible change up to a concentration as high as about 0.326 M for LEP and 0.204 M for LET. The change in surface tension above this concentration is not discussed here. This behavior is indicative of solubilization of LET and LEP in DMSO and the absence of adsorption of these species at the DMSO/air interface. Significantly, in the solvent mixture, the surface tension vs concentration profile starts to resemble the profile of these amphiphiles in neat water. The surface tension of these surfactants in the solvent mixture shows a progressive decrease followed by a constant value region (Figure 13). The constant surface tension value of both LET and LEP increases progressively with an increase in DMSO concentration in the solvent mixture. The concentration corresponding to the onset of constant surface tension is regarded as the cmc. The important results on cmc and constant surface tension depicted in Figure 13a and 13b are presented in Table 6. Both LET and LEP show a progressive increase in cmc with DMSO concentration in the solvent mixture. Significantly, cmc increases by 1 order of magnitude with the addition of DMSO to 25% in water, and thereafter, as DMSO is increased in the mixture, cmc increases only by small increments. For example, the cmc of LET is increased by about 1 order of magnitude, from 4 × 10-5 M to 2.11 × 10-4 M; similarly, the cmc of LEP changes from 1.26 × 10-4 to 1.05 × 10-3 M in the a solvent mixture of water and DMSO in a weight ratio of 3:1. These results suggest that the adsorption and micellization characteristics of both LET and LEP in the solvent mixtures are very different from those in simple solvent. The decrease in polarity effected with DMSO addition to the solvent mixture promotes solubility of LET and LEP rather than adsorption at the interface and hence the observed results. 4. Summary LET and LEP are shown to exhibit different conformations of aggregated structure that depend on the chemical architecture

Figure 12. 2D NOESY spectrum of (a) LET and (b) LEP at 200 mM in DMSO-d6 for a mixing time of 700 ms.


Figure 13. Plot of γ vs concentration of (A) LET and (B) LEP. a-e: 100, 75, 50, 25, and 0% (by volume) water in DMSO.

TABLE 6: CMC (Μ) of LET and LEP in Water, DMSO, and Their Mixtures, Temperature 28 °C system

LET cmc (M)

LEP cmc (M)

water 75% water in DMSO (v/v) 50% water in DMSO (v/v) 25% water in DMSO (v/v) DMSO

4.00 × 10-5 2.11 × 10-4 4.01 × 10-4 8.30 × 10-4 -

1.26 × 10-4 1.05 × 10-3 4.07 × 10-3 6.03 × 10-3 -

and polarity of the solvent. Our conclusions are drawn from detailed 1H NMR spectral investigations. For this study, DMSOd6-D2O solvent mixtures consisting of 0-100% D2O (by volume) are chosen to generate polarity on a broad scale. LET and LEP in D2O at 200 mM predominantly consist of micellar structures and show distinct differences in 1H NMR spectral features. 1H NMR spectrum of LEP in D2O is characterized by two signals in the region of δ of 0.70-1.80 ppm that evidence shielding of some part of the alkyl chain protons attributed to the ring current effect. This could infer proximity of a part of the aliphatic chain to the aromatic ring in the micelle of LEP. On the contrary, the appearance of well-separated signals due to β CH2, methyl, and alkyl chain protons in the region of δ ) 0.80 to 1.80 ppm in LET indicates the absence of ring current effect. Thus, the aromatic ring and aliphatic chain are far apart in the micelles of LET in D2O. In DMSO-d6, quite interestingly, the spectra of LET and LEP show strong similarity to one

J. Phys. Chem. B, Vol. 114, No. 43, 2010 13701 another with respect to the separate signals for the various protons of the aliphatic chain and aromatic ring. This indicates that both LET and LEP in DMSO-d6 exhibit similar conformations wherein the aromatic ring and aliphatic chain are oriented far away from each other. In the solvent mixtures of DMSO-d6 and D2O, the polarity of the solvent changes effectively. In LET, the resonance signals due to various protons remain the same, i.e., there is no distinct coalescence or splitting of lines as observed with LEP. However, with an increase in D2O in the solvent mixture, a progressive increase in half height width could be observed for protons of almost all segments of LET. These results suggest that in the solvent mixture, the aggregation process is promoted with retention of conformation. On the other hand, in LEP, visible coalescence of resonance signals occurs with an increase in concentration of D2O in the solvent mixture, i.e., with the increase in polarity of the solvent mixtures in comparison to simple DMSO. These changes are considerable at above 25 vol% D2O in the solvent mixture, the composition at which large changes in solvent characteristics are established. The progressive coalescence of β CH2 and γ CH2 protons with those of middle methylene chain protons and methyl protons, respectively, and the ortho and meta protons of aromatic chain, is observed with an increase in D2O volume in the solvent mixture. This suggests the promotion of proximity of the aromatic ring to the aliphatic chain with an increase in polarity of the solvent mixtures. The line broadening and surface tension measurements suggest that D2O-DMSO-d6 solvent mixtures favor the micellization process. It could be thus deduced that on increasing the polarity of the solvent, as could be effected with addition of water, a micellization process occurs that is accompanied by the folding of the aromatic ring in LEP. Thus, proximity of the aromatic ring to the aliphatic chain is effected. The conformation is substantiated from 2D NOSEY measurements. The presence of cross-peaks between aromatic and aliphatic protons for LEP in D2O and the absence of such crosspeaks for LEP in DMSO or LET in D2O and DMSO all support the proposed conformation, locating the aromatic ring and aliphatic chain far apart from each other under these conditions. The experimental results on the conformation are corroborated with molecular models. The discovery that the conformation is dependent upon the chemical architecture and polarity of solvent is significant in view of the implications on microstructure characteristics and interface-specific applications. Acknowledgment. The authors thank Dr. B.S.R. Reddy, Scientist G, CLRI India, for his constant encouragement. The authors thank Dr. B.V.N., Phani Kumar for technical support and valuable suggestions for NMR measurement. The support of RC CLRI is acknowledged. R.V. thanks CSIR for the senior research fellowship. Supporting Information Available: Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, Q. F.; Wei, W.; Lu, M.; Sun, F.; Li, J.; Zhang, Y. C. Catal. Lett. 2009, 131, 485. (2) Branco, L. C.; Ferreira, F. C.; Santos, J. L.; Crespo, J. G.; Afonso, C. A. M. AdV. Synth. Catal. 2008, 350, 2086. (3) Itoh, J.; Fuchibe, K.; Akiyama, T. Synthesis 2006, 4075. (4) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407. (5) Zhang, D. B.; Qi, L. M.; Ma, J. M.; Cheng, H. M. AdV. Mater. 2002, 14, 1499. (6) Jiang, C. L.; Wang, Y. F. Mater. Chem. Phys. 2009, 113, 531. (7) Cheng, Y. Y.; Wu, Q. L.; Li, Y. W.; Hu, J. J.; Xu, T. W. J. Phys. Chem. B 2009, 113, 8339.

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