Conformational Mobility in Alkyl-Chains of an Anchored Bilayer

Bangalore 560012, India. ReceiVed: ... the NMR indicating a much greater extent of disorder. .... greater detail using variable contact time CP-MAS an...
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2007, 111, 495-500 Published on Web 12/19/2006

Conformational Mobility in Alkyl-Chains of an Anchored Bilayer R. Suresh,† N. Venkataraman,† S. Vasudevan,*,† and K. V. Ramanathan*,‡ Department of Inorganic and Physical Chemistry and NMR Research Centre, Indian Institute of Science, Bangalore 560012, India ReceiVed: NoVember 6, 2006; In Final Form: December 7, 2006

The conformation of alkyl chains in an anchored bilayer formed by the intercalation of cetyl-trimethyl ammonium ions in layered CdPS3 has been investigated by 13C NMR and infrared spectroscopy at different temperatures. The two techniques return widely differing estimates of gauche conformational disorder with the NMR indicating a much greater extent of disorder. Reconciliation of this difference requires that the observed features in the NMR be dynamically averaged. We show from infrared spectroscopy that the nature of disorder in the anchored bilayer is essentially in the form of kinks (gauche-trans-gauche sequences), and if these are mobile along the length of the alkyl chain on time scales faster than 3.5 ms, they would result in the dynamic averaging of features in the NMR.

Introduction One of the most important structural characteristics of a biological membrane is that it is like a two-dimensional liquid; the constituent molecules can undergo thermal movement within the membrane without altering its overall morphology. This fluid-like nature of the membrane is largely due to the conformational freedom of phospholipid molecules and particularly gauche-trans isomerization of the alkyl chains. Nothing approaching an accurate molecular-level description of the conformation and dynamics of the hydrocarbon chains in bilayers is, however, currently available. This in part is because these systems, especially the lipid bilayers, have the experimental drawback that they are essentially liquid-like and hence not amenable for study by powerful solid-state spectroscopic techniques. Long chain amphiphilic surfactant molecules can be introduced within the interlamellar region of layered inorganic host lattices to form anchored alkyl chain bilayers within the galleries.1-3 The intercalated bilayer bears a striking resemblance to lipid bilayers that are an integral feature of biomembranes. However, unlike lipid bilayers where individual molecules can undergo lateral diffusion and also flip-flop between layers, the anchored bilayer is characterized by the total absence of translational mobility. The degrees of freedom of the alkyl chains of the anchored bilayer are restricted to changes in conformation. Here we report a comparison of infrared and 13C NMR spectroscopic measurements on the conformation of alkyl chains in an anchored bilayer formed by the ion-exchange intercalation of cetyl trimethyl ammonium in ions in layered CdPS3.4 This system was chosen because the density of the anchored alkyl chains is high enough for it to form a bilayer, rather than a * Authors to whom correspondence may be addressed. (S.V.) E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/ 0683. (K.V.R.) E-mail: [email protected]. Tel: +91-80-2293-3299. † Department of Inorganic and Physical Chemistry. ‡ NMR Research Centre.

10.1021/jp067298l CCC: $37.00

monolayer, and yet below the critical density for phase transitions, that have been observed in many other anchored surfactant systems, to occur.5,6 Vibrational spectroscopy, particularly infrared, and NMR spectroscopy have been used extensively to establish conformation in a variety of alkyl-chain assemblies: self-assembled monolayers, nanoparticles, surfactant intercalated solids, and, of course, lipid bilayers and membranes. Vibrational spectroscopy can provide information on both local as well as global conformation of alkyl chains.7 NMR techniques, on the other hand, are sensitive to the local chemical environment and conformation and additionally can provide information on the dynamics and mobilities in alkyl chain assemblies.8-10 The results of these independent studies are highly informative of the microscopic state of the assembly, although interrelated have, in most cases, not been correlated. Here we highlight the fact that estimates of disorder in the anchored bilayer by the two techniques, infrared and 13C NMR, are very different, with the NMR indicating a much larger degree of disorder. It is shown that this is a consequence of dynamical averaging and the very different time scales associated with the two techniques. The layered host lattice chosen for the present study cadmium thiophosphate, CdPS3, is representative of many layered inorganic solids, e.g., the mica-type silicate clays, and has similar host-guest chemistry. The long chain cationic surfactant, cetyl trimethyl ammonium (CTA), CH3(CH2)15N+(CH3)3, may be introduced in the galleries of CdPS3 by ion-exchange intercalation to form Cd0.83PS3(CTA)0.34. The intercalated surfactant forms a tilted bilayer with the cationic head group anchored at the negatively charged, cadmium deficient, Cd0.83PS3 layer, with a mean head-to-head distance of 9 Å (Figure 1).3 An all-trans alkyl chain would occupy a cylindrical volume in space with cross-sectional diameter ∼4 Å. There is, therefore, sufficient space for the alkyl chains in the anchored bilayer to exercise conformational freedom. Infra red and Raman spectroscopic studies have indeed shown that, although a majority of the © 2007 American Chemical Society

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Figure 1. Model representing the tilted bilayer arrangement of alkyl chains in Cd0.83PS3(CTA)0.34.

Figure 2. 13C MAS NMR of the anchored bilayer, Cd0.83PS3(CTA)0.34, at different temperatures (a) with cross-polarization (CP) and (b) without CP. The numbering of the carbon atoms along the length of the chain is indicated. (c) The intensity ratio of the 33 ppm resonance in the 13C MAS NMR without CP, I33/(I33 + I35), at different temperatures.

methylene units of the bilayer adopt the trans configuration at room temperature, gauche defects are also present.4,11 It had also been shown using variable contact time cross-polarization magic angle spinning NMR and 2D wide-line separation spectroscopy (2D-WISE) that there is a gradient of conformational mobilities as one proceed from the immobile head toward the tail.12 Experimental Section CdPS3 was prepared from elements following the procedure reported in the literature13 and treated with a 4 M aqueous solution of KCl in the presence of 0.1 M EDTA and 1 M K2CO3/KHCO3 as buffer, to give Cd0.83PS3K0.34(H2O). This compound has a lattice spacing of 9.4 Å corresponding to a lattice expansion of 2.8 Å as compared to the pristine host CdPS3. The interlamellar potassium ions were further ionexchanged with cetyl trimethyl ammonium (CTA) ions, by refluxing in an aqueous solution of CTA bromide at 50 °C for 6 h, to give Cd0.83PS3(CTA)0.34. Completion of ion exchange was ascertained by the appearance of a new series of 00l reflections in the diffraction pattern with a lattice spacing 33 Å, an expansion of 26.4 corresponding to the formation of Cd0.83PS3(CTA)0.34.13 C MAS NMR spectra were recorded on a

Bruker DSX-300 FT-NMR spectrometer at a Larmor frequency 75.47 MHz with a magic angle spinning (MAS) speed of 8 kHz. 13C CP-MAS spectra were acquired with a proton π/2 pulse length of 3.5 µs and 5 s recycle delay time with 512 transients being accumulated. Single-pulse (SP) Bloch decay experiments were done using the same proton π/2 pulse length and recycle delay time as used for CP/MAS experiments. 13C chemical shifts are quoted in ppm with respect to TMS. Infra red spectra were recorded on a Bruker IFS 55 spectrometer. Sample temperatures could be varied between 360 K and 50 K using a Lake Shore close cycle cryostat. Powder X-ray diffraction patterns at different temperatures were acquired on a Shimadzu XD-D1 diffractometer with a home-built heating arrangement using Cu KR radiation. Results And Discussions The solid-state 13C CP/MAS NMR spectra of the anchored bilayer recorded at different temperatures is shown in Figure 2a. At 295 K, seven carbon resonances are observed, which in order of increasing upfield shifts are C1, CN, C14, C4-C13, C2, C3,15, and C16 (the numbering of the carbon atoms and assignments are shown in Figure 1a). The resonance of the methylene carbons (C4-C13) in the interior of the chains shows

Letters two peaks one at ∼35 ppm and the other as a shoulder at ∼33 ppm. This upfield shift of ∼2 ppm for the C4-C13 carbons is due to the γ-gauche effect. 10,14 The presence of gauche conformers in an alkyl chain is known to produce upfield shifts of 4-5 ppm as compared to an all-trans chain. This shift is attributed to differences in the shielding of a methylene carbon depending on whether the three intervening bonds separating it from the γ-substituent (Cγ) atom are trans or gauche. From the ratio of the integrated intensities of the resonances at 33 and 35 ppm, it is possible to estimate the number of methylene units experiencing the γ-gauche effect and hence the number of gauche bonds. This quantity has been used extensively to quantify the extent of gauche disorder in alkyl-chain assemblies.15,16 With increase in temperature, the upfield shifted resonance at 33 ppm grows in intensity at the expense of the 35 ppm peak. By 310 K, it is the peak at 33 ppm, characteristic of the carbon atoms that experience the γ-gauche effect, that is the dominant feature. This change is a reflection of the increased gauche disorder at higher temperatures. The other significant change with temperature is the total disappearance of the C16 resonance above 300 K. The intensity of a resonance peak in a 13C CPMAS NMR experiment depends, among other things, on the efficacy of the cross-polariaztion (CP), the mechanism of which involves heteronuclear 13C-1H dipolar coupling. In the presence of rapid motion, this dipolar coupling vanishes, and transfer of magnetization (1H f 13C) by CP no longer exists.17 The disappearance of C16 resonance above 300 K may be attributed to the higher mobility of the surfactant “tail”, which lies midway in the bilayer. We had shown earlier that in the anchored bilayer there is a gradient of conformational mobilities with the tail C16 highly mobile even at room temperature.12 The change in the relative intensities of the resonance of the C4-C13 with temperature is rather surprising. At 295 K, the upfield shifted 33 ppm resonance appears only as a pronounced shoulder, but over a temperature range of just about 15 K, there is a dramatic change in the intensities of the 35 and the 33 ppm peaks; the former being completely absent above 310 K. This would mean that above 310 K all of the carbons from C4 to C13 experience the γ-gauche effect, which in turn, implies that one out of two or half the bonds between C4 to C13 are gauche! It may be noted that for a methylene chain with no constraints the expected population of gauche bonds at 310 K, assuming a gauche-trans energy difference of 500 cal/mol,18 is ∼30%. Although these results appear unusual there are many similar reports in the literature, wherein the upfield resonance, due to the γ-gauche effect, grows rapidly in intensity over a narrow temperature rise and is the dominant feature at temperatures just above ambient.15,16 The 13C NMR spectra of the anchored bilayer at different temperatures was also recorded without cross-polarization (Figure 2b). The results are similar to the measurements with cross-polarization; the 35 ppm resonance is completely absent by 315 K (Figure 2c). There are, however, differences in the spectra measured with and without CP, especially in the resonance of the “tail” C16 carbon. At room-temperature, two resonance peaks at 18.7 and 17.8 ppm are observed, whereas in the corresponding CP spectra, only a single resonance at 18.2 ppm. As the temperature is increased, the intensity of the 18.7 ppm peak in the single-pulse spectra (Figure 1b) decreases and that of the 17.8 ppm peak increases. Between 305 and 310 K, there is a reversal in intensities, and above 310 K, only the 17.8 ppm resonance remains. This splitting of resonances for the tail carbon atom indicates that even at room temperature there are

J. Phys. Chem. C, Vol. 111, No. 2, 2007 497 two kinds of tail carbons: one that experiences that the γ-gauche effect and the other that does not. With increase in temperature, the population of the former grows and by 315 K is the only resonance observed. The fact that this resonance is not observed in the CP measurements clearly indicates that the tails are mobile. The other interesting feature is that at 305 and 310 K the relative ratio of the intensities of the 35 and 33 ppm resonances is higher in the spectrum with CP as compared to the single pulse spectrum. This suggests that the carbons experiencing the gauche effect are less mobile compared to the all-trans carbons. In most polymeric systems and in alkyl chain assemblies it is usually the opposite that is observed: the carbons associated with conformational disorder are more mobile.9 This observation for the anchored bilayer has been investigated in greater detail using variable contact time CP-MAS and 2DWISE measurements that we report elsewhere. Here we wish to address the question as to why the intensity of the 33 ppm resonance increases so dramatically over a 15 K interval. There are two possible reasons why the intensity of the 33 ppm resonance dominates at 315 K and above. There could be a structural phase transition leading to increased disorder. This may, however, be ruled out. X-ray diffraction patterns recorded in this temperature interval, 290-320 K, showed no change (see the Supporting Information). The second possibility is that the 33 ppm peak is an averaged value due to a dynamic averaging of trans and gauche states. Typically, the carbons that experience the γ-gauche effect due to a trans-gauche conformation are expected to show an upfield shift of ∼4.5 ppm as compared to the trans-trans conformation.10,14 The observed shift in the anchored bilayer is much smaller, 2 ppm, which could be an indication of dynamical averaging. It is, however, not possible to verify this possibility by recording the NMR spectra at lower temperatures where exchange and hence dynamical averaging would be absent as the intensity of the upfield shifted 33 ppm resonance in the CP-MAS NMR spectra is negligible below 295 K. Here we address this issue by obtaining an independent estimation of conformational disorder using infrared spectroscopy. Vibrational spectroscopy has been widely used to monitor the conformation and its changes in alkyl chain assemblies.7 The position of the methylene symmetric, νsym (CH2), and asymmetric, νasym (CH2), modes are known to shift to higher frequencies with increased conformational disorder.19,20 For an all-trans alkyl chain, as in the case of crystalline n-alkanes, the symmetric and antisymmetric stretching frequencies are in the range of 2846-2850 and 2916-2920 cm-1, respectively. With increasing number of gauche conformers, as in the hightemperature liquid phases of the n-alkanes, the range shifts to 2855-2865 and 2925-2935 cm-1 respectively. In the anchored bilayer, Cd0.83PS3(CTA)0.34, the methylene symmetric and asymmetric stretching modes appear at 2846 and 2915 cm-1, respectively, at 50 K, and with an increase in temperature, they gradually shift to higher frequencies and by 360 K are 2851 and 2921 cm-1, respectively, indicating an increase in conformational disorder (see the Supporting Information). The values at high temperatures, however, are well within the range expected for an alkyl chain assembly where a majority of the bonds are in the trans conformation and are in apparent contradiction with the 13C NMR measurements that indicate that a majority of the carbon atoms experience the γ-gauche effect. Although the methylene stretching modes are sensitive to chain conformation, it is not possible to obtain quantitative information on the extent of conformational disorder from the position of these modes. This information may, however, be

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Figure 3. (a) Wagging progression bands of the anchored bilayer, at different temperatures. The wagging progression bands are indicated. (b) The fraction of all-trans chains in the anchored bilayer at different temperatures calculated from the ratio of the sum of the intensities of the wagging bands, indicated in the panel on the left, to that at 70 K. The solid line is a guide to the eye.

obtained from an analysis of the progression bands in the infrared that are characteristic of the global conformation of the methylene chain. The progression bands arise from the coupling of the vibrational modes of methylene units that are in the trans registry.21,22 These bands correspond to modes delocalized over the length of the chain, and their spacing and frequency depend on the number of coupled trans methylene units. Appropriate for the present study are the progression bands arising from the coupling of wagging modes of trans methylene units since they appear in a spectral region of the infrared free of interfering bands. A previous analysis of the progression bands of Cd0.83PS3(CTA)0.34 had shown that they could be assigned by considering all 15-CH2 units of the CTA chain to be in the trans registry and that most chains in the anchored bilayer adopt a planar all-trans conformation at low temperatures (50 K).3,11 An increase in temperature induces conformational disorder in the chains. The presence of a single gauche bond in an alltrans ordered chain/segment is sufficient to decouple the vibrational modes and such chains no longer contribute to the intensity of the progression bands. The intensity of the progression bands is, therefore, directly proportional to the concentration of all-trans chains in the ensemble and the ratio of the integrated intensities of the progression bands at two temperatures, a direct measure of the ratio of the concentration of all-trans chains at the two temperatures.23 The wagging progression bands of the alkyl chains of the anchored surfactant bilayer at different temperatures are shown in Figure 3a. There is no change in the positions or spacing of the progression bands with temperature; the only change being in the intensities of these bands. The intensities do not change below 75 K, and hence, it is reasonable to assume that all intercalated methylene chains adopt an alltrans conformation at this temperature. The ratio of the integrated intensity of the progression bands at any temperature with respect to that at 50 K is, therefore, the fraction of the all-trans

chains present in the sample at that temperature. The temperature variation of this ratio (Figure 3b) directly characterizes the thermal evolution of all-trans conformational order in the bilayer. It may be seen from Figure 3b that at 300 K ∼20% of the chains of the anchored bilayer retain an all-trans conformation or, in other words, do not have a single gauche bond and it is only above this temperature that the progression bands in the infrared due to all-trans chains disappear. The NMR, on the other hand, shows that by 310 K almost 90% of the methylene carbons between C4-C13 experience the γ-gauche effect. Interestingly, the temperatures at which the progression bands, the signature of all-trans chains in the anchored bilayer, are no longer seen in the infrared is close to that where the upfield shifted 33 ppm resonance of the C4-C13 carbons in the 13C NMR spectra is the dominant feature. There is thus an apparent contradiction between the thermal evolution of conformational disorder as obtained from infrared (Figure 3b) which is a measure of chains having all 15-CH2 units in the trans registry and the intensity changes associated with the trans/gauche peaks in the NMR (Figure 2c) which provide an estimate of the C4-C13 carbons that experience the γ-gauche effect. The two results may be reconciled by proposing that the intensity of the 33 ppm resonance in the NMR spectra is not a measure of the concentration of gauche bonds but rather of the number of alkyl chains having a gauche bond. This would then require that the 33 ppm resonance in the 13C NMR spectra arises due to dynamical averaging. In order to identify the nature of the gauche defects, we have examined the methylene wagging modes between 1300 and 1400 cm-1 that are sensitive to specific localized conformational sequences that contain a gauche bond.24 A band at 1341 cm-1 indicates a penultimate bond oriented such that the terminal methyl group is in the gauche conformation relative to the methylene groups three carbon atoms away (end-gauche). A peak at 1354 cm-1 is due to two adjacent gauche bonds (double-

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Figure 4. (a) Infrared spectra in the methylene wagging region of the anchored bilayer, Cd0.83PS3(CTA)0.34, recorded at different temperatures. The spectra are normalized with respect to the 1377 cm -1 CH3 symmetric bending mode. (b) The intensities of the peaks corresponding to the “kink” 1364 cm -1 and end-gauche 1341 cm -1 conformations at different temperatures.

gauche), and a peak at 1364 cm-1 arises from gauche-transgauche′ (g-t-g′) sequence or a “kink”. The population of these localized conformational sequences is proportional to the normalized intensities of the peaks in the infrared. The methylene wagging modes of the anchored bilayer at different temperatures are shown in Figure 4a. The spectra have been normalized using the intensity of the 1377 cm-1 methyl umbrella deformation mode as an internal standard. With increase in temperature the features due to the end-gauche (1341 cm-1) and kink (1364 cm-1) grow in intensity but saturate above 320 K (Figure 4b). In lipid bilayers, the intensity of these modes have been analyzed using the rotationally isomeric state (RIS) model to obtain quantitative estimates of the concentration of each type of defect sequence.25 In the anchored bilayer, the RIS model is not applicable as there is a gradient of conformational mobilities, and it is therefore not possible to obtain quantitative estimates. Nevertheless, the intensities are directly proportional to the concentration, and the fact that the intensities saturate suggests that there is not more than one kink per chain. This is understandable, since the chains are anchored, space available per chain is limited, and the structure cannot sustain more than one kink per chain without breaking-up. It is for this reason that the population of the double-gauche defect, too, is insignificant even at 360 K. The end-gauche and kink are related; an end-gauche is a kink that has moved to the end of the chain. The difference in estimates of gauche disorder from the infrared and NMR measurements is a reflection of the different time-scales associated with the two spectroscopic techniques. The infrared involves shorter time-scales and provides an instantaneous snapshot of the conformation of the chain. It shows that even at 300 K there are chains that do not have a single gauche defect, and it is only above this temperature that there are no planar all-trans chains in the anchored bilayer. The gauche defects are kinks and their occurrence probably limited to one per chain. The NMR, on the other hand, indicates that all carbons between C4 and C16 experience the γ-gauche effect at similar temperatures. This may be realized if the kinks once formed are mobile on NMR time-scales. It may be noted that the lateral area of the chain is the same irrespective of the position of the kink, unlike for a isolated gauche defect, and

consequently, the energies are the same. Mobility would be entropically favorable and would, in part, defray the cost of creating a kink. How fast do the kinks move? We can place a lower limit by assuming that the expected upfield shift for the γ-gauche effect is 4 ppm (300 Hz) so that dynamic averaging would require motion faster than 3.5 ms. In conclusion, a model that invokes a kink, mobile along the length of the chain, can reconcile the apparently contradictory estimates of disorder from NMR and infrared spectroscopy measurements. Supporting Information Available: (1) X-ray diffraction patterns of the anchored bilayer, Cd0.83PS3(CTA)0.34, recorded at different temperatures. (2) Methylene stretching modes in the infrared spectrum of the anchored bilayer, Cd0.83PS3(CTA)0.34 at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lagaly, G. Angew. Chem., Int. Ed. Engl. 1976, 15, 575. (2) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (3) Venkataraman, N. V.; Vasudevan, S. Proc. Ind. Acad. Sci 2001, 113, 539. (4) Venkataraman, N. V.; Vasudevan, S. J. Phys. Chem. B 2001, 105, 1805. (5) Barman, S.; Venkataraman, N. V.; Vasudevan, S.; Seshadri, R. J. Phys. Chem. B 2003, 107, 203. (6) Osman, M. A.; Ploetze, M.; Skrabal, P. J. Phys. Chem. B 2004, 108, 2580. (7) Zerbi, G.; Del Zoppo, M. In Modern Polymer Spectroscopy; Zerbi, G., Ed.; Wiley-VCH: Germany, 1999. (8) Neumann-Singh, S.; Garibay-Villanueva, J.; Muller, K. J. Phys. Chem. B 2004, 108, 1906. (9) Badia, A.; Lennox, B. R.; Reven, L. Acc. Chem. Res. 2000, 33, 475. (10) Bovey, F. A.; Mirau, P. A. NMR of Polymers; Academic Press: San Diego, CA, 1996. (11) Venkataraman, N. V.; Vasudevan, S. J. Phys. Chem. B 2001, 105, 7639. (12) Suresh, R.; Vasudevan, S.; Ramanathan, K. V. Chem. Phys. Lett. 2003, 371, 118 (13) Klingen, V. W.; Ott, R.; Hahn, H. Z. Anorg. Allg. Chem. 1973, 396, 271. (14) Tonelli, A.; Schilling, F. C. Acc. Chem. Res. 1981, 14, 233. (15) Wang, L.; Exarhos, J. G.; Liu, J. AdV. Mater. 1999, 11, 1331 (16) Khimayak, Z. Y.; Klinowski, J. Phys. Chem. Chem. Phys. 2001, 3, 616. (17) Kolodziejski, W.; Klinowski, J. Chem. ReV. 2002, 102, 613.

500 J. Phys. Chem. C, Vol. 111, No. 2, 2007 (18) Flory, P. J. Statistical Mechanics of Chain Molecules; WileyInterscience: New York, 1969. (19) MacPhail, R. A.; Straus, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (20) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (21) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411.

Letters (22) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (23) Barman, S.; Vasudevan, S. J. Phys. Chem B 2006, 110, 22407. (24) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (25) Snyder, R. G.; Tu, K.; Klein, M. K.; Mendelssohn, R.; Strauss, H. L.; Sun, W. J. Phys. Chem. B 2002, 106, 334.