Article pubs.acs.org/JPCA
Unprecedented Relationship Between the Size of Spherical Chiral Micellar Aggregates and Their Specific Optical Rotations R. Vijay,† Geetha Baskar,‡ A. B. Mandal,‡ and Prasad L. Polavarapu*,† †
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States CSIR-Central Leather Research Institute (CLRI), Adyar, Chennai 600020, India
‡
S Supporting Information *
ABSTRACT: Transmission electron microscopy (TEM) images and fluorescence quenching methods indicated that lauryl ester of L-phenylalanine (LEP) and lauryl ester of L-tyrosine (LET) form spherical chiral micelles in the 50−200 mM range and their size increases with concentration. The number of molecules present in these spherical chiral aggregates varied from 80 to 160 for LEP and 80−100 for LET. The specific optical rotation, representing circular birefringence, for LEP at 405 nm and 32 °C is found to increase linearly from 37 deg cc g−1 dm−1 for an isolated molecule to 56 deg cc g−1 dm−1 for ∼200 nm size aggregate. A similar trend was found for temperatures up to 70 °C and at other visible wavelengths. A linear relation between specific optical rotation and the size of aggregate is also observed for LET. Circular dichroism, as measured in both the visible and infrared wavelength regions, however did not reveal any concentration dependent changes. The unique sensitivity uncovered for specific optical rotation as a function of the size of spherical chiral aggregates is unprecedented and opens new areas of enquiry for physical chemists.
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INTRODUCTION Chiroptical spectroscopic techniques probe differential refractive index, absorption, or scattering of circularly polarized light from chiral molecules as reflected respectively in their specific optical rotation (or circular birefringence), circular dichroism, or Raman optical activity properties. Numerous developments have taken place recently, in the instrumentation for chiroptical spectroscopic measurements.1−9 These modern instruments render the applications to chiral molecules10,11 much more facile than ever before. However the literature pertaining to use of modern chiroptical spectroscopic instruments to characterize chiral amphiphiles is scarce.12−21 The early work22−26 dates back to several decades ago, where signal quality was far less than that available currently27,28 to derive useful information and studies were limited only to concentrations that are near the critical micelle concentration (CMC). Surfactants are indispensable in a variety of applications.29 The use of surfactant solutions on an industrial scale, for example as soft templates in the design of nanoparticles30−36 demand routine assessment of the size of the aggregates as they can change with time, conditions of preparation, or storage.37−42 Further, as the aggregates are as such polydisperse, different measurements yield different aggregation numbers.43−45 For example, the aggregation number of 0.1 M sodium dodecyl sulfate solution at 30 °C varies from 50 to 80, with an average of 62, a Gaussian distribution centered on the value 62, with a variance of 13.46 Therefore advanced practical applications of surfactant solutions demand development of methods to quickly assess the size of the aggregates in the surfactant solutions prepared from different batches on a routine basis. The methods available for studying the micellar © 2013 American Chemical Society
size, or the aggregation number, include nuclear magnetic resonance, fluorescence quenching, small-angle neutron scattering, gel permeation chromatography, transmission electron microscopy (TEM), and small-angle X-ray scattering.29 These techniques are time-consuming and expensive and require expertise to make meaningful interpretation of the data. Static (SLS) and dynamic light scattering (DLS) methods are convenient techniques to characterize the micellar aggregates, but they suffer from erratic results due to the inseparable scattering of light by contaminants like dust particles. In this article we report an unprecedented observation that circular birefringence is a manifestation of the size (or aggregation number) of spherical chiral micelles.
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EXPERIMENTAL SECTION Lauryl ester of L-phenylalanine (LEP) and lauryl ester of L-tyrosine (LET) were synthesized and characterized according to the methods reported elsewhere.47,48 The optical rotation measurements were made at five different wavelengths (405, 436, 546, 589, and 633 nm) using an Autopol IV polarimeter (Rudolph Research Analytical, Flanders, NJ, USA), with reproducibility of 0.003°, using either a 2 dm or 0.5 dm cylindrical cell with quartz windows. The optical rotation values of LEP solutions were measured in the concentration range of 0.001−200 mM at room temperature. At concentrations less than 0.01 mM, LEP solution did not register any optical rotation. At a concentration of 0.01 mM a small optical rotation Received: February 12, 2013 Revised: April 8, 2013 Published: April 10, 2013 3791
dx.doi.org/10.1021/jp401544g | J. Phys. Chem. A 2013, 117, 3791−3797
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value between 0.003 and 0.005°, just around instrumental uncertainty, was measured for wavelengths 405−633 nm. At a concentration in the range of 0.05−5 mM, the LEP solutions appeared translucent and optical rotation values were fluctuating and unstable. At a concentration beyond 10 mM, LEP solutions are optically clear and stable measurable optical rotation values were obtained. All measured optical rotations were converted to, and reported here as, specific rotations (specific rotation is the more commonly used terminology for specific optical rotation). Because CMC of LEP and LET are47 respectively 1.3 × 10−4 and 3.8 × 10−5 M, the specific rotation values reported here for 50−200 mM represent those for postCMC solutions. Electronic circular dichroism (ECD) measurements were made using a JASCO 720 spectrometer using a 0.1 mm, 1 mm, or 1 cm quartz cell. Vibrational circular dichroism (VCD) measurements were made using ChiralIR spectrometer using fixed path length (100 or 50 μm) cells with BaF2 windows. ECD and VCD spectral measurements for LET at room temperature could not be undertaken because its Kraft temperature is ∼25 °C. For the same reason, the LET solution was prepared for TEM measurements at ∼50 °C. TEM measurements were obtained using a CM20, operated at an acceleration voltage of 200 kV. A drop of surfactant solution consisting of the aggregates was placed onto a ultrathin carbonon-Formvar TEM grid (Ted Pella, Inc.). About 1 min after deposition, the grid was blotted with filter paper to remove surface solution. Negative staining was performed by using a droplet of a 1 wt % phosphotungstic acid solution. The aggregation numbers of LEP and LET at different concentrations and temperatures were obtained from timeresolved fluorescence quenching (TRFQ) method as reported elsewhere.48 Although the shape of aggregates was not known at the time, the TEM images obtained in the current work for LEP and LET (see Figure 1) showed the formation of spherical micelles. Thus the aggregation numbers of LEP and LET determined from TRFQ method48 are now assigned for spherical micelles of LEP and LET.
Figure 1. TEM images of the aggregates of LEP at 50 (A) and 200 mM (B), both at room temperature, with respective scale bars at 100 and 500 nm and of LET at 100 mM (solution prepared at 50 °C) with scale bar at 500 nm (C).
Scheme 1. Chemical Structures of Lauryl Ester of (A) LTyrosine and (B) L-Phenylalanine
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RESULTS AND DISCUSSION The chemical structures of two well characterized amino acid based chiral amphiphiles, namely LEP and LET,48 used in this study are shown in Scheme 1. The TEM images obtained for aqueous solutions of LEP at 50 and 200 mM, which are 2−3 orders of magnitude higher than its CMC (1.3 × 10−4 M48), clearly demonstrate that LEP forms spherical micelles. Moreover, the size of the LEP micelles is seen to grow from ∼50 to 200 nm in the concentration range of 50−200 mM. The polydispersity information derived from TEM images is given in Table 1. The specific rotations, reflecting circular birefringence, of LEP micellar solutions at 405 nm for various concentrations in the range of 50−200 mM and temperatures in the range of 32− 70 °C are shown in Figure 2. The specific rotation of LEP micellar solutions is observed to increase with an increase in concentration (Figure 2A). It is important to note that the specific rotations are expected to be independent of chiral solute concentration,49 but this assumption can break down in the presence of intermolecular interactions.27,28,50,51 Thus the concentration dependent increase in specific rotation of LEP reflects intermolecular interactions arising from within the microscopic aggregates. The slope of [α]405 vs concentration (obtained from fitting the data to a linear equation) is ∼0.10, 0.09, 0.08, 0.07, and 0.06 at 32, 40, 50, 60, and 70 °C,
Table 1. Number-Weighted Particle Sizea (dn), Weight− Weighted Particle Sizeb (dw), and Polydispersity Indexc (PDI) of LEP and LET Micelles from TEM Micrographs dn (nm) dw (nm) PDI
LEP (50 mM)
LEP (200 mM)
LET (200 mM, 50 °C)
53.2 56.9 1.07
236.7 243.1 1.03
233.8 248.2 1.06
a
dn = (∑idini)/(∑ini). bdw = (∑idi2ni)/(∑idini), where di is the size of the ith micelle. cPDI = dw/dn, a measure of the heterogeneity of micelle size.
respectively. Therefore the change in [α]405 with concentration is higher at lower temperatures than that at higher temperatures. Extrapolation of [α]405 to zero concentration yields intrinsic rotation, reflecting the value for single solvated 3792
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Figure 2. Dependence of the specific rotation and aggregation number of lauryl ester of L-phenylalanine on concentration and temperature.
Figure 3. Dependence of the specific rotation at (A) 405, (B) 436, (C) 546, (D) 589, and (E) 633 nm on the aggregation number of lauryl ester of Lphenylalanine.
surfactant molecule,52 which for LEP is ∼39 at 70 °C and ∼37 at 32 °C. The specific rotation of LEP micellar solutions is observed to decrease with increase in temperature (Figure 2B). The slope of [α]405 vs temperature is ∼−0.16, −0.15, and −0.09 at 200, 150, and 100 mM, respectively, and is nearly zero at 50 mM (note that at 50 mM the observed rotation at 405 nm changes only by 0.005° in going from 32 to 70 °C, so the variations among individual data points at 50 mM are not beyond the instrumental uncertainty of 0.003°). The change in [α]405
with temperature is larger at higher concentrations than that at lower concentrations. As the TEM images (Figure 1) of LEP indicate the formation of spherical micelles, the aggregation numbers determined for LEP by fluorescence quenching method reflect the size of spherical micelles. The trends in [α]405 vs concentration/ temperature for LEP micellar solutions correlate with corresponding trends in the aggregation number (N) (Figure 2C,D). From Figure 2C, one can see that N generally increases with an increase in concentration at all temperatures. With 3793
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increasing T, N is seen to decrease (Figure 2D) for 50, 100, 150, and 200 mM solutions. The individual variations for [α]405 and N, as a function of concentration/temperature, can be cross-correlated to ascertain the dependence of specific rotation on the size of spherical chiral micelles. The plot of [α]405 vs N is shown in Figure 3. At 32 and 40 °C it is clear that specific rotation increases linearly with N (correlation coefficients are 0.99 and 0.999, respectively). At 50, 60, and 70 °C, [α]405 vs N curves can be approximated to be linear (correlation coefficients are 0.98, 0.98 and 0.95, respectively), although a slight bend giving the appearance of a hump in the middle (which lies within the uncertainty (5%) in determination of N), is apparent for data at 70 °C. In the linear fit of [α]405 vs N, the slope is 0.23, 0.27, 0.21, 0.16 and 0.14 at 32, 40, 50, 60, and 70 °C, respectively. These observations indicate that [α]405 varies approximately linearly with aggregation number, and the change in [α]405 with N is larger at lower temperatures than at higher temperatures. One may extrapolate the trends seen in Figure 3 to suggest that [α]405 may become independent of N in the limit of very high temperature. The behavior of [α]λ at other wavelengths with N (also shown in Figure 3) is identical to that shown for [α]405. Figures 1−3 present clear evidence that circular birefringence, as reflected by specific rotation, is a manifestation of the size of spherical chiral micelles at concentrations well above CMC. ECD spectra of aqueous LEP solutions were measured at room temperature in the concentration range of 0.01−200 mM (Figure 4). No ECD bands are apparent for the premicellar solution at a concentration of 0.01 mM. At 0.05, 0.1, and 0.5
mM, where optical rotation could also be registered, a band centered at ∼217 nm is visible. At 1 mM, where the solution was slightly translucent and optical rotation was fluctuating, the ECD band became noisy. At higher concentrations of 10−200 mM, where the solutions are clear, ECD bands centered around 217 nm could be measured with higher signal-to-noise ratio. However, ECD spectra plotted as molar extinction coefficient vs wavelength in the concentration range of 10−200 mM overlay on top of each other and no significant differences could be discerned. It is possible that ECD spectra may show changes if vacuum ultraviolet region (
