Amphiphilic Lauryl Ester Derivatives from Aromatic ... - ACS Publications

J. Phys. Chem. B 2009, 113, 13959–13970

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Amphiphilic Lauryl Ester Derivatives from Aromatic Amino Acids: Significance of Chemical Architecture in Aqueous Aggregation Properties R. Vijay,† Jasmeet Singh,‡ Geetha Baskar,*,† and Radha Ranganathan*,‡ Industrial Chemistry Laboratory, Central Leather Research Institute, Adyar, Chennai 600020, India, and Department of Physics and Center for Supramolecular Studies, California State UniVersity Northridge, Northridge, California 91330-8268 ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: September 1, 2009

Lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) were, in a previous interface adsorption study, found to adopt very different interfacial conformations. The present study is an investigation of their aqueous aggregation properties with the goal of elucidating the effects of the presence in LET and absence in LEP of the phenolic OH group on their aqueous aggregate structures and micellar conformations of the surfactant monomers. The measured properties included aggregation numbers from time-resolved fluorescence quenching (TRFQ), interface hydration index and microviscosity by electron spin resonance (ESR), chemical shifts of 1 H resonance lines by NMR, and Krafft temperatures and enthalpies of structural transitions by differential scanning calorimetry (DSC). The TRFQ, ESR, and NMR experiments were conducted at various temperatures from 23 to 70 °C for various surfactant concentrations from 0.050 to 0.200 M. Markedly different temperature dependences of aggregation number and 1H NMR chemical shifts are exhibited by LET and LEP micelles. LET and LEP form ionic micelles. The aggregation number of LEP decreases as is characteristic of ionic micelles, but that of LET increases slightly with temperature. The changes with temperature in the NMR chemical shifts and width of the resonance lines are significantly greater for the various LEP protons than for those of LET. The differences in these properties and other fluorescence decay characteristics of fluorophores incorporated into the micelles could be attributed to the difference in the micellar conformations of LET and LEP which are postulated to be similar to that at oil-water interfaces. The phenolic group is hypothesized to be in the micelle-water interface as part of the headgroup in LET micelles, and its location does not change with temperature. On the other hand, in LEP micelles, the phenyl ring is folded into the core overlapping with the flexible hydrophobic chains. The resulting closer proximity between the phenyl ring and the flexible hydrocarbon chain causes interdependence of the phenyl ring and chain proton resonances, leading to the observed temperature dependence of the chemical shifts in LEP. The TRFQ and ESR data are combined together in a molecular space-filling model, referred to as the polar shell model, to derive the geometrical properties of the micelle. The DSC scans in the temperature range 10-55 °C showed the presence of distinctly different endotherms for LET and LEP. The Krafft temperatures, KT, and the enthalpies were determined. The higher KT and broader peak of the DSC endotherm of LET as compared to LEP are attributed to the stabilization of fiberlike structures below the Krafft temperature due to its chirality and the hydrogen bonding capability of the phenolic OH and also to the ion-dipole interactions. Thus, all of the observed differences between LET and LEP could be attributed to the difference in their chemical architecture. 1. Introduction The growing environmental concerns in industry, bilology, and medicine demand the design of novel eco-friendly materials and methodologies. The design of amphiphilic materials from amino acids and the methodologies involving these materials in water medium are well recognized as one of the approaches. Among various amino acids, aromatic amino acids are significant especially in view of their UV visible absorption characteritics and reactivity.1 We have chosen to design surfactants from tyrosine that can provide additional functional characteristics due to the phenolic group. They offer scope for generation of industrially significant eco-friendly polyphenolic materials. The polymerization of a phenolic group and that of tyrosine, employing enzymes, has been well demonstrated.2,3 The potential applications of mono ester derivatives of a peptide from † ‡

Central Leather Research Institute. California State University Northridge.

tyrosine in surface characterization, drug delivery, sterilization, and fabrication techniques have been well documented.4 The significance of micellar solutions of amphiphilic decyl esters of tyrosine in promoting controlled enzymatic polymerization reaction in contrast to that of the non-amphiphilic compounds that form insoluble cross-linked products has been demonstrated in recent work by Marx et al.5,6 The interfacial organized structures provided by the C-18 ester of various amino acids including tyrosine have been shown to favor polycondensation reactions under alkaline conditions.7 In the context of our efforts8,9 to design new surfactants, and understand the structure property correlation of interfacial organized structures that perform as novel microreactors in aqueous medium, we designed the amphiphile from lauryl ester of L-tyrosine (LET), in our recent work.10 The free amino group in the ester derivative is amenable for various chemical modifications apart from protonation using different counterions. With an aim to understand the influence of structural features,

10.1021/jp905384y CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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Figure 1. Chemical structural representation of (a) LET and (b) LEP according to the deduced orientations at the oil-water interface.10

especially the phenolic ring of tyrosine, we carried out, in previous work, comparative investigations with lauryl ester of L-phenylalanine (LEP) on their monomeric organization at different interfaces and aggregated structures by optical microscopy and scanning electron microscopy (SEM).10 The conclusion from the data on oil-water and air-water interfacial tension experiments was that LET exhibits more efficient adsorption and greater packing (greater number of molecules per unit surface area) than LEP.10 The surface area per molecule calculated from the interfacial tension vs surfactant concentration gave a value of 50.61 Å2 for LET and 64.62 Å2 for LEP.10 Molecular modeling calculations of the energies of surface molecular conformations at the oil-water interface confirmed these numbers. Figure 1 shows the chemical structures of LET and LEP and the deduced orientations at the oil-water interface. The data further showed that the portion of the LET monomer containing the phenolic ring pointed out of the oil and into the water, that is, the ring was in the water. The OH group in LET prefers contact with water and is responsible for pulling the aromatic ring out of the oil and keeping the molecule in that conformation. On the other hand, in LEP, the phenyl ring was folded down into the oil because the aromatic ring, not having the hydrophilic OH group to pull it into the water, tends to fold into the oil. The present interest is to understand how this principle operates in the structural formations of LET and LEP micelles and in general the differences between the LET and LEP micellar aggregation processes due to the difference in their chemical architecture. Toward that goal, physicochemical characterization of the aggregation behavior of LET and LEP was undertaken. Time-resolved fluorescence quenching (TRFQ), electron spin resonance (ESR), and 1H NMR were used to determine the micellar aggregate properties at various temperatures ranging from 23 to 70 °C for each of the surfactant concentrations: 0.050, 0.100, 0.150, and 0.200 M. The measured properties included aggregation numbers using TRFQ, interface hydration index and microviscosity using ESR, and chemical shifts of 1H resonance lines. Differential scanning calorimetry (DSC) measurements were conducted in the temperature range 5-60 °C to determine the Krafft temperatures (KT) of LET and LEP and the enthalpies associated with the observed endotherms. LET and LEP exhibit

Vijay et al. visibly different DSC endotherms, and LET has a higher Kraft temperature than LEP. Marked differences were observed between LET and LEP aggregate properties, particularly in their 1 H NMR spectra. These differences could be explained in the light of the different conformations adopted by the LET and LEP monomers in the micelle. The TRFQ and ESR data were combined together in a molecular space-filling model, referred to as the polar shell model, to derive the geometrical features of the aggregate.11,12 In these calculations, the micellar organization of the aromatic rings of LET and LEP are hypothesized to be similar to their organization at the oil-water interface. The experimental results and the derived conformational features of the monomers in the aggregate are then discussed in the context of this hypothesis. The experimental observations corroborate the hypothesis and thereby the polar shell model itself. In view of the difference in Krafft temperatures, the micellar models are developed and their structures discussed as a function of temperature starting from 32 °C for LEP and 40 °C for LET. TRFQ in the manner applied to investigate micelles is naturally meaningful only in the micellar phase, whereas ESR and NMR need not be restricted to micellar structures alone. Some points to be kept in mind with regard to LET and LEP are that their solutions in water have a pH of about 3.0. Considering the pKa of about 9.04 and 9.09 of the NH2 group in the parent amino acid, viz., tyrosine and phenyl alanine,13 these surfactants at pH 3.0 are almost fully protonated and exhibit cationic character due to NH3+ with chloride as the counterion, as shown in Figure 1.5 The cmc of LET is 37.8 µM, and that of LEP is 126 µM at 28 °C.10 The positioning of the two hydrophobic parts, the aromatic ring and the tail on either side of the polar protonated amino headgroup, could account for the low values of cmc that are rather more typical of nonionic surfactants. Below 32 °C, LET does not form optically clear solutions, because of the formation of fibril structures stabilized by the network of intermicellar hydrogen bonding facilitated by the phenolic OH and its chirality.14-16 At temperatures greater than 32 °C, the hydrogen bonds weaken, leading to clear solutions with the possibility of formation of small spherical micelles.16 2. Materials and Methods 2.1. Chemicals. Lauryl esters of tyrosine (LET) and phenylalanine (LEP) were synthesized according to the method reported elsewhere.10,17 The fluorescent probe pyrene (optical grade, 99%) and the quencher 3,4-dimethyl benzophenone (DMBP, 99%), employed in the TRFQ studies, were obtained from Aldrich Chemicals and used as received. The 5-doxyl stearic acid methyl ester (5DSE; 99% Sigma) was the spin probe used in the ESR experiments. Sodium 4,4-dimethyl-4-silapentane-1-sulfonate (DSS, 97% Aldrich) was used as the internal standard in NMR experiments. 2.2. Micelle Solution Preparation. The solution concentrations investigated ranged from 0.050 to 0.200 M. All solutions were prepared in double distilled water except in NMR experiments where D2O (D 99.9%, MSD Isotopes, Canada) was used as the solvent. The pyrene concentration in the samples prepared for TRFQ experiments was kept at about one hundredth the concentration of micelles. This ensures that the fraction of micelles with two or more pyrenes is negligible. The quencher (DMBP) concentration was kept at about 1.3[micelles]. The micelle concentration was estimated from the known aggregation numbers for other surfactants with similar tail length, e.g., DPS and SDS micelles.11,18 The appropriate amounts of stock

Aqueous Aggregation Properties of LET and LEP

J. Phys. Chem. B, Vol. 113, No. 42, 2009 13961

solutions each of pyrene and DMBP in ethanol were added to a clean glass vial. The resulting mixture was vortexed thoroughly to produce a clear solution which was then dried under dry N2 flux to produce a film of pyrene and DMBP. Thereafter, the required amount of the LET or LEP and water was added to the dry film to achieve the final concentrations. The solution was stirred overnight to ensure the complete solubilization of pyrene and DMBP in surfactant micellar solution. The samples for ESR were similarly prepared with 5DSE as the spin probe at a concentration of 0.003[total surfactant]. 2.3. Time-Resolved Fluorescence Quenching (TRFQ). In the TRFQ method, fluorescence probes (pyrene) and quenchers, being themselves hydrophobic, are dispersed in micelles and the quenched time decay of the pyrene fluorescence is measured. The Infelta-Tachiya model describes the quenched decay of pyrene fluorescence, F(t), in micellar solutions according to12,18-24

F(t) ) F(0) exp[-A2t - A3{1 - exp(A4t)}]

(1)

The fitting parameters A2, A3, and A4 are

A2 ≡

[

kq 1 1 ) + k-〈n〉 T1 τ0 kq + k-

[

(2)

]

(3)

1 ) kq + kT2

(4)

kq A3 ) 〈n〉 kq + kA4 ≡

]

2

The constants in the above equations are sample parameters and fluorescence decay properties of the lifetime of pyrene fluorescence in the absence of quenchers, τ0; pyrene fluorescence lifetime in the presence of quenchers, T1; quenching rate constant, kq, for intramicellar quenching of excited pyrene by a single quencher; the exit rate constant, k-, for exit from the micelle of probes and/or quencher, and the average number of quenchers per micelle, 〈n〉. After obtaining the lifetime τ0 in the absence of quenchers and fitting the quenched decay to eq 1, the decay characteristics 〈n〉, kq, and k- can be calculated from A2, A3, and A4 and τ0 using24

kq ) A3

A42 1 + A3A4 A2 τ0

k- ) A4 - kq

〈n〉 ) A3

A42 kq2

(5)

(6)

where CT is the total surfactant concentration and cmc is the critical micelle concentration. Equation 8 comes about because the number of quenchers per micelle, 〈n〉 ) [Q]/[micelles] and [micelles] ) (CT - cmc)/N. The cmc of LET is 37.8 µM, and that of LEP is 126 µM at 28 °C.10 These values may be neglected in comparison to the surfactant concentrations of the present measurements. There are a few considerations to be kept in mind while employing the micellar quenching model to the decay curve: (1) The model applies when (i) micelle size is monodisperse and (ii) probes and quenchers are assumed to occupy the micelles according to Poisson statistics. If either of these conditions is not satisfied, then the derived values of N and kq will depend on quencher concentration. (2) There are two time scales involved in the decay. One is T1 ) A2-1, and the other is T2 ) A4-1. For micellar quenching methods of investigating aggregation numbers, T2 must be less than T1; that is, the quencher must encounter the probe within the lifetime of the probe fluorescence. Reliable fits are obtained typically when T1 g 2T2. (3) If k- is ,kq and ,1/τ0, it means that the quenching reaction (encounter between pyrene and quencher) is faster than the rate at which the quencher exits the micelle. The rate constant k- can then be treated as zero, and the decay parameters simplify to

T1 ) τ0 ;

A3 ) 〈n〉 ;

A4 ) kq ) 1/T2

(9)

A proper application of TRFQ in the investigation of micelles involves conducting decay experiments in the absence of quenchers to obtain τ0 and at more than one quencher concentration to ascertain that N and kq do not depend on quencher concentration and T1 g 2T2. The TRFQ data reported in this work are restricted to those sample concentrations and temperatures which meet these demands. If the fit value obtained for T1 is equal to τ0 (then it also means that T1 does not depend on quencher concentration), T1 g 2T2, and A3 and T2 do not depend on quencher concentration, then the aggregation number is given by eq 8 with 〈n〉 ) A3. If T1 is less than τ0, then there is intermicellar quencher migration; that is, kcannot be neglected. This situation entails the use of eqs 5-7. TRFQ measurements were conducted on LET and LEP micelles at temperatures ranging from 32 to 70 °C and at 0.050 M e [surfactant] e 0.200 M. The fluorescence decay curves of pyrene were obtained by time-correlated single photon counting using an FL900 lifetime measurement spectrometer of Edinburgh Analytical Instruments (EAI) with nanosecond flash lamp excitation. The decay curves, corrected for instrument response, were fitted to the Infelta-Tachiya model (eq 1) using the Level 2 analysis software of EAI. 2.4. Electron Spin Resonance (ESR). Spin probe ESR methods were used to determine the hydration index, denoted by H, of the aggregate/water interface. H is the polarity of the interface and is given by the volume, VOH, in the interface occupied by OH dipoles from the interfacial water as well as those from the surfactant itself.25,26 H is measured and stated as a fraction of the total interface volume, Vshell.

(7) H)

VOH Vshell - Vdry ) Vshell Vshell

(10)

The aggregation number, N, is then given by

N ) 〈n〉

CT - cmc [Q]

(8)

where Vdry is the volume in the polar shell inaccessible to water. In the simple continuum model that has thus far proved successful,12,25-30 Vdry is the sum of the volumes of the headgroups, the counterions, and any hydrocarbons from the

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alkyl chains of the surfactants that occupy the shell (see polar shell model in section 2.7 below).12 Spin probes are sensitive and informative monitors of their environment.31 Surfactants with a nitroxide free radical attached to one of the carbons of the hydrocarbon tail incorporated into micelles are excellent probes of the hydration of the interface.25,26 This is because the polar nitroxide group points in to the interface and undergoes rapid rotational motion, sampling all regions of the polar layer.32 The ESR spectra of spin probes comprise three sharp, well-resolved lines resulting from the hyperfine coupling interaction of the unpaired electron and the nitrogen nucleus. Magnetic parameters extracted from the ESR spectrum can be interpreted on the basis of molecular structures and orientations, nature of the environment, and molecular motions.33 Special spectral fitting methods developed in our laboratories and the development of exceptionally linear and reproducible modern magnetic field sweeps permit measurement of line positions and line widths in the ESR spectrum with milligauss precision.34-36 The ESR spectra were taken at X-band using a Bruker ESP 300 E spectrometer equipped with a Bruker variable temperature unit (model B-VT-2000). The details of sample configuration for ESR have been described previously.26 The temperature of the sample was measured with an Omega temperature indicator (model DP41-TC-S2). Five ESR spectra were acquired for each sample. Fittings of the experimental ESR lines to a Lorentzian-Gaussian sum function were performed. This yields the position of the resonance fields of the three ESR lines with a precision of a few milligauss, and also separates the Lorentzian and Gaussian contributions of the spin label ESR lines. The spacing, A+, between the low-field and central lines in the ESR is a linear function of the hydration index H and for 5DSE the relation is26

A+(H) ) 14.210 + 1.552H

(11)

Thus, values of H can be found from the nitrogen hyperfine coupling constant A+. The calibration curve (eq 11) was derived from measurements on water/methanol mixtures at 25 °C. The intrinsic variation of A+ is only about 5 mG over the range 25-45 °C, and eq 11 may be used for this range.25,27 The ESR spectral linewidths yield the rotational correlation times of the nitroxide probe from which the microviscosity of the polar shell can be calculated.28 The rotational correlation time, τ, of the spin probe given by the width of the ESR lines that is obtained by fitting is used to calculate the microviscosity, η, from the Debye-Stokes-Einstein equation.37,38

τ)

4πηr3 3kT

(12)

where r is the hydrodynamic radius of the spin label ()4.68 Å for 5DSE),26,39 k is the Boltzmann constant, and T is the sample temperature. The details of the instrument and the method are given in previous publications and are not repeated here.23,26,28,40 ESR measurements were conducted for temperatures in the range 25-70 °C. 2.5. NMR. 1H NMR techniques are well-known in micellar characterization studies.41-45 This technique has been successfully exploited in surfactant systems consisting of aromatic groups wherein the ring currents significantly influence the magnetization of neighboring protons, and thus provide useful information on the neighboring groups in the micellar structure.

In this work, 1H NMR spectra were measured on a Bruker AC400E spectrometer at a frequency of 400 MHz. The temperature during spectrum acquisition was controlled with a Bruker variable temperature unit (model B-VT-2000). All chemical shifts were measured relative to sodium 4,4-dimethyl4-silapentane-1-sulfonate (DSS), which acted as an internal standard. Deuterium oxide (D2O) was used as the solvent instead of water to weaken the water signal for all solutions. Experiments were performed as a function of concentration of the surfactants, LET and LEP, and at different temperatures from 23 to 65 °C for each of the concentrations in order to determine the influence of temperature and concentration on the micellar structures. The changes in chemical shift (∆δ) from these measurements were considered for detailed analysis. Additional 1 H NMR measurements were performed at 500 MHz on a JEOL ECA 500 NMR spectrometer for selected concentrations and temperatures. 2.6. Differential Scanning Calorimetry (DSC). The behavior of micellar assemblies and their stability at different temperatures are important especially to draw information on the suitable temperature range for their application. DSC measurements are known to shed information on the thermal response of the surfactant solutions. Ionic surfactants are characterized by a Krafft temperature, KT, only above which investigations on micellar structures are meaningful. LET and LEP surfactants are ionic in nature, and we have chosen to investigate their thermal behavior and measure KT using DSC. The DSC measurements were performed with a TA calorimeter (model DSC Q200) in the temperature range 5-60 °C using 30 µL Tzero hermetic pans. Solutions of LET at different concentrations were prepared by dissolving the appropriate amount of solid in water. Heating to about 45 °C was necessary to bring about complete dissolution. A comparative DSC measurement was performed for LEP at a concentration of 0.200 M. LEP dissolved spontaneously at ambient temperature. Accurately weighed surfactant solution corresponding to 25 µL of solution was transferred into the hermetic pan, sealed, and maintained at 5 °C for 30 min for complete equilibration. All measurements were carried out keeping the reference pan empty. The solution was heated at a rate of 1 °C/min. The temperature corresponding to the onset of the endotherm in the DSC trace is considered to be KT. The enthalpy (∆H) associated with the endotherm was estimated by an integration of the area under the peak using the supported Universal Analysis 2000 software. 2.7. Polar Shell Model. The microstructure of a micelle of aggregation number N is an oil core of the hydrocarbon tails enclosed by a shell of the hydrophilic headgroups.11 This shell is the interface region between the core and water and contains the headgroups, counterions, and water.11,28,29 The interface is thus a polar region with a polarity given by the number of OH dipoles per unit volume of the polar shell. The model is simply that of molecular space filling of a sphere with certain numbers of surfactant and water molecules both of which are experimentally determined. The conformation of the monomer in the micelle adds more complexity than implied by this statement. This is so in general, because the core in micellar aggregates is liquid and the tails are flexible. It is improbable that the tails would all be straight and rigid. The flexibility and motion of the tails lead to some folding and cause some portion of the tail to be present in the shell.46-49 Thus, now instead of the entire hydrocarbon portion being in the core, a fraction f is in the core and the remaining (1 - f ) is in the shell. However, this is not an assumption because the model does allow for the possibility that f ) 1. The value of f * 1 is rather an outcome



Figure 2. Calculations of the polar shell thickness, s, for (a) LET micelle as the sum of the height of the triangle and the height of the base of this triangle from the point of contact of the hydrocarbon tail and (b) LEP micelle as the distance of polar -NH3+ group from the point of contact of the hydrocarbon tail. All of the bond lengths were calculated by using ChemDraw 3D Ultra 6.0. Schematic illustration of arrangements of the monomers at the interface and flexibility of tails (c) LET and (d) LEP micelle.

of the calculations which therefore confirms the liquid nature of the core. The fraction (1 - f) is the extent of folding. If f ) 1, there is no folding. For a spherical micelle of core radius Rcore and shell thickness s, this is expressed by

4 Vcore ) NVhc f ) πR3core 3

(13)

where Vhc is the volume of the hydrocarbon portion of the surfactant that forms the core of the micelle. Noting that the composition of the interface is given by N headgroups (each of volume Vhg), counterions (volume Vci), the OH dipoles of water, N(1 - f)Vhc, the volume Vshell can be written as the sum of these component volumes

Vshell ) NVhc(1 - f) + NVhg + VOH + βNVci ) 4 π{(Rcore + s)3 - R3core} (14) 3 The counterion (chloride) association factor is denoted by β, so that the number of counterions in the shell is βN. VOH may be replaced with HVshell using the defining equation, eq 10, for H. In the case of surfactants with an OH group, VOH includes the contribution from the water dipoles as well as the OH group of the surfactant headgroup and eq 14 is modified to

Vshell ) NVhc(1 - f) + N(Vhg - VOH) + HVshell + 4 βNVci ) π{(Rcore + s)3 - R3core} (15) 3

where VOH is the volume of phenolic OH. Equations 13 and 14 or 15 are a set of coupled equations. The variables N, H, and β are experimentally determined numbers. In the present calculations, a value of 0.7, typical of ionic micelles, was used for β.29,50-52 The parameters are the various molecular volumes which were determined from their van der Waals radii.53 In surfactants with a simple architecture that allows the division of the molecule into a tail part that is hydrophobic and a hydrophilic headgroup at one end of the tail, the assignment of which of the groups belong in the interface and which ones in the core is straightforward. The polar shell thickness, s, is simply the length of the hydrophilic portion. In the present case, the assignment is more complex particularly in the case of LET because of the phenolic group. In order to proceed with the structural model for the micelle, the hypothesis employed here is that the orientation of LET at the micelle-water interface is similar to the oil-water interface investigated in the adsorption studies.10 In LET, the OH group pulls the aromatic ring into the micelle-water interface. The headgroup therefore includes the phenolic group and the ionic part. An angle of 45° is ascribed for the angle between the plane of the ring and the tangential plane at the surface of the spherical core (Figure 2a and c). It is the compromise between the preferences of the phenolic group for a perpendicular (due to the hydrophilic OH) and parallel orientation (due to the hydrophobic aromatic ring) with respect to the surface. The orientation of headgroups with respect to the micelle surface is reasoned to be the balance resulting from competing forces that favor orientation either perpendicular or parallel to the surface.54,55 The thickness, s, for LET micelles is calculated as the sum of the height of the triangle shown in Figure 2a and the height of the base of this triangle from the point of contact of the hydrocarbon tail. In LEP, the ring, being flexible at the point of attachment, is bent down toward the core

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TABLE 1: Polar Shell Thickness (s) in Å and Volumes in Å3 of Headgroup, Hydrophobic Tails, and Other Groups Used in the Polar Shell Model Calculationsa s Vhg Vhc VOH Vci

LET

LEP

7.90 167.8 214.8 16.0 22.4

4.52 66.5 294.2

nw )

22.4

The volumes were calculated according to the method of Zhao et al.53

3. Results 3.1. DSC. Comparative DSC traces of LET solutions at different concentrations and that of 0.200 M LEP are shown in Figure 3. The DSC traces of LET and LEP solutions show a distinct endothermic peak in the temperature range 10-55 °C that is indicative of complete solubilization upon formation of micellar assemblies for T > 17 °C for LEP and T g 40 °C for LET. LEP shows a sharp narrow peak in contrast to LET wherein the endotherm is very broad. The results on KT and ∆H associated with the endotherm are presented in Table 4. KT was measured as 25.83 ( 0.58 °C at all concentrations of LET, and that of LEP was found to be comparatively much lower at 17.05 °C. ∆H values were calculated as 24.9 ( 0.2 kJ/mol for LET solutions of different concentrations and that for LEP as 24.06 kJ/mol. Visual inspection of LET and LEP solutions revealed crystal formation upon cooling below 25 and 17 °C, respectively. These temperatures agree closely with the respective KT measured from DSC. 3.2. TRFQ. The unquenched and quenched pyrene fluorescence decay was measured for the concentrations and temperatures stated in section 2.2. Exponential fits to this decay yield τ0. The value of τ0 itself was much shorter than the 180 ns typical in micelles in aerated aqueous media at 25 °C.56 This is because of quenching of pyrene fluorescence by the aromatic group. The quenched fluorescence decay was fitted to eq 1. Sample plots of the unquenched and quenched decay and the fit are shown in Figure 4. Tables 2 and 3 are records of the numerical results of the fits. In LET micelles, T1 was almost equal to τ0 with the difference within the error of (8%. In LEP, T1 is considerably smaller than τ0. This means that the intermicellar migration rate constant of the quencher, k-, is higher in LEP than in LET. For quenched decays, T1 was g2T2 at all of the investigated concentrations and temperatures in LET, but for LEP, this was so only for [LEP] e 0.100 M at all temperatures and for T g 40 °C for [LEP] ) 0.150 and 0.200 M. The aggregation numbers calculated using eq 8 are plotted as functions of temperature and concentration in Figure 5. 3.3. ESR. The hydration index, H, variations with temperature and concentration are shown in Figure 6a and b. The number of water molecules per headgroup, nw, calculated using

Figure 3. DSC trace in the temperature range 10-55 °C of (a) 0.100, (b) 0.150, and (c) 0.200 M LET and (d) 0.200 M LEP.

and put together with the tail to form the core. The LEP polar shell thickness, s, is the length shown in Figure 2. The volume and shell thickness parameters are listed in Table 1. The only unknown now is the fraction f. Application of eqs 13-15 with the data on N and H and the volumes and s from Table 1 yields f. The geometrical properties of Rcore, s, and the micelle radius Rmic are now completely defined. Numerical values of Vshell can be easily calculated using eq 14 or 15. For LEP, the hydration in terms of the number of water molecules per headgroup can then be obtained using

VOH VwN

(16)

(for LEP)

(17)

(for LET)

where Vw is the molecular volume of water and VOH is the volume of OH in the phenolic ring of LET (Table 1). The equations for LET and LEP are different because in LET the phenolic OH also contributes to the measured hydration index H, whereas in LEP all of the H is due to the OH of water. When calculating the actual number of water molecules in the interface, the number of phenolic OH must be excluded in the case of LET.

a

nw )

VOH - VOHN VwN

In the case of LET, which has an OH group in the headgroup region, eq 16 is modified to TABLE 2: Pyrene Fluorescence Lifetimes in LET in Nanosecondsa [LET] ) 0.050 M

[LET] ) 0.100 M

[LET] ) 0.150 M

[LET] ) 0.200 M

T (°C)

τ0

T1

T2

τ0

T1

T2

τ0

T1

T2

τ0

T1

T2

32 40 50 60 70

76.5 65.6 57.6 47.7 43.2

71.1 64.1 55.7 48.9 43

34.7 29.4 22.7 13.8 11

78.6 69.7 59.3 51.1 46.6

74.7 68.9 60.1 52.3 46.5

36.5 30.8 19.9 13.7 10.8

86.1 73.1 62 54.5 47.3

78.3 71.4 64.4 57.1 50.9

38.9 31.8 20.4 14.6 9.06

86.4 75 65.5 57 49.9

80.1 75.1 70.3 57.5 51.7

40.9 30.9 22.1 16.8 11.5

τ0, in the absence of quenchers; T1, in the presence of quenchers; and the inverse of the quenching rate constant, A4-1 ) T2, for the various surfactant concentrations and temperatures. a



TABLE 3: Pyrene Fluorescence Lifetime in LEP in Nanosecondsa [LEP] ) 0.050 M

[LEP] ) 0.100 M

[LEP] ) 0.150 M

[LEP] ) 0.200 M

T (°C)

τ0

T1

T2

τ0

T1

T2

τ0

T1

T2

τ0

T1

T2

32 40 50 60 70

165 146 133 122 111

157 142 125 107 97.3

28.7 21.1 15.2 11.6 10.4

147 133 120 105 94.4

98.9 119 109 104 95.8

38.55 27.16 18.39 13.26 10.54

136 125 116 102

94.1 92.9 92 88.7

35.9 29.3 21.6 16.1

123 116 107

81.7 80.6 79.9

37.3 31.7 25.7

τ0, in the absence of quenchers; T1, in the presence of quenchers; and the inverse of the quenching rate constant, A4-1 ) T2, for the various surfactant concentrations and temperatures. a

TABLE 4: Krafft Temperature, KT, and Enthalpy of Micellization, ∆H, at Various Surfactant Concentrations [surfactant] (M)

KT (°C)

∆H (kJ/mol)

[LET] ) 0.100 [LET] ) 0.150 [LET] ) 0.200 [LEP] ) 0.200

25.25 26.41 25.25 17.05

24.66 25.05 24.66 24.06

eqs 16 and 17 is presented in Figure 6c and d. Hydration index and nw increase with temperature. The microviscosity, shown in Figure 7, decreases with temperature and is also sensitive to concentration in LEP more than in LET. The microviscosities were fit to a dependence of the form

η ) η0 exp(-E*/kT)

(18)

where η is the microviscosity at temperature T and E* is the activation energy. The fit values of E* at 0.050, 0.100, 0.150,

Figure 5. Variation of the aggregation number, N ((5%), of (a) LET and (b) LEP micelles with temperature, T, at concentrations of 0.050, 0.100, 0.150, and 0.200 M.

Figure 4. Fluorescence decay of unquenched and quenched pyrene in (a) LET and (b) LEP micelles at 50 °C for a surfactant concentration of 0.050 M. The lines are fits to eq 1.

and 0.200 M surfactant concentrations are 24.73, 24.64, 24.77, and 24.98 kJ/mol for LET micelles and 23.30, 26.05, 24.74, and 23.40 kJ/mol for LEP micelles, respectively. These numbers are similar to the values obtained for alkylammonium dodecylsulfate micelles.57 3.4. Polar Shell Model Calculations. The fractional distribution of the hydrophobic portion of the amphiphile between the interface and the core changes with temperature according to Figure 8. For LET, the fraction in the core increases with temperature for all concentrations. For LEP, this fraction changes by about 2% or less between 32 and 70 °C for all concentrations. Note that in LET the fraction is that of the hydrocarbon tail only, whereas in LEP it is that of the hydrocarbon and the ring. Experiments and simulations on hydrocarbon chain packing in micelles conclude the presence of some hydrocarbon in the interface.46-49 The values of f obtained here from calculations constrained by the measured aggregation number and hydration provide some quantitative information on hydrocarbon chain packing. The polar shell model calculations are for spherical shapes. TRFQ by itself does not distinguish between ellipsoidal and

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Figure 6. Variation of the micelle hydration index, H, of (a) LET and (b) LEP and number of water molecules per headgroup, nw, in (c) LET and (d) LEP micelles with temperature, T, at concentrations of 0.050, 0.100, 0.150, and 0.200 M.

spherical shapes. The values of T2 obtained by TRFQ in this work are from 11 to 40 ns (Tables 2 and 3). These are typical of globular and not cylindrical micelles.11,58 For an ellipsoid of semiminor axes a and b, a ) b, and axial ratio rx, the micelle volume is 4/3πrxa3. The present calculations are for an equivalent sphere of volume 4/3πRc3. We also carried out the calculations for ellipsoidal shapes with an axial ratio of 2 as a test case to obtain the effect on f. The changes in f are at most 5%. For example, for [LET] ) 0.050 M at T ) 70 °C, where N ) 101, f changes from 0.83 for rx ) 1 to 0.79 for rx ) 2. 3.5. NMR. The representative spectra of LET and LEP for a concentration of 0.100 M in D2O measured at 35 °C at 500 MHz are presented in Figure 9, with peak assignments for the different protons as labeled. The peak assignments for the different protons in LET are made by reference to published data on the related compound of LET, viz., methyl ester of tyrosine.59 The same reference compound holds good for LEP as well in view of the structural similarity with LET except for the phenolic group. In both LET and LEP, the peak assignments closely agree with calculations made using Chemdraw Ultra 8.0. A simple comparative evaluation of the spectra of LET vs LEP indicates a distinct difference in their features especially with respect to the line widths at all temperatures. It is interesting to observe that the line width of aromatic and aliphatic protons in LEP is broader in comparison to LET. This is indicative of restricted mobility of these groups in LEP micelles. The correlation between line width and mobility in micellar solutions has been well documented.43-45 1 H NMR spectral measurements were performed on LET and LEP solutions at different temperatures ranging from 23 to 65

°C at each of the concentrations 0.050, 0.100, 0.150, and 0.200 M. The spectra were analyzed in some detail to understand the effects of aromatic groups on the neighboring protons. Proton labeling in the structure of LET and LEP is shown in Figure 9. The chemical shift (δ) position of protons of different groups in LET and LEP changes with an increase in temperature (Figures 10 and 11). The change in δ at any temperature T with respect to the position at 23 °C is denoted as ∆δ ()δ(23 °C) δ(T )). Accordingly, a positive ∆δ is indicative of upfield shift and a negative value that of downfield shift. The temperature dependences of the change in chemical shifts (∆δ) of proton resonances of terminal methyl (a), chain methylene groups (b), benzyl headgroup (d), methine (f ), and phenyl ring protons (g ) in LET and LEP micelles are presented in Figure 11. Due to the exchange of proton with deuterium of D2O, signals from phenolic OH and headgroup ammonium protons were not observed. In LEP, the tail protons show positive ∆δ that are indicative of upfield shift, in contrast to that of LET that show a negligible change with an increase in temperature (Figure 11a). This is suggestive of the strong shielding effect of tail protons in LEP (a and b). Interestingly, distinct deshielding effects (-∆δ, downfield shift) are observed in the cases of methine (f ), benzyl (d), and aromatic (g) protons in LEP (Figure 11b) in contrast to the LET protons that show a comparatively small change in ∆δ. The observed differences between LET and LEP in the experiments presented are discussed in the following section, in the context of the chemical architecture of the monomer from which micellar and aggregate structural differences arise.



Figure 7. Variation of the microviscosity, η, for (a) LET and (b) LEP micelles with temperature, T, at concentrations of 0.050, 0.100, 0.150, and 0.200 M. The lines are fits to η ) η0 exp(-E*/kT ).

4. Discussion The conformations of LET and LEP are drastically different at an oil-water interface because the OH in LET seeks water and therefore carries the aromatic ring with it, forming an interfacial region containing the phenolic ring, whereas in LEP the phenyl ring stays folded into the oil. These conformational principles were extended as a hypothesis to develop the polar shell model of the micelle. The OH and aromatic ring were placed in the shell in LET. The aromatic ring was folded into the hydrophobic core in LEP. The results of the individual techniques presented in the previous section are now discussed in the context of this hypothesis and also the main structural difference between LET and LEP, namely, the phenolic OH in LET. The DSC endotherms of single chain surfactant solutions record the heat of dissolution from solid hydrated crystals to micelles.60 The Krafft temperature reflects the solubility of monomer surfactants in the presence of hydrated surfactant crystals.The narrow endotherm peak in LEP is indicative of a single step transition process as against the broad peak in LET that appears to be made up of multiple steps (Figure 3). Various factors such as the nature and hydration of head groups and counterions, hydrophobicity of tail group, and headgroup interactions (including hydrogen bonding, dipole-dipole, and ion-dipole) strongly influence KT.61-63 In the present cases of LET and LEP, the counterion, namely, chloride, is the same. Thus, the counterion contribution to hydration would be the

Figure 8. Variation of the fractional distribution of the hydrophobic portion, f, of the amphiphile between the interface and the core in (a) LET and (b) LEP micelles, with temperature, T, at concentrations of 0.050, 0.100, 0.150, and 0.200 M.

same for LET and LEP. We may then consider the possibility of attributing the higher KT and broader endotherm in LET to its phenolic OH. The scope for intermicellar hydrogen bonding due to phenolic OH in LET combined with chirality can lead to the formation of large fiberlike structures, as is known for surfactants of this nature.10,14-16,64 Thus, LET would require higher temperatures than LEP to break the larger fibrous structure to form small micelles. Suspensions of LET in water below about 35 °C were observed to transform from a gelatinous and fibrous appearance to optically clear solutions upon addition of urea. This is a qualitative confirmation of the presence of hydrogen bonding because urea is well-known to weaken water structure by breaking hydrogen bonds.65-68 In addition, interheadgroup ion-dipole interaction between OH and NH3+ could also account for the higher KT.61 Thus, hydrogen bonding, chirality, and ion-dipole interactions can together account for the broader endotherm and higher KT of LET. The DSC results establish that, for temperatures higher than about 40 °C, both LET and LEP aggregates are micelles. Between 23 and 40 °C, LEP is in micellar form but LET is in fiber form. However, the NMR resonance lines of LEP are broader than in LET at all of the temperatures investigated, 23-65 °C. The broader NMR peaks in LEP than in LET (Figure 9) point to a more hindered environment for the LEP protons

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Figure 9. Proton labeling and 1H NMR spectrum of 0.100 M solution of (A) LET and (B) LEP in D2O at 500 MHz (T ) 35 °C).

due to the presence of the phenyl ring in the hydrophobic region.69,70 Broadening as a result of restricted mobility due to inclusion of aromatic compounds in bilayers and micelles has been observed.71,72 The broader resonances in LEP for T between 23 and 40 °C must mean that the protons experience less mobility in LEP than in LET even though LEP is in liquid micellar form and LET is in fibril form. In these forms, the bulk fluidity of LET and LEP may be different, but locally, as sensed by NMR, LET protons are less restricted. This is because the phenyl ring in LET is in the interface and does not affect the tail protons. Another indication that the local mobility changes rather continuously through the transition in LET is that the microviscosity (Figure 7) obtained from ESR, also a local probe, decreases continuously with temperature. Microviscosity as well as 1H NMR linewidths are quite sensitive to transitions like chain melting, but no sudden changes in these properties are observed in LET.72,73 The NMR chemical shift dependence on temperature differs starkly between LET and LEP aggregates (Figure 11). The conformational difference between LET and LEP is again invoked to understand these behavior variations. The magnetic anisotropic effects due to aromatic ring currents are well-known to induce shielding of neighboring protons.41,74,75 In LEP micelles, the closer proximity than in LET of the ring to the tails causes the various proton resonances to be more sensitive to the ring currents. Thus, the temperature induced increased motion of the monomers subject the LEP protons to greater consequences than in LET (Figure 11a). Proximity to the ring causes an upfield shift in the position, δ, of the peaks of LEP tail protons (δ decreases and ∆δ increases) with temperature indicative of increased shielding with temperature. Such an effect has been well demonstrated in micellar systems with aromatic rings.76 Concomitant change in the environment around LEP-g protons of the phenyl ring was also observed but as a downfield shift. From this, it follows that the phenyl ring, being hydrophobic in nature, moves further into the interior of the micelle and closer to the tail, causing upfield shifts. No such magnetic environmental changes are indicated for LET protons, where negligibly small ∆δ values were obtained. In LET

micelles, the interface location of the phenyl ring due to the polar OH group at the para-position has little to no effect on ∆δ. The aggregation numbers of LEP micelles are in general greater than LET micelles (Figure 5). The fraction f in the core of an LEP micelle is that of the entire hydrophobic part including the hydrocarbon tail and the aromatic ring, whereas in LET it is only the fraction of the tail. The hydrophobic volume available to form the micelle core is thus greater for LEP than for LET, leading to the greater aggregation numbers observed for LEP than LET micelles. Also, the increase with concentration at any given temperature is also greater for LEP, suggestive of a greater degree of ionic character for this micelle. The growth with surfactant concentration is a characteristic of ionic micelles and has been shown to depend on the aqueous counterion concentration.22,77-79 This greater change in LEP is reflected as a greater variation of microviscosity with concentration in LEP compared to LET (Figure 6). There is also a difference in the temperature dependence of the aggregation numbers between LET and LEP (Figure 5). The aggregation numbers of LEP decrease and those of LET increase with temperature. It is known that aggregation numbers of ionic micelles decrease with temperature and those of nonionic micelles increase.79 From the behavior of N vs T, it would seem that LEP shows characteristics of ionic micelles and LET that of nonionic micelles. This difference can again be understood in terms of the presence of the aromatic ring and OH in the polar shell of LET by virtue of which hydrogen bond and attractive inter-headgroup ion-dipole interactions between the OH and NH3+ are present in the polar shell. These interactions allow for a growth with T for reasons as in nonionic micelles (where typically hydrogen bond and attractive dipole-dipole interactions are present). The lifetime, T1, of pyrene fluorescence in LEP micelles in the presence of quenchers is less than that in the absence of quenchers, τ0, and decreases with an increase in quencher concentration (Table 3). This indicates that the quencher exits the micelle at a rate that is not negligible compared to the quenching rate constant (section 3.1). In LET, T1 is almost equal to τ0 or at most differs by 8% (Table 2). The higher quencher



Figure 11. Change in 1H chemical shifts (∆δ) of LET and LEP micelles vs temperature observed for a concentration of 0.200 M for the signals. (a) δ: a and b. (b) δ: d, f, and g.

Figure 10. Variable temperature 1H NMR spectra of 0.100 M LEP solution in D2O for (A) tail protons a and b and (B) aromatic ring protons g at temperatures of 35, 45, and 55 °C measured at 500 MHz.

exit rate in LEP could be a consequence of the increased rigidity and hindrance due to the combined presence of the aromatic rings and the tails in the core of the micelle as compared to the LET core which is rendered more fluid by the presence of only the flexible tails. Decrease of fluorescence lifetime in micelles containing aromatic rings is known to occur.80 5. Summary and Conclusions The difference in the chemical architectures of the lauryl ester derivatives of tyrosine and phenylalanine brings about significant differences in their aqueous aggregate structures and also in the thermal properties of their structural transitions. The experimental results in this work are interpreted in the light of the difference in the chemical architectures of the surfactants. The physical insight obtained is that the molecular basis for all of the observed differences in aggregate properties is linked, respectively, to the presence and absence of phenolic OH in LET and LEP. Below the Krafft temperature, LET forms networked, ordered fiberlike structures due to a combination of its chirality and

hydrogen bonding facilitated by the phenolic OH, whereas LEP forms rods.10,14,15,64 The present DSC investigations showed that these structures give rise to a higher Krafft temperature and a broader endothermic transition to micellar structures in LET than in LEP. The micellar structures at the higher temperatures were investigated by a combination of TRFQ, ESR, and NMR. The concept of the micelle as an oil-like liquid core enveloped by a polar shell is of general consensus and well-known. The model postulating the presence of the phenolic OH and phenyl ring in the polar interface in LET micelles and the folding of the ring in toward the core in LEP micelles is supported by the results of the different spectroscopic experiments employed. The simple geometrical calculations in this work elucidate some features of the hydrocarbon chain packing, namely, the presence of some deterministic fraction (1 - f ) of the hydrocarbon tail in the interface, indicating that the tails are flexible. The quantity f may be treated as a hydrocarbon chain packing parameter. This is a property that is of interest in molecular modeling of micelles.46-49 The difference in micellar monomer conformations between LET and LEP is shown to have significant impact in the dimensions of micellar shell and core. The phenolic OH in LET favors packing of aromatic ring and protonated amino group in the micellar interface that brings about larger micellar interface thickness. LET that bears cationic charge exhibits characteristics of nonionic surfactants as inferred from cmc and variation of aggregation number with temperature. It is significant that this

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single surfactant could provide synergistic effects of cationic and nonionic surfactants, which is very rare. The physical insights into the aggregate structures of these two new surfactants, LET and LEP, drawn from this study are expected to have significant impact especially in the design of high performing surfactants for drug delivery systems, controlled reactions including polymerization reaction, and nanoparticle generation. Acknowledgment. The authors J.S. and R.R. acknowledge the support of NIH through grant S06 GM048680 for this project and the Offices of Research and Sponsored Projects of the California State University Northridge (CSUN). The authors R.V. and G.B. thank Dr. A. B. Mandal, Director, and Dr. B. S. R. Reddy, Director Grade Scientist, Head, Industrial Chemistry Laboratory, CLRI, for their support and permission to publish this paper. G.B. is thankful to DG CSIR and Dr. T. Ramasami, Secretary, DST, and former Director, CLRI, for the approval of a collaborative project, 5/258/2003-TNBP, between CLRI, India, and CSUN, USA. G.B. acknowledges the financial support from DST, India, through grant SR-S1-PC 14. R.V. thanks CSIR for the senior research fellowship. References and Notes (1) Biochemistry, Protein Folding, 3rd ed.; Meyers, R. A., Ed.; Academic Press,: New York, 2004; p 179. (2) Kobayashi, S.; Uyama, H.; Kimura, S. Chem. ReV. 2001, 101, 3793. (3) Kobayashi, S.; Higashimura, H. Prog. Polym. Sci. 2003, 28, 1015. (4) Bourke, S. L.; Kohn, J. AdV. Drug DeliVery ReV. 2003, 55, 447. (5) Marx, K. A.; Alva, K. S.; Sarma, R. Mater. Sci. Eng., C 2000, 11, 155. (6) Marx, K. A.; Lee, J. S.; Sung, C. Biomacromolecules 2004, 5, 1869. (7) Fukuda, K.; Shibasaki, Y.; Nakahara, H.; Liu, M.-h. AdV. Colloid Interface Sci. 2000, 87, 113. (8) Gaspar, L. J. M.; Baskar, G. Biomacromolecules 2006, 7, 1318. (9) Gaspar, L. J. M.; Baskar, G.; Reddy, B. S. R.; Ranganathan, R.; Peric, P. Langmuir, 2004, 20, 9029. (10) Vijay, R.; Angayarkanny, S.; Baskar, G. Colloids Surf., A 2008, 317, 643. (11) Singh, J. S.; Miller, J.; Ranganathan, R. J. Phys. Chem. B 2007, 111 (31), 9317. (12) Ranganathan, R.; Peric, M.; Medina, R.; Garcia, U.; Bales, B. L.; Almgren, M. Langmuir 2001, 17, 6765. (13) CRC Handbook of Chemistry and Physics, 82nd ed.; CRC Press: 2001. (14) Fuhrhop, J. H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (15) Fuhrhop, J. H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861. (16) Imae, T.; Takahashi, Y.; Muramatsu, H. J. Am. Chem. Soc. 1992, 114, 3414. (17) Suzuki, M.; Sano, M.; Kimura, M.; Hanabusa, K.; Shirai, H. Eur. Polym. J. 1999, 35, 1079. (18) Ranganathan, R.; Peric, M.; Bales, B. L. J. Phys. Chem. B 1998, 102, 8436. (19) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (20) Tachiya, M. J. Chem. Phys. 1983, 78, 5282. (21) Gehlen, M. H.; De Schryver, F. C. Chem. ReV. 1993, 93 (1), 199. (22) Ranganathan, R.; Tran, L.; Bales, B. L. J. Phys. Chem. B 2000, 104, 2260. (23) Ranganathan, R.; Giongo, C.; Bakshi, M. S.; Bales, B. L.; Hajdu, J. Chem. Phys. Lipids 2005, 135, 93. (24) Alonso, E. O.; Quina, F. H. Langmuir 1995, 11 (7), 2459. (25) Bales, B. L.; Howe, A. M.; Pitt, A. R.; Roe, J. A.; Griffiths, P. C. J. Phys. Chem. B 2000, 104, 264. (26) Bales, B. L.; Messina, L.; Vidal, A.; Peric, M.; Nascimento, O. R. J. Phys. Chem. B 1998, 102, 10347. (27) Bales, B. L.; Shahin, A.; Lindblad, C.; Almgren, M. J. Phys. Chem. B 2000, 104, 256. (28) Bales, B. L.; Ranganathan, R.; Griffiths, P. C. J. Phys. Chem. B 2001, 105 (31), 7465. (29) Bales, B. L.; Zana, R. J. Phys. Chem. B 2002, 106 (8), 1926. (30) Ranganathan, R.; Vautier-Giongo, C.; Bales, B. L. J. Phys. Chem. B 2003, 107, 10312. (31) Spin Labeling: Theory and Applications; Berliner, L. J., Ed.; Academic Press: New York, 1976; Vol. 1.

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