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Oct 2, 2016 - The effect of negatively charged DNA nucleotide (AMP) addition into the cationic and anionic SAILs is more prominent for the cationic ch...
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Probing the Interaction between a DNA Nucleotide (Adenosine-5′Monophosphate Disodium) and Surface Active Ionic Liquids by Rotational Relaxation Measurement and Fluorescence Correlation Spectroscopy Arpita Roy, Pavel Banerjee, Rupam Dutta, Sangita Kundu, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB India S Supporting Information *

ABSTRACT: This article demonstrates the interaction of a deoxyribonucleic acid (DNA) nucleotide, adenosine-5′-monophosphate disodium (AMP) with a cationic surface active ionic liquid (SAIL) 1-dodecyl-3-methylimidazoium chloride (C12mimCl), and an anionic SAIL, 1-butyl-3-methylimidazolium n-octylsulfate ([C4mim][C8SO4]). Dynamic light scattering (DLS) measurements and 1H NMR (nuclear magnetic resonance) studies indicate that substantial interaction is taking place among the DNA nucleotide (AMP) and the SAILs. Moreover, cryogenic transmission electron microscopy (cryoTEM) suggests that SAILs containing micellar assemblies are transformed into larger micellar assemblies in the presence of DNA nucleotides. Additionally, the rotational motion of two oppositely charged molecules, rhodamine 6G perchlorate (R6G) and fluorescein sodium salt (Fl-Na), have been monitored in these aggregates. The rotational motion of R6G and Fl-Na differs significantly between SAILs micelles and SAILs-AMP containing larger micellar aggregates. The effect of negatively charged DNA nucleotide (AMP) addition into the cationic and anionic SAILs is more prominent for the cationic charged molecule R6G than that of anionic probe Fl-Na due to the favorable electrostatic interaction between the AMP and cationic R6G. Moreover, the influence of the anionic DNA nucleotide on the cationic and anionic SAIL micelles is monitored through the variation of the lateral diffusion motion of oppositely charged probe molecules (R6G and Fl-Na) inside these aggregates. This variation in diffusion coefficient values also suggests that the interaction pattern of these oppositely charged probes are different within the SAILs-nucleotide containing aggregates. Therefore, both rotational and translational diffusion measurements confirm that the DNA nucleotide (AMP) renders more rigid microenvironment within the micellar solution of SAILs.

1. INTRODUCTION Self-assembly of amphiphilic molecules is the main driving force for the formation of a wide variety of nanostructures using different building units. Soft1 and biological2,3 materials have gained great attention due to their self-association characteristics. Actually, the self-assembling property of biological materials such as nucleic acids and proteins is mainly controlled by weak interactions between them. Particularly, deoxyribonucleic acid (DNA) can be considered as a model in which weak interactions are responsible to induce self-association. In DNA molecules, the specific interactions between the nucleobases along with some unspecific stacking interaction provide the stability toward its double helix structure. Bioinspired materials with self-assembling properties have been designed to build different systems that mimic nature.4−7 In this context, the nucleoamphiphile systems gain attention because of their molecular recognition properties,8 self-assembling properties9−11 at the air−water interface,12 or Langmuir−Blodgett films.13 Investigation on the interaction between nucleolipids © XXXX American Chemical Society

with organized assemblies (micelles or vesicles) have been reported.8 Nucleotidesurfactant systems linked through noncovalent bonds have also been investigated aiming toward molecular recognition. Oda and co-workers14 found that upon the addition of complementary nucleoside bases, the cationic gemini surfactants solution containing nucleotides as counterions undergo morphological transitions and turns into hydrogels. It was also investigated that the nucleotides guanine-5′-monophosphate-disodium (GMP) and adinosine5′-monophosphate-disodium (AMP) form complexes with nonchiral monocationic surfactants by electrostatic interaction.11 Wang et al.15 used negatively charged nucleotide GMP to form complexes with the surfactants through favorable electrostatic interaction. While the above-mentioned systems involve covalent and electrostatic interactions between Received: July 27, 2016 Revised: September 24, 2016

A

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Langmuir Scheme 1. Chemical Structure of Different Materials Used in Various Experiments

AMP, a large and sudden increase in the size of the SAIL micelles is monitored in the aqueous solution of individual SAILs. The detailed morphological characterizations of the newly formed large aggregates composed of SAILs in combination with the DNA nucleotide (AMP) are carried out by a dynamic light scattering (DLS) study and cryotransmission electron microscopy (cryo-TEM). In addition, we have also investigated the motion of probe molecules during this microstructural variation to monitor the dynamical properties of this SAILs-AMP containing aggregates. The dynamical properties of probe molecules in various selfassemblies depend upon its location and interaction with the surrounding microenvironments.33−36 In organized assemblies, the solubilization sites and the mode of interaction of differently charged probe molecules depend upon the nature and charge of the aggregates. In the SAILs-AMP aggregates, the microstuctural nature will be different compared to that of the normal SAIL forming micelles. Hence, it would be interesting to notice the motion of two differently charged probes during the transition of smaller sized cationic and anionic SAIL micelles into the larger SAIL-AMP aggregates due to the interaction with a DNA nucleotide AMP. For this purpose, we have chosen two structurally similar oppositely charged solutes: rhodamine 6G (R6G, cationic) and fluorescein sodium salt (FlNa, anionic). Finally, fluorescence correlation spectroscopic (FCS) techniques have been employed to monitor the modulation of translational diffusion of a cationic solute (R6G) and an anionic probe (Fl-Na) as a consequential effect of the SAILs- nucleotide (AMP) interactions.

nucleotide and surfactant components, in this study we have characterized a new system in which a DNA nucleotide and the surface active ionic liquid (SAIL) amphiphiles are complexed through electrostatic as well as π−π interactions. The approach presented herein involves a monoanionic nucleotide adinosine5′-monophosphate-disodium (AMP) which is separately complexed with cationic and anionic surface active ionic liquids in aqueous solutions. Room temperature ionic liquids (RTILs) bearing long alkyl chain and display surface activity are known as surface active ionic liquids (SAILs). These SAILs are able to form various types of self-assemblies.16−20 SAILs are generally well-known as a novel class of amphiphilic molecules with uniqueness toward the physiochemical properties of ionic liquids, enhancing the use of their self-assembly properties in chemistry, material, and biological sciences.21−23 Recently, researchers have extensively investigated to get a better insight into the structure, properties, and aggregation behavior of SAILs in comparison with common surfactants.24−26 Besides, oppositely charged surfactants or additive molecules sometimes promote the aggregation of SAILs in aqueous solution into vesicular assemblies.19,27−29 In recent times, researchers have shown interest in the photophysical and dynamical phenomenon of various fluorophores in nanoaggregates formed by RTILs.19,30,31 It is also reported that the surface active properties of the imidazolium moiety containing ionic liquids are superior to common ionic surfactants containing similar alkyl parts.32 However, to the best of our knowledge, this is the first report where interaction between SAIL and a DNA nucleotide, adinosine-5′-monophosphate-disodium (AMP) has been investigated. In this article, we have investigated the microstructural variation of a cationic SAIL, 1-dodecyl-3-methylimidazoium chloride (C12mimCl), and an anionic SAIL, 1-butyl-3methylimidazolium n-octylsulfate ([C4mim][C8SO4]) separately in the presence of the DNA nucleotide (AMP). In aqueous medium, C12mimCl and [C4mim][C8SO4] molecules form small micellar assemblies; however, with the addition of

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 1-Dodecyl-3-methylimidazoium chloride (C12mimCl) and 1-butyl-3-methylimidazolium n-octylsulfate ([C4mim][C8SO4]) were purchased from Kanto Chemicals (98% purity) and Sigma-Aldrich, respectively. Adinosine-5′-monophosphatedisodium (AMP) (99% purity) was obtained from SRL (India). Rhodamine 6G perchlorate (R6GClO4) and fluorescein sodium salt (Fl-Na) (all LASER grades) were obtained from Exciton and SigmaB

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Figure 1. Intensity-size distribution of (a) 40 mM C12mimCl and C12mimCl+AMP, and (b) 40 mM [C4mim][C8SO4] and [C4mim][C8SO4]+AMP containing nanoaggregates at various concentrations of AMP. Aldrich, respectively. All these materials were used as received. During the preparation of micellar solutions, triply distilled Milli-Q water was used. The concentration of C12mimCl and [C4mim][C8SO4] was 40 mM. The saturation concentration (190 mM) of AMP is taken as the highest concentration throughout the various experiments. The structures of C12mimCl, [C4mim][C8SO4], Fl-Na, R6G, and AMP are given in Scheme 1. 2.2. Solution Preparation. The stock solution of 40 mM concentration of C12mimCl and [C4mim][C8SO4] was prepared in phosphate buffer solution (pH 7.4). A specific volume of micellar solution of C12mimCl and [C4mim][C8SO4] was taken into separate glass bottles and then, the required amount of AMP was added to get the required concentration. 2.3. Instrumentations. 2.3.1. Steady State Absorption and Emission Measurements. By using a Shimadzu spectrophotometer (model number UV-2450) and Hitachi (model number F-7000) spectrofluorimeter, the UV and fluorescence spectra of Fl-Na and R6G were collected. 2.3.2. Time Resolved Fluorescence Anisotropy Study. For timeresolved anisotropy measurement, a time correlated single photon counting (TCSPC) picosecond set up was utilized. A picosecond diode laser (IBH, UK, Nanoled) of 440 nm was used as excitation source. The Hamamatsu microchannel plate photomultiplier tube (MCP PMT) (3809U) was used during the measurement. 2.3.3. Fluorescence Correlation Spectroscopy (FCS) Study. For FCS study, we have used a DCS 120 Confocal Laser Scanning Microscope (CLSM) system (Becker & Hickl DCS-120) with an inverted optical microscope of Zeiss (Carl Zeiss, Germany). The correlation function G(τ) has been defined as37

G(τ ) =

⟨δF(t )δF(t + τ )⟩ ⟨F(t )⟩2

component containing the dodecyl chain is very intuitive and may act as surfactant and may form a micelle like conventional cationic surfactant CTAC.40However, the aggregation phenomena of [C4mim][C8SO4] is not that simple. It is reported that in aqueous solution, dual transitions occurs in the physical properties of [C4mim][C8SO4].41 It has been corroborated that anionic aggregation has occurred in the first transition and that the second transition represents restructuring of the anionic aggregates. During the second transition, the penetration of the alkyl chain containing the imidazolium cation has occurred toward the micellar core.41Figure 1a,b depicts the intensity-size distribution profiles of C12mimCl and [C4mim][C8SO4] containing aggregates, respectively. The micellar size of SAILs is dependent upon the relative occupancy and nature of the counterions.40 In case of anionic micelle [C4mim][C8SO4], the counterion [C4mim]+ is larger than that of the Cl− anion of [C12mim]Cl. Besides, the [C4mim]+ cation remains embedded in the micellar core region of [C4mim][C8SO4],41 whereas the counterion Cl− anion of [C12mim]Cl remains on the micellar interface. Hence, in spite of the fact that the two micelles have different numbers of carbon atoms on the chains of their cationic part, the total number of carbons contributing toward the hydrophobic domain of the micelle is the same. The average diameter of C12mimCl and [C4mim][C8SO4] micelles is around ∼3.6 nm; however, due to the presence of few larger aggregates, a minor peak population in the larger diameter range is also noticed. However, with the addition of AMP (190 mM), a significant change in the size distribution profiles for both the SAIL micelles has been observed. For C12mimCl-AMP and [C4mim][C8SO4]-AMP, the average diameter becomes ∼475 nm and ∼462 nm, respectively, which indicates the formation of larger SAILs-AMP aggregates. Size enhancement of aggregates can be induced by the formation of large structures like vesicles27 or the change of micelle structure and composition.42,43In the latter situation, one example is reported by Rai et al., who discussed the size enhancement of aggregates in the aqueous solution mixed with imidazolium-based IL and SDS or SDBS (sodium dodecyl benzenesulfonate).43 They observed that the size of aggregates increased much more in the IL + SDBS + water system than that in the IL + SDS + water system, which is mainly attributed to the benzene ring in SDBS and the generated cation−π interaction. There are other literature reports where the π−π interaction along with the electrostatic interaction plays a crucial role in the morphology of the

(2)

where δF(t) represents the fluctuation of fluorescence signal F(t) as deviations from the temporal average of the signal ⟨F⟩ at time t. Therefore, δF(t) = F(t) − ⟨F⟩. The details of the all instrumentation are given in Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Structural Characterizations of SAILs and AMP Containing Nanoaggregates. 3.1.1. Dynamic Light Scattering (DLS) and Cryo-TEM Measurements. DLS measurements have been performed to elucidate the microstructural changes of C12mimCl and [C4mim][C8SO4] micelles due to the addition of DNA nucleotide AMP. The cmc’s (critical micelle concentrations) of C12mimCl and [C4mim][C8SO4] are ∼14.22 mM and ∼31 mM, respectively.38,39 The aggregation phenomena of C12mimCl and [C4mim][C8SO4] in aqueous media is different from each other. For C12mimCl, the cationic C

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Figure 2. Cryo-TEM images of aggregates composed of C12mimCl+190 mM AMP (a,b) and [C4mim][C8SO4]+ 190 mM AMP (c,d).

aggregates.44−46 In our case, the size enhancement can be explained by the cation−π interaction between the imidazolium ring of the SAILs and the π electron cloud of the adenosine moiety of the AMP. Also, there is a strong electrostatic interaction between the positively charged imidazolium rings of the SAILs with the negatively charged phosphate moiety of the DNA nucleotide (AMP). Hence, the cation−π interaction between the imidazolium ring of the SAILs and the adenosine moiety of AMP makes the headgroups to be separated compared to the neat SAILs micelle which renders the headgroup area to be enlarged. Besides, few number of adenosine moieties may be extended toward the hydrophobic core region of the SAILs micelle due to the favorable hydrophobic interaction. Similar relocations of bulk molecules into the micellar aggregates are reported in some studies.42,43 Therefore, the enhanced headgroup area along with the relocation of the bulk aromatic AMP molecules inside the micellar assemblies from the micellar surface results in a large increment in size of the SAILs micelles. The formation of larger micellar assemblies of SAILs-AMP aggregates is further observed from the cryo-TEM measurement (Figure 2). Moreover, one can get confirmation about the shape and morphology of the SAILs-AMP assemblies by cryo-TEM measurement. The cryo-TEM images indicate that the interaction of the SAILs and DNA nucleotide AMP results in the formation of large spherical micellar aggregates. The cryoTEM images also suggest that the size of the C12mimCl-AMP and [C4mim][C8SO4]-AMP aggregates lies within ∼80−470 nm. The histogram analysis of the cryo-TEM micrograph using Image J software is shown in Figure S1, Supporting Information. DLS measurement provides information regarding the diameter of a spherical particle by means of a distribution profile. Hence, there will be some distribution of particle size as also seen from cryo-TEM measurements. However, cryo-TEM is a number based particle size measurement tool, whereas DLS is usually an intensity based one which is actually governed by Rayleigh’s approximation (intensity is propotional to r6). Hence, practically these two measurements cannot be compared. There is a literature report where the results of cryo-TEM and scattering study are not correlated.47Their obtained vesicle size from the cryo-TEM measurement is smaller than that obtained from the scattering measurement. In spite of this technical and principal difference, these two techniques corroborate about the size enhancement of the SAILs micelles in the presence of DNA nucleotide AMP. 3.1.2. NMR Studies. The 1H NMR technique has been further applied to understand the interaction of nucleotide (AMP) with the SAILs [C12mim]Cl and [C4mim][C8SO4] in deuterium oxide solutions. 1H NMR measurement can provide information to understand the synergistic interaction between the surface active material and a DNA nucleotide resulting in

the formation of larger aggregates. Figures S2 and S3 (Supporting Information) show the NMR spectra of SAILs solutions upon addition of AMP. From these figures, it is evident that the NMR peaks responsible for the protons in the aromatic imidazolium moiety of [C12mim]Cl and [C4mim][C8SO4] are shifted upfield as the SAIL micelles are interacting with the DNA nucleotide AMP. A significant change in the chemical shifts of the aromatic protons of SAILs is taking place as the aromatic ring current of the imidazolium ring bearing circulating π electrons experiences a change upon introduction of a negatively charged AMP molecule. This is a clear indication of π−π interactions between the imidazolium moiety of the SAILs and the aromatic ring of the AMP. In addition to this, there is also a possibility of H bonding between H atoms of the imidazolium ring of SAILs and the negatively charged phosphate group of AMP.44,45 The synergic effect between the SAILs and AMP is governed by the cation−π interaction and electrostatic attraction along with the hydrophobic interactions. Only the aromatic protons (H2,4,5) of SAILs and Hg,f protons of AMP are shifted upfield. From this information, it is inferred that AMP molecules are intercalated in the hydrophobic microdomain of SAIL micelles near the headgroup region. In the case of Hb,c,d,e,h protons on the pentose backbone, hardly any upfield shift is noticed, and thus, it is corroborated that the effect of the imidazolium ring currents is negligible here.46 Previously, it was reported that there is significant upfield shift of the protons of the ILs, and N-butyl imidazolium chloride (N-BImCl) and 1-butyl-3-methyl imidazolium chloride, (HMImBr) have been observed in the presence of submicellar concentration of CTAB (cetyltrimethylammonium bromide).48 They have proposed that the inclusion of CTAB molecules into the hydrogen bond network of ILs disrupts some of the hydrogen bonds so that the observed mean value of chemical shifts goes toward higher fields. We have also observed that the 1H NMR peaks of the aromatic protons of AMP shifted upfield. The upfield shift of all the aromatic protons of AMP suggests the interaction of nucleotide with the SAILs assemblies. Roy et al.49 also observed a similar kind of upfield shift of the 1H NMR peaks of the aromatic protons of RNA nucleoside during its interaction with the aqueous solution of cyclodextrin. 3.2. Photophysical and Dynamical Properties of R6G and Fl-Na in SAILs-AMP Containing Aggregates. 3.2.1. UV−Vis Absorption Studies. For the photophysical studies in these SAILs-DNA nucleotide aggregates, we have chosen one cationic probe R6G and FlNa as the anionic one. The concentrations of R6G and FlNa used for the spectral measurements are ∼5 μM. It is reported that at a high concentration of FlNa (1 × 10−4 M), H-aggregate forms over cationic CTAB micelles due to the electrostatic factor.50 However, at lower concentration of FlNa (micromolar level), D

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Figure 3. Variation in emission spectra of (a) Fl-Na and (b) R6G in phosphate buffer, SAILs, and SAILs+190 mM AMP containing nanoaggregates (λex = 440 nm).

Figure 4. Variation in time-resolved anisotropy of R6G in (a) buffer, [C4mim][C8SO4] and [C4mim][C8SO4]+190 mM AMP and (b) buffer, C12mimCl and C12mimCl+190 mM AMP containing nanoaggregates.

respectively. However, in the [C4mim][C8SO4] micelle, the absorption maxima of R6G comes around ∼532 nm, and in [C4mim][C8SO4]-AMP assemblies, it is ∼534 nm. Similar to UV−vis profiles, the emission spectral profiles of R6G are also slightly red-shifted with the addition of nucleotide AMP in C12mimCl and [C4mim][C8SO4] micellar solutions (Figure 3a,b). On the other hand, in aqueous solution, the absorption peak of Fl-Na appears around ∼490 nm. It is red-shifted by ∼12 nm in C12mimCl and ∼2 nm in [C4mim][C8SO4] micellar solutions. Here, the larger red shift in the cationic SAIL micelle is due to the favorable electrostatic interaction of the anionic probe (fluorescein) with the cationic SAIL micelles. It implies that the excited state of probe (fluorescein) is more stabilized by the binding to C12mimCl micelles compared to the ground state.54 It is observed that there is a slight red shift in both the absorption and fluorescence spectra of Fl-Na in the anionic [C4mim][C8SO4] micellar assembly, which is in contrast with the normal anionic surfactant system (i.e., SDS, sodium dodecyl sulfate). In normal anionic surfactant SDS, no changes in spectral properties were detected for Fl-Na. The high aqueous solubility and electrostatic repulsion between the anionic fluorescein and anionic SDS micelles are responsible for this observtion.54 The observed red shift of fluorescein in the anionic SAIL micelle [C4mim][C8SO4] is due to the interaction

there is no sign of aggregate formation in CTAB micelles. It is also reported that FlNa forms aggregates in aqueous media at concentrations greater than 1 × 10−4 M.51,52 In another study, it has been reported that as the concentration of R6G approaches greater than 5 × 10−5 M, R6G molecules begin to form aggregates in the solution, and nanostructures efficiently form on the surface of the DNA double helix.53 However, we have used a much lower concentration (5 × 10−5 M) of the two probes so that any kind of aggregate formation can be eliminated, and we assured that the SAILs-DNA nucleotide complexes do not change upon interaction with the probes. Several literature reports are available where R6G and FlNa are extensively used in micromolar concentration for fluorescence measurements.35,36,59 To ensure about the intactness of the aggregates, we have checked DLS measurements which indicate that the aggregates remain intact in the presence of probe molecules (Figure S4, Supporting Information). The UV−vis absorption and emission spectra of R6G and Fl-Na have been monitored in SAILs micelle and SAILs-AMP organized assemblies. The normalized absorption spectra of R6G and Fl-Na in water, SAILs, and SAILs-AMP aggregates are shown in Figure S5 (Supporting Information). In aqueous solution, the absorption maximum of R6G is around ∼525 nm. In C12mimCl micelle and C12mimCl-AMP aggregates, the absorption peak of R6G is red-shifted to ∼531 and ∼535 nm, E

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Figure 5. Variation in time-resolved anisotropy of Fl-Na in (a) buffer, [C4mim][C8SO4] and [C4mim][C8SO4]+190 mM AMP and (b) buffer, C12mimCl and C12mimCl+190 mM AMP containing nanoaggregates.

Table 1. Time-Resolved Fluorescence Anisotropy Decay Parameters of R6G and Fl-Na in the SAIL-Nucleotide System (AMP = 190 mM) (λex = 440 nm) system

a

τfast(afast) (ns)

τslow(aslow) (ns)

⟨τr⟩ (ns)a

viscosity (cP)b

R6G in buffer [C4mim][C8SO4] [C4mim][C8SO4]+AMP C12mimCl C12mimCl+AMP

0.26 0.31 0.45 0.37 1.03

(1.00) (0.53) (0.58) (0.52) (0.74)

0.97 1.27 1.43 7.37

(0.47) (0.42) (0.48) (0.26)

0.26 0.62 0.80 0.88 2.67

0.92 0.96 1.08 0.97 1.15

Fl-Na in buffer [C4mim][C8SO4] [C4mim][C8SO4]+AMP C12mimCl C12mimCl+AMP

0.15 0.21 0.33 0.47 1.08

(1.00) (0.86) (0.91) (0.47) (0.66)

0.91 0.99 1.37 3.51

(0.14) (0.09) (0.53) (0.34)

0.15 0.31 0.39 0.95 1.91

0.92 0.96 1.08 0.97 1.15

Experimental error ∼5%. bExperimental error ∼10%.

of anionic probe fluorescein with the [C4mim]+ moiety of the [C4mim][C8SO4] system. However, the addition of negatively charged AMP causes very little red shift in the absorption and emission spectra of Fl-Na in both the SAILs-AMP aggregates. In contrast, as R6G is a cationic probe, after the addition of negatively charged AMP into the SAILs forming micellar assemblies, it interacts electrostatically with the SAILs-AMP aggregates. However, Fl-Na is an anionic probe; hence, it has a lower effect compared to that of the cationic probe R6G upon AMP addition into the SAILs micellar aggregates. 3.2.2. Time-Resolved Anisotropy Measurements. To gain further insight about the location of Fl-Na and R6G molecules due to the interaction of the SAIL micelles with the DNA nucleotide AMP, time-resolved anisotropy decays of both the probe molecules have been monitored. The anisotropy decays are biexponential, and average rotational relaxation time is estimated using the following equation:55 ⟨τr ⟩ = aslowτslow + afast τfast

the compactness of the SAILs micelles is enhanced due to the incorporation of the DNA nucleotide AMP into the SAILs micellar assemblies. This is the consequence of the cation−π interaction between the imidazolium cation of the SAILs and the π electron cloud of the adenosine moiety of the AMP. Besides this, a strong electrostatic interaction is taking place between the positively charged imidazolium rings of the SAILs with the negatively charged phosphate moiety of the DNA nucleotide (AMP). Because of these combined effects, the rotational motion of the two probes (R6G and Fl-Na) becomes restricted in these SAILs-AMP containing aggregates. However, the extent of restriction of the two probes is different in the SAILs-AMP assemblies due to the different modes of interaction of R6G and Fl-Na with these aggregates. In bulk water, R6G exhibits single exponential decay with a rotational time constant of ∼260 ps.35 However, the reorientation time of R6G in both the SAIL micellar assemblies ([C4mim][C8SO4] and C12mimCl) become biexponential. Another interesting point to be noted is that the average rotational time constant of R6G further enhances significantly with the addition of AMP into the both cationic and anionic SAILs assemblies. The average reorientation time constant of R6G in the C12mimCl-AMP and [C4mim][C8SO4]-AMP aggregate becomes ∼2.67 and ∼0.80 ns, respectively. On the contrary, for the anionic fluorophore Fl-Na, the situation is quite different. In both the anionic [C4mim][C8SO4] and cationic C12mimCl micellar solutions, the anisotropy decay is biexponential in nature. The biexponential

(3)

where τslow and τfast are the slow and fast components of the decay time of the probe molecule; aslow and afast designate the relative magnitude of these components; and ⟨τr⟩ represents the average rotational time constant of the probes. The anisotropy decays of R6G and Fl-Na in SAILs-AMP systems are shown in in Figure 4 and Figure 5, respectively. The corresponding anisotropy decay parameters of both molecules in micellar and SAILs-AMP aggregates are given in Table 1 and Table S1, (Supporting Information). In this study, F

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Figure 6. FCS traces of R6G in (a) buffer, [C4mim][C8SO4] and [C4mim][C8SO4]+190 mM AMP and (b) buffer, C12mimCl and C12mimCl+190 mM AMP containing nanoaggregates.

with the internal motion of probe (τe), the cone angle (θ0), and wobbling diffusion coefficient (DW). The rotational parameters of R6G and Fl-Na molecules are tabulated in Table S1 (Supporting Information), and the useful equations are given in the Supporting Information. As described in Table S1 (Supporting Information), the overall rotation motion, τM, is very high in SAILs-nucleotide assemblies compared with the value of τslow and τfast values of the R6G and Fl-Na molecules. Therefore, the contribution of the overall rotational motion of SAILs-nucleotide aggregates to anisotropy decay is almost negligible. Moreover, τD values are similar to τslow, indicating that the translational diffusion of the probe molecules is effectively represented by the τslow values. The wobbling diffusion coefficient, DW, of both R6G and Fl-Na decreases with the addition of DNA nucleotide into the micellar assemblies. This can be accounted for by an increase in rigidity and compactness of the SAILs-nucleotide assemblies with nucleotide content. In [C4mim][C8SO4]-AMP and C12mimClAMP systems, the order parameter S values are between 0.3 and 0.63. This observation clearly indicates the incorporation of probe molecules inside the rigid environment of SAILsnucleotide aggregates, as its value ranges from “0” (unrestricted motion) to “1” (restricted motion). Now, the microviscosity (ηmic) inside the pseudophase of aggregates has been determined using the Stokes−Einstein−Debye equation:57,58

nature of the anisotropy decay of Fl-Na in cationic C12mimCl containing micellar aggregates is because of the variable location of Fl-Na in this micellar assembly. As Fl-Na is negatively charged, it undergoes favorable electrostatic interaction with the cationic SAIL containing micellar aggregate compared to that with the anionic [C4mim][C8SO4] micellar system. Hence, fluorescein has some population at the surface of the C12mimCl micelle, and it is also solubilized in the inner part of the Stern region of the cationic micellar aggregate.56 The slower component of rotational diffusion arises from the probe molecules residing at the Stern layer of the micelle, and the faster one corresponds to the population at the surface of the micellar assemblies. Because of the lack of major electrostatic interaction with the anionic SAIL micelle, the major population of Fl-Na resides toward the surface of the [C4mim][C8SO4] micelles. Hence, in case of the anionic micelle, the contribution of the faster component is many times higher than that of the slower one. However, the formation of more compact larger aggregates due to the incorporation of the DNA nucleotide into both cationic and anionic SAILs renders the rotational motion of fluorescein slower compared to that experienced in the micellar aggregates. Therefore, the probable location of two probes in these newly formed SAIL-AMP aggregates can be ascertained by the anisotropy measurements. The shorter component represents the population of probe molecules present at the micelle−water interface and the larger component corresponds to those probe molecules solubilized at the headgroup region or polar surface region of the SAIL-AMP assemblies. The negative charge on AMP facilitates electrostatic interaction with the cationic R6G; hence, R6G interacts more with the headgroup region of the SAIL-AMP aggregates than Fl-Na which is an anionic probe. An appropriate understanding about the rotational motion of a molecule can be gained by applying two-step and wobbling in cone models. By using these models, we were able to evaluate the respective parameters correlating the motion of probe molecules in these SAILs-nucleotide assemblies, assuming that the fast and slow motions are separable. The anisotropy decay can be correlated to the wobbling motion of probe molecules, translational or lateral diffusion along the surface, and overall rotational motion of the aggregates. In accordance with the two-step model, the slow component of rotational relaxation time (τslow) is associated with lateral diffusion of the probe (τD) along the inner surface of the aggregates and overall rotational motion of aggregates (τM). According to the wobbling in a cone model, the fast component of the rotational motion is linked

⟨τr ⟩ =

ηmic V kT

(4)

where V is the volume of the probe molecule, and ⟨τr⟩ and T represent average rotational time and absolute temperature, respectively. The volume of R6G and Fl-Na was taken as 414 Å3 and 295.8 Å3, respectively, which was calculated using Edward’s volume increment method.59 The bulk viscosities of the medium are given in Table 1. The results assured us that the microviscosities obtained in SAILs-nucleotide mixed systems are quite larger than bulk viscosity. The strong interaction between the imidazolium moiety of the SAILs and phosphate of the AMP as well as the cation−π interaction between the imidazolium ring of the SAILs and adenosine moiety of the DNA nucleotide increases the compactness and rigidity of the SAILs-nucleotide systems. The microviscosity experienced by R6G in [C4mim][C8SO4]-AMP and C12mimClAMP containing aggregates increases up to ∼11.10 and ∼37.10 cP, respectively. Previously, this type of large microviscosity is also observed in the case of a vesicular system.55 G

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Figure 7. FCS traces of Fl-Na in (a) buffer, [C4mim][C8SO4] and [C4mim][C8SO4]+ 190 mM AMP and (b) buffer, C12mimCl and C12mimCl+190 mM AMP containing nanoaggregates.

3.2.3. Fluorescence Correlation Spectroscopic (FCS) Measurements. In recent times, FCS is used extensively as a powerful tool to investigate molecular interaction and diffusion in biomolecules, surfactant aggregates, chemical kinetics, ionic liquid microheterogeneity, etc. in near single molecular level.60−67 In FCS, the autocorrelation function is generated using the fluctuations of fluorescence intensity inside a small observation volume of ∼1 fL.60 The FCS technique has been employed to gain information concerning the lateral diffusion motion of a probe molecule in various organized assemblies.61−67 The motive behind this investigation is to get insight into how the lateral diffusion of two oppositely charged molecules alter due to the interaction between the SAIL micelles and DNA nucleotide. To monitor this phenomenon, we have studied the lateral diffusion of a cationic probe R6G and an anionic probe Fl-Na in the SAILs forming micelles and SAILs-nucleotide self-assemblies. The FCS traces of R6G and Fl-Na in [C4mim][C8SO4], C12mimCl micellar solution and SAILs-AMP aggregates are shown in Figure 6 and Figure 7, respectively. In SAILsnucleotide systems, a multiple-species fitting equation has been used to fit the autocorrelation traces for both the probes. The diffusion coefficients (Dt) of both R6G and Fl-Na are substantially lower in SAILs micelles and SAILs-nucleotide aggregates than that in bulk water. As R6G and Fl-Na are hydrophilic in nature, it is quite obvious that a fraction of the probe molecules are solubilized in the micelle−water interface and that some population is also there in the headgroup region of the SAIL micelles or SAILs-nucleotide assemblies. Hence, the Dt values of the respective probes in water have been fixed as one component, and then, the other component is obtained by fitting the FCS traces using bimodal equation. This obtained component represents the diffusion of entrapped R6G and FlNa inside micelle or SAILs-nucleotide assemblies. The diffusion constants of R6G in cationic C12mimCl and anionic [C4mim][C8SO4] micelles are ∼95 μm2/s and 214 μm2/s, respectively (Table 2). Moreover, with the addition of nucleotide AMP into the respective micellar solutions, the diffusion coefficient (Dt) of R6G decreases in both the SAILsAMP systems. With the addition of nucleotide into the C12mimCl and [C4mim][C8SO4] micellar solutions, the smaller micellar aggregates are transformed into more compact and larger SAILS-nucleotide aggregates. R6G being a cationic probe interacts favorably with both the SAIL-nucleotide aggregates ([C4mim][C8SO4]-AMP and C12mimCl-AMP) as the nucleotide (AMP) is negatively charged. Hence, due to the enhanced

Table 2. Diffusion Coefficients (Dt) of R6G and Fl-Na in SAIL-Nucleotide System (AMP = 190 mM) probe

system

Dt (μm2/s)a

R6G

phosphate buffer C12mimCl C12mimCl+AMP [C4mim][C8SO4] [C4mim][C8SO4]+AMP phosphate buffer [C4mim][C8SO4] [C4mim][C8SO4]+AMP C12mimCl C12mimCl+AMP

426 (100%) 95 (74%) 57 (46%) 214 (75%) 141 (51%) 430 (100%) 236 (28%) 155 (24%) 85 (79%) 62 (77%)

Fl-Na

a

Experimental error ∼3%.

electrostatic interaction in the SAILs-nucleotide assemblies, the lateral diffusion of cationic probe R6G becomes slower compared to that of the SAILs micelles. The diffusion constants of R6G in C12mimCl-AMP and [C4mim][C8SO4]-AMP assemblies are ∼57 μm2/s and 141 μm2/s, respectively (Table 2). However, the scenario toward Fl-Na is quite different. In phosphate buffer medium, the lateral diffusion coefficient of FlNa is 430 μm2/s. Upon solubilization in the micellar assemblies of SAILs, the lateral diffusion coefficient of Fl-Na in cationic SAIL assembly decreases several times than that of the aqueous solution. Fl-Na is negatively charged; hence, it has favorable electrostatic interactions with the cationic SAIL C12mimCl containing the micellar aggregate, whereas it lacks the anionic [C4mim][C8SO4] micellar assembly. Therefore, the lateral diffusion of Fl-Na becomes slower in the cationic C12mimCl micellar aggregate than that of anionic SAIL system. Moreover, formation of the rigid SAILs-nucleotide aggregate results in further reduction of lateral diffusion motion of Fl-Na in these aggregates. Therefore, the different extents of reduction of translational diffusion motion of R6G and Fl-Na in these various self-assemblies relies on the different charges of the probe molecules as well as of the SAILs C12mimCl and [C4mim][C8SO4]. The variation in diffusion coefficient values and their relative contribution clearly differentiate the microenvironment between micelle and the SAILs-nucleotide aggregates in SAILsnucleotide assemblies. H

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4. CONCLUSIONS In this article, the interaction between a DNA nucleotide (AMP) and a cationic (C12mimCl) and an anionic ([C4mim][C8SO4]) imidazolium containing surface active ionic liquid has been revealed. The formation of larger sized aggregates in SAILs-nucleotide solution mixtures has been confirmed by cryo-TEM images. 1H NMR studies further ensure the interaction of DNA nucleotide and SAILs. We have also monitored the rotational motion of a cationic probe, R6G, and an anionic probe, Fl-Na, to monitor the photophysical aspects of these two differently charged probes inside these newly formed SAILs-nucleotide aggregates. The variation in microheterogeneity of SAILs micellar and SAILs-nucleotide aggregates influences the binding of Fl-Na and R6G molecules. The anisotropy and FCS measurements have unveiled that SAILs micelles are transformed into more compact and rigid SAILs-nucleotide aggregates with the incorporation of AMP. The lateral and rotational diffusion motion of R6G inside SAIL micelles increases significantly in the presence of AMP due to the increased electrostatic interaction with the SAILsnucleotide aggregates than that of micelles. However, the scenario around the surrounding environment of anionic probe Fl-Na differs with that of R6G. The AMP addition into the SAILs exerts a lesser effect on the overall rotational and lateral diffusion of Fl-Na compared to R6G in these aggregates. Overall, the observed results also indicate that SAILs-nucleotide containing large aggregates are stable systems. Therefore, the ability of the DNA nucleotide to form larger aggregates upon binding with the SAIL micelles and capability to encapsulate negatively and positively charged molecules make these newly characterized stable SAILs-nucleotide aggregates a possible vehicle to have great potential in material science and biological applications.

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Mr. Sandip Dey, and Mr. Chiranjit Biswas of IICB, Kolkata for their help in obtaining Cryo-TEM images.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02794. Detailed instrumentations of DLS, cryo-TEM, NMR, anisotropy measurements, FCS, some important equations, table of analytical rotational parameters, numbersize distribution plot, 1H NMR spectra of different systems, DLS plot of SAILs-+AMP containing nanoaggregates in the presence of R6G and FlNa, and UV spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 91-3222-283332. Fax: 91-3222-255303. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S. gratefully acknowledges SERB (Grant No: IR/S1/LU001/2013 dated 24/03/2015), Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Government of India for providing generous research grants. A.R., P.B., R.D., and S.K. are thankful to CSIR for their research fellowships. We thank Dr. Jayati Sengupta, I

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K

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