Article pubs.acs.org/JPCB
Cosolubilization of Coumarin30 and Warfarin in Cationic, Anionic, and Nonionic Micelles: A Micelle−Water Interfacial Charge Dependent FRET Aijaz Ahmad Dar* and Oyais Ahmad Chat Department of Chemistry, University of Kashmir, Srinagar-190006, Jammu and Kashmir India S Supporting Information *
ABSTRACT: Solubilization of structurally varied coumarins, viz., Warfarin (WF; a 4hydroxy coumarin) and Coumarin30 (C30, a 7-amino coumarin) individually and in mixed states (cosolubilization) within the aqueous surfactant self-assemblies of varying architectures has been explored, exploiting steady-state, time-resolved fluorimetric, and spectrophotometric techniques. Cosolubilization studies within micelles, which have rarely been done in the literature, were specifically undertaken with the aim of understanding the effect of micelles on their photophysical phenomena when simultaneously present within these nanocarriers and assess their prospective use as an efficient FRET pair. WF solubilizes within CTAB micelles, whereas little or no solubilization is observed in Brij30 and SDS micelles. On the other hand, C30 solubilizes deep into the palisade layer of CTAB micelles, between negatively charged head groups in SDS micelles and between OE groups in Brij30 micelles. C30 and WF maintain their solubilization sites during cosolubilization. In SDS and Brij30 micelles, an increase in WF causes fluorescence quenching of C30 molecules, while in CTAB, an increase in WF causes an increase in fluorescence of C30 by excited WF molecules indicating FRET between the two molecules.
1. INTRODUCTION Benzo-α-pyrones, commonly called coumarins, have been widely used as efficient fluorescent indicators at physiological pH1 and as fluorescent probes to determine the rigidity and fluidity of living cells and their surrounding medium.2,3 The spectrophysics of coumarins has been found to be strongly dependent on the surrounding media, nature of substituents, and their position in the coumarin ring.4−6 These observations have led to the synthesis of a vast number of coumarin derivatives, demonstrating the interesting photophysical properties they exhibit such as intramolecular charge transfer (ICT)7 and twisted intramolecular charge transfer (TICT)8 relaxation processes. A number of spectroscopic and theoretical studies reported in the literature have been employed to characterize the photophysical behavior of such substances.9 Warfarin (WF), a 4-hydroxy coumarin shown in Scheme 1, is an oral anticoagulant drug that has been in wide use in the treatment of venous as well as arterial thromboembolism such as myocardial infarction and stroke and in agriculture as a rodenticide and pesticide.10,11 Among a large number of analytical techniques, fluorescence spectroscopy is one of the most efficient and simplest methods to monitor WF,12−14 though environmentally dependent structural complexity limits its direct fluorimetric detection.15−18 The sensitivity of such detection is, therefore, enhanced by sensitized fluorescence in organized media.19 Coumarin30 (C30), a 7-amino coumarin shown in Scheme 1, has been used in constructing mixed dye lasers having performance better than the single-dye ones.20 © XXXX American Chemical Society
The effect of medium on the emission properties of coumarin dyes is important for their lasing action and detection. Due to their low aqueous solubility and quenching properties of water for their emission characteristics, such a medium is always avoided. However, aqueous media have been of interest due to the anticipated benefits of reduced thermal gradients for water compared to typical organic solvents under lasing conditions and in the biological processes where water is an important medium. To this end, the solubilization of normally insoluble coumarins in water with the aid of surfactants or cyclodextrins, has been studied.21−26 The important result of these studies is that the rapid nonradiative decay that mitigates fluorescence for many coumarin dyes (especially for a medium of pure water) is suppressed when the dye is sequestered in less polar domain of such amphiphilic aggregates. Dye mixtures are quite useful wherein one dye with considerable absorption at pumping wavelength acts as booster to another dye when the booster emission band strongly overlaps with the absorption band of another dye provided the dye molecules are in close proximity with each other.27 Nature uses self-assembly designs in a perfect and exquisite manner, compartmentalizing chromophores within small distances to maximize interactions (like energy transfer) between them and causing segregation to avoid destructive and undesirable Received: December 1, 2014 Revised: August 5, 2015
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DOI: 10.1021/jp511978h J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
our knowledge, neither have such cosolubilization studies, using coumarins, been reported nor has this pair been established as a prospective FRET pair, which, it is believed, will be highly useful for the studies involving drug−drug or protein−drug interaction in biosystems.
Scheme 1. Structure of Coumarins and Surfactants
2. EXPERIMENTAL SECTION 2.1. Materials. 3-(a-Phenyl-b-acetylethyl)-4-hydroxycoumarin, Warfarin (98%), Coumarin30 (99%), Cetyltrimethylammonium bromide (CTAB, 99%), sodiumdodecyl sulfate (SDS, 99%) and Brij30 (99%) were all Sigma-Aldrich products and used as received (Scheme 1). All the solvents used were spectrochemical grade. The stock solutions of CTAB, SDS, and Brij30 were prepared in 10 mM phosphate buffer of pH 7.4 with double-distilled water and utilized to prepare the samples of desired concentrations. Stock solutions of high concentration of WF and C30 were prepared in methanol of spectrochemical grade, and when added to the surfactant solution, the amount added was always less than 2% by volume of the main solution. 2.2. Methods. The absorption spectra were recorded in a JASCO V530 spectrophotometer, and fluorescence spectra were measured in a Varian Cary eclipse fluorimeter. The fluorescence decays have been recorded on an IBH Fluorocube TCSPC spectrometer at a resolution of 7 ps/channel and at magic angle polarization with respect to the excitation light. The excitation sources used were 295 nm (fwhm = 700 ps), 340 nm (fwhm = 700 ps), and 406 nm (fwhm = 200 ps). The PL decays were analyzed by using IBH DAS 6.2 software. The data are fitted, using the iterative reconvolution method, to a sum of exponentials I(t ) = I(0) ∑ ai exp( −t /τi) i
(1)
where I(t) and I(0) are the PL intensities at time t and zero after excitation by the pulse of light. ai is the amplitude of the ith component.
interactions.28 In the laboratory, such a process can easily be mimicked using a mixture of model fluorophores in micellar media. Nanoscopic micelles have been found to be extremely important in simulating the complex environmental conditions prevalent in biological assemblies and usually assumed as simple biomimetic models.29−31 Since there is a considerable spectral overlap of emission spectrum of WF with the excitation/absorption spectrum of C30, studying the nature of interactions between these chromophores within the biomimetic nanoassemblies of ionic and nonionic micelles could be relevant in understanding the activity of ligands and drugs in the human body for the development of effective fluorescent labels and targeted drug delivery.32−38 A good number of instances have been devoted to study the different aspects of fluorescence resonance energy transfer (FRET) process between dye molecules in vesicles, micelles, and reverse micelles,39−53 though with much less emphasis on the solubilization phenomenon of dye molecules within these nanoassemblies. Therefore, the main motivation behind the present work was to (i) investigate the photophysical properties of WF and C30 in the nonionic, anionic, and cationic micellar solutions for enhanced fluorescence, being scarcely reported in literature; (ii) study simultaneous solubilization (cosolubilization) within these nanocarriers with the aim of finding the effect of coumarin structure on their cosolubilization; and (iii) find whether the two will together emerge as a potential donor− acceptor FRET pair for sensitized fluorescence. To the best of
3. RESULTS AND DISCUSSION 3.1. Interaction of Warfarin (WF) with Surfactant SelfAssemblies. WF is a fluorescent coumarin derivative of clinical potency as anticoagulant drug.54−56 Valente et al.56,57 have reported a series of isomers of WF (Scheme 1) that coexist in solution, the isomeric ratio of which is a function of solvent polarity, pH, and redox properties. Karlson et al.16 demonstrated that open chain isomer in protonated/deprotonated forms gives a broad S0−S1 absorption band in the region of 310−360 nm, while as for cyclic hemiketal isomer, it lies between 250 and 305 nm. Moreover, this demonstrates that the cyclic hemiketal isomer predominates in aprotic nonpolar solvents, showing fluorescence emission maxima at around 360 nm while as open chain protonated/deprotonated forms predominate in protic polar solvents, showing a broad featureless and highly red-shifted band at around 400 nm. Figure 1a shows the absorption and emission spectra of WF in heptane (aprotic, nonpolar solvent), 1-butanol (low polarity, high viscosity protic solvent), and aqueous phosphate buffer solution (PBS) at pH 7.4 (polar solvent). Clearly, both cyclic hemiketal and open chain forms exist in heptane with predominance of the former, owing to absorption band at around 280 and 310 nm and fluorescence emission maximum at around 360 nm. The appearance of a broad S0−S1 absorption band in region 310−350 nm in butanol and aqueous B
DOI: 10.1021/jp511978h J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
the solvent. Therefore, high viscosity of butanol increases the quantum yield of WF due to suppression of its vibronic modes that otherwise provide pathways for nonradiative transitions between excited and ground states. WF is known to display two fluorescence lifetimes, viz., τ1 < 0.1 ns and τ2 = 0.5−1.6 ns dependent on the solvent character.16,17,60 The short lifetime originates from the neutral isomeric forms of WF in both polar and nonpolar organic solvents, whereas longer lifetime appears due to deprotonated open side chain form of WF which exists only in polar environment. Figure 1b shows the picosecond decay profile of WF at excitation of 295 nm in aqueous buffered solution, heptane, and 1-butanol, while the fitted lifetime values are collected in Table 1. We observed a single dominating decay time τ1 < 0.1 ns in n-heptane and aqueous buffer as solvents. WF has been shown to possess very short lifetime (
