Article pubs.acs.org/Langmuir
Photophysical Behavior of 8‑Anilino-1-Naphthalenesulfonate in Vesicles of Pulmonary Surfactant Dipalmitoylphosphatidylcholine (DPPC) and Its Sensitivity toward the Bile Salt−Vesicle Interaction Monalisa Mohapatra and Ashok K. Mishra* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *
ABSTRACT: The photophysical behavior of 8-anilino-1-naphthalenesulphonate (ANS) in vesicles of dipalmitoylphosphatidylcholine (DPPC), a pulmonary surfactant, has been carried out in a detailed manner. ANS shows notable variations in fluorescence intensity, lifetime, and anisotropy parameters as it gets into the vesicle. It was found that ANS partitions well into the DPPC bilayer membrane with an estimated partition coefficient of ∼2.0 × 105. Among the various fluorescence parameters of ANS, fluorescence anisotropy was found to be most responsive to the temperature induced phase change of the bilayer membrane. These interesting fluorescence parameters of ANS were then used to study the hydration of lipid bilayer membrane by submicellar concentration of bile salts. From the steady-state fluorescence intensity and dynamic fluorescence lifetime analyses it is clear that ANS is able to probe the submicellar concentration (≤1 mM) of bile salt induced hydration of lipid bilayer membrane that accompanies expulsion of ANS from the bilayer to the aqueous bulk phase. Lower-temperature shift in the phase transition of DPPC bilayer indicates that fluorescence anisotropy of ANS is sensitive enough to the bile salt induced perturbation in the packed acyl chains of DPPC bilayer and modification in the membrane fluidity. In presence of sodium deoxycholate (NaDC) and sodium cholate (NaC) in DPPC vesicles, ANS experiences restriction in rotational mobility which is evident from the variation in steady-state fluorescence anisotropy and fluorescence anisotropy decay parameters.
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INTRODUCTION An appropriate fluorescent molecular probe for organized media shows remarkable changes in its one or more fluorescence parameters such as transition maximum, intensity, decay parameters and anisotropy as it gets into the organized media.1 The sensitivity of polarity sensitive fluorescent molecular probe 8-anilino-1-naphthalenesulphonate (ANS) makes it a good fluorescent molecular probe for organized and aggregated systems like micelles, proteins, lipid bilayer membranes, polymeric gels etc.2−6 Additionally, compared with biological molecules, the small size of ANS enables the study around its microenvironment with little interference. The photophysical behavior of ANS and its derivatives in various solvents has been thoroughly studied by Kosower and coworkers.7,8 It has been suggested that the absorption of light leads to excitation of ANS to a local excited state S1,np(nonplanar (NP) geometry) in less polar solvents which then undergoes transition accompanied by electron transfer to the more stable charge transfer (CT) state Sl,ct in high polar solvents. The CT state in polar medium decays to the ground state S0,np via another electron transfer process resulting in quenched fluorescence.7 An intense blue fluorescence of ANS in less polar solvents and a weak green fluorescence in aqueous solutions have been observed for the first time by Weber and Laurence.9 The intense blue fluorescence in hydrophobic © 2013 American Chemical Society
environment is mostly due to the restricted rotational motion of the phenylamino group of ANS. Figure 1 shows the molecular structure and excited state dynamics of ANS.
Figure 1. Molecular structure and excited state dynamics of ANS.
Pulmonary surfactants are essential components of respiratory mechanics. The human pulmonary surfactants consist of approximately 80% phospholipids. Phosphatidylcholine (PC) is the most abundant phospholipid, among which DPPC is the major constituent of pulmonary surfactant.10,11 The biosurfactants (bile salts) are the indispensable component of Received: June 22, 2013 Revised: August 9, 2013 Published: August 9, 2013 11396
dx.doi.org/10.1021/la402355j | Langmuir 2013, 29, 11396−11404
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fluorescence anisotropy, emission maximum, fluorescence lifetime and fluorescence anisotropy decay parameters of ANS have been used for the study.
hepatobiliary systems; liver, gall bladder and bile ducts etc. To facilitate the dissolution and bioavailability of insoluble drugs, the mixed micelles of bile salts and phosphatidylcholine have been widely used in oral drug delivery.12 In addition, phosphatidylcholines play important role to prevent the slight toxicity of bile salts on gastrointestinal epithelia and membranes. The interaction between bile salts and phosphatidylcholine bilayers is crucial during several physiological processes such as lipid digestion and absorption, where bile salts serve as solubilizers for dietary lipids in the gastrointestinal tract.11,13 With respect to their interaction the approximate physiological concentrations of bile salts are of interest; proximal small intestine (10 mM), distal ileum (2 mM), gall capillaries (5 mM), gall bladder (10−50 mM), portal vein (0.1 mM), peripheral blood (0.02 mM), plasma (
