Chemistry and Physics of Lipids 154 (2008) 38–45 Contents lists available at ScienceDirect Chemistry and Physics of Lipids journal homepage: www.elsevier.com/locate/chemphyslip Photophysics of a -carboline based non-ionic probe in anionic and zwitterionic liposome membranes Paramita Das, Deboleena Sarkar, Nitin Chattopadhyay ∗ Department of Chemistry, Jadavpur University, Calcutta 700032, India article info Article history: Received 13 November 2007 Received in revised form 27 February 2008 Accepted 3 March 2008 Available online 13 March 2008 Keywords: Liposome Lipid–water interface Surface charge Micropolarity Fluorescence quenching Limiting anisotropy abstract Interaction of a biologically active -carboline based non-ionic probe, 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), with the liposomal vesicles of dimyristoyl-l--phosphatidylcholine (DMPC) and dimyristoyl-l--phosphatidylglycerol (DMPG) has been demonstrated using steady-state and time-resolved fluorescence and fluorescence anisotropy techniques. Polarity sensitive intramolecular charge transfer of AODIQ shows a large hypsochromic shift along with an enhancement in the fluorescence quantum yield and fluorescence lifetime in the bilayer membranes compared to those in aqueous buffer solution. Polarity of the immediate vicinity of the probe in the lipid environments has been determined. The fluorometric, quenching and micropolarity determination studies reveal that the fluorophore pene- trates deeper in the zwitterionic DMPC membrane compared to the anionic DMPG vesicle. Enhancement in the rotational relaxation time of AODIQ in liposomal membranes suggests that the fluorophore exists in motionally restricted environments. © 2008 Elsevier Ireland Ltd. All rights reserved. 1. Introduction In recent years, a variety of research has been performed successfully to study the incorporation of various fluorophores into the liposome membranes (Nassar et al., 1998; Sujatha and Mishra, 1998; Lakowicz, 1999; Chattopadhyay, 2003; Mukherjee and Chattopadhyay, 2005; Moyano et al., 2006; Esquembre et al., 2007). Lipid vesicles are closed systems consisting of one or more lamellae containing the amphiphiles. In vesicles, the hydrophobic part of the amphiphiles orients in a way to form hydrophobic inte- rior of the bilayer and the hydrophilic portion remains in contact with the aqueous phase (Lasic, 1993). Liposomes serve as valuable model for biological membranes and correspond to the environ- ment in which many drugs, proteins and enzymes display their activity. Since the structure of the biological membranes is com- plicated due to a wide diversity in their composition, one uses synthetic liposomes that mimic the structure and geometry of the cell membrane (Ashgarian and Schelly, 1999; Moyano et al., 2006). Liposome has important applications in the field of drug load- ing, diagnostics, food industry, and immunology. Lipid vesicles are immensely used as selective barriers, nanocapsules and to protect the entrapped molecules (Needham and Dewhirst, 2001; Wen-Hua and Regen, 2005). ∗ Corresponding author. Fax.: 91 33 2414 6266. E-mail address: pcnitin@yahoo.com (N. Chattopadhyay). Interaction of drug molecules with lipid membrane is the prime concern in membrane biochemistry. To characterize interaction behavior, location of the probe molecules of the membrane, differ- ent spectroscopic techniques have been extensively used, among which fluorescence technique is perhaps the most sensitive one (Lakowicz, 1999; Mukherjee and Chattopadhyay, 2005; Moyano et al., 2006; Esquembre et al., 2007). It is known that the local microenvironment surrounding a probe molecule influences its electronic structure and hence its photophysics (Mallick et al., 2004, 2005a; Das et al., 2007a,b). Changes in the local microenviron- ment can be monitored spectroscopically and allows elucidation of the influence of the immediate environment on the molecule to identify specific interactions (Pansenkiewicz-Gierula et al., 1997; Duportail et al., 2001). At physiological pH (7.4), lipid vesicles mod- ify microenvironment around the dye appreciably compared to that in the bulk aqueous phase (Sujatha and Mishra, 1998; Rodrigues et al., 2002). Penetration of the probe into the bilayer membrane from the bulk water modifies the photoprocesses since the polar- ity and viscosity in the immediate environments around the probe are quite different from those of the bulk water (Lakowicz, 1999; Moyano et al., 2006). It is thus important to study the liposomal microenvironments and the modifications of the photophysics of the probe in the liposome membranes. In a recent work (Mallick et al., 2003) we reported the fluoro- metric behavior of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ) with dual emission from, viz., a locally excited (LE) state and a charge transfer (CT) state. The CT emission was 0009-3084/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.chemphyslip.2008.03.003