Full Length Article Investigation on ionic states of 1,2-Dipalmitoyl-sn-glycero-3- phosphorylcholine (DPPC) using organic laser dyes: A FRET study Arpan Datta Roy, Jaba Saha, D. Dey, D. Bhattacharjee, Syed Arshad Hussain n Thin Film and Nanoscience Laboratory, Department of Physics, Tripura University, Suryamaninagar 799022, Tripura, India article info Article history: Received 12 May 2016 Received in revised form 24 October 2016 Accepted 25 December 2016 Available online 31 December 2016 Keywords: DPPC pH Cell membrane FRET Dyes abstract Fluorescence Resonance Energy Transfer (FRET) between two organic dyes Fluorescein and Rhodamine 6G were successfully investigated in aqueous solution in presence and absence of 1,2-Dipalmitoyl-sn- glycero-3-phosphorylcholine (DPPC) at different pH. Spectroscopic studies suggest that both the dyes were present mainly as monomer in solution. FRET occurred from Fluorescein to Rhodamine 6G in so- lutions. Energy transfer efficiency increases in presence of DPPC and the maximum efficiency was 59.3% when the concentration of DPPC was 1.4 Â 10 À4 M at ambient condition. pH plays a crucial role in this investigation as the energy transfer efficiency was found to change in presence of DPPC at different pH. It has been demonstrated that with proper calibration it is possible to use the present system under in- vestigation to realize various ionic states of DPPC by observing the change in FRET efficiency between these two dyes. & 2017 Elsevier B.V. All rights reserved. 1. Introduction DPPC is a phospholipid consisting of two palmitic acids [1,2]. It is the major constituent of pulmonary surfactant, most responsible for the surface tension lowering properties of surfactant [3]. It is also the only surface active component of lung surfactant capable of lowering surface tension to near zero levels which is utmost important for proper functioning of the heart. There exists con- siderable potential for the lung to affect the heart. Because they share the thoracic cavity, changes in intrathoracic pressure ac- companying lung inflation are transmitted directly to the heart. In addition, all of the blood leaving the right ventricle must traverse the pulmonary vascular bed, so changes in pulmonary vascular resistance may greatly affect right ventricular function [4]. DPPC is used for research purposes in studying liposomes, lipid bilayers, model biological membrane, in the formation of reconstituted HDL particles and cell signaling [5–8]. For this reason the bulk of the research concerning various states of phospholipids are related to understanding the bio membrane processes. One of the main factors which influence molecular process like cell cycle progression, cell proliferation as well as differentiation is the environmental acidity or pH of living cells and tissues. Con- sequently, malignant transformation, oncogenesis, metastasis and angiogenesis are largely affected by environmental acidity [9]. The environmental acidity also greatly influences the response of cancer cells to various treatments [9]. The microenvironment in tumor on both animal and human is acidic as compared with that in normal tissues because of elevated in anaerobic as well as aerobic glycolysis in tumors [9–11]. Wike-Hooley et al. reported that electrode-evaluated human tumor pH is on average, lower than the pH of normal tissues [10], whereas, Song et al. reported the rigorous investigation on the intra and extracellular pH in solid tumor cells [9]. The pH gradient difference between tumor and normal tissues provides a strong rationale for the design and evaluation of the efficacy of drugs [10]. A number of studies have been reported drug partitioning into artificial lipid vesicles and cells as a function of pH. DPPC is a zwitterionic phospholipid which is a representative lipid of the biological cell membrane [12] i.e., in physiological pH range of human body; DPPC mainly re- mains in zwitterionic form. But pH in the microenvironment is different in biological and pathological situations such as cancer, as well as different organelles (the liposomes are acidic). As ionic states of DPPC is a function of pH [13–15], so there may be a possibility that DPPC changes its ionic states in abnormal biolo- gical condition of our body. Further research may be helpful to realize this fact more precisely. Surfactant abnormalities often result in very severe consequences, even death [16]. Ionic nature of DPPC plays a crucial role in drug-membrane biophysical interac- tion studies [12]. Thus change of ionic states of DPPC can be treated as an alarming indicator of growing abnormalities in cell membranes. It is worthwhile to mention in this context that the ionic states of DPPC play a crucial role with various drug interac- tions in drug delivery system [17,18]. Consequently, by observing Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2016.12.040 0022-2313/& 2017 Elsevier B.V. All rights reserved. n Corresponding author. E-mail addresses: sa_h153@hotmail.com, sahussain@tripurauniv.in (S.A. Hussain). Journal of Luminescence 185 (2017) 42–47