Chemical Physics Letters 613 (2014) 115–121 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al hom epage: www.elsevier.com/locate/cplett Interaction of fluorescence dyes with 5-fluorouracil: A photoinduced electron transfer study in bulk and biologically relevant water Jagannath Kuchlyan, Debasis Banik, Niloy Kundu, Arpita Roy, Nilmoni Sarkar ∗ Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India a r t i c l e i n f o Article history: Received 21 July 2014 In final form 26 August 2014 Available online 1 September 2014 a b s t r a c t The interactions of widely used chemotherapeutic drug, 5-fluorouracil (5FU) with coumarin dyes have been investigated for the first time using steady-state and time-resolved fluorescence spectroscopic mea- surements. The fluorescence quenching along with the decrease in lifetimes of excited state of coumarin derivatives with gradual addition of 5FU is explained by photoinduced electron transfer (PET) mecha- nism. Our studies were performed in bulk water and confined water of AOT (aerosol OT) reverse micelle to investigate the effect of confinement on PET dynamics. The feasibility of PET reaction for coumarin-5FU systems is investigated calculating the standard free energy changes using the Rehm–Weller equation. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The 5-substituted pyrimidines belong to a class of base ana- logues having biological importance. Among them, 5FU is one of the oldest drugs used in chemotherapy, for the treatment of liver, lung, bladder, colon, skin, breast, pancreatic and head and neck cancers [1,2]. The antitumor activity of 5-FU is based on the inhi- bition of DNA synthesis by competitive inhibition of thymidylate synthetase, which is the target enzyme for the drug [1]. However, its clinical use has been restricted by its systemic toxicities [3]. Thus it is important to know the activity of 5FU in biologically rel- evant systems. Photoinduced electron transfer reaction between the base pairs of the DNA double helix explores -stack-mediated electron transfer [4]. Strong stacking interactions between donor and acceptor result in fast electron-transfer kinetics as the close contact between donor and acceptor favours ET process. More- over, Miranda et al. studied adsorption behaviour of 5FU on Au (III) surface and they proposed that -stacking is enhanced when halogens are incorporated into the uracil structure [5]. It was also reported that uracil and its derivatives containing substituents ( F, Cl, Br, CH 3 etc.) at 5-position have a significant -electron charge density in the rings [6]. These investigations provide evi- dence for electron donating ability of 5FU in photoinduced electron transfer reaction. Recent literature reports on electron affinities, ionization energies, oxidation potential and protonation dynamics ∗ Corresponding author. E-mail addresses: nilmoni@chem.iitkgp.ernet.in, nilmoni1@rediffmail.com (N. Sarkar). of individual nucleic-acid bases have predicted the photoinduced interactions [7,8]. PET have been studied in detail by photophys- ical and photochemical behaviour of free fluorescence dyes of coumarin derivatives [9], acridine [10], rhodamine [11], 3,3 ′ ,4,4 ′ - benzophenone tetracarboxylic acid [12], and oxazine [13]. In these cases the fluorescent dyes acted as electron acceptor in their excited state and nucleobases acted as electron donor in their ground state. Donor to acceptor photoinduced electron transfer (PET) is a very common reaction in chemistry and biology [9,10,14]. There are many reports regarding the investigation of photoinduced electron transfer reaction in reverse micelle [15], micelle [16], noisome [17], ionic liquid [18], cyclodextrins [19], Protein-surfactant complexes [20] etc. Water plays a vital role in many biological phenomena such as electron transfer and proton transfer. The confined water of AOT (aerosol OT, sodium dioctyl sulfosuccinate) reverse micelle resembles with biological water [21]. The surfactant, AOT, is well characterized and is commonly used for the preparation of reverse micelles as it can solubilize large amount of water. Aqueous AOT reverse micelles consist of water, surfactants, and nonpo- lar solvents with appropriate ratios. The inner water pool of AOT/heptane/water reverse micelles is surrounded by polar head groups of surfactant molecules oriented towards the water pool and the nonpolar tail parts pointed outside, towards bulk hep- tane [22]. Inside AOT reverse micelles, the radius of the water pool is approximately 2w 0 Å, where w 0 denotes the molar ratio of water and AOT surfactant [23]. Substantial amount of water is solubilized in AOT reverse micelles; w 0 ranges from 0 to 70 for many systems [24]. Recent investigations have increased to under- stand the nature of confined water since, confined water shows http://dx.doi.org/10.1016/j.cplett.2014.08.062 0009-2614/© 2014 Elsevier B.V. All rights reserved.