From molecular modelling to photophysics of neutral oligo- and polyfluorenes incorporated into phospholipid bilayers† M. J. Tapia, * a M. Monteser ´ ın, a H. D. Burrows, b J. A. S. Almeida, b A. A. C. C. Pais, b J. Pina, b J. S. Seixas de Melo, b S. Jarmelo b and J. Estelrich c The combination of various experimental techniques with theoretical simulations has allowed elucidation of the mode of incorporation of fluorene based derivatives into phospholipid bilayers. Molecular dynamics (MD) simulations on a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayer, with benzene (B), biphenyl (BP), fluorene (F) and tri-(9,9-di-n-octylfluorenyl-2,7-diyl), TF, have provided insights into the topography of these molecules when they are present in the phospholipid bilayer, and suggest marked differences between the behavior of the small molecules and the oligomer. Further information on the interaction of neutral fluorenes within the phospholipid bilayer was obtained by an infrared (IR) spectroscopic study of films of DMPC and of the phospholipid with PFO deuterated specifically on its alkyl chains (DMPC–PFO-d 34 ). This was complemented by measurements of the effect of F, TF and two neutral polymers: polyfluorene poly(9,9-di-n-octylfluorenyl-2,7-diyl), PFO, and poly(9,9-di-n-dodecylfluorenyl-2,7-diyl), PFD, on the phospholipid phase transition temperature using differential scanning calorimetry (DSC). Changes in liposome size upon addition of F and PFO were followed by dynamic light scattering. In addition, the spectroscopic properties of F, TF, PFO and PFD solubilised in DMPC liposomes (absorption, steady-state and time-resolved fluorescence) were compared with those of the same probes in typical organic solvents (chloroform, cyclohexane and ethanol). Combining the insight from MD simulations with the results at the molecular level from the various experimental techniques suggests that while the small molecules have a tendency to be located in the phospholipid head group region, the polymers are incorporated within the lipid bilayers, with the backbone predominantly orthogonal to the phospholipid alkyl chains and with interdigitation of them and the PFO alkyl chains. Introduction Liposomes are synthetic lipid vesicles consisting of phospho- lipid bilayers. They are normally prepared from hydrated phospholipid lms at temperatures above the gel to liquid phase transition, which depends on the lipid alkyl chain length. 1 In the process of liposome formation, they can entrap both water and lipid soluble compounds, while polyelectrolytes can be adsorbed on their surface through electrostatic interac- tions. 1 In addition, liposomes, which typically have sizes on the hundred nanometer scale, can be considered as good carriers for various molecules, and can solubilize neutral compounds, 2 including polymers, 3–5 to form aqueous dispersions. 5,6 The variety of interactions makes liposomes interesting carriers of drugs for therapeutic applications, 1,7–9 including photodynamic therapy. 10 In addition, their structural relationship to cell membranes makes liposomes useful models for understanding membrane biophysics, including the way guest molecules are incorporated into phospholipid bilayers. 1 For example, this can allow us to gain an insight into the interaction of polycyclic aromatic hydrocarbons with membranes to understand how such pollutants penetrate and interact with human skin. The use of uorescent aromatic compounds, such as uorene, allows the sensitivity of methods such as steady-state and time- resolved uorescence, and uorescence anisotropy to probe the a Departamento de Qu´ımica, Universidad de Burgos, Plaza Misael Ba˜ nuelos s/n, 09001 Burgos, Spain. E-mail: mjtapia@ubu.es b Centro de Qu´ımica de Coimbra (CQC), Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal c Facultat de Farm` acia, Universitat de Barcelona, Avda. Joan XXIII s/n 08028 Barcelona, Catalonia, Spain † Electronic supplementary information (ESI) available: The area per lipid and bilayer thickness for the fully hydrated DMPC bilayer in the absence and presence of a uorophore are shown. Experimental DSC thermograms for aqueous DMPC and DMPC/probe ratio 2 : 1. The normalized absorption and emission spectra of F, TF, PFD and PFO in chloroform, in cyclohexane and in ethanol are compared separately for each solvent (effect of the compound structure). The normalized absorption and emission spectra of F in cyclohexane, chloroform, ethanol and DMPC are compared. The same gures were prepared for TF, PFO and PFD (effect of the environment for each spectroscopic probe). See DOI: 10.1039/c4sm02145b Cite this: Soft Matter, 2015, 11, 303 Received 25th September 2014 Accepted 5th November 2014 DOI: 10.1039/c4sm02145b www.rsc.org/softmatter This journal is © The Royal Society of Chemistry 2015 Soft Matter, 2015, 11, 303–317 | 303 Soft Matter PAPER Published on 05 November 2014. Downloaded by Universidade de Coimbra on 13/05/2015 14:59:56. View Article Online View Journal | View Issue