Molecular Packing in Langmuir Monolayers Composed of a Phosphatidylcholine and a Pyrene Lipid Denise Gradella Villalva, † Marco Diociaiuti, ‡ Luisa Giansanti,* ,§ Manuela Petaccia, § Neva Bes ̌ ker, ⊥ and Giovanna Mancini # † Dipartimento di Chimica, Universita ̀ degli Studi di Roma “Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy ‡ Dipartimento di Tecnologia e Salute, Istituto Superiore di Sanita ̀ , Viale Regina Elena 299, 00161 Roma, Italy § Dipartimento di Scienze Fisiche e Chimiche, Universita ̀ degli Studi dell’Aquila, Via Vetoio, 67100 Coppito, AQ, Italy ⊥ CINECA, SCAISuper Computing Applications and Innovation Department, Via dei Tizii, 6, 00185, Rome, Italy # CNRIstituto di Metodologie Chimiche, Via Salaria km 29.300, 00016 Monterotondo Scalo, Roma, Italy ABSTRACT: Pyrene lipids are useful tools to investigate membrane organization and intracellular lipid trafficking. The molecular interactions controlling the organization of lipid monolayers composed of a cationic amphiphile tagged with a pyrene residue and a saturated or unsaturated phospholipid, namely, 1,2-dimyristoyl-sn-glycero-3-phos- phocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine, were inves- tigated by Langmuir trough isotherms to understand how the molecular structure of the components and their relative amount affect the physicochemical properties of lipid monolayers. The obtained results show that the cationic headgroups and unsaturation of hydrophobic chains strongly affect the organization of the lipid monolayer as a function of the amount of components. On the other hand, the presence of the pyrene moiety does not seem to have a marked influence on the interaction within lipid assembly. ■ INTRODUCTION Lipid molecules containing pyrene (Pyr-Ls) in their molecular structure are powerful probes largely used in membrane biophysics, biochemistry, and cell biology to study lateral arrangement, fluidity, and phase transitions of lipid bilayers, membrane fusion, lipid conformation, and lipid trafficking in living cells, because of the versatility of pyrene as a chromophore and a fluorophore. 1-4 It is well-known that Pyr-Ls form excimers in a concentration-dependent manner; however, phase separation, in terms of both lipid composition and coexistence of solid and fluid phases, can affect this phenomenon because a fraction of the pyrene probe can be isolated and not available to the formation of excimers. 5 As a consequence, the determination of lateral miscibility of Pyr-L in lipid bilayers is crucial information to identify the effective fraction of pyrene probe that actually can form excimers. It is often assumed that the lateral distribution of lipid is not affected by the presence of a pyrene residue in the lipid skeleton. However, this is not always true because even subtle changes in the structure of hydrophobic chains can have a significant effect on lipid organization in membranes. 1 Actually, the polycyclic aromatic ring of pyrene is much more rigid and bulky with respect to an alkyl chain; 1 thus its presence can drastically alter the organization of the lipid bilayer in which it is embedded. Besides the steric perturbation in the hydrophobic region, the pyrene moiety might tend to remain at the lipid/water interface due to specific interactions with lipid headgroups, such as cation-π interaction, thus affecting the headgroup hydration level. 6 In its turn the hydration level and the interactions between lipid polar headgroups signi ficantly affect the properties and the thermodynamics of lipid bilayer/water interface. 7 With these premises, it is obvious that in the case of lipid assemblies containing Pyr-Ls, the spectroscopic evidence has to be interpreted with caution. Herein we report on the investigation of mixed lipid monolayers composed of the cationic amphiphile 1, charac- terized by a pyrrolidinium headgroup and a hydrocarbon tail tagged with a pyrene moiety, 6 and either a saturated or unsaturated phospholipid, namely, 1,2-dimyristoyl-sn-glycero-3- phosphocholine (DMPC) or 1,2-dioleoyl-sn-glycero-3-phos- phocholine (DOPC) (Chart 1). The lipid monolayers, which can be considered as two-dimension membrane models, were investigated for mixed systems at different molar ratios by Langmuir compression isotherms to determine how and to what extent the organization and the properties of lipid bilayer are influenced by the molecular structure and the amount of lipid components. Received: December 3, 2015 Revised: January 21, 2016 Published: January 22, 2016 Article pubs.acs.org/JPCB © 2016 American Chemical Society 1126 DOI: 10.1021/acs.jpcb.5b11836 J. Phys. Chem. B 2016, 120, 1126-1133