Published: July 12, 2011 r2011 American Chemical Society 10109 dx.doi.org/10.1021/jp203532c | J. Phys. Chem. B 2011, 115, 10109–10119 ARTICLE pubs.acs.org/JPCB Interaction of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl-Labeled Fatty Amines with 1-Palmitoyl, 2-Oleoyl-sn-glycero-3-phosphocholine Bilayers: A Molecular Dynamics Study Hugo A. L. Filipe, † Maria Jo ~ ao Moreno, †,‡ and Luís M. S. Loura* ,†,§ † Centro de Química de Coimbra, Universidade de Coimbra, Largo D. Dinis, Rua Larga, 3004-535 Coimbra, Portugal ‡ Departamento de Química, Faculdade de Ci ^ encias e Tecnologia, Universidade de Coimbra, Largo D. Dinis, Rua Larga, 3004-535 Coimbra, Portugal § Faculdade de Farm acia, Universidade de Coimbra, P olo das Ci ^ encias da Sa ude, Azinhaga de Santa Comba, 3000-548 Coimbra, Portugal b S Supporting Information ’ INTRODUCTION The interaction of amphiphilic molecules with lipid bilayers is a very important step that determines their location inside the cells and the rate of permeation across the different hydrophobic barriers in the cell or organism. Molecules with pharmacological or biological activity are usually amphiphilic due to their need to cross different cell membranes before they reach the target site and become active molecules. The establishment of quantitative relationships between the amphiphile structure and its rate of permeation through biomembranes is therefore an important tool in the rational development of new drugs, and this has been addressed by us 1À5 and other authors (see, e.g., refs 6À14). In addition to applications in the enhancement of drug bioavailability, the interaction between amphiphilic molecules with different lengths in the alkyl or acyl chain and lipid bilayers is also of fundamental importance for the understanding of the consequences of protein modification by acylation. 8,11,15,16 This is a prevalent cell regulation process where the association between the protein and a given membrane is enhanced, leading to an increased activity via its local concentration or inhibition by sequestration. 16,17 In the last years, this research team has been characterizing in detail the interaction of amphiphiles with serum proteins and lipid bilayers regarding both equilibrium and kinetic parameters. 1À5,18 More recently, the homologous series of fatty amines labeled with the fluorescent moiety 7-nitrobenz-2-oxa- 1,3-diazol-4-yl (NBD 19 ) has been studied, and the effect of the length of the alkyl chain on their solubility in aqueous media, binding to serum proteins, and partitioning to lipid bilayers 1,20 has been characterized. The results obtained revealed nonmo- notonic behavior along the homologous series. This has prompted us to characterize their interaction with lipid bilayers using molecular dynamics (MD) simulations, so as to gain molecular detail on the interactions established. The interaction of fluorescent molecules with lipid bilayers is also a very important tool in the characterization of the lipid bilayer itself. 21 For that purpose, in addition to equilibrium and kinetic parameters for the interaction, it is necessary to know the Received: April 15, 2011 Revised: July 7, 2011 ABSTRACT: A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled fatty amines of varying alkyl chain length, NBD-C n , inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics (MD) simulations. For all amphiphiles, the NBD fluorophore locates near the glycerol backbone/carbonyl region of POPC and establishes stable hydrogen bonding with POPC ester oxygen atoms. Small differences observed in the transverse location of the fluoro- phore correlate with other calculated parameters and with small discrepancies recently measured in the photophysical properties of the molecules. The longer-chained NBD-C n amphiphiles show significant mass density near the bilayer midplane, and the chains of these derivatives interdigitate to some extent the opposite bilayer leaflet. This phenomenon leads to a slower lateral diffusion for the longer-chained derivatives (n > 12). Effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain-labeled NBD probes. The molecular details obtained by this work allow the rationalization of the nonmonotonic behavior, recently obtained experimentally, for the photophysical parameters of the amphiphiles and the kinetic and thermodynamic parameters for their interaction with the POPC membranes.