Intestinal fatty acid binding protein (IFABP) is thought to participate in the intracellular transport of fatty acids (FAs). Fatty acid transfer from IFABP to phospholipid membranes is proposed to occur during protein-membrane collisional interactions. In this study, we analyzed the participation of electrostatic and hydrophobic interactions in the collisional mechanism of FA transfer from IFABP to membranes. Using a fluorescence resonance energy transfer assay, we examined the rate and mechanism of transfer of anthroyloxy-fatty acid analogs a) from IFABP to phospholipid membranes of different composition; b) from chemically modified IFABPs, in which the acetylation of surface lysine residues eliminated positive surface charges; and c) as a function of ionic strength. The results show clearly that negative charges on the membrane surface and positive charges on the protein surface are important for establishing the "collisional complex," during which fatty acid transfer occurs. In addition, changes in the hydrophobicity of the protein surface, as well as the hydrophobic volume of the acceptor vesicles, also influenced the rate of fatty acid transfer. Thus, ionic interactions between IFABP and membranes appear to play a primary role in the process of fatty acid transfer to membranes, and hydrophobic interactions can also modulate the rates of ligand transfer.Instituto de Investigaciones Bioquímicas de La Plata