Alzheimer's disease is characterized by the deposition of aggregates of the beta-amyloid peptide (A beta) in the brain. A potential therapeutic strategy for Alzheimer's disease is the use of synthetic beta-sheet breaker peptides, which are capable of binding A beta but unable to become part of a beta-sheet structure, thus inhibiting the peptide aggregation. Many studies suggest that membranes play a key role in the All aggregation; consequently, it is strategic to investigate the interplay between beta-sheet breaker peptides and A beta in the presence of lipid bilayers. In this work, we focused on the effect of the beta-sheet breaker peptide acetyl-LPFFD-amide, iA beta 5p, on the interaction of the A beta(25-35) fragment with lipid membranes, studied by Electron Spin Resonance spectroscopy, using spin-labeled membrane components (either phospholipids or cholesterol). The ESR results show that iA beta 5p influences the A beta(25-35) interaction with the bilayer through a cholesterol-mediated mechanism: iA beta 5p withholds cholesterol in the inner hydrophobic core of the bilayer, making the interfacial region more fluid and capable to accommodate A beta(25-35). As a consequence, iA beta 5p prevents the A beta(25-35) release from the lipid membrane, which is the first step of the beta-amyloid aggregation process. (C) 2011 Elsevier Inc. All rights reserved.