Alzheimers disease is one of the most common types of degenerative dementia.As a considerable cause of Alzheimers disease, neurotoxic plaques composed of 39 to 42 residue-long amyloid beta peptides (Aβs) have been found in the patients brain in large quantity.A previous study found that erythrosine B (ER), a red color food dye approved by Food and Drug Administration (FDA), inhibits the formation of Aβ fibril and Aβ-associated cytotoxicity in vitro.Here, in an attempt to elucidate the inhibition mechanism, we performed molecular dynamics simulations to demonstrate the conformational change of Aβ40 induced by either one or two ER molecules in atomistic detail.During the simulation, the ER bound to the surfaces of N-terminus region of Aβ40 under the presence of both one ER and two ER molecules.The observed stacking of the ER and the aromatic side chains near the N-terminus region suggests a possible inhibition mechanism in which disturbing the inter-chain stacking of phenylalanines destabilizes β-sheet enriched in Aβ fibrils.Binding of ER at C-terminus region of Aβ40 was also obtained from a series of simulations with two ER molecules consistently.The bound ER blocks water molecules and thereby help stabilizing alpha-helical structure at the C-terminus.Regardless of the number of ER molecules used, the interruption of the formation of the salt-bridge between aspartic acid at position 23 and lysine at position 28was occurred in all systems.Our findings can help better understanding of the current and upcoming treatment studies for Alzheimers disease by suggesting inhibition mechanism of ER on the conformational transition of Aβ40 at the molecular level.